LT3758IDD#PBF - Linear Technology Corporation

LT3758/LT3758A

3758afd

ApplicAtions

n Automotive

n Telecom

n Industrial

n Wide Input Voltage Range: 5.5V to 100V

n Positive or Negative Output Voltage Programming

with a Single Feedback Pin

n Current Mode Control Provides Excellent Transient

Response

n Programmable Operating Frequency (100kHz to

1MHz) with One External Resistor

n Synchronizable to an External Clock

n Low Shutdown Current < 1µA

n Internal 7.2V Low Dropout Voltage Regulator

n Programmable Input Undervoltage Lockout with

Hysteresis

n Programmable Soft-Start

n Small 10-Lead DFN (3mm × 3mm) and

MSOPE Packages

typicAl ApplicAtion

Description

High Input Voltage,

Boost, Flyback, SEPIC and

Inverting Controller

The LT

3758/LT3758A are wide input range, current

mode, DC/DC controllers which are capable of generating

either positive or negative output voltages. They can be

configured as either a boost, flyback, SEPIC or inverting

converter. The LT3758/LT3758A drive a low side external

N-channel power MOSFET from an internal regulated 7.2V

supply. The fixed frequency, current-mode architecture

results in stable operation over a wide range of supply

and output voltages.

The operating frequency of LT3758/LT3758A can be set

with an external resistor over a 100kHz to 1MHz range,

and can be synchronized to an external clock using the

SYNC pin. A minimum operating supply voltage of 5.5V,

and a low shutdown quiescent current of less than 1µA,

make the LT3758/LT3758A ideally suited for battery-

powered systems.

The LT3758/LT3758A feature soft-start and frequency

foldback functions to limit inductor current during start-up

and output short-circuit. The LT3758A has improved load

transient performance compared to the LT3758.

12V Output Nonisolated Flyback Power Supply

FeAtures

SENSE

LT3758

VIN

DSN

VIN

36V TO

72V CIN

2.2µF

100V

X7R

63.4k

200kHz

GATE

FBX

GND INTVCC

SHDN/UVLO

SYNC

0.022µF

100V T1

1,2,3

(SERIES)

4,5,6

(PARALLEL)

44.2k

0.47µF

100pF 10k

10nF

0.030Ω 15.8k

105k

CVCC

4.7µF

10V

X5R

VOUT

12V

1.2A

3758 TA01

COUT

47µF

X5R

6.2k

5.1Ω

1N4148

L, LT , LT C , LT M , Linear Technology, the Linear logo and Burst Mode are registered trademarks

and No RSENSE and ThinSOT are trademarks of Linear Technology Corporation. All other

trademarks are the property of their respective owners. Patents pending.

LT3758/LT3758A

3758afd

pin conFigurAtion

Absolute MAxiMuM rAtings

VIN, SHDN/UVLO (Note 7) ......................................100V

INTVCC ....................................................VIN + 0.3V, 20V

GATE ........................................................ INTVCC + 0.3V

SYNC ..........................................................................8V

VC, SS .........................................................................3V

RT ............................................................................................... 1.5V

SENSE .................................................................... ±0.3V

FBX ................................................................. –6V to 6V

(Note 1)

TOP VIEW

DD PACKAGE

10-LEAD (3mm × 3mm) PLASTIC DFN

1VIN

SHDN/UVLO

INTVCC

GATE

SENSE

FBX

SYNC

TJMAX = 125°C, θJA = 43°C/W

EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB

FBX

SYNC

VIN

SHDN/UVLO

INTVCC

GATE

SENSE

TOP VIEW

MSE PACKAGE

10-LEAD PLASTIC MSOP

TJMAX = 150°C, θJA = 40°C/W

EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB

orDer inForMAtion

LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE

LT3758EDD#PBF LT3758EDD#TRPBF LDNK 10-Lead (3mm × 3mm) Plastic DFN –40°C to 125°C

LT3758IDD#PBF LT3758IDD#TRPBF LDNK 10-Lead (3mm × 3mm) Plastic DFN –40°C to 125°C

LT3758EMSE#PBF LT3758EMSE#TRPBF LTDNM 10-Lead (3mm × 3mm) Plastic MSOP –40°C to 125°C

LT3758IMSE #PBF LT3758IMSE#TRPBF LTDNM 10-Lead (3mm × 3mm) Plastic MSOP –40°C to 125°C

LT3758HMSE#PBF LT3758HMSE#TRPBF LTDNM 10-Lead (3mm × 3mm) Plastic MSOP –40°C to 150°C

LT3758MPMSE #PBF LT3758MPMSE#TRPBF LTDNM 10-Lead (3mm × 3mm) Plastic MSOP –55°C to 150°C

LT3758AEDD#PBF LT3758AEDD#TRPBF LGGS 10-Lead (3mm × 3mm) Plastic DFN –40°C to 125°C

LT3758AIDD#PBF LT3758AIDD#TRPBF LGGS 10-Lead (3mm × 3mm) Plastic DFN –40°C to 125°C

LT3758AEMSE#PBF LT3758AEMSE#TRPBF LTGGK 10-Lead (3mm × 3mm) Plastic MSOP –40°C to 125°C

LT3758AIMSE#PBF LT3758AIMSE#TRPBF LTGGK 10-Lead (3mm × 3mm) Plastic MSOP –40°C to 125°C

LT3758AHMSE#PBF LT3758AHMSE#TRPBF LTGGK 10-Lead (3mm × 3mm) Plastic MSOP –40°C to 150°C

LT3758AMPMSE#PBF LT3758AMPMSE#TRPBF LTGGK 10-Lead (3mm × 3mm) Plastic MSOP –55°C to 150°C

Consult LT C Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container.

For more information on lead free part marking, go to: http://www.linear.com/leadfree/

For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/

Operating Junction Temperature Range (Notes 2, 8)

LT3758E/LT3758AE ........................... –40°C to 125°C

LT3758I/LT3758AI ............................. –40°C to 125°C

LT3758H/LT3758AH .......................... –40°C to 150°C

LT3758MP/LT3758AMP ..................... –55°C to 150°C

Storage Temperature Range

DFN .................................................... –65°C to 125°C

MSOP ................................................ –65°C to 150°C

Lead Temperature (Soldering, 10 sec)

MSOP ............................................................... 300°C

LT3758/LT3758A

3758afd

electricAl chArActeristics

The l denotes the specifications which apply over the full operating temp-

erature range, otherwise specifications are at TA = 25°C. VIN = 24V, SHDN/UVLO = 24V, SENSE = 0V, unless otherwise noted.

PARAMETER CONDITIONS MIN TYP MAX UNITS

VIN Operating Range 5.5 100 V

VIN Shutdown IQSHDN/UVLO = 0V

SHDN/UVLO = 1.15V

0.1 1

µA

VIN Operating IQVC = 0.3V, RT = 41.2k 1.75 2.2 mA

VIN Operating IQ with Internal LDO Disabled VC = 0.3V, RT = 41.2k, INTVCC = 7.5V 350 400 µA

SENSE Current Limit Threshold l100 110 120 mV

SENSE Input Bias Current Current Out of Pin –65 µA

Error Amplifier

FBX Regulation Voltage (VFBX(REG)) FBX > 0V (Note 3)

FBX < 0V (Note 3)

1.569

–0.816

1.6

–0.800

1.631

–0.784

FBX Overvoltage Lockout FBX > 0V (Note 4)

FBX < 0V (Note 4)

FBX Pin Input Current FBX = 1.6V (Note 3)

FBX = –0.8V (Note 3)

–10

70 100

Transconductance gm (∆IVC /∆FBX) (Note 3) 230 µS

VC Output Impedance (Note 3) 5 MΩ

VFBX Line Regulation (∆VFBX/[∆VIN • VFBX(REG)]) FBX > 0V, 5.5V < VIN < 100V (Notes 3, 6)

FBX < 0V, 5.5V < VIN < 100V (Notes 3, 6)

0.006

0.005

0.025

0.03

%/V

VC Current Mode Gain (∆VVC /∆VSENSE) 5.5 V/V

VC Source Current VC = 1.5V –15 µA

VC Sink Current FBX = 1.7V

FBX = –0.85V

µA

Oscillator

Switching Frequency RT = 41.2k to GND, FBX = 1.6V

RT = 140k to GND, FBX = 1.6V

RT = 10.5k to GND, FBX = 1.6V

270 300

100

1000

330 kHz

kHz

RT Voltage FBX = 1.6V 1.2 V

Minimum Off-Time 220 ns

Minimum On-Time 220 ns

SYNC Input Low 0.4

SYNC Input High 1.5

SS Pull-Up Current SS = 0V, Current Out of Pin –10 µA

Low Dropout Regulator

INTVCC Regulation Voltage l7 7.2 7.4 V

INTVCC Undervoltage Lockout Threshold Falling INTVCC

UVLO Hysteresis

4.3 4.5

0.5

4.7 V

INTVCC Overvoltage Lockout Threshold 17.5 V

INTVCC Current Limit VIN = 100V

VIN = 20V

11 16

22 mA

INTVCC Load Regulation (∆VINTVCC / VINTVCC) 0 < IINTVCC < 10mA, VIN = 8V –1 –0.4 %

INTVCC Line Regulation (∆VINTVCC / [∆VIN • VINTVCC]) 8V < VIN < 100V 0.005 0.02 %/V

Dropout Voltage (VIN – VINTVCC) VIN = 6V, IINTVCC = 10mA 500 mV

LT3758/LT3758A

3758afd

electricAl chArActeristics

The l denotes the specifications which apply over the full operating temp-

erature range, otherwise specifications are at TA = 25°C. VIN = 24V, SHDN/UVLO = 24V, SENSE = 0V, unless otherwise noted.

PARAMETER CONDITIONS MIN TYP MAX UNITS

INTVCC Current in Shutdown SHDN/UVLO = 0V, INTVCC = 8V 16 µA

INTVCC Voltage to Bypass Internal LDO 7.5 V

Logic Inputs

SHDN/UVLO Threshold Voltage Falling VIN = INTVCC = 8V l1.17 1.22 1.27 V

SHDN/UVLO Input Low Voltage IVIN Drops Below 1µA 0.4 V

SHDN/UVLO Pin Bias Current Low SHDN/UVLO = 1.15V 1.7 2 2.5 µA

SHDN/UVLO Pin Bias Current High SHDN/UVLO = 1.33V 10 100 nA

Gate Driver

tr Gate Driver Output Rise Time CL = 3300pF (Note 5), INTVCC = 7.5V 22 ns

tf Gate Driver Output Fall Time CL = 3300pF (Note 5), INTVCC = 7.5V 20 ns

Gate Output Low (VOL) 0.05 V

Gate Output High (VOH) INTVCC

–0.05

Note 1: Stresses beyond those listed under Absolute Maximum Ratings

may cause permanent damage to the device. Exposure to any Absolute

Maximum Rating condition for extended periods may affect device

reliability and lifetime.

Note 2: The LT3758E/LT3758AE are guaranteed to meet performance

specifications from the 0°C to 125°C junction temperature. Specifications

over the –40°C to 125°C operating junction temperature range are

assured by design, characterization and correlation with statistical process

controls. The LT3758I/LT3758AI are guaranteed over the full –40°C to

125°C operating junction temperature range. The LT3758H/LT3758AH are

guaranteed over the full –40°C to 150°C operating junction temperature

range. High junction temperatures degrade operating lifetimes. Operating

lifetime is derated at junction temperatures greater than 125°C. The

LT3758MP/LT3758AMP are 100% tested and guaranteed over the full

–55°C to 150°C operating junction temperature range.

Note 3: The LT3758/LT3758A are tested in a feedback loop which servos

VFBX to the reference voltages (1.6V and –0.8V) with the VC pin forced

to 1.3V.

Note 4: FBX overvoltage lockout is measured at VFBX(OVERVOLTAGE) relative

to regulated VFBX(REG).

Note 5: Rise and fall times are measured at 10% and 90% levels.

Note 6: SHDN/UVLO = 1.33V when VIN = 5.5V.

Note 7: For VIN below 6V, the SHDN/UVLO pin must not exceed VIN.

Note 8: The LT3758/LT3758A include overtemperature protection that

is intended to protect the device during momentary overload conditions.

Junction temperature will exceed the maximum operating junction

temperature when overtemperature protection is active. Continuous

operation above the specified maximum operating junction temperature

may impair device reliability.

LT3758/LT3758A

3758afd

TEMPERATURE (°C)

–75 –50

1580

1585

REGULATED FEEDBACK VOLTAGE (V)

1590

1605

1600

0 50 75

1595

–25 25 100 150125

3758 G01

VIN = 100V

VIN = 24V

VIN = 8V

VIN = INTVCC = 5.5V

SHDN/UVLO = 1.33V

TEMPERATURE (°C)

REGULATED FEEDBACK VOLTAGE (mV)

–802

–800

–798

–790

–792

–794

–804

–796

3758 G02

–75 –50 0 50 75–25 25 100 150125

VIN = 100V

VIN = 24V

VIN = 8V

VIN = INTVCC = 5.5V

SHDN/UVLO = 1.33V

typicAl perForMAnce chArActeristics

Positive Feedback Voltage

vs Temperature, VIN

Negative Feedback Voltage

vs Temperature, VIN

Quiescent Current

vs Temperature, VIN

TA = 25°C, unless otherwise noted.

–75 –50 0 50 75–25 25 100 150125

TEMPERATURE (°C)

1.5

QUIESCENT CURRENT (mA)

1.6

1.9

1.8

1.7

3758 G03

VIN = 100V

VIN = 24V

VIN = INTVCC = 5.5V

Dynamic Quiescent Current

vs Switching Frequency

RT vs Switching Frequency

Normalized Switching

Frequency vs FBX

FBX VOLTAGE (V)

–0.8

NORMALIZED FREQUENCY (%)

120

–0.4 0 0.4 0.8

3758 G06

1.2 1.6

100

SWITCHING FREQUENCY (kHz)

IQ(mA)

300 500 600 700

100 200 400 900800 1000

3758 G04

CGATE = 3300pF

SWITCHING FREQUENCY (kHz)

RT (kΩ)

100

1000

300 500 600 700

100 200 400 900800 1000

3758 G05

LT3758/LT3758A

3758afd

Switching Frequency

vs Temperature

SENSE Current Limit Threshold

vs Temperature

SENSE Current Limit Threshold

vs Duty Cycle

SHDN/UVLO Threshold

vs Temperature

SHDN/UVLO Current vs Voltage

SHDN/UVLO Hysteresis Current

vs Temperature

–75 –50 0 50 75–25 25 100 150125

TEMPERATURE (°C)

100

SENSE THRESHOLD (mV)

105

110

115

120

3758 G08

DUTY CYCLE (%)

SENSE THRESHOLD (mV)

105

20 40 8060

115

100

110

100

3758 G09

SHDN/UVLO VOLTAGE (V)

SHDN/UVLO CURRENT (µA)

20 6040 80

100

3758 G11

–75 –50 0 50 75–25 25 100 150125

TEMPERATURE (°C)

1.6

SHDN/ UVLO

(µA)

1.8

2.0

2.2

2.4

3758 G12

–75 –50 0 50 75–25 25 100 150125

TEMPERATURE (°C)

270

SWITCHING FREQUENCY (kHz)

280

290

300

310

330

3758 G07

320

RT = 41.2K

–75 –50 0 50 75–25 25 100 150125

TEMPERATURE (°C)

1.18

SHDN/UVLO VOLTAGE (V)

1.22

1.24

1.26

1.28

1.20

3758 G10

SHDN/UVLO FALLING

SHDN/UVLO RISING

typicAl perForMAnce chArActeristics

TA = 25°C, unless otherwise noted.

LT3758/LT3758A

3758afd

typicAl perForMAnce chArActeristics

TA = 25°C, unless otherwise noted.

INTVCC Line Regulation

INTVCC Dropout Voltage

vs Current, Temperature

INTVCC vs Temperature

INTVCC Minimum Output

Current vs VIN INTVCC Load Regulation

–75 –50 0 50 75–25 25 100 150125

TEMPERATURE (°C)

7.0

INTV

(V)

7.1

7.2

7.3

7.4

3758 G13

VIN (V)

INTV

VOLTAGE (V)

7.25

7.20

20 30 5010 40 60 70 80 100

7.15

7.10

7.30

3758 G16

10 100

TJ = 150°C

VIN (V)

3758 G14

INTV

CURRENT (mA)

INTVCC = 6V

INTVCC = 4.7V

INTVCC LOAD (mA)

6.8

7.1

7.2

7.3

10 20 25

6.9

515

3758 G15

INTVCC VOLTAGE (V)

VIN = 8V

0 42 6 8 10

INTVCC LOAD (mA)

DROPOUT VOLTAGE (mV)

500

600

300

400

200

100

1000

900

800

700

3758 G17

150°C

25°C

0°C

–55°C

75°C

VIN = 6V

125°C

Gate Drive Rise

and Fall Time vs INTVCC Typical Start-Up Waveforms

FBX Frequency Foldback

Waveforms During Overcurrent

INTVCC (V)

TIME (ns)

612 15

3758 G19

CL = 3300pF

RISE TIME

FALL TIME

2ms/DIV

SEE TYPICAL APPLICATION: 18V TO 72V INPUT,

24V OUTPUT SEPIC CONVERTER

VOUT

10V/DIV

IL1A + IL1B

1A/DIV

3758 G20

VIN = 48V

50µs/DIV

VOUT

20V/DIV

VSW

50V/DIV

IL1A + IL1B

2A/DIV

3758 G21

VIN = 48V

SEE TYPICAL APPLICATION: 18V TO 72V INPUT,

24V OUTPUT SEPIC CONVERTER

Gate Drive Rise

and Fall Time vs CL

CL (nF)

TIME (ns)

5 1510 20 25 30

3758 G18

RISE TIME

INTVCC = 7.2V

FALL TIME

LT3758/LT3758A

3758afd

pin Functions

VC (Pin 1): Error Amplifier Compensation Pin. Used to

stabilize the voltage loop with an external RC network.

FBX (Pin 2): Positive and Negative Feedback Pin. Re-

ceives the feedback voltage from the external resistor

divider across the output. Also modulates the switching

frequency during start-up and fault conditions when FBX

is close to GND.

SS (Pin 3): Soft-Start Pin. This pin modulates compensation

pin voltage (VC) clamp. The soft-start interval is set with

an external capacitor. The pin has a 10µA (typical) pull-up

current source to an internal 2.5V rail. The soft-start pin

is reset to GND by an undervoltage condition at SHDN/

UVLO, an INTVCC undervoltage or overvoltage condition

or an internal thermal lockout.

RT (Pin 4): Switching Frequency Adjustment Pin. Set the

frequency using a resistor to GND. Do not leave this pin

open.

SYNC (Pin 5): Frequency Synchronization Pin. Used to

synchronize the switching frequency to an outside clock.

If this feature is used, an RT resistor should be chosen to

program a switching frequency 20% slower than the SYNC

pulse frequency. Tie the SYNC pin to GND if this feature is

not used. SYNC is bypassed when FBX is close to GND.

SENSE (Pin 6): The Current Sense Input for the Control

Loop. Kelvin connect this pin to the positive terminal of

the switch current sense resistor in the source of the NFET.

The negative terminal of the current sense resistor should

be connected to GND plane close to the IC.

GATE (Pin 7): N-Channel MOSFET Gate Driver Output.

Switches between INTVCC and GND. Driven to GND when

IC is shut down, during thermal lockout or when INTVCC

is above or below the overvoltage or UV thresholds,

respectively.

INTVCC (Pin 8): Regulated Supply for Internal Loads and

Gate Driver. Supplied from VIN and regulated to 7.2V (typi-

cal). INTVCC must be bypassed with a minimum of 4.7µF

capacitor placed close to pin. INTVCC can be connected

directly to VIN, if VIN is less than 17.5V. INTVCC can also

be connected to a power supply whose voltage is higher

than 7.5V, and lower than VIN, provided that supply does

not exceed 17.5V.

SHDN/UVLO (Pin 9): Shutdown and Undervoltage Detect

Pin. An accurate 1.22V (nominal) falling threshold with

externally programmable hysteresis detects when power

is okay to enable switching. Rising hysteresis is generated

by the external resistor divider and an accurate internal

2µA pull-down current. An undervoltage condition resets

sort-start. Tie to 0.4V, or less, to disable the device and

reduce VIN quiescent current below 1µA.

VIN (Pin 10): Input Supply Pin. Must be locally bypassed

with a 0.22µF, or larger, capacitor placed close to the pin.

Exposed Pad (Pin 11): Ground. This pin also serves as the

negative terminal of the current sense resistor. The exposed

pad must be soldered directly to the local ground plane.

LT3758/LT3758A

3758afd

block DiAgrAM

Figure 1. LT3758 Block Diagram Working as a SEPIC Converter

R3R4

FBX

1.22V

2.5V

CDC

CIN

VOUT

COUT2

COUT1

CVCC

INTVCC

VIN

RSENSE

VISENSE

•+

VIN

IS1

2µA

SHDN/UVLO

INTERNAL

REGULATOR

AND UVLO

TSD

165˚C

A10

17.5V

5V UP

4.5V DOWN

UVLO

IS2

10µA

IS3

CC1

CC2

DRIVER

SLOPE

SENSE

GND

GATE

110mV

SR1

–

CURRENT

LIMIT

RAMP

GENERATOR

7.2V LDO

•

–

R O

2.5V

CSS

SYNC

1.28V

1.2V

FBX

1.6V

–0.8V

–

3 5 4

–

RAMP

PWM

COMPARATOR

FREQUENCY

FOLDBACK

100kHz-1MHz

OSCILLATOR

FREQ

PROG

3758 F01

–

+Q1

1.72V

–0.88V

–

A11

A12

–

G4 G3

8k D3

LT3758/LT3758A

3758afd

ApplicAtions inForMAtion

Main Control Loop

The LT3758 uses a fixed frequency, current mode control

scheme to provide excellent line and load regulation. Op-

eration can be best understood by referring to the Block

Diagram in Figure 1.

The start of each oscillator cycle sets the SR latch (SR1) and

turns on the external power MOSFET switch M1 through

driver G2. The switch current flows through the external

current sensing resistor RSENSE and generates a voltage

proportional to the switch current. This current sense

voltage VISENSE (amplified by A5) is added to a stabilizing

slope compensation ramp and the resulting sum (SLOPE)

is fed into the positive terminal of the PWM comparator A7.

When SLOPE exceeds the level at the negative input of A7

(VC pin), SR1 is reset, turning off the power switch. The

level at the negative input of A7 is set by the error amplifier

A1 (or A2) and is an amplified version of the difference

between the feedback voltage (FBX pin) and the reference

voltage (1.6V or –0.8V, depending on the configuration).

In this manner, the error amplifier sets the correct peak

switch current level to keep the output in regulation.

The LT3758 has a switch current limit function. The current

sense voltage is input to the current limit comparator A6.

If the SENSE pin voltage is higher than the sense current

limit threshold VSENSE(MAX) (110mV, typical), A6 will reset

SR1 and turn off M1 immediately.

The LT3758 is capable of generating either positive or

negative output voltage with a single FBX pin. It can be

configured as a boost, flyback or SEPIC converter to gen-

erate positive output voltage, or as an inverting converter

to generate negative output voltage. When configured as

a SEPIC converter, as shown in Figure 1, the FBX pin is

pulled up to the internal bias voltage of 1.6V by a volt-

age divider (R1 and R2) connected from VOUT to GND.

Comparator A2 becomes inactive and comparator A1

performs the inverting amplification from FBX to VC. When

the LT3758 is in an inverting configuration, the FBX pin

is pulled down to –0.8V by a voltage divider connected

from VOUT to GND. Comparator A1 becomes inactive and

comparator A2 performs the noninverting amplification

from FBX to VC.

The LT3758 has overvoltage protection functions to

protect the converter from excessive output voltage

overshoot during start-up or recovery from a short-circuit

condition. An overvoltage comparator A11 (with 20mV

hysteresis) senses when the FBX pin voltage exceeds the

positive regulated voltage (1.6V) by 8% and provides a

reset pulse. Similarly, an overvoltage comparator A12

(with 10mV hysteresis) senses when the FBX pin voltage

exceeds the negative regulated voltage (–0.8V) by 11%

and provides a reset pulse. Both reset pulses are sent to

the main RS latch (SR1) through G6 and G5. The power

MOSFET switch M1 is actively held off for the duration of

an output overvoltage condition.

Programming Turn-On and Turn-Off Thresholds with

the SHDN/UVLO Pin

The SHDN/UVLO pin controls whether the LT3758 is

enabled or is in shutdown state. A micropower 1.22V

reference, a comparator A10 and a controllable current

source IS1 allow the user to accurately program the supply

voltage at which the IC turns on and off. The falling value

can be accurately set by the resistor dividers R3 and R4.

When SHDN/UVLO is above 0.4V, and below the 1.22V

threshold, the small pull-down current source IS1 (typical

2µA) is active.

The purpose of this current is to allow the user to program

the rising hysteresis.

The Block Diagram of the comparator

and the external resistors is shown in Figure 1. The typical

falling threshold voltage and rising threshold voltage can

be calculated by the following equations:

VVIN,FALLING =1.22 •

(R3 +R4)

VVIN,RISING =2µA •R3+VIN,FALLING

For applications where the SHDN/UVLO pin is only used

as a logic input, the SHDN/UVLO pin can be connected

directly to the input voltage VIN through a 1k resistor for

always-on operation.

LT3758/LT3758A

3758afd

ApplicAtions inForMAtion

INTVCC Regulator Bypassing and Operation

An internal, low dropout (LDO) voltage regulator produces

the 7.2V INTVCC supply which powers the gate driver, as

shown in Figure 1. The LT3758 contains an undervoltage

lockout comparator A8 and an overvoltage lockout com-

parator A9 for the INTVCC supply. The INTVCC undervoltage

(UV) threshold is 4.5V (typical), with 0.5V hysteresis, to

ensure that the MOSFETs have sufficient gate drive voltage

before turning on. The logic circuitry within the LT3758 is

also powered from the internal INTVCC supply.

The INTVCC overvoltage threshold is set to be 17.5V

(typical) to protect the gate of the power MOSFET. When

INTVCC is below the UV threshold, or above the overvolt-

age threshold, the GATE pin will be forced to GND and the

soft-start operation will be triggered.

The INTVCC regulator must be bypassed to ground im-

mediately adjacent to the IC pins with a minimum of 4.7µF

ceramic capacitor. Good bypassing is necessary to supply

the high transient currents required by the MOSFET gate

driver.

In an actual application, most of the IC supply current is

used to drive the gate capacitance of the power MOSFET.

The on-chip power dissipation can be a significant concern

when a large power MOSFET is being driven at a high fre-

quency and the VIN voltage is high. It is important to limit

the power dissipation through selection of MOSFET and/

or operating frequency so the LT3758 does not exceed its

maximum junction temperature rating. The junction tem-

perature TJ can be estimated using the following equations:

TJ = TA + PIC • θJA

TA = ambient temperature

θJA = junction-to-ambient thermal resistance

PIC = IC power consumption

= VIN • (IQ + IDRIVE)

IQ = VIN operation IQ = 1.6mA

IDRIVE = average gate drive current = f • QG

f = switching frequency

QG = power MOSFET total gate charge

The LT3758 uses packages with an Exposed Pad for en-

hanced thermal conduction. With proper soldering to the

Exposed Pad on the underside of the package and a full

copper plane underneath the device, thermal resistance

(θJA) will be about 43°C/W for the DD package and 40°C/W

for the MSE package. For an ambient board temperature of

TA = 70°C and maximum junction temperature of 125°C,

the maximum IDRIVE (IDRIVE(MAX)) of the DD package can

be calculated as:

IDRIVE(MAX) =(TJ

−

TA)

(θJA •VIN )−IQ=1.28W

VIN

−1.6mA

The LT3758 has an internal INTVCC IDRIVE current limit

function to protect the IC from excessive on-chip power

dissipation. The IDRIVE current limit decreases as the VIN

increases (see the INTVCC Minimum Output Current vs VIN

graph in the Typical Performance Characteristics section).

If IDRIVE reaches the current limit, INTVCC voltage will fall

and may trigger the soft-start.

Based on the preceding equation and the INTVCC Minimum

Output Current vs VIN graph, the user can calculate the

maximum MOSFET gate charge the LT3758 can drive at

a given VIN and switch frequency. A plot of the maximum

QG vs VIN at different frequencies to guarantee a minimum

4.7V INTVCC is shown in Figure 2.

Figure 2. Recommended Maximum QG vs VIN at Different

Frequencies to Ensure INTVCC Higher Than 4.7V

VIN (V)

QG (nC)

10 100

3758 F02

120

140

100

300kHz

1MHz

LT3758/LT3758A

3758afd

ApplicAtions inForMAtion

As illustrated in Figure 2, a trade-off between the operating

frequency and the size of the power MOSFET may be needed

in order to maintain a reliable IC junction temperature.

Prior to lowering the operating frequency, however, be

sure to check with power MOSFET manufacturers for their

most recent low QG, low RDS(ON) devices. Power MOSFET

manufacturing technologies are continually improving, with

newer and better performance devices being introduced

almost yearly.

An effective approach to reduce the power consumption

of the internal LDO for gate drive is to tie the INTVCC pin

to an external voltage source high enough to turn off the

internal LDO regulator.

If the input voltage VIN does not exceed the absolute

maximum rating of both the power MOSFET gate-source

voltage (VGS) and the INTVCC overvoltage lockout threshold

voltage (17.5V), the INTVCC pin can be shorted directly

to the VIN pin. In this condition, the internal LDO will be

turned off and the gate driver will be powered directly

from the input voltage VIN. With the INTVCC pin shorted to

VIN, however, a small current (around 16µA) will load the

INTVCC in shutdown mode. For applications that require

the lowest shutdown mode input supply current, do not

connect the INTVCC pin to VIN.

In SEPIC or flyback applications, the INTVCC pin can be

connected to the output voltage VOUT through a blocking

diode, as shown in Figure 3, if VOUT meets the following

conditions:

1. VOUT < VIN (pin voltage)

2. VOUT < 17.5V

3. VOUT < maximum VGS rating of power MOSFET

A resistor RVCC can be connected, as shown in Figure 3, to

limit the inrush current from VOUT. Regardless of whether

or not the INTVCC pin is connected to an external voltage

source, it is always necessary to have the driver circuitry

bypassed with a 4.7µF low ESR ceramic capacitor to

ground immediately adjacent to the INTVCC and GND pins.

Operating Frequency and Synchronization

The choice of operating frequency may be determined

by on-chip power dissipation, otherwise it is a trade-off

between efficiency and component size. Low frequency

operation improves efficiency by reducing gate drive cur-

rent and MOSFET and diode switching losses. However,

lower frequency operation requires a physically larger

inductor. Switching frequency also has implications for

loop compensation. The LT3758 uses a constant-frequency

architecture that can be programmed over a 100kHz to

1000kHz range with a single external resistor from the

RT pin to ground, as shown in Figure 1. The RT pin must

have an external resistor to GND for proper operation of

the LT3758. A table for selecting the value of RT for a given

operating frequency is shown in Table 1.

Table 1. Timing Resistor (RT) Value

SWITCHING FREQUENCY (kHz) RT (kΩ)

100 140

200 63.4

300 41.2

400 30.9

500 24.3

600 19.6

700 16.5

800 14

900 12.1

1000 10.5

The operating frequency of the LT3758 can be synchronized

to an external clock source. By providing a digital clock

signal into the SYNC pin, the LT3758 will operate at the

SYNC clock frequency. If this feature is used, an RT resistor

should be chosen to program a switching frequency 20%

slower than SYNC pulse frequency. It is recommended the

SYNC pulse have a minimum pulse width of 200ns. Tie

the SYNC pin to GND if this feature is not used.

Figure 3. Connecting INTVCC to VOUT

CVCC

4.7µF

VOUT

3758 F03

INTVCC

GND

LT3758 RVCC

DVCC

LT3758/LT3758A

3758afd

ApplicAtions inForMAtion

Duty Cycle Consideration

Switching duty cycle is a key variable defining converter

operation. As such, its limits must be considered. Minimum

on-time is the smallest time duration that the LT3758 is

capable of turning on the power MOSFET. This time is

generally about 220ns (typical) (see Minimum On-Time

in the Electrical Characteristics table). In each switching

cycle, the LT3758 keeps the power switch off for at least

220ns (typical) (see Minimum Off-Time in the Electrical

Characteristics table).

The minimum on-time and minimum off-time and the

switching frequency define the minimum and maximum

switching duty cycles a converter is able to generate:

Minimum duty cycle = minimum on-time • frequency

Maximum duty cycle = 1 – (minimum off-time • frequency)

Programming the Output Voltage

The output voltage VOUT is set by a resistor divider, as

shown in Figure 1. The positive and negative VOUT are set

by the following equations:

VOUT,POSITIVE =1.6V •1+R2











VOUT,NEGATIVE =–0.8V •1+R2











The resistors R1 and R2 are typically chosen so that

the error caused by the current flowing into the FBX pin

during normal operation is less than 1% (this translates

to a maximum value of R1 at about 158k).

In the applications where VOUT is pulled up by an external

positive power supply, the FBX pin is also pulled up through

the R2 and R1 network. Make sure the FBX does not exceed

its absolute maximum rating (6V). The R5, D2, and D3 in

Figure 1 provide a resistive clamp in the positive direction.

To ensure FBX is lower than 6V, choose sufficiently large

R1 and R2 to meet the following condition:

6V •1+R2









+3.5V •R2

8kΩ>VOUT(MAX)

where VOUT(MAX) is the maximum VOUT that is pulled up

by an external power supply.

Soft-Start

The LT3758 contains several features to limit peak switch

currents and output voltage (VOUT) overshoot during

start-up or recovery from a fault condition. The primary

purpose of these features is to prevent damage to external

components or the load.

High peak switch currents during start-up may occur in

switching regulators. Since VOUT is far from its final value,

the feedback loop is saturated and the regulator tries to

charge the output capacitor as quickly as possible, resulting

in large peak currents. A large surge current may cause

inductor saturation or power switch failure.

The LT3758 addresses this mechanism with the SS pin.

As shown in Figure 1, the SS pin reduces the power

MOSFET current by pulling down the VC pin through

Q2. In this way the SS allows the output capacitor to

charge gradually toward its final value while limiting the

start-up peak currents. The typical start-up waveforms

are shown in the Typical Performance Characteristics

section. The inductor current IL slewing rate is limited by

the soft-start function.

Besides start-up (with SHDN/UVLO), soft-start can also

be triggered by the following faults:

1. INTVCC > 17.5V

2. INTVCC < 4.5V

3. Thermal lockout

Any of these three faults will cause the LT3758 to stop

switching immediately. The SS pin will be discharged by

Q3. When all faults are cleared and the SS pin has been

discharged below 0.2V, a 10µA current source IS2 starts

charging the SS pin, initiating a soft-start operation.

The soft-start interval is set by the soft-start capacitor

selection according to the equation:

TSS =CSS •1.25V

10µA

LT3758/LT3758A

3758afd

ApplicAtions inForMAtion

FBX Frequency Foldback

When VOUT is very low during start-up or a GND fault on

the output, the switching regulator must operate at low

duty cycles to maintain the power switch current within

the current limit range, since the inductor current decay

rate is very low during switch off time. The minimum on-

time limitation may prevent the switcher from attaining a

sufficiently low duty cycle at the programmed switching

frequency. So, the switch current will keep increasing

through each switch cycle, exceeding the programmed

current limit. To prevent the switch peak currents from

exceeding the programmed value, the LT3758 contains

a frequency foldback function to reduce the switching

frequency when the FBX voltage is low (see the Normal-

ized Switching Frequency vs FBX graph in the Typical

Performance Characteristics section).

During frequency foldback, external clock synchroniza-

tion is disabled to prevent interference with frequency

reducing operation.

Thermal Lockout

If LT3758 die temperature reaches 165°C (typical), the

part will go into thermal lockout. The power switch will

be turned off. A soft-start operation will be triggered. The

part will be enabled again when the die temperature has

dropped by 5°C (nominal).

Loop Compensation

Loop compensation determines the stability and transient

performance. The LT3758/LT3758A use current mode

control to regulate the output which simplifies loop com-

pensation. The LT3758A improves the no-load to heavy

load transient response, when compared to the LT3758.

New internal circuits ensure that the transient from not

switching to switching at high current can be made in a

few cycles. The optimum values depend on the converter

topology, the component values and the operating condi-

tions (including the input voltage, load current, etc.). To

compensate the feedback loop of the LT3758/LT3758A,

a series resistor-capacitor network is usually connected

from the VC pin to GND. Figure 1 shows the typical VC

compensation network. For most applications, the capacitor

should be in the range of 470pF to 22nF, and the resistor

should be in the range of 5k to 50k. A small capacitor is

often connected in parallel with the RC compensation

network to attenuate the VC voltage ripple induced from the

output voltage ripple through the internal error amplifier.

The parallel capacitor usually ranges in value from 10pF to

100pF. A practical approach to design the compensation

network is to start with one of the circuits in this data sheet

that is similar to your application, and tune the compensa-

tion network to optimize the performance. Stability should

then be checked across all operating conditions, including

load current, input voltage and temperature.

SENSE Pin Programming

For control and protection, the LT3758 measures the

power MOSFET current by using a sense resistor (RSENSE)

between GND and the MOSFET source. Figure 4 shows a

typical waveform of the sense voltage (VSENSE) across the

sense resistor. It is important to use Kelvin traces between

the SENSE pin and RSENSE, and to place the IC GND as

close as possible to the GND terminal of the RSENSE for

proper operation.

Figure 4. The Sense Voltage During a Switching Cycle

3758 F04

VSENSE(PEAK)

∆VSENSE = χ • VSENSE(MAX)

VSENSE

DTS

VSENSE(MAX)

LT3758/LT3758A

3758afd

ApplicAtions inForMAtion

Due to the current limit function of the SENSE pin, RSENSE

should be selected to guarantee that the peak current sense

voltage VSENSE(PEAK) during steady state normal operation

is lower than the SENSE current limit threshold (see the

Electrical Characteristics table). Given a 20% margin,

VSENSE(PEAK) is set to be 80mV. Then, the maximum

switch ripple current percentage can be calculated using

the following equation:

c = ∆VSENSE

80mV −0.5 •∆VSENSE

c is used in subsequent design examples to calculate in-

ductor value. ∆VSENSE is the ripple voltage across RSENSE.

The LT3758 switching controller incorporates 100ns timing

interval to blank the ringing on the current sense signal

immediately after M1 is turned on. This ringing is caused

by the parasitic inductance and capacitance of the PCB

trace, the sense resistor, the diode, and the MOSFET. The

100ns timing interval is adequate for most of the LT3758

applications. In the applications that have very large and

long ringing on the current sense signal, a small RC filter

can be added to filter out the excess ringing. Figure 5

shows the RC filter on the SENSE pin. It is usually suf-

ficient to choose 22Ω for RF LT and 2.2nF to 10nF for CFLT.

Keep RF LT ’s resistance low. Remember that there is 65µA

(typical) flowing out of the SENSE pin. Adding RF LT will

affect the SENSE current limit threshold:

VSENSE_ILIM = 110mV – 65µA • RF LT

APPLICATION CIRCUITS

The LT3758 can be configured as different topologies. The

first topology to be analyzed will be the boost converter,

followed by the flyback, SEPIC and inverting converters.

Boost Converter: Switch Duty Cycle and Frequency

The LT3758 can be configured as a boost converter for

the applications where the converter output voltage is

higher than the input voltage. Remember that boost con-

verters are not short-circuit protected. Under a shorted

output condition, the inductor current is limited only by

the input supply capability. For applications requiring a

step-up converter that is short-circuit protected, please

refer to the Applications Information section covering

SEPIC converters.

The conversion ratio as a function of duty cycle is:

VOUT

VIN

1−D

in continuous conduction mode (CCM).

For a boost converter operating in CCM, the duty cycle

of the main switch can be calculated based on the output

voltage (VOUT) and the input voltage (VIN). The maximum

duty cycle (DMAX) occurs when the converter has the

minimum input voltage:

DMAX =VOUT −VIN(MIN)

VOUT

Discontinuous conduction mode (DCM) provides higher

conversion ratios at a given frequency at the cost of reduced

efficiencies and higher switching currents.

Boost Converter: Inductor and Sense Resistor Selection

For the boost topology, the maximum average inductor

current is:

IL(MAX) =IO(MAX) •1

1−DMAX

Then, the ripple current can be calculated by:

∆IL=c•IL(MAX) =c•IO(MAX) •1

1−DMAX

Figure 5. The RC Filter on the SENSE Pin

CFLT

3758 F05

LT3758

RFLT

RSENSE

SENSE

GATE

GND

LT3758/LT3758A

3758afd

ApplicAtions inForMAtion

The constant c in the preceding equation represents the

percentage peak-to-peak ripple current in the inductor,

relative to IL(MAX).

The inductor ripple current has a direct effect on the choice

of the inductor value. Choosing smaller values of ∆IL

requires large inductances and reduces the current loop

gain (the converter will approach voltage mode). Accepting

larger values of ∆IL provides fast transient response and

allows the use of low inductances, but results in higher input

current ripple and greater core losses. It is recommended

that c fall within the range of 0.2 to 0.6.

Given an operating input voltage range, and having chosen

the operating frequency and ripple current in the inductor,

the inductor value of the boost converter can be determined

using the following equation:

L=VIN(MIN)

∆IL•f•DMAX

The peak and RMS inductor current are:

IL(PEAK) =IL(MAX) •1+c











IL(RMS) =IL(MAX) •1+c2

Based on these equations, the user should choose the

inductors having sufficient saturation and RMS current

ratings.

Set the sense voltage at IL(PEAK) to be the minimum of the

SENSE current limit threshold with a 20% margin. The

sense resistor value can then be calculated to be:

RSENSE =80mV

IL(PEAK)

Boost Converter: Power MOSFET Selection

Important parameters for the power MOSFET include the

drain-source voltage rating (VDS), the threshold voltage

(VGS(TH)), the on-resistance (RDS(ON)), the gate to source

and gate to drain charges (QGS and QGD), the maximum

drain current (ID(MAX)) and the MOSFET’s thermal

resistances (RθJC and RθJA).

The power MOSFET will see full output voltage, plus a

diode forward voltage, and any additional ringing across

its drain-to-source during its off-time. It is recommended

to choose a MOSFET whose BVDSS is higher than VOUT by

a safety margin (a 10V safety margin is usually sufficient).

The power dissipated by the MOSFET in a boost converter is:

PFET = I2L(MAX) • RDS(ON) • DMAX + 2 • V2OUT • IL(MAX)

• CRSS • f/1A

The first term in the preceding equation represents the

conduction losses in the device, and the second term, the

switching loss. CRSS is the reverse transfer capacitance,

which is usually specified in the MOSFET characteristics.

For maximum efficiency, RDS(ON) and CRSS should be

minimized. From a known power dissipated in the power

MOSFET, its junction temperature can be obtained using

the following equation:

TJ = TA + PFET • θJA = TA + PFET • (θJC + θCA)

TJ must not exceed the MOSFET maximum junction

temperature rating. It is recommended to measure the

MOSFET temperature in steady state to ensure that absolute

maximum ratings are not exceeded.

Boost Converter: Output Diode Selection

To maximize efficiency, a fast switching diode with low

forward drop and low reverse leakage is desirable. The

peak reverse voltage that the diode must withstand is

equal to the regulator output voltage plus any additional

ringing across its anode-to-cathode during the on-time.

The average forward current in normal operation is equal

to the output current, and the peak current is equal to:

ID(PEAK) =IL(PEAK) =1+c









•IL(MAX)

It is recommended that the peak repetitive reverse voltage

rating VRRM is higher than VOUT by a safety margin (a 10V

safety margin is usually sufficient).

The power dissipated by the diode is:

PD = IO(MAX) • VD

and the diode junction temperature is:

TJ = TA + PD • RθJA

LT3758/LT3758A

3758afd

ApplicAtions inForMAtion

Figure 6. The Output Ripple Waveform of a Boost Converter

VOUT

(AC)

tON

∆VESR

RINGING DUE TO

TOTAL INDUCTANCE

(BOARD + CAP)

∆VCOUT

3758 F06

tOFF

The output capacitor in a boost regulator experiences high

RMS ripple currents, as shown in Figure 6. The RMS ripple

current rating of the output capacitor can be determined

using the following equation:

IRMS(COUT) ≥IO(MAX) •DMAX

1−DMAX

Multiple capacitors are often paralleled to meet ESR

requirements. Typically, once the ESR requirement is

satisfied, the capacitance is adequate for filtering and has

the required RMS current rating. Additional ceramic capaci-

tors in parallel are commonly used to reduce the effect of

parasitic inductance in the output capacitor, which reduces

high frequency switching noise on the converter output.

Boost Converter: Input Capacitor Selection

The input capacitor of a boost converter is less critical

than the output capacitor, due to the fact that the inductor

is in series with the input, and the input current wave-

form is continuous. The input voltage source impedance

determines the size of the input capacitor, which is typi-

cally in the range of 10µF to 100µF. A low ESR capacitor

is recommended, although it is not as critical as for the

output capacitor.

The RMS input capacitor ripple current for a boost con-

verter is:

IRMS(CIN) = 0.3 • ∆IL

FLYBACK CONVERTER APPLICATIONS

The LT3758 can be configured as a flyback converter

for the applications where the converters have multiple

outputs, high output voltages or isolated outputs. Figure

7 shows a simplified flyback converter.

The flyback converter has a very low parts count for mul-

tiple outputs, and with prudent selection of turns ratio, can

have high output/input voltage conversion ratios with a

desirable duty cycle. However, it has low efficiency due to

the high peak currents, high peak voltages and consequent

power loss. The flyback converter is commonly used for

an output power of less than 50W.

The RθJA to be used in this equation normally includes the

RθJC for the device plus the thermal resistance from the

board to the ambient temperature in the enclosure. TJ must

not exceed the diode maximum junction temperature rating.

Boost Converter: Output Capacitor Selection

Contributions of ESR (equivalent series resistance), ESL

(equivalent series inductance) and the bulk capacitance

must be considered when choosing the correct output

capacitors for a given output ripple voltage. The effect of

these three parameters (ESR, ESL and bulk C) on the output

voltage ripple waveform for a typical boost converter is

illustrated in Figure 6.

The choice of component(s) begins with the maximum

acceptable ripple voltage (expressed as a percentage of

the output voltage), and how this ripple should be divided

between the ESR step ∆VESR and the charging/discharg-

ing ∆VCOUT. For the purpose of simplicity, we will choose

2% for the maximum output ripple, to be divided equally

between ∆VESR and ∆VCOUT. This percentage ripple will

change, depending on the requirements of the applica-

tion, and the following equations can easily be modified.

For a 1% contribution to the total ripple voltage, the ESR

of the output capacitor can be determined using the fol-

lowing equation:

ESRCOUT ≤0.01•VOUT

ID(PEAK)

For the bulk C component, which also contributes 1% to

the total ripple:

COUT ≥IO(MAX)

0.01•VOUT •f

LT3758/LT3758A

3758afd

ApplicAtions inForMAtion

The flyback converter can be designed to operate either

in continuous or discontinuous mode. Compared to con-

tinuous mode, discontinuous mode has the advantage of

smaller transformer inductances and easy loop compen-

sation, and the disadvantage of higher peak-to-average

current and lower efficiency.

to the number of variables involved. The user can choose

either a duty cycle or a turns ratio as the start point. The

following trade-offs should be considered when select-

ing the switch duty cycle or turns ratio, to optimize the

converter performance. A higher duty cycle affects the

flyback converter in the following aspects:

• Lower MOSFET RMS current ISW(RMS), but higher

MOSFET VDS peak voltage

• Lower diode peak reverse voltage, but higher diode

RMS current ID(RMS)

• Higher transformer turns ratio (NP/NS)

The choice,

D+D2 =1

(for discontinuous mode operation with a given D3) gives

the power MOSFET the lowest power stress (the product

of RMS current and peak voltage). The choice,

D+D2 =2

(for discontinuous mode operation with a given D3) gives

the diode the lowest power stress (the product of RMS

current and peak voltage). An extreme high or low duty

cycle results in high power stress on the MOSFET or diode,

and reduces efficiency. It is recommended to choose a

duty cycle between 20% and 80%.

Figure 7. A Simplified Flyback Converter

RSENSE

NP:NS

VIN

CIN CSN

VSN

SUGGESTED

RCD SNUBBER

VDS

ISW

3758 F07

GATE

GND

LT3758

SENSE

–

RSN

DSN

–

COUT

Figure 8. Waveforms of the Flyback Converter

in Discontinuous Mode Operation

3758 F08

ISW

VDS

DTSD2TSD3TS

ISW(MAX)

ID(MAX)

Flyback Converter: Switch Duty Cycle and Turns Ratio

The flyback converter conversion ratio in the continuous

mode operation is:

VOUT

VIN

=NS

•D

1−D

Where NS/NP is the second to primary turns ratio.

Figure 8 shows the waveforms of the flyback converter

in discontinuous mode operation. During each switching

period TS, three subintervals occur: DTS, D2TS, D3TS.

During DTS, M is on, and D is reverse-biased. During

D2TS, M is off, and LS is conducting current. Both LP and

LS currents are zero during D3TS.

The flyback converter conversion ratio in the discontinu-

ous mode operation is:

VOUT

VIN

=NS

•D

According to the preceding equations, the user has relative

freedom in selecting the switch duty cycle or turns ratio to

suit a given application. The selections of the duty cycle

and the turns ratio are somewhat iterative processes, due

LT3758/LT3758A

3758afd

ApplicAtions inForMAtion

Flyback Converter: Transformer Design for

Discontinuous Mode Operation

The transformer design for discontinuous mode of opera-

tion is chosen as presented here. According to Figure 8,

the minimum D3 (D3MIN) occurs when the the converter

has the minimum VIN and the maximum output power

(POUT). Choose D3MIN to be equal to or higher than 10%

to guarantee the converter is always in discontinuous

mode operation. Choosing higher D3 allows the use of

low inductances but results in higher switch peak current.

The user can choose a DMAX as the start point. Then, the

maximum average primary currents can be calculated by

the following equation:

ILP(MAX) =ISW(MAX) =POUT(MAX)

DMAX •VIN(MIN) •h

where h is the converter efficiency.

If the flyback converter has multiple outputs, POUT(MAX)

is the sum of all the output power.

The maximum average secondary current is:

ILS(MAX) =ID(MAX) =IOUT(MAX)

where

D2 = 1 – DMAX – D3

the primary and secondary RMS currents are:

ILP(RMS) =2•ILP(MAX) •DMAX

ILS(RMS) =2•ILS(MAX) •D2

According to Figure 8, the primary and secondary peak

currents are:

ILP(PEAK) = ISW(PEAK) = 2 • ILP(MAX)

ILS(PEAK) = ID(PEAK) = 2 • ILS(MAX)

The primary and second inductor values of the flyback

converter transformer can be determined using the fol-

lowing equations:

LP=D2MAX •V2IN(MIN) •h

2•POUT(MAX) •f

LS=D22•(VOUT +VD)

2•IOUT(MAX) •f

The primary to second turns ratio is:

=LP

Flyback Converter: Snubber Design

Transformer leakage inductance (on either the primary or

secondary) causes a voltage spike to occur after the MOS-

FET turn-off. This is increasingly prominent at higher load

currents, where more stored energy must be dissipated.

In some cases a snubber circuit will be required to avoid

overvoltage breakdown at the MOSFET’s drain node. There

are different snubber circuits, and Application Note 19 is

a good reference on snubber design. An RCD snubber is

shown in Figure 7.

The snubber resistor value (RSN) can be calculated by the

following equation:

RSN =2•

V2SN −VSN •VOUT •NP

I2SW(PEAK) •LLK •f

where VSN is the snubber capacitor voltage. A smaller

VSN results in a larger snubber loss. A reasonable VSN is

2 to 2.5 times of:

VOUT •NP

LT3758/LT3758A

3758afd

ApplicAtions inForMAtion

LLK is the leakage inductance of the primary winding, which

is usually specified in the transformer characteristics. LLK

can be obtained by measuring the primary inductance with

the secondary windings shorted. The snubber capacitor

value (CCN) can be determined using the following equation:

CCN =VSN

∆VSN •RCN •f

where ∆VSN is the voltage ripple across CCN. A reasonable

∆VSN is 5% to 10% of VSN. The reverse voltage rating of

DSN should be higher than the sum of VSN and VIN(MAX).

Flyback Converter: Sense Resistor Selection

In a flyback converter, when the power switch is turned on,

the current flowing through the sense resistor (ISENSE) is:

ISENSE = ILP

Set the sense voltage at ILP(PEAK) to be the minimum of

the SENSE current limit threshold with a 20% margin. The

sense resistor value can then be calculated to be:

RSENSE =80mV

ILP(PEAK)

Flyback Converter: Power MOSFET Selection

For the flyback configuration, the MOSFET is selected with

a VDC rating high enough to handle the maximum VIN, the

reflected secondary voltage and the voltage spike due to

the leakage inductance. Approximate the required MOSFET

VDC rating using:

BVDSS > VDS(PEAK)

where

VDS(PEAK) = VIN(MAX) + VSN

The power dissipated by the MOSFET in a flyback con-

verter is:

PFET = I2M(RMS) • RDS(ON) + 2 • V2DS(PEAK) • IL(MAX) •

CRSS • f/1A

The first term in this equation represents the conduction

losses in the device, and the second term, the switching

loss. CRSS is the reverse transfer capacitance, which is

usually specified in the MOSFET characteristics.

From a known power dissipated in the power MOSFET, its

junction temperature can be obtained using the following

equation:

TJ = TA + PFET • θJA = TA + PFET • (θJC + θCA)

TJ must not exceed the MOSFET maximum junction

temperature rating. It is recommended to measure the

MOSFET temperature in steady state to ensure that absolute

maximum ratings are not exceeded.

Flyback Converter: Output Diode Selection

The output diode in a flyback converter is subject to large

RMS current and peak reverse voltage stresses. A fast

switching diode with a low forward drop and a low reverse

leakage is desired. Schottky diodes are recommended if

the output voltage is below 100V.

Approximate the required peak repetitive reverse voltage

rating VRRM using:

VRRM >NS

•VIN(MAX) +VOUT

The power dissipated by the diode is:

PD = IO(MAX) • VD

and the diode junction temperature is:

TJ = TA + PD • RθJA

The RθJA to be used in this equation normally includes the

RθJC for the device, plus the thermal resistance from the

board to the ambient temperature in the enclosure. TJ must

not exceed the diode maximum junction temperature rating.

Flyback Converter: Output Capacitor Selection

The output capacitor of the flyback converter has a similar

operation condition as that of the boost converter. Refer

to the Boost Converter: Output Capacitor Selection section

for the calculation of COUT and ESRCOUT.

The RMS ripple current rating of the output capacitors

in discontinuous operation can be determined using the

following equation:

IRMS(COUT),DISCONTINUOUS ≥IO(MAX) •4−(3 •D2)

3•D2

LT3758/LT3758A

3758afd

ApplicAtions inForMAtion

Flyback Converter: Input Capacitor Selection

The input capacitor in a flyback converter is subject to

a large RMS current due to the discontinuous primary

current. To prevent large voltage transients, use a low

ESR input capacitor sized for the maximum RMS current.

The RMS ripple current rating of the input capacitors in

discontinuous operation can be determined using the

following equation:

IRMS(CIN),DISCONTINUOUS ≥POUT(MAX)

VIN(MIN) •h•4−(3 •DMAX )

3•DMAX

SEPIC CONVERTER APPLICATIONS

The LT3758 can be configured as a SEPIC (single-ended

primary inductance converter), as shown in Figure 1. This

topology allows for the input to be higher, equal, or lower

than the desired output voltage. The conversion ratio as

a function of duty cycle is:

VOUT +VD

VIN

1−D

in continuous conduction mode (CCM).

In a SEPIC converter, no DC path exists between the input

and output. This is an advantage over the boost converter

for applications requiring the output to be disconnected

from the input source when the circuit is in shutdown.

Compared to the flyback converter, the SEPIC converter

has the advantage that both the power MOSFET and the

output diode voltages are clamped by the capacitors (CIN,

CDC and COUT), therefore, there is less voltage ringing

across the power MOSFET and the output diodes. The

SEPIC converter requires much smaller input capacitors

than those of the flyback converter. This is due to the fact

that, in the SEPIC converter, the inductor L1 is in series

with the input, and the ripple current flowing through the

input capacitor is continuous.

SEPIC Converter: Switch Duty Cycle and Frequency

For a SEPIC converter operating in CCM, the duty cycle

of the main switch can be calculated based on the output

voltage (VOUT), the input voltage (VIN) and the diode

forward voltage (VD).

The maximum duty cycle (DMAX) occurs when the converter

has the minimum input voltage:

DMAX =VOUT

VIN(MIN) +VOUT +VD

SEPIC Converter: Inductor and Sense Resistor Selection

As shown in Figure 1, the SEPIC converter contains two

inductors: L1 and L2. L1 and L2 can be independent, but can

also be wound on the same core, since identical voltages

are applied to L1 and L2 throughout the switching cycle.

For the SEPIC topology, the current through L1 is the

converter input current. Based on the fact that, ideally, the

output power is equal to the input power, the maximum

average inductor currents of L1 and L2 are:

IL1(MAX) =IIN(MAX) =IO(MAX) •DMAX

1−DMAX

IL2(MAX) =IO(MAX)

In a SEPIC converter, the switch current is equal to IL1 +

IL2 when the power switch is on, therefore, the maximum

average switch current is defined as:

ISW(MAX) =IL1(MAX) +IL2(MAX) =IO(MAX) •1

1−DMAX

and the peak switch current is:

ISW(PEAK) =1+c









•IO(MAX) •1

1−DMAX

The constant c in the preceding equations represents the

percentage peak-to-peak ripple current in the switch, rela-

tive to ISW(MAX), as shown in Figure 9. Then, the switch

ripple current ∆ISW can be calculated by:

∆ISW = c • ISW(MAX)

The inductor ripple currents ∆IL1 and ∆IL2 are identical:

∆IL1 = ∆IL2 = 0.5 • ∆ISW

The inductor ripple current has a direct effect on the

choice of the inductor value. Choosing smaller values of

∆IL requires large inductances and reduces the current

loop gain (the converter will approach voltage mode).

LT3758/LT3758A

3758afd

ApplicAtions inForMAtion

Accepting larger values of ∆IL allows the use of low in-

ductances, but results in higher input current ripple and

greater core losses. It is recommended that c falls in the

range of 0.2 to 0.6.

Figure 9. The Switch Current Waveform of the SEPIC Converter

3758 F09

∆ISW = χ • ISW(MAX)

ISW

DTS

ISW(MAX)

where

cL1 =∆IL1

IL1(MAX)

IL2(RMS) =IL2(MAX) •1+c2L2

where

cL2 =∆IL2

IL2 (MAX)

Based on the preceding equations, the user should choose

the inductors having sufficient saturation and RMS cur-

rent ratings.

In a SEPIC converter, when the power switch is turned on,

the current flowing through the sense resistor (ISENSE) is

the switch current.

Set the sense voltage at ISENSE(PEAK) to be the minimum

of the SENSE current limit threshold with a 20% margin.

The sense resistor value can then be calculated to be:

RSENSE =80 mV

ISW(PEAK)

SEPIC Converter: Power MOSFET Selection

For the SEPIC configuration, choose a MOSFET with a

VDC rating higher than the sum of the output voltage and

input voltage by a safety margin (a 10V safety margin is

usually sufficient).

The power dissipated by the MOSFET in a SEPIC con-

verter is:

PFET = I2SW(MAX) • RDS(ON) • DMAX

+ 2 • (VIN(MIN) + VOUT)2 • IL(MAX) • CRSS • f/1A

The first term in this equation represents the conduction

losses in the device, and the second term, the switching

loss. CRSS is the reverse transfer capacitance, which is

usually specified in the MOSFET characteristics.

For maximum efficiency, RDS(ON) and CRSS should be

minimized. From a known power dissipated in the power

Given an operating input voltage range, and having chosen

the operating frequency and ripple current in the inductor,

the inductor value (L1 and L2 are independent) of the SEPIC

converter can be determined using the following equation:

L1=L2 =VIN(MIN)

0.5 •∆ISW •f•DMAX

For most SEPIC applications, the equal inductor values

will fall in the range of 1µH to 100µH.

By making L1 = L2, and winding them on the same core,

the value of inductance in the preceding equation is re-

placed by 2L, due to mutual inductance:

L=VIN(MIN)

∆ISW •f•DMAX

This maintains the same ripple current and energy storage

in the inductors. The peak inductor currents are:

IL1(PEAK) = IL1(MAX) + 0.5 • ∆IL1

IL2(PEAK) = IL2(MAX) + 0.5 • ∆IL2

The RMS inductor currents are:

IL1(RMS) =IL1(MAX) •1+c2L1

LT3758/LT3758A

3758afd

ApplicAtions inForMAtion

MOSFET, its junction temperature can be obtained using

the following equation:

TJ = TA + PFET • θJA = TA + PFET • (θJC + θCA)

TJ must not exceed the MOSFET maximum junction

temperature rating. It is recommended to measure the

MOSFET temperature in steady state to ensure that absolute

maximum ratings are not exceeded.

SEPIC Converter: Output Diode Selection

To maximize efficiency, a fast switching diode with a low

forward drop and low reverse leakage is desirable. The

average forward current in normal operation is equal to

the output current, and the peak current is equal to:

ID(PEAK) =1+c









•IO(MAX) •1

1−DMAX

It is recommended that the peak repetitive reverse voltage

rating VRRM is higher than VOUT + VIN(MAX) by a safety

margin (a 10V safety margin is usually sufficient).

The power dissipated by the diode is:

PD = IO(MAX) • VD

and the diode junction temperature is:

TJ = TA + PD • RθJA

The RθJA used in this equation normally includes the RθJC

for the device, plus the thermal resistance from the board,

to the ambient temperature in the enclosure. TJ must not

exceed the diode maximum junction temperature rating.

SEPIC Converter: Output and Input Capacitor Selection

The selections of the output and input capacitors of the

SEPIC converter are similar to those of the boost converter.

Please refer to the Boost Converter: Output Capacitor

Selection and Boost Converter: Input Capacitor Selection

sections.

SEPIC Converter: Selecting the DC Coupling Capacitor

The DC voltage rating of the DC coupling capacitor (CDC,

as shown in Figure 1) should be larger than the maximum

input voltage:

VCDC > VIN(MAX)

CDC has nearly a rectangular current waveform. During

the switch off-time, the current through CDC is IIN, while

approximately –IO flows during the on-time. The RMS

rating of the coupling capacitor is determined by the fol-

lowing equation:

IRMS(CDC) >IO(MAX) •VOUT +VD

VIN(MIN)

A low ESR and ESL, X5R or X7R ceramic capacitor works

well for CDC.

INVERTING CONVERTER APPLICATIONS

The LT3758 can be configured as a dual-inductor inverting

topology, as shown in Figure 10. The VOUT to VIN ratio is:

VOUT −VD

VIN

= − D

1−D

in continuous conduction mode (CCM).

Figure 10. A Simplified Inverting Converter

RSENSE

CDC

VIN

CIN

COUT VOUT

3758 F10

GATE

GND

LT3758

SENSE

–

+–

Inverting Converter: Switch Duty Cycle and Frequency

For an inverting converter operating in CCM, the duty

cycle of the main switch can be calculated based on the

negative output voltage (VOUT) and the input voltage (VIN).

The maximum duty cycle (DMAX) occurs when the converter

has the minimum input voltage:

DMAX =VOUT −VD

VOUT −VD−VIN(MIN)

LT3758/LT3758A

3758afd

Inverting Converter: Inductor, Sense Resistor, Power

MOSFET, Output Diode and Input Capacitor Selections

The selections of the inductor, sense resistor, power

MOSFET, output diode and input capacitor of an invert-

ing converter are similar to those of the SEPIC converter.

Please refer to the corresponding SEPIC converter sections.

Inverting Converter: Output Capacitor Selection

The inverting converter requires much smaller output

capacitors than those of the boost, flyback and SEPIC

converters for similar output ripples. This is due to the fact

that, in the inverting converter, the inductor L2 is in series

with the output, and the ripple current flowing through the

output capacitors are continuous. The output ripple voltage

is produced by the ripple current of L2 flowing through

the ESR and bulk capacitance of the output capacitor:

∆VOUT(P–P) = ∆IL2 •ESRCOUT +1

8•f•COUT











After specifying the maximum output ripple, the user can

select the output capacitors according to the preceding

equation.

The ESR can be minimized by using high quality X5R or

X7R dielectric ceramic capacitors. In many applications,

ceramic capacitors are sufficient to limit the output volt-

age ripple.

The RMS ripple current rating of the output capacitor

needs to be greater than:

IRMS(COUT) > 0.3 • ∆IL2

Inverting Converter: Selecting the DC Coupling Capacitor

The DC voltage rating of the DC coupling capacitor (CDC,

as shown in Figure 10) should be larger than the maximum

input voltage minus the output voltage (negative voltage):

VCDC > VIN(MAX) – VOUT

CDC has nearly a rectangular current waveform. During

the switch off-time, the current through CDC is IIN, while

approximately –IO flows during the on-time. The RMS

rating of the coupling capacitor is determined by the fol-

lowing equation:

IRMS(CDC) >IO(MAX) •DMAX

1−DMAX

A low ESR and ESL, X5R or X7R ceramic capacitor works

well for CDC.

Board Layout

The high speed operation of the LT3758 demands careful

attention to board layout and component placement. The

Exposed Pad of the package is the only GND terminal of

the IC, and is important for thermal management of the

IC. Therefore, it is crucial to achieve a good electrical and

thermal contact between the Exposed Pad and the ground

plane of the board. For the LT3758 to deliver its full output

power, it is imperative that a good thermal path be pro-

vided to dissipate the heat generated within the package.

It is recommended that multiple vias in the printed circuit

board be used to conduct heat away from the IC and into

a copper plane with as much area as possible.

To prevent radiation and high frequency resonance prob-

lems, proper layout of the components connected to the

IC is essential, especially the power paths with higher di/

dt. The following high di/dt loops of different topologies

should be kept as tight as possible to reduce inductive

ringing:

• In boost configuration, the high di/dt loop contains

the output capacitor, the sensing resistor, the power

MOSFET and the Schottky diode.

• In flyback configuration, the high di/dt primary loop

contains the input capacitor, the primary winding, the

power MOSFET and the sensing resistor. The high di/

dt secondary loop contains the output capacitor, the

secondary winding and the output diode.

• In SEPIC configuration, the high di/dt loop contains

the power MOSFET, sense resistor, output capacitor,

Schottky diode and the coupling capacitor.

• In inverting configuration, the high di/dt loop contains

power MOSFET, sense resistor, Schottky diode and the

coupling capacitor.

ApplicAtions inForMAtion

LT3758/LT3758A

3758afd

ApplicAtions inForMAtion

Check the stress on the power MOSFET by measuring its

drain-to-source voltage directly across the device terminals

(reference the ground of a single scope probe directly to

the source pad on the PC board). Beware of inductive

ringing, which can exceed the maximum specified voltage

rating of the MOSFET. If this ringing cannot be avoided,

and exceeds the maximum rating of the device, either

choose a higher voltage device or specify an avalanche-

rated power MOSFET.

The small-signal components should be placed away

from high frequency switching nodes. For optimum load

regulation and true remote sensing, the top of the output

voltage sensing resistor divider should connect indepen-

dently to the top of the output capacitor (Kelvin connec-

tion), staying away from any high dV/dt traces. Place the

divider resistors near the LT3758 in order to keep the high

impedance FBX node short.

Figure 11 shows the suggested layout of the 10V to 40V

input, 48V output boost converter in the Typical Applica-

tions section.

Figure 11. Suggested Layout of the 10V to 40V Input, 48V Output

Boost Converter in the Typical Applications Section

VIN

3758 F11

VOUT

VIAS TO GROUND

PLANE

COUT1

COUT2

CIN

CSS

CVCC

CC1

CC2

LT3758

LT3758/LT3758A

3758afd

ApplicAtions inForMAtion

Table 2. Recommended Component Manufacturers

VENDOR COMPONENTS WEB ADDRESS

AVX Capacitors avx.com

BH Electronics Inductors,

Transformers

bhelectronics.com

Coilcraft Inductors coilcraft.com

Cooper Bussmann Inductors bussmann.com

Diodes, Inc Diodes diodes.com

Fairchild MOSFETs fairchildsemi.com

General Semiconductor Diodes generalsemiconductor.

com

International Rectifier MOSFETs, Diodes irf.com

IRC Sense Resistors irctt.com

Kemet Tantalum Capacitors kemet.com

Magnetics Inc Toroid Cores mag-inc.com

Microsemi Diodes microsemi.com

Murata-Erie Inductors, Capacitors murata.co.jp

Nichicon Capacitors nichicon.com

On Semiconductor Diodes onsemi.com

Panasonic Capacitors panasonic.com

Pulse Inductors pulseeng.com

Sanyo Capacitors sanyo.co.jp

Sumida Inductors sumida.com

Taiyo Yuden Capacitors t-yuden.com

TDK Capacitors, Inductors component.tdk.com

Thermalloy Heat Sinks aavidthermalloy.com

Tokin Capacitors nec-tokinamerica.com

Toko Inductors tokoam.com

United Chemi-Con Capacitors chemi-com.com

Vishay/Dale Resistors vishay.com

Vishay/Siliconix MOSFETs vishay.com

Würth Elektronik Inductors we-online.com

Vishay/Sprague Capacitors vishay.com

Zetex Small-Signal Discretes zetex.com

Recommended Component Manufacturers

Some of the recommended component manufacturers

are listed in Table 2.

LT3758/LT3758A

3758afd

typicAl ApplicAtions

10V to 40V Input, 48V Output Boost Converter

Efficiency vs Output Current

SENSE

LT3758

VIN

10V TO 40V CIN

4.7µF

50V

X7R

×2VOUT

48V

0.012Ω

41.2k

300kHz

GATE

FBX

GND INTVCC

SHDN/UVLO

SYNC

200k

32.4k

CSS

0.68µF

CC2

100pF

10k

CC1

10nF

18.7µH

3758 TA02a

464k

15.8k

CVCC

4.7µF

10V

X5R

COUT2

4.7µF

50V

X7R

×4

COUT1

100µF

63V

CIN, COUT2: MURATA GRM32ER71H475KA88L

COUT1: PANASONIC ECG EEV-TG1J101UP

D1: VISHAY SILICONIX 30BQ060

L1: PULSE PB2020.223

M1: VISHAY SILICONIX SI7460DP

OUTPUT CURRENT (A)

0.001

EFFICIENCY (%)

100

0.01 0.1

3758 TA02b

VIN = 40V

VIN = 24V

VIN = 10V

Start-Up Waveforms

5ms/DIV

VOUT

20V/DIV

IL1

2A/DIV

3758 TA02c

VIN = 24V

LT3758/LT3758A

3758afd

typicAl ApplicAtions

12V Output Nonisolated Flyback Power Supply

Efficiency vs Output Current Start-Up Waveform

SENSE

LT3758

VIN

DSN

VIN

36V TO 72V

CIN

2.2µF

100V

X7R

63.4k

200kHz

GATE

FBX

GND INTVCC

SHDN/UVLO

SYNC

0.022µF

100V T1

1,2,3

(SERIES)

4,5,6

(PARALLEL)

44.2k

0.47µF

100pF 10k

10nF

0.030Ω

5.1Ω

1N4148

15.8k

105k

CVCC

4.7µF

10V

X5R

VOUT

12V

1.2A

3758 TA03a

COUT

47µF

X5R

6.2k

CIN: MURATA GRM32ER72A225KA35L

T1: COILTRONICS VP2-0066

M1: VISHAY SILICONIX SI4848DY

D1: ON SEMICONDUCTOR MBRS360T3G

DSN: VISHAY SILICONIX ES1D

COUT: MURATA GRM32ER61C476ME15L

OUTPUT CURRENT (A)

0.01

EFFICIENCY (%)

100

0.1 1

3758 TA03b

VIN = 48V

5ms/DIV

VOUT

5V/DIV

3758 TA03c

VIN = 48V

Frequency Foldback Waveforms

When Output Short-Circuit

20µs/DIV

VOUT

5V/DIV

VSW

50V/DIV

3758 TA03d

VIN = 48V

LT3758/LT3758A

3758afd

10ms/DIV

VOUT1,

VOUT2

20V/DIV

3758 TA04b

VIN = 12V VOUT1

VOUT2

2µs/DIV

VSW

50V/DIV

VOUT2

1V/DIV

(AC)

VOUT1

1V/DIV

(AC)

3758 TA04c

typicAl ApplicAtions

VFD (Vacuum Fluorescent Display) Flyback Power Supply

Start-Up Waveforms Switching Waveforms

SENSE

LT3758

VIN

9V TO 16V CIN

22µF

25V

63.4k

200kHz

GATE

FBX

GND INTVCC

SHDN/UVLO

SYNC

COUT2

2.2µF

100V

X7R

1, 2, 3

178k

32.4k

0.47µF

47pF 10k

10nF

0.019Ω

0.5W 1.62k

95.3k

CVCC

4.7µF

10V

X5R

VOUT

96V

80mA

VOUT2

64V

40mA

3758 TA04a

COUT1

1µF

100V

X7R

SWM1

CIN: MURATA GRM32ER61E226KE15L

COUT1: MURATA GRM31CR72A105K01L

COUT2: MURATA GRM32ER72A225KA35L

D1: VISHAY SILICONIX ES1D

D2: VISHAY SILICONIX ES1C

M1: VISHAY SILICONIX Si4100DY

T1: COILTRONICS VP1-0102

(*PRIMARY = 3 WINDINGS IN PARALLEL)

220pF

22Ω

LT3758/LT3758A

3758afd

typicAl ApplicAtions

36V to 72V Input, 3.3V Output Isolated Telecom Power Supply

SENSE

LT3758

VIN

INTVCC

BAV21W

FDC2512

0.03Ω

VIN

36V TO 72V CIN

2.2µF

100V

X7R

63.4k

200kHz

GATE

FBX

GND

SHDN/UVLO

SYNC

0.022µF

100V

4.7µF

25V

X5R

44.2k

0.47µF

VOUT+

3.3V

VOUT-

3758 TA05a

5.6k

16k

4.7µF

25V

X5R

10Ω

274Ω

BAS516

PS2801-1

BAS516

UPS840

0.47µF

LT4430

47nF

1µF

VIN

GND

OPTO

COMP

OC 0.5V FB

47pF

22.1k

100kBAT54CWTIG

2200pF

250VAC

100pF

COUT

100µF

6.3V

×3

PA1277NL

OUTPUT CURRENT (A)

0.01

EFFICIENCY (%)

100

0.1 1

3758 TA05b

VIN = 36V VIN = 72V

VIN = 48V

Efficiency vs Output Current

LT3758/LT3758A

3758afd

typicAl ApplicAtions

18V to 72V Input, 24V Output SEPIC Converter

SENSE

LT3758A

VIN

18V TO 72V CIN2

2.2µF

100V

X7R

CDC

2.2µF

100V

X7R, ×2VOUT

24V

0.025Ω

41.2k

300kHz

GATE

FBX

GND INTVCC

SHDN/UVLO

SYNC

•

232k

20k

0.47µF

10nF

10k

L1A

L1B

CIN1: PANASONIC EEE2AA100UP

CIN2, CDC: TAIYO YUDEN HMK325B7225KN-T

COUT1: MURATA GRM31CR61E106KA12L

COUT2: KEMET T495X336K035AS

3758 TA06a

280k

20k

COUT1

10µF

25V

X5R

×4

CVCC

4.7µF

10V

X5R

L1A, L1B: COILTRONICS DRQ127-470

M1: FAIRCHILD SEMICONDUCTOR FDMS2572

D1: ON SEMICONDUCTOR MBRS3100T3G

CIN1

10µF

100V

COUT1

33µF

35V

×2

Efficiency vs Output Current

OUTPUT CURRENT (A)

0.001

EFFICIENCY (%)

100

0.01 0.1

3758 TA06b

VIN = 72V

VIN = 48V

VIN = 18V

Load Step Waveform

500µs/DIV

VOUT

2V/DIV

AC-COUPLED

IOUT

1A/DIV 0A

3758 TA06c

VIN = 48V

Start-Up Waveform

2ms/DIV

VOUT

10V/DIV

IL1A + IL1B

1A/DIV

3758 TA06d

VIN = 48V

50µs/DIV

VSW

50V/DIV

VOUT

20V/DIV

IL1A + IL1B

2A/DIV

3758 TA06e

VIN = 48V

Frequency Foldback Waveforms

When Output Short-Circuit

LT3758/LT3758A

3758afd

typicAl ApplicAtions

10V to 40V Input, –12V Output Inverting Converter

SENSE

LT3758A

VIN

10V TO 40V CIN2

4.7µF

50V

X7R

×2

CDC

2.2µF

100V

X7R, ×2

0.015Ω

41.2k

300kHz

GATE

FBX

GND INTVCC

SHDN/UVLO

SYNC

•

0.47µF 10k

L1A

3758 TA07a

CVCC

4.7µF

10V

X5R

CIN1

4.7µF

50V

×2

COUT2

47µF

20V

×2

200k

32.4k

6.8nF

•

L1B

VOUT

–12V

105k

7.5k

COUT1

22µF

16V

X5R

×4

CIN1: KEMET T495X475K050AS

CIN2, CDC: MURATA GRM32ER71H475KA88L

COUT1: MURATA GRM32ER61C226KE20

COUT2: KEMET T495X476K020AS

D1: VISHAY SILICONIX 30BQ060

L1A, L1B: COILTRONICS DRQ127-150

M1: VISHAY SILICONIX SI7850DP

Efficiency vs Output Current

OUTPUT CURRENT (A)

0.001

EFFICIENCY (%)

100

0.01 0.1 1

3758 TA07b

VIN = 40V

VIN = 24V

VIN = 10V

Load Step Waveforms

500µs/DIV

VOUT

1V/DIV

AC-COUPLED

IOUT

1A/DIV 0A

3758 TA07c

VIN = 24V

Start-Up Waveforms

5ms/DIV

VOUT

5V/DIV

IL1A + IL1B

2A/DIV

3758 TA07d

VIN = 24V

50µs/DIV

VSW

20V/DIV

VOUT

10V/DIV

IL1A + IL1B

5A/DIV 3758 TA07e

VIN = 24V

Frequency Foldback Waveforms

When Output Short-Circuit

LT3758/LT3758A

3758afd

pAckAge Description

3.00 ±0.10

(4 SIDES)

NOTE:

1. DRAWING TO BE MADE A JEDEC PACKAGE OUTLINE M0-229 VARIATION OF (WEED-2).

CHECK THE LTC WEBSITE DATA SHEET FOR CURRENT STATUS OF VARIATION ASSIGNMENT

2. DRAWING NOT TO SCALE

3. ALL DIMENSIONS ARE IN MILLIMETERS

4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE

MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE

5. EXPOSED PAD SHALL BE SOLDER PLATED

6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE

TOP AND BOTTOM OF PACKAGE

0.40 ± 0.10

BOTTOM VIEW—EXPOSED PAD

1.65 ± 0.10

(2 SIDES)

0.75 ±0.05

R = 0.125

TYP

2.38 ±0.10

(2 SIDES)

106

PIN 1

TOP MARK

(SEE NOTE 6)

0.200 REF

0.00 – 0.05

(DD) DFN REV C 0310

0.25 ± 0.05

2.38 ±0.05

(2 SIDES)

RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS

1.65 ±0.05

(2 SIDES)2.15 ±0.05

0.50

BSC

0.70 ±0.05

3.55 ±0.05

PACKAGE

OUTLINE

0.25 ± 0.05

0.50 BSC

DD Package

10-Lead Plastic DFN (3mm × 3mm)

(Reference LTC DWG # 05-08-1699 Rev C)

PIN 1 NOTCH

R = 0.20 OR

0.35 × 45°

CHAMFER

LT3758/LT3758A

3758afd

pAckAge Description

MSOP (MSE) 0911 REV H

0.53 ±0.152

(.021 ±.006)

SEATING

PLANE

0.18

(.007)

1.10

(.043)

MAX

0.17 –0.27

(.007 – .011)

TYP

0.86

(.034)

REF

0.50

(.0197)

BSC

1234 5

4.90 ±0.152

(.193 ±.006)

0.497 ±0.076

(.0196 ±.003)

REF

8910

3.00 ±0.102

(.118 ±.004)

(NOTE 3)

3.00 ±0.102

(.118 ±.004)

(NOTE 4)

NOTE:

1. DIMENSIONS IN MILLIMETER/(INCH)

2. DRAWING NOT TO SCALE

3. DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS.

MOLD FLASH, PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.152mm (.006") PER SIDE

4. DIMENSION DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSIONS.

INTERLEAD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.152mm (.006") PER SIDE

5. LEAD COPLANARITY (BOTTOM OF LEADS AFTER FORMING) SHALL BE 0.102mm (.004") MAX

6. EXPOSED PAD DIMENSION DOES INCLUDE MOLD FLASH. MOLD FLASH ON E-PAD

SHALL NOT EXCEED 0.254mm (.010") PER SIDE.

0.254

(.010) 0° – 6° TYP

DETAIL “A”

GAUGE PLANE

5.23

(.206)

MIN

3.20 – 3.45

(.126 – .136)

0.889 ±0.127

(.035 ±.005)

RECOMMENDED SOLDER PAD LAYOUT

1.68 ±0.102

(.066 ±.004)

1.88 ±0.102

(.074 ±.004)

0.50

(.0197)

BSC

0.305 ± 0.038

(.0120 ±.0015)

TYP

BOTTOM VIEW OF

EXPOSED PAD OPTION

1.68

(.066)

1.88

(.074)

0.1016 ±0.0508

(.004 ±.002)

DETAIL “B”

CORNER TAIL IS PART OF

THE LEADFRAME FEATURE.

FOR REFERENCE ONLY

NO MEASUREMENT PURPOSE

0.05 REF

0.29

REF

MSE Package

10-Lead Plastic MSOP, Exposed Die Pad

(Reference LTC DWG # 05-08-1664 Rev H)

LT3758/LT3758A

3758afd

Information furnished by Linear Technology Corporation is believed to be accurate and reliable.

However, no responsibility is assumed for its use. Linear Technology Corporation makes no representa-

tion that the interconnection of its circuits as described herein will not infringe on existing patent rights.

revision history

REV DATE DESCRIPTION PAGE NUMBER

A 3/10 Deleted Bullet from Features and Last Line of Description

Updated All Sections to Include H-Grade and Military Grade

Deleted Vendor Telephone Information from Table 2 in Applications Information Section

Revised TA04 and TA04c in Typical Applications

Replaced Related Parts List

2 to 7

B 5/10 Revised last sentence of SYNC Pin description

Updated Block Diagram

Revised value in last sentence of Programming Turn-on and Turn-off Thresholds in the SHDN/UVLO Pin Section

Revised penultimate sentence of Operating Frequency and Synchronization section

C 5/11 Revised MP-grade temperature range in Absolute Maximum Ratings and Order Information

Revised Note 2

Revised formula in Applications Information

D 07/12 Added LT3758A version Throughout

Updated Block Diagram 9

Updated Programming the Output Voltage section 13

Updated Loop Compensation section 14

Updated the schematic and Load Step Waveforms in the Typical Applications section 31, 32

LT3758/LT3758A

3758afd

Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417

(408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com

 LINEAR TECHNOLOGY CORPORATION 2009

LT 0712 REV D • PRINTED IN USA

typicAl ApplicAtions

relAteD pArts

8V to 72V Input, 12V Output SEPIC Converter

SENSE

LT3758

VIN

8V TO 72V CIN

2.2µF

100V

X7R

×2

CDC

2.2µF

100V

X7R, ×2VOUT

12V

0.012Ω

Si7456DP

41.2k

300kHz

GATE

FBX

GND INTVCC

SHDN/UVLO

SYNC

•

154k

32.4k

0.47µF

10nF

10k

L1A

L1B

MBRS3100T3G

3758 TA08a

105k

15.8k

1% COUT2

10µF

16V

X5R

×4

COUT1

47µF

20V

×2

CVCC

4.7µF

10V

X5R

L1A, L1B: COILTRONICS DRQ127-220

Efficiency vs Output Current

VIN = 8V

OUTPUT CURRENT (A)

EFFICIENCY (%)

100

3758 TA08b

0.001 0.01 0.1 1 10

VIN = 72V

VIN = 42V

PART NUMBER DESCRIPTION COMMENTS

LT3757A Boost, Flyback, SEPIC and Inverting Controller 2.9V ≤ VIN ≤ 40V, Current Mode Control, 100kHz to 1MHz Programmable

Operation Frequency, 3mm × 3mm DFN-10 and MSOP-10E Packages

LT3759 Boost, SEPIC and Inverting Controller 1.6V ≤ VIN ≤ 42V, Current Mode Control, 100kHz to 1MHz Programmable

Operation Frequency, MSOP-12E Packages

LT3957A Boost, Flyback, SEPIC and Inverting Controller

with 5A, 40V Switch

3V ≤ VIN ≤ 40V, Current Mode Control, 100kHz to 1MHz Programmable Operation

Frequency, 5mm × 6mm QFN Package

LT3958 Boost, Flyback, SEPIC and Inverting Controller

with 3.3A, 84V Switch

5V ≤ VIN ≤ 80V, Current Mode Control, 100kHz to 1MHz Programmable Operation

Frequency, 5mm × 6mm QFN Package

LT3573/LT3574/

LT3575

40V Isolated Flyback Converters Monolithic No-Opto Flybacks with Integrated 1.25A/0.65A/2.5A Switch

LT3511/LT3512 100V Isolated Flyback Converters Monolithic No-Opto Flybacks with Integrated 240mA/420mA Switch

LT3798 Offline Isolated No Opto-Coupler Flyback

Controller with Active PFC

VIN and VOUT Limited Only by External Components, MSOP-16 Package

LT3799/LT3799-1 Offline Isolated Flyback LED Controllers with

Active PFC

VIN and VOUT Limited Only by External Components, MSOP-16 Package