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FEATURES DESCRIPTION
BLOCK DIAGRAM
Pinnumbersreferto24-pinpackages.
UC2849
UC3849
SLUS360C – JULY 1995 – REVISED AUGUST 2007
SECONDARY SIDE AVERAGE CURRENT MODE CONTROLLER
•Practical Secondary Side Control of Isolated
The UC3849 family of average current-modePower Supplies
controllers accurately accomplishes secondary sideaverage current mode control. The secondary-side•1 MHz Operation
output voltage is regulated by sensing the output•Differential AC Switching Current Sensing
voltage and differentially sensing the ac switching•Accurate Programmable Maximum Duty Cycle
current. The sensed output voltage drives a voltageerror amplifier. The ac switching current, monitored•Multiple Chips Can be Synchronized to
by a current sense resistor, drives a high bandwidth,Fastest Oscillator
low offset current sense amplifier. The outputs of the•Wide Gain Bandwidth Product (70 MHz, Acl
voltage error amplifier and current sense amplifier>10) Current Error and Current Sense
differentially drive a high bandwidth, integratingAmplifiers
current error amplifier. The sawtooth waveform at the•Up to Ten Devices Can Easily Share a
current error amplifier output is the amplified andinverted inductor current sensed through the resistor.Common Load
This inductor current down-slope compared to thePWM ramp achieves slope compensation, whichgives an accurate and inherent fast transientresponse to changes in load.
Please be aware that an important notice concerning availability, standard warranty, and use in critical applications of TexasInstruments semiconductor products and disclaimers thereto appears at the end of this data sheet.
PRODUCTION DATA information is current as of publication date.
Copyright © 1995–2007, Texas Instruments IncorporatedProducts conform to specifications per the terms of the TexasInstruments standard warranty. Production processing does notnecessarily include testing of all parameters.
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DESCRIPTION (cont.)
CONNECTION DIAGRAMS
DIL-24, SOIC-24 (TopView)
NandDWPackages
QPackage
PLCC-28(TopView)
ABSOLUTE MAXIMUM RATINGS
(1)
RECOMMENDED OPERATING CONDITIONS
UC2849
UC3849
SLUS360C – JULY 1995 – REVISED AUGUST 2007
The UC3849 features load share, oscillator synchronization, undervoltage lockout, and programmable outputcontrol. Multiple chip operation can be achieved by connecting up to ten UC3849 chips in parallel. The SHAREbus and CLKSYN bus provide load sharing and synchronization to the fastest oscillator respectively. TheUC3849 is an ideal controller to achieve high power, secondary side average current mode control.
VALUE UNIT
Supply Voltage (V
CC
) 20 VOutput current source or sink 0.3 AAnalog input voltages –0.3 to 7
VILIM, KILL, SEQ, ENBL, RUN –0.3 to 7CLKSYN current source 12RUN current sink 15
mASEQ current sink 20RDEAD current sink 20Share bus voltage (voltage with respect to GND) 0 to 6.2ADJ voltage (voltage with respect to GND) 0.9to 6.3 VVVEE (voltage with respect to GND) –1.5Storage temperaturee –65 to 150Junction temperature –65 to 150 °CLead temperature (soldering, 10 sec.) 300
(1) All voltages with respect to VEE except where noted; all currents are positive into, negative out of the specified terminal.
MIN MAX UNIT
Input voltage 8 20 VSink/source output current 250 mA
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ELECTRICAL CHARACTERISTICS
(1)
UC2849
UC3849
SLUS360C – JULY 1995 – REVISED AUGUST 2007
RECOMMENDED OPERATING CONDITIONS (continued)
MIN MAX UNIT
Timing resistor (RT) 1 200 k ΩTiming capacitor (CT) 75 2000 pF
Unless otherwise stated these specifications apply for T
A
= –40 °C to 85 °C for UC2849; and 0 °C to 70 °C for UC3849; V
CC
=12 V, VEE = GND, Output no load, C
T
= 345 pF, RT = 4530 Ω, RDEAD = 511 Ω, R
CLKSYN
= 1 k Ω, T
A
= T
J
.
PARAMETER TEST CONDITIONS MIN TYP MAX UNIT
Current Sense Amplifier
Ib 0.5 3 μAT
A
= 25 °C 3V
IO
mVOver Temperature 5Avo 60 90 dBGBW
(2)
Acl = 1, R
IN
= 1 k Ω, CC = 15 pF, f = 200 kHz
(3)
4.5 7 MHzV
OL
I
O
= 1 mA, voltage above VEE 0.5I
O
= 0 mA 3.8 VV
OH
I
O
= –1 mA 3.5CMRR –0.2 < Vcm < 6.5 V 80
dBPSRR 10 V < VCC < 20 V 80
Current Error Amplifier
Ib 0.5 3 μAV
IO
3 20 mVAvo 60 90 dBGBW
(2)
Acl = 1, R
IN
= 1 k Ω, CC = 15 pF, f = 200 kHz
(3)
4.5 7 MHzV
OL
I
O
= 1 mA, voltage above VEE 0.5I
O
= 0 mA 3.8 VV
OH
I
O
= –1 mA 3.5CMRR –0.1 < Vcm < 6.5 V 80
dBPSRR 10 V < VCC < 20 V 80
Voltage Error Amplifier
Ib 0.5 3 μAV
IO
2 5 mVAvo 60 90 dBGBW
(2)
f = 200 kHz 4.5 7 MHzV
OL
I
O
= 175 μA, voltage above VEE 0.3 0.6
VV
OH
ILIM > 3 V 2.85 3 3.15V
OH
– ILIM Tested ILIM = 0.5 V, 1.0 V, 2.0 V –100 100 mVCMRR –0.1 < Vcm < 6.5 V 80
dBPSRR 10 V < VCC < 20 V 80
(1) Unless otherwise specified all voltages are with respect to GND. Currents are positive into, negative out of the specified terminal.(2) Ensured by design not 100% tested in production.(3) If a closed loop gain greater than 1 is used, the possible GBW will increase by a factor of ACL + 10; where ACL is the closed loop gain.
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UC2849
UC3849
SLUS360C – JULY 1995 – REVISED AUGUST 2007
ELECTRICAL CHARACTERISTICS (continued)Unless otherwise stated these specifications apply for T
A
= –40 °C to 85 °C for UC2849; and 0 °C to 70 °C for UC3849; V
CC
=12 V, VEE = GND, Output no load, C
T
= 345 pF, RT = 4530 Ω, RDEAD = 511 Ω, R
CLKSYN
= 1 k Ω, T
A
= T
J
.
PARAMETER TEST CONDITIONS MIN TYP MAX UNIT
2X Amplifier and Share Amplifier
V offset (b; y = mx + b) 20 mVGAIN (m; y = mx + b) Slope with AV
OUT
= 1 V and 2 V 1.98 2.02 VGBW
(2)
100 kHzR
SHARE
VCC = 0, V
SHARE
/I
SHARE
200 k ΩNegative supply is VEE, GND Open,Total offset –75 0 75 mVVAO = GNDV
OL
VAO = voltage amplifier Vol, volts above VEE 0.05 0.45 0.6I
O
= 0 mA, ILIM = 3 V, VAO = voltage amp V
OH
5.7 6 6.3 VV
OH
I
O
= –1mA, ILIM = 3 V, VAO = voltage amp V
OH
5.7 6 6.3
Adjust Amplifier
V
IO
40 60 80 mVgm I
OUT
= –10 μA to 10 μA, V
OUT
= 3.5 V, C
ADJ
= 1 μF –1 mSI
OUT
= 0 0.9 1 1.1V
OL
I
OUT
= 50 μA 0.85 1 1.15
VI
OUT
= 0 , V
SHARE
= 6.5 V 5.7 6 6.3V
OH
I
OUT
= –50 μA, V
SHARE
= 6.5 V 5.7 6 6.3
Oscillator
Frequency 450 500 550 kHzMax duty cycle 80% 85% 90%OSC range amplitude 2 2.5 2.8 V
Clock Driver/SYNC (CLKSYN)
V
OL
0.02 0.23.6 VV
OH
R
CLKSYN
= 200 Ω3.2I
SOURCE
25 mAR
CLKSYN
V
CC
= 0, V
CLKSYN/ICLKSYN
10 k ΩV
TH
1.5 V
VREF Comparator
Turn-on threshold 4.72
VHysteresis 0.4
4
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UC2849
UC3849
SLUS360C – JULY 1995 – REVISED AUGUST 2007
ELECTRICAL CHARACTERISTICS (continued)Unless otherwise stated these specifications apply for T
A
= –40 °C to 85 °C for UC2849; and 0 °C to 70 °C for UC3849; V
CC
=12 V, VEE = GND, Output no load, C
T
= 345 pF, RT = 4530 Ω, RDEAD = 511 Ω, R
CLKSYN
= 1 k Ω, T
A
= T
J
.
PARAMETER TEST CONDITIONS MIN TYP MAX UNIT
VCC Comparator
Turn-on threshold 7.9 8.3 9.5
VHysteresis 0.4
KILL Comparator
Voltage threshold 3 V
Sequence Comparator
Voltage threshold 2.5
VSEQ SAT 0.25
ENABLE Comparator
Voltage threshold 2.5
VRUN SAT 0.25
Reference
T
A
= 25 °C 4.95 5 5.05VREF VVCC = 15 V 4.9 5.1Line regulation 10 < VCC < 20 3 15
mVLoad regulation 0 < I
O
< 10 mA 3 15Short circuit I VREF = 0 V 30 60 90 mA
Output Stage
Rise time C
L
= 100 pF 10 20
nsFall time C
L
= 100 pF 10 20VCC > 11 V, I
O
= –10 mA 8.0 8.4 8.8V
OH
I
O
= –200 mA 7.8
VI
O
= 200 mA 3.0V
OL
I
O
= 10 mA 0.5
Virtual Ground
VEE is externally supplied, GND is floating and used asV
GND
-VEE 0.2 0.75 Vsignal GND
Icc
Icc (run) 21 33 mA
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Pin Descriptions
Frequency [1
TCHARGE )TDISCHARGE
(1)
Maximum Duty Cycle [TCHARGE
TCHARGE )TDISCHARGE
(2)
UC2849
UC3849
SLUS360C – JULY 1995 – REVISED AUGUST 2007
ADJ: The output of the transconductance (gm = –1 ms) amplifier adjusts the control voltage to maintain equalcurrent sharing. The chip sensing the highest output current will have its output clamped to 1 V. A resistordivider between VREF and ADJ drives the control voltage (VA+) for the voltage amplifier. Each slave unit's ADJvoltage increases (to a maximum of 6 V) its control voltage (VA+) until its load current is equal to the master.The 60-mV input offset on the gm amplifier specifies that the unit sensing the highest load current is chosen asthe master. The 60-mV offset ensures by design to be greater than the inherent offset of the gm amplifier andthe buffer amplifier. While the 60-mV offset represents an error in current sharing, the gain of the current and 2Xamplifiers reduces it to only 30 mV. This pin needs a 1- μF capacitor to compensate the amplifier.to the master.
CA–: The inverting input to the current error amplifier. This amplifier needs a capacitor between CA– and CAOto set its dominant pole.
CAO: The output of the current error amplifier which is internally clamped to 4 V. It is internally connected to theinverting input of the PWM comparator.
CS–, CS+: The inverting and non-inverting inputs to the current sense amplifier. This amplifier is not internallycompensated so the user must compensate externally to attain the highest GBW for the application.
CLKSYN: The clock and synchronization pin for the oscillator. This is a bidirectional pin that can be used tosynchronize several chips to the fastest oscillator. Its input synchronization threshold is 1.4 V. The CLKSYNvoltage is 3.6 V when the oscillator capacitor (CT) is being discharged, otherwise it is 0 V. If the recommendedsynchronization circuit is not used, a 1 k Ωor lower value resistor from CLKSYN to GND may be needed toincrease fall time on CLKSYN pin.
CSO: The output of the current sense amplifier which is internally clamped to 4 V.
ENBL: The active low input with a 2.5-V threshold enables the output to switch. SEQ and RUN are driven lowwhen ENBL is above its 2.5-V threshold.
GND: The signal ground used for the voltage sense amplifier, current sense amplifier, current error amplifier,voltage reference, 2X amplifier, and share amplifier. The output sink transistor is wired directly to this pin.
KILL: The active low input with a 3.0-V threshold stops the output from switching. Once this function is activatedRUN must be cycled low by driving KILL above 3.0 V and either resetting the power to the chip (VCC) orresetting the ENBL signal.
ILIM: A voltage on this pin programs the voltage error amplifier’s Voh clamp. The voltage error amplifier outputrepresents the average output current. The Voh clamp consequently limits the output current. If ILIM is tied toVREF, it defaults to 3.0 V. A voltage less than 3.0 V connected to ILIM clamps the voltage error amplifier at thisvoltage and consequently limits the maximum output current.
OSC: The oscillator ramp pin which has a capacitor (CT) to ground and a resistor (RDEAD) to the RDEAD pinprograms its maximum duty cycle by programming a minimum dead time. The ramp oscillates between 1.2 V to3.4 V when an RDEAD resistor is used. The maximum duty cycle can be increased by connecting RDEAD toOSC which changes the oscillator ramp to vary between 0.2 V and 3.5 V. In order to ensure zero duty cycle inthis configuration VEE should not be connected to GND.
The charge time is approximately T
CHARGE
= R
T
•C
T
when the RDEAD resistor is used.
The dead time is approximately T
DISCHARGE
= 2 •RDEAD •C
T
.
The C
T
capacitance should be increased by approximately 40 pF to account for parasitic capacitance.
OUT: The output of the PWM driver. It has an upper clamp of 8.5 V. The peak current sink and source are 250mA. All UVLO, SEQ, ENBL, and KILL logic either enable or disable the output driver.
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VREF
2 RT
(3)
UC2849
UC3849
SLUS360C – JULY 1995 – REVISED AUGUST 2007
RDEAD: The pin that programs the maximum duty cycle by connecting a resistor between it and OSC. Themaximum duty cycle is decreased by increasing this resistor value which increases the discharge time. Thedead time, the time when the output is low, is 2 •RDEAD •C
T
. The C
T
capacitance should be increased byapproximately 40 pF to account for parasitic capacitance.
RT: This pin programs the charge time of the oscillator ramp. The charge current is
The charge time is approximately T
CHARGE
≈R
T
•C
T
when the RDEAD resistor is used.
The dead time is approximately T
DISCHARGE
≈2•RDEAD •C
T
.
RUN: This is an open collector logic output that signifies when the chip is operational. RUN is pulled high toVREF through an external resistor when VCC is greater than 8.4 V, VREF is greater than 4.65 V, SEQ is greaterthan 2.5 V, and KILL lower than 3.0 V. RUN connected to the VA+ pin and to a capacitor to ground adds an RCrise time on the VA+ pin initiating a soft start.
SEQ: The sequence pin allows the sequencing of startup for multiple units. A resistor between VREF and SEQand a capacitor between SEQ and GND creates a unique RC rise time for each unit which sequences the outputstartup.
SHARE: The nearly dc voltage representing the average output current. This pin is wired directly to all SHAREpins and is the load share bus.
VA+, VA–: The inverting and non-inverting inputs to the voltage error amplifier.
VAO: The output of the voltage error amplifier. Its Voh is clamped with the ILIM pin.
VCC: The input voltage of the chip. The chip is operational between 8.4 V and 20 V.
VEE: The negative supply to the chip which powers the lower voltage rail for all amplifiers. The chip isoperational if VEE is connected to GND or if GND is floating. When voltage is applied externally to VEE, GNDbecomes a virtual ground because of an internal diode between VEE and GND. The GND current flows throughthe forward biased diode and out VEE. GND is always the signal ground from which the voltage reference andall amplifier inputs are referenced.
VREF: The reference voltage equal to 5.0 V.
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Circuit Block Description
PWM Oscillator
IRT [2.5 V
RT
UC2849
UC3849
SLUS360C – JULY 1995 – REVISED AUGUST 2007
The oscillator block diagram with external connections is shown in Figure 1 . A resistor (R
T
) connected to pin RTsets the linear charge current;
Figure 1. Oscillator Block with External Connections
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VAOCURRENTCOMMAND
3.4V
1.2V
3.6V
1.4V (THRESHOLD)
8.5V
0V
OSC
CLKSYN
OUT
MINIMUMOUTPUTLOWTIME
75
100
50
0
25
1 10 100 1k 10k 100k 1M
R -
TW
C =345pF
T
READ=51
READ=5000
READ=511
1 10 100 1k 10k 100k 1M
R -
TW
CT =345pF
1nF
2.2nF
4.7nF
READ=500 W
1000
10k
100
10
1
f-Frequency-Hz
UC2849
UC3849
SLUS360C – JULY 1995 – REVISED AUGUST 2007
The timing capacitor (C
T
) is linearly charged with the charge current forcing the OSC pin to charge to a 3.4-Vthreshold. After exceeding this threshold, the RS flip-flop is set driving CLKSYN high and RDEAD low whichdischarges C
T
. This discharge time with the RC time delay of 2 •C
T
•RDEAD is the minimum output low time.OSC continues to discharge until it reaches a 1.2 -V threshold and resets the RS flip-flop which repeats thecharging sequence as shown in Figure 2 . Equations to approximate frequency and maximum duty cycle arelisted under the OSC pin description. Figure 3 and Figure 4 graphs show measured variation of frequency andmaximum duty cycle with varying RT, CT, and RDEAD component values.
As shown in Figure 5 , several oscillators are synchronized to the highest free running frequency by connecting100-pF capacitors in series with each CLKSYN pin and connecting the other side of the capacitors togetherforming the CLKSYN bus. The CLKSYN bus is then pulled down to ground with a resistance of approximately 10kΩ. Referring to Figure 1 , the synchronization threshold is 1.4 V. The oscillator blanks any synchronization pulsethat occurs when OSC is below 2.5 V. This allows units, once they discharge below 2.5 V, to continue throughthe current discharge and subsequent charge cycles whether or not other units on the CLKSYN bus are stillsynchronizing. This requires the frequency of all free running oscillators to be within 40% of each other to assuresynchronization.
Figure 2. Oscillator and PWM Output Waveform
Figure 3. Output Frequency Figure 4. Maximum Duty Cycle
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Grounds, Voltage Sensing and Current Sensing
100pF
100pF
100pF
100pF
CLKSYN
CLKSYN
CLKSYN
CLKSYN
20
20
20
20
OSC1
OSC2
OSC3
OSC10
C
L
K
S
Y
N
B
U
S
10kW
UC2849
UC3849
SLUS360C – JULY 1995 – REVISED AUGUST 2007
The voltage is sensed directly at the load. Proper load sharing requires the same sensed voltage for each powersupply connected in parallel. Referring to Figure 6 , the positive sense voltage (VSP) connects to the voltageerror amplifier inverting terminal (VA–), the return lead for the on-chip reference is used as the negative sense(VSM). The current is sensed across the shunt resistor, R
S
.
Figure 6 shows one recommended voltage and current sensing scheme when VEE is connected to GND. Thesignal ground is the negative sense point for the output voltage and the positive sense point for the outputcurrent. The voltage offset on the current sense amplifier is not needed if VEE is separated from GND. VEE isthe negative supply for the current sense amplifier. When it is separated from GND, it extends the current senseamplifier's common mode input voltage range to include VEE which is approximately –0.7 V below ground. Theresistor R
ADJ
is used for load sharing. The unit which is the master will force V
ADJ
to 1.0 V. Therefore, theregulated voltage being sensed is actually:
Figure 5. Oscillator Synchronization
Connection Diagram
Figure 6. Voltage and Current Sense VEETied to GND
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VSP *VSM +ǒVREF *VADJǓ ǒRADJ
R1 )RADJǓ)VADJ
(4)
VSM +0 V, VADJ +1 V(master), VREF +5 V
(5)
VSP +4 ǒRADJ
R1 )RADJǓ)1 V
(6)
Phase
AVO
qm=50º
200
160
120
80
40
0
10 100 1k 10k 100k 1M 10M 100M
f-Frequency-Hz
Gain-dB
Startup and Shutdown
UC2849
UC3849
SLUS360C – JULY 1995 – REVISED AUGUST 2007
The ADJ pin voltage on the slave chips will increase forcing their load currents to increase to match the master.
The ac frequency response of the voltage error amplifier is shown in Figure 7 .
Figure 7. AC Frequency Response of theVoltage Error Amplifier
Isolated power up can be accomplished using the UCC1889. Application Note U-149 is available for additionalinformation.
The UC3849 offers several features that enhance startup and shutdown. Soft start is accomplished byconnecting RUN to VA+ and a capacitor to ground. The resulting RC rise time on the VA+ pin initiates a softstart. It can also be accomplished by connecting RUN to ILIM. When RUN is low it commands zero load current,assuring a soft start. The undervoltage lockout (UVLO) is a logical AND of ENBL < 2.5 V, SEQ > 2.5 V, VCC >8.4 V and VREF > 4.65 V. The block diagram shows that the thresholds are set by comparators. By placing anRC divider on the SEQ pin, the enabling of multiple chips can be sequenced with different RC time constants.Similarly, different RC time constants on the ENBL pins can sequence shutdown. The UVLO keeps the outputfrom switching; however the internal reference starts up with VCC less than 8.4 V. The KILL input shuts downthe switching of the chip. This can be used in conjunction with an overvoltage comparator for overvoltageprotection. In order to restart the chip after KILL has been initiated, the chip must be powered down and thenback up. A pulse on the ENBL pin also accomplishes this without actually removing voltage to the VCC pin.
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Load Sharing
Current Control Loop
Frequency(0 dB) +1
2pRINV CCOMP
(7)
20.8 +20 log 1
11
(8)
UC2849
UC3849
SLUS360C – JULY 1995 – REVISED AUGUST 2007
Load sharing is accomplished similar to the UC1907. The sensed current for the UC3849 has an ac componentthat is amplified and then averaged. The voltage error amplifier output is the current command signalrepresenting the average output load current. The ILIM pin programs the upper clamp voltage of this amplifierand consequently the maximum load current. A gain of 2 amplifier connected between the voltage error amplifieroutput and the share amplifier input increases the current share resolution and noise margin. The averagecurrent is used as an input to a source only load share buffer amplifier. The output of this amplifier is the currentshare bus. The device with the highest sensed current will have the highest voltage on the current share busand consequently act as the master. The 60-mV input offset ensures that the unit sensing the highest loadcurrent is chosen as the master.
The adjust amplifier is used by the remaining (slave) devices to adjust their respective references high in orderto balance each device's load current. The master's ADJ pin will be at its 1.0-V clamp and connected back to thenon-inverting voltage error amplifier input through a high value resistor. This requires the user to initiallycalculate the control voltage with the ADJ pin at 1.0 V.
VREF can be adjusted 150 mV to 300 mV which compensates for 5% unit to unit reference mismatch andexternal resistor mismatch. R
ADJ
typically is 10 to 30 times larger than R1. This also attenuates the overallvariation of the ADJ clamp of 1 V ±100 mV by a factor of 10 to 30, contributing only a 3 mV to 10 mV additionaldelta to VREF. Refer to the UC3907 Application Note U-130 for further information on parallel power supply loadsharing.
The current sense amplifier (CSA) is designed specifically for the task of sensing and amplifying the inductorripple current at frequencies up to 1 MHz. The CSA's input offset voltage (VIO) is trimmed to less than 1 mV tominimize error of the average current signal. This amplifier is not internally compensated allowing the user tooptimally choose the zero crossing bandwidth.on on parallel power supply load sharing.
R
INV
is the input resistance at the inverting terminal CS– C
COMP
is the capacitance between C
S
– and CSO.
Although it is only unity gain stable for a GBW of 7 MHz, the amplifier is typically configured with a differentialgain of at least 10, allowing the amplifier to operate at 70 MHz with sufficient phase margin. A closed loop gainof 10 attenuates the output by 20.8 dB to the inverting terminal assuring stability. The amplifier’s gain fed backinto the inverting terminal is less than unity at 7 MHz, where the phase margin begins to roll off. See Figure 8 fortypical Bode plot.
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200
160
120
80
40
0
10 100 1k 10k 100k 1M 10M 100M
f-Frequency-Hz
Phase
qm=70º
A -
V O b
A =OpenLoopGain
VO
A =-10
VC
Gain-dB
RS+VRS
Max ILOAD
(9)
CSGAIN +VILIM
VRS
(10)
GCA +VS fs
ǒVOńLǓ RS CSGAIN
;
(11)
UC2849
UC3849
SLUS360C – JULY 1995 – REVISED AUGUST 2007
Figure 8. Current Sense Amplifier andCurent Error Amplifier Bode Plot
The gain of the differential current sense amplifier (CS
GAIN
) is calculated by knowing the maximum load current.The maximum voltage across the shunt resistor (R
S
) divided by R
S
is the maximum load current. By amplifyingthe voltage across RS, V
RS
, to be equal to the voltage error amplifier Voh, the current control loop keeps theload from exceeding its current limit. Voh is set at 3.0 V if ILIM is connected to VREF. The maximum currentlimit clamp can be reduced by reducing the voltage at ILIM to less than 3.0 V as described in the ILIM pindescription.
The current error amplifier (CEA) also needs its loop compensated by the user with the same criteria as thecurrent sense amplifier. This amplifier is essentially the same wide bandwidth amplifier without the input offsetvoltage trim. The zero crossing can also be approximately calculated with Equation 7 . The gain bandwidth of thecurrent loop is optimized by matching the inductor downslope (V
O
/L) to the oscillator ramp slope (V
S
•f
S
).Subharmonic oscillation problems are avoided by keeping the amplified inductor downslope less than theoscillator ramp slope.
The following equation determines the current error amplifier gain (GCA):
where CS
GAIN
and R
S
are defined by Equation 9 and Equation 10 ,Vs is the oscillator peak to peak voltage,fs is the oscillator frequency,V
O
is the output voltage,and L is the inductance.
Additional Information about average current mode control can be found in Unitrode Application Note U-140.
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Design Example
5
6
C1
1.0mF
7
8
1
2
3
4
9
10
11
12
20
19
18
17
24
23
22
21
16
15
14
13
ADJ
ILIM
SHARE
OSC
VA-
VA+
RDEAD
RT
VAO
CA-
CLKSYN
VEE
CAO
CS+
GND
OUT
CS-
CSO
VCC
RUN
ENBL
SEQ
VREF
KILL
R1
500kW
C4
345pF
R2
5kW
R4
10kW
R3
2kW
C5
0.1mF
C5
0.1mF
+20V
+20
R14, 10kW
C3, 100pF
R17, 10kW
R13, 1kW
R18, 5kW
R6
15kW
10kW
R9
10kW
R11, 1kW
R10, 10kW
C2 100pF
R12
10 kW
R7
10kW
+
VISENSE
UC2849
UC3849
SLUS360C – JULY 1995 – REVISED AUGUST 2007
Figure 9 is an open loop test that lets the user test the circuit blocks discussed without having to build an entirecontrol loop. The pulse width can be varied by either the V
ADJ
or the VI
SENSE
inputs. Figure 10 shows an isolatedpower supply using the UC3849 secondary side average current mode controller.
Figure 9. Open Loop Circuit
14
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UC2849
UC3849
SLUS360C – JULY 1995 – REVISED AUGUST 2007
Figure 10. UC3849 Application Diagram
15Submit Documentation Feedback
PACKAGING INFORMATION
Orderable Device Status (1) Package
Type Package
Drawing Pins Package
Qty Eco Plan (2) Lead/Ball Finish MSL Peak Temp (3)
UC2849DW ACTIVE SOIC DW 24 25 Green (RoHS &
no Sb/Br) CU NIPDAU Level-2-260C-1 YEAR
UC2849DWG4 ACTIVE SOIC DW 24 25 Green (RoHS &
no Sb/Br) CU NIPDAU Level-2-260C-1 YEAR
UC2849DWTRG4 ACTIVE SOIC DW 24 TBD Call TI Call TI
UC2849N ACTIVE PDIP N 24 15 Green (RoHS &
no Sb/Br) CU NIPDAU N / A for Pkg Type
UC2849NG4 ACTIVE PDIP N 24 15 Green (RoHS &
no Sb/Br) CU NIPDAU N / A for Pkg Type
UC3849DW ACTIVE SOIC DW 24 25 Green (RoHS &
no Sb/Br) CU NIPDAU Level-2-260C-1 YEAR
UC3849DWG4 ACTIVE SOIC DW 24 25 Green (RoHS &
no Sb/Br) CU NIPDAU Level-2-260C-1 YEAR
UC3849DWTR ACTIVE SOIC DW 24 2000 Green (RoHS &
no Sb/Br) CU NIPDAU Level-2-260C-1 YEAR
UC3849DWTRG4 ACTIVE SOIC DW 24 2000 Green (RoHS &
no Sb/Br) CU NIPDAU Level-2-260C-1 YEAR
UC3849NG4 ACTIVE PDIP N 24 TBD Call TI Call TI
UC3849Q ACTIVE PLCC FN 28 37 TBD Call TI Level-2-220C-1 YEAR
UC3849QTR ACTIVE PLCC FN 28 750 TBD Call TI Level-2-220C-1 YEAR
(1) The marketing status values are defined as follows:
ACTIVE: Product device recommended for new designs.
LIFEBUY: TI has announced that the device will be discontinued, and a lifetime-buy period is in effect.
NRND: Not recommended for new designs. Device is in production to support existing customers, but TI does not recommend using this part in
a new design.
PREVIEW: Device has been announced but is not in production. Samples may or may not be available.
OBSOLETE: TI has discontinued the production of the device.
(2) Eco Plan - The planned eco-friendly classification: Pb-Free (RoHS), Pb-Free (RoHS Exempt), or Green (RoHS & no Sb/Br) - please check
http://www.ti.com/productcontent for the latest availability information and additional product content details.
TBD: The Pb-Free/Green conversion plan has not been defined.
Pb-Free (RoHS): TI's terms "Lead-Free" or "Pb-Free" mean semiconductor products that are compatible with the current RoHS requirements
for all 6 substances, including the requirement that lead not exceed 0.1% by weight in homogeneous materials. Where designed to be soldered
at high temperatures, TI Pb-Free products are suitable for use in specified lead-free processes.
Pb-Free (RoHS Exempt): This component has a RoHS exemption for either 1) lead-based flip-chip solder bumps used between the die and
package, or 2) lead-based die adhesive used between the die and leadframe. The component is otherwise considered Pb-Free (RoHS
compatible) as defined above.
Green (RoHS & no Sb/Br): TI defines "Green" to mean Pb-Free (RoHS compatible), and free of Bromine (Br) and Antimony (Sb) based flame
retardants (Br or Sb do not exceed 0.1% by weight in homogeneous material)
(3) MSL, Peak Temp. -- The Moisture Sensitivity Level rating according to the JEDEC industry standard classifications, and peak solder
temperature.
Important Information and Disclaimer:The information provided on this page represents TI's knowledge and belief as of the date that it is
provided. TI bases its knowledge and belief on information provided by third parties, and makes no representation or warranty as to the
accuracy of such information. Efforts are underway to better integrate information from third parties. TI has taken and continues to take
reasonable steps to provide representative and accurate information but may not have conducted destructive testing or chemical analysis on
incoming materials and chemicals. TI and TI suppliers consider certain information to be proprietary, and thus CAS numbers and other limited
information may not be available for release.
In no event shall TI's liability arising out of such information exceed the total purchase price of the TI part(s) at issue in this document sold by TI
to Customer on an annual basis.
PACKAGE OPTION ADDENDUM
www.ti.com 19-Jun-2007
Addendum-Page 1
TAPE AND REEL BOX INFORMATION
Device Package Pins Site Reel
Diameter
(mm)
Reel
Width
(mm)
A0 (mm) B0 (mm) K0 (mm) P1
(mm) W
(mm) Pin1
Quadrant
UC3849DWTR DW 24 SITE 41 330 24 10.85 15.8 2.7 12 24 Q1
UC3849QTR FN 28 SITE 41 330 24 12.95 12.95 5.0 16 24 Q1
PACKAGE MATERIALS INFORMATION
www.ti.com 8-Oct-2007
Pack Materials-Page 1
Device Package Pins Site Length (mm) Width (mm) Height (mm)
UC3849DWTR DW 24 SITE 41 346.0 346.0 41.0
UC3849QTR FN 28 SITE 41 346.0 346.0 41.0
PACKAGE MATERIALS INFORMATION
www.ti.com 8-Oct-2007
Pack Materials-Page 2
MECHANICAL DATA
MPDI006B – SEPTEMBER 2001 – REVISED APRIL 2002
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
N (R–PDIP–T24) PLASTIC DUAL–IN–LINE
0.020 (0,51) MIN
0.021 (0,53)
0.015 (0,38)
0.100 (2,54)
1
24
0.070 (1,78) MAX 12
13
1.222 (31,04) MAX
0.125 (3,18) MIN
0’–15’
0.010 (0,25) NOM
0.425 (10,80) MAX
Seating Plane
0.200 (5,08) MAX
0.360 (9,14) MAX
0.010 (0,25)
4040051–3/D 09/01
NOTES: A. All linear dimensions are in inches (millimeters).
B. This drawing is subject to change without notice.
C. Falls within JEDEC MS–010
MECHANICAL DATA
MPDI008 – OCTOBER 1994
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
N (R-PDIP-T**) PLASTIC DUAL-IN-LINE PACKAGE
24 PIN SHOWN
12
Seating Plane
0.560 (14,22)
0.520 (13,21)
13
0.610 (15,49)
0.590 (14,99)
524840
0.125 (3,18) MIN
2.390
(60,71)
(62,23)(53,09)
(51,82)
2.040
2.090 2.450 2.650
(67,31)
(65,79)
2.590
0.010 (0,25) NOM
4040053/B 04/95
A
0.060 (1,52) TYP
1
24
322824
1.230
(31,24)
(32,26) (36,83)
(35,81)
1.410
1.450
1.270
PINS **
DIM
0.015 (0,38)
0.021 (0,53)
A MIN
A MAX 1.650
(41,91)
(40,89)
1.610
0.020 (0,51) MIN
0.200 (5,08) MAX
0.100 (2,54)
M
0.010 (0,25) 0°–15°
NOTES: A. All linear dimensions are in inches (millimeters).
B. This drawing is subject to change without notice.
C. Falls within JEDEC MS-011
D. Falls within JEDEC MS-015 (32 pin only)
MECHANICAL DATA
MPLC004A – OCTOBER 1994
1
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
FN (S-PQCC-J**) PLASTIC J-LEADED CHIP CARRIER
4040005/B 03/95
20 PIN SHOWN
0.026 (0,66)
0.032 (0,81)
D2/E2
0.020 (0,51) MIN
0.180 (4,57) MAX
0.120 (3,05)
0.090 (2,29)
D2/E2
0.013 (0,33)
0.021 (0,53)
Seating Plane
MAX
D2/E2
0.219 (5,56)
0.169 (4,29)
0.319 (8,10)
0.469 (11,91)
0.569 (14,45)
0.369 (9,37)
MAX
0.356 (9,04)
0.456 (11,58)
0.656 (16,66)
0.008 (0,20) NOM
1.158 (29,41)
0.958 (24,33)
0.756 (19,20)
0.191 (4,85)
0.141 (3,58)
MIN
0.441 (11,20)
0.541 (13,74)
0.291 (7,39)
0.341 (8,66)
18
19
14
13
D
D1
13
9
E1E
4
8
MINMAXMIN
PINS
**
20
28
44
0.385 (9,78)
0.485 (12,32)
0.685 (17,40)
52
68
84 1.185 (30,10)
0.985 (25,02)
0.785 (19,94)
D/E
0.395 (10,03)
0.495 (12,57)
1.195 (30,35)
0.995 (25,27)
0.695 (17,65)
0.795 (20,19)
NO. OF D1/E1
0.350 (8,89)
0.450 (11,43)
1.150 (29,21)
0.950 (24,13)
0.650 (16,51)
0.750 (19,05)
0.004 (0,10)
M
0.007 (0,18)
0.050 (1,27)
NOTES: A. All linear dimensions are in inches (millimeters).
B. This drawing is subject to change without notice.
C. Falls within JEDEC MS-018
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