AVAILABLE
Functional Diagrams
Pin Configurations appear at end of data sheet.
Functional Diagrams continued at end of data sheet.
UCSP is a trademark of Maxim Integrated Products, Inc.
For pricing, delivery, and ordering information, please contact Maxim Direct
at 1-888-629-4642, or visit Maxim’s website at www.maximintegrated.com.
DESCRIPTION
The DS2715 is well suited for cost-sensitive charger
applications where the battery pack is either internal
or external to the application. It has been optimized for
safe and reliable charging of 1 to 10 NiMH cells in
series. The internal gain block can be selected as
either a comparator or transconductance amplifier for
charge current regulation. This makes the DS2715
configurable as a switched DC charger, a linear
current regulator, or a switchmode current source.
The DS2715 pre-conditions severely depleted cells
before entering full charge mode. It terminates full
charge using the dT/dt technique, which requires an
external sensing thermistor. Over-temperature, under-
temperature, and over-voltage detection prevents
charging under unsafe conditions. A user selectable
charge timer allows charge rates from 0.167C to 2C.
FAST-CHARGE, TOPOFF and DONE modes are
included for highly reliable, safe charging of NiMH
cells. Discharge mode allows the DS2715 to enter a
low power sleep state while the cell pack is being
discharged.
FEATURES
Charges 1 to 10 NiMH Cells
FAST-CHARGE at up to a 2C Rate
PRECHARGE and TOPOFF Charge Modes Help
Cell Conditioning
Load Detection Allows the DS2715 to Enter Low
Power Sleep Mode (Less than 10µA) while the
Cell Pack is Discharged
dT/dt Charge Termination Eliminates Cell Charge
Stress
Monitors Voltage, Temperature, and Time for
Safety and Secondary Termination
Regulates Current through Either Linear Control
or Switch-Mode Control
LED Output Displays Charge State
Small 16-Pin SO Package
APPLICATIONS
Portable DVD Players
Portable Television Sets
Handheld Gaming
Test Equipment
Handheld POS Terminals
PIN CONFIGURATION
Table 1. ORDERING INFORMATION
+ Denotes a lead(Pb)-free/RoHS-compliant package.
T&R = Tape and reel.
OPERATIONAL DIAGRAM
PART MARKING PIN-PACKAGE
DS2715Z+ D2715 16 SO
DS2715Z+T&R D2715 16 SO
DS2715BZ+ D2715B 16 SO
DS2715BZ+T&R D2715B 16 SO
DS2715
NiMH Battery Pack Charge Controlle
r
www.maxim-ic.com
See Table 1 for Ordering Information.
16 SO
CBIAS
VCH
VSS
LED1
DNC
VSS
CTG
CTG
VBATT
THM
RT
VDD
SNS-
SNS+
DIV
MODE
19-4592; 4/09
DS2715: NiMH Battery Pack Charge Controller
ABSOLUTE MAXIMUM RATINGS*
Voltage on VDD and VCH Pins with Respect to VSS -0.3V to +18V
Voltage on LED1 Pin -0.3V to +18V
Voltage on SNS- -0.3V to +0.3V
Voltage on CBIAS -0.3V to 6V
Voltage on all Other Pins -0.3V to VCbias
Continuous Sink Current VCH and LED1 28mA
Operating Temperature Range -20°C to +85°C
Storage Temperature Range -55°C to +125°C
Soldering Temperature See IPC/JEDEC J-STD-020
Human Body Model (HBM) ESD Limit of VCH Pin 500V
HBM ESD Limit of all Other Pins 2KV
*This is a stress rating only and functional operation of the device at these or any other conditions above those indicated in the operation
sections of this specification is not implied. Exposure to absolute maximum rating conditions for extended periods of time may affect reliability.
RECOMMENDED DC OPERATING CONDITIONS
(4.5V VDD 16.5V; TA = 0C to +70C.)
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
Supply Voltage VDD (Note 1) 4.5 16.5 V
LED1 Voltage VLED (Note 1) 0.0 16.5 V
Mode Voltage VMODE (Note 1) 0.0 VCbias V
VCH Voltage VVCH (Note 1) 0.0 16.5 V
CBIAS Capacitor Range CCbias .02 .15 μF
RT Resistor Range RRt 20 240 KΩ
DC ELECTRICAL CHARACTERISTICS
(4.5V VDD 16.5V, TA = 0C to +70C, unless otherwise noted.)
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
Linear Mode, VDD = 16.5V 1.3 1.6 mA
Operating Current
(Note 2) IDDA Comparator Mode,
VDD = 16.5V 150 250 μA
Idle Current IDDS V
DD < VUVLO 10 μA
Discharge Current IDDD Discharge latch set (Note 2) 200 μA
UVLO Threshold VUVLO V
DD Rising (Note 3) 3.8 3.9 4.0 V
UVLO Hysteresis VUVLO-HYS V
DD Falling 35 mV
VCH Sink Current IOL-Vch V
OL = 1.5V 20 mA
LED1 Sink Current IOL-LED V
OL = 1.0V 20 mA
Leakage Current, VCH,
LED1 ILKG Pin Inactive or Device Idle -1 +1 μA
THM Pin Leakage
Current ILKG-THM -1 +1 μA
VBATT Pin Leakage
Current ILKG-Vbatt -50 +50 nA
CBIAS Voltage VCbias 0 < ICbias < 0.4mA 3.9 4.0 4.3 V
DIV Pin Load Current IDiv 500 uA
Current Sense Amplifier
Gain GERR 100μA < I Vch < 20mA 5 6.25 7.5 Ω-1
DS2715: NiMH Battery Pack Charge Controller
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
Current Sense
Comparator Gain GCOMP (Note 7) 10 Ω-1
AC ELECTRICAL CHARACTERISTICS
(4.5V VDD 16.5V, TA = 0C to +70C, unless otherwise noted.)
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
UVLO Debounce Time tUVLO 10 μs
Current Sense
Comparator Propagation
Delay
tCOMP (Note 7) 250 ns
Discharge Detect
Propagation Delay tDD From detection of current
reversal 1 μs
Return To Normal
Function (Op-Amp or
Comparator Mode)
tRNF Time from reset of discharge
latch 1 μs
RT Timing Accuracy tRt (Note 4) -10 +10 %
Internal Clock Accuracy tBASE -10 +10 %
ELECTRICAL CHARACTERISTICS: CHARGING
(4.5V VDD 16.5V, TA = 0C to +70C, unless otherwise noted.)
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
FAST-CHARGE
Comparator Threshold VFC FAST-CHARGE -127 -121 -115 mV
FAST-CHARGE
Comparator Hysteresis VHYS-FC FAST-CHARGE 31 28 25 mV
TOPOFF and
PRECHARGE
Comparator Threshold
VTO TOPOFF and PRECHARGE -38 -33 -28 mV
TOPOFF and
PRECHARGE
Comparator Hysteresis
VHYS-TO TOPOFF and PRECHARGE 10 8 6 mV
Discharge Latch Reset
Threshold VDCHG-RST -15 -10 -5 mV
Discharge Latch Set
Threshold VDCHG-SET Reverse current through
sense resistor 5 10 15 mV
Low Battery Detect
Threshold VLB From presence detect into
PRECHARGE 0.95 1.0 1.05 V
Cell Detect Threshold VDET 1.50 1.55 1.60 V
No Cell Detect Threshold VOPEN 1.60 1.65 1.70 V
Presence Detect
Threshold Hysteresis VHYS-PD 90 100 110 mV
2.88 2.92 2.96 V
Minimum Charge Temp VTHM-MIN (Note 5, 6) 0 ºC
1.28 1.32 1.36 V
Maximum Charge Temp VTHM-MAX (Note 5, 6) 45 ºC
1.12 1.16 1.20 V
Over Temp VTHM-STOP (Note 5, 6) 50 ºC
dT/dt Detect TTERM 0.425 0.5 0.575 ºC/min
DS2715: NiMH Battery Pack Charge Controller
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
dT/dt Blanking Time tBLANK 3.85 4.3 4.75 Min
PRECHARGE Timeout tPC-TO 30 Min
FAST-CHARGE Timer
Range tFC 0.5 6 Hours
DS2715 1:2
TOPOFF to FAST-
CHARGE Duration Ratio
DS2715B 1:4
Note 1: Voltages relative to VSS.
Note 2: Does not include current through VCH, RT, and DIV pins.
Note 3: Below this voltage no I/O pins are active.
Note 4: Does not include tolerance of RT resistor.
Note 5: VBIAS and resistor tolerances must be added to determine actual threshold.
Note 6: Specified temperature thresholds are only valid if recommended thermistor types are used.
Note 7: Specification is guaranteed by design.
DETAILED PIN DESCRIPTION
PIN NAME DESCRIPTION
1 CBIAS Bypass for Internal Voltage Regulator
2 VCH Cell Stack Charge Control Output
3 VSS Chip Supply Return and Local Ground Reference
4 LED1 Charging Indicator Output
5 DNC Do Not Connect
6 VSS Chip Supply Return and Local Ground Reference
7 CTG Connect to Local Ground (VSS)
8 CTG Connect to Local Ground (VSS)
9 MODE
Mode Select. Connect to VSS for linear mode of operation or CBIAS for comparator
mode of operation.
10 DIV
Thermistor Divider. Stable output to form a resistor divider for measuring
temperature on THM
11 SNS+ Positive Current Sense. Connect to the charge source side of the sense resistor
12 SNS- Negative Current Sense. Connect to the cell stack side of the sense resistor
13 VDD Chip Supply Input: +4.5V to +16.5V range
14 RT Failsafe Timeout. Timeout is selected by an external resistor from RT to VSS
15 THM
Thermistor Input. Connect to a thermistor located in the cell pack and a divider
resistor from the Div pin
16 VBATT Battery Voltage Sense Input. Connect to a divider from the positive terminal of the
cell stack to measure the voltage of a single cell
DS2715: NiMH Battery Pack Charge Controller
Figure 1. BLOCK DIAGRAM
DS2715: NiMH Battery Pack Charge Controller
Figure 2. STATE DIAGRAM
Discharge Latch set
Discharge latch set
DS2715: NiMH Battery Pack Charge Controller
DETAILED DESCRIPTION
Charge Cycle Overview
The DS2715 regulates the charge of up to 10 NiMH cells in a series configuration. With the mode select pin, the
DS2715 can be configured to regulate either as an error amplifier in linear mode or as a comparator in switched
mode. A charge cycle begins in one of two ways: with the application of power to the DS2715 while the cell pack is
already inserted, or with the detection of cell insertion after power-up.
After cell presence confirmation, PRECHARGE qualification occurs to prevent fast charging of deeply depleted
cells or charging under extreme temperature conditions. Pre-charging is performed at a reduced rate of
approximately ¼ the FAST-CHARGE current until each cell reaches 1V. The algorithm then proceeds to the FAST-
CHARGE phase. Fast charging continues as long as the cell pack temperature is less than 50C based on the
THM voltage, and the cell voltage as measured at the VBATT pin in the middle of the resistor divider remains below
1.65V, indicating the cell pack is still present. Fast charging terminates normally by measuring the cell pack’s
thermal rate of change, dT/dt. When the cell pack’s thermal rate of change exceeds 0.5°C per minute, the DS2715
enters TOPOFF. The DS2715 has an internal charge timer as secondary overcharge protection if the charge is not
terminated properly by the dT/dt method. The charge termination timer duration is user selectable from 30 minutes
up to 6 hours by an external resistor on the RT pin.
Following a normally terminated or timed-out FAST-CHARGE phase, the DS2715 enters TOPOFF. It remains in
this state for one-half (one-quarter for DS2715B) of the period of the FAST-CHARGE timeout as selected by the
external resistor on RT. After the TOPOFF charge timer expires, the DONE phase continues indefinitely until the
cell pack is removed from the charger or DISCHARGE mode is entered. For the standard application circuit
configuration, when a load drawing at least Vdchg-set across the sense resistor is attached to the cell pack, the
DS2715 switches to DISCHARGE mode. All charge functions are disabled and the regulation FET is driven on to
allow the cell pack to discharge. Throughout the charging process, the open-drain LED1 output indicates the
charge status to the user.
Undervoltage Lockout (Reset)
The UVLO circuit serves as a power-up and brownout detector by monitoring VDD to prevent charging until VDD
rises above VUVLO, or when VDD drops below VUVLO - VUVLO-HYS. If UVLO is active, charging is prevented, the state
machine is forced to the RESET state, and all charge timers are reset. A 10s deglitch circuit provides noise
immunity. Once VDD reaches an acceptable operating voltage, the DS2715 enters the PRESENCE state.
DS2715: NiMH Battery Pack Charge Controller
PRESENCE
The DS2715 enters the PRESENCE state whenever VDD > VUVLO and VBATT > VOPEN indicating that the charge
source is present, but no cell is available to charge. The DS2715 will remain in the PRESENCE state until a cell is
inserted into the circuit causing the voltage on VBATT to fall below 1.55V (VDET) and the cell temperature is inside a
valid charging range between 0°C and 45°C (TTHM-MIN and TTHM-MAX when used with recommended thermistor and
resistor values). If both of these conditions are met, the DS2715 will enter PRE-CHARGE. If cells are inserted, but
the temperature is outside the valid charging range, the DS2715 will remain in the PRESENCE state until the cell
temperature falls within the valid charging range.
PRE-CHARGE
The DS2715 enters the PRECHARGE state when a valid cell voltage is applied to VBATT and the cell temperature
as measured by the DS2715 thermistor circuit is within the valid charging range. PRE-CHARGE mode has a 4
second filter to suppress noise on VBATT caused by cell insertion that may cause a premature return to PRESENCE
state. The DS2715 precharges the cell by regulating the voltage drop across the sense resistor to -33mV (VTO) in
linear mode or -29mV (VTO + 0.5 x VHYS-TO) in comparator mode. The polarity of the voltage drop across the sense
resistor is referenced to the polarity relationship indicated by the SNS+ and SNS- pins on the device. Precharging
will last until the cell voltage measured by VBATT exceeds 1.0V (VBATT > VLB), at which time the DS2715 will enter
the FAST-CHARGE state. If the cell voltage does not exceed VLB within 30 minutes or if the cell temperature
exceeds 50°C at any time during PRECHARGE, the DS2715 enters the FAULT state. If at any time during
PRECHARGE the voltage on VBATT exceeds 1.65V (VOPEN), the DS2715 determines that the cell pack has been
removed and returns to the PRESENCE state.
FAST-CHARGE
In FAST-CHARGE mode, the DS2715 regulates the voltage across the sense resistor to -121mV (VFC) in linear
mode or about -107mV (VFC + 0.5 x VHYS-FC) in comparator mode. LED1 indicates the cell pack is being charged.
During FAST-CHARGE, the DS2715 constantly measures the rate of change of the cell temperature (dT/dt). When
the cell pack’s dT/dt exceeds 0.5°C per minute (TTERM) the DS2715 enters the TOPOFF state. The DS2715 ignores
changes in the cell temperature caused by charge initiation for the first 4.3 minutes (tBLANK). As secondary
overcharge protection, the DS2715 will terminate FAST-CHARGE and enter TOPOFF based on a time delay set
by the external resistor on the RT pin. This resistor value can set the secondary charge termination delay to
anywhere from 30 minutes up to 6 hours. If the cell temperature exceeds 50°C at any time during FAST-CHARGE,
the DS2715 enters the DONE state. If at any time during FAST-CHARGE the voltage on VBATT exceeds 1.65V
(VOPEN), the DS2715 determines that the cell pack has been removed and returns to the PRESENCE state.
TOPOFF
In TOPOFF mode, the DS2715 regulates the voltage across the sense resistor to -33mV (VTO) in linear mode or -
29mV (VTO + 0.5 x VHYS-TO) in comparator mode. LED1 indicates the cell pack is being charged. The charge timer is
reset and restarted with a time-out period of one-half (one-quarter for DS2715B) the fast-charge duration. When
the charge timer expires or if the measured temperature exceeds 50°C, the charger enters the DONE state.
DONE/Maintenance
The DS2715 enters the DONE state whenever the charge completes normally or if the measured cell temperature
exceeds 50°C during the charge. While in the done state VCH is driven to high impedance to prevent further
regulated charging of the cell pack and LED is driven high-impedance to indicate no charging is taking place. A
maintenance charge can be applied to the cells by providing a one-way resistive path from the charge source to the
cell pack bypassing the regulating transistor. See the example in the circuit of Figure 3. The DS2715 remains in the
DONE state until a cell voltage greater than 1.65V (VOPEN) is detected on VBATT indicating cell pack removal, the Rt
pin is floated (see SUSPEND function), or DISCHARGE mode is entered.
FAULT
The DS2715 enters FAULT if PRECHARGE is unable to charge the cell above 1.0V (VLB) before the 30 minute
PRECHARGE timeout (tPC-TO) or if the cell temperature exceeds 50°C during PRECHARGE. In the fault state, VCH
is high impedance and LED1 output pulses to indicate the fault condition. The DS2715 remains in FAULT until a
cell voltage greater than 1.65V (VOPEN) is detected on VBATT indicating the cell pack has been removed. The
DS2715 then enters the PRESENCE state and waits for the next cell insertion.
SUSPEND
DS2715: NiMH Battery Pack Charge Controller
Suspension of charge activity is possible by floating the RT pin. The state machine and all timers are reset to their
presence test conditions when suspending from a charge mode. The DISCHARGE mode is not affected by the
SUSPEND function. The Vch output is high-impedance for charging modes, and operates as normal for
DISCHARGE mode during SUSPEND. The SUSPEND function is useful for resetting the DS2715 in applications
where the batteries are not removed from the circuit and DISCHARGE mode is not utilized. It also allows for a
means to stop charging by the application circuit, such as with a microcontroller signal for example.
DISCHARGE Mode
When the DS2715 detects a discharge current voltage drop of VDCHG-SET or greater across the sense resistor,
charging is terminated and the DS2715 enters the DISCHARGE state. Initially, the discharge current must flow
through the parasitic diode of the PFET regulating transistor until the DS2715 switches to DISCHARGE mode.
While in this mode, voltage sensing, thermal sensing, and the LED1 output are disabled. The VCH pin is driven low
to fully bias the charge control transistor into a low impedance state and allow the pack to be discharged. Current
drain of the DS2715 drops to IDDD. The DS2715 remains in DISCHARGE mode until a charge current across the
sense resistor causes a sense voltage of at least VDCHG-RST or the device is power cycled. When either of these
events occur, the DS2715 enters the PRESENCE state to begin a new charge cycle.
LED1 Output
Open-drain output LED1 pulls low to indicate charge status. When inactive, the output is high impedance. LED1
displays the state of charge and the charge results. The LED1 pin drives low in a 1Hz, 50% duty cycle “blink”
pattern to indicate cells are charging. LED1 blinks at 4Hz, 50% duty cycle to signal a charging fault has occurred.
The LED1 pin remains in a high-impedance state when no cells are present or the discharge latch is set. Table 2
summarizes the LED operation for each charge condition.
Table 2. LED DISPLAY PATTERNS BASED ON CHARGE STATE
CHARGE STATE
NO
BATTERY CHARGING DONE FAULT DISCHARGE
MODE
LED1 High-Z Blinks at 1HZ,
50% duty cycle High-Z Blinks at 4Hz,
50% duty cycle High-Z
High-Z = High Impedance
CURRENT REGULATION
Three basic modes of charging operation are supported by the DS2715: Offline switching through an optocoupler,
linear regulation, and DC input switched mode. The offline switching method requires a voltage clamp on the
regulated output. Mode of operation is selected through the Mode pin. Connecting the Mode pin to VSS configures
the analog block as a transconductance amplifier for linear mode of operation. Connecting the Mode pin to the
Cbias pin configures the DS2715 as a comparator for switched mode of operation.
Current-Sense Amplifier Mode
An error amplifier block provides several options to regulate the charge current. The 20mA open-drain output VCH
can drive a PMOS or PNP pass element for linear regulation, or the output can drive an optocoupler for isolated
feedback to a primary-side PWM controller. PMOS is the preferred device type when the pass element will also be
used as a discharge path to the load. This is because sufficient transconductance with both polarities of drain
current is easily realized compared to the difficulty in achieving this with BJT types. The SNS- pin is a remote-
sense return and should be connected to the battery ground side of the sense resistor using a separate isolated
conductor. During FAST-CHARGE, an error signal between the current-sense signal (across the sense resistor)
and the internal reference is produced so the voltage across the sense resistor is maintained at VFC in a closed-
loop circuit. During PRECHARGE and TOPOFF, the voltage across the sense resistor is maintained at VTO.
Current-Sense Comparator Mode
The comparator in the DS2715 switches the open drain VCH pin between ON and OFF when the sense resistor
voltage reaches high and low thresholds. The VCH output is capable of driving a PNP bipolar or a PMOS transistor,
enabling the use of a switch-mode power stage. PMOS is again preferred when the pass element will also be used
DS2715: NiMH Battery Pack Charge Controller
as a discharge path to the load. Hysteresis on the comparator input provides the difference between the ON and
OFF state thresholds. In a closed-loop regulation circuit, the comparator regulates voltage across the sense
resistor as referenced to the SNS pins to a DC average of:
VSNS = VFC + 0.5 x VHYS-FC = -0.107V during FAST-CHARGE
VSNS = VTO + 0.5 x VHYS-TO = -0.029V during TOPOFF
DS2715: NiMH Battery Pack Charge Controller
Figure 3. Ideal Comparator Input and Charge Control Output Waveforms
|Vsns|
Time
|Vsns(DC)|
|VFC|
|VFC + VHYS-FC|
VCH
VOL
VPullup
Not drawn to scale
Charge Rate Selection
The charge rate is determined by an external sense resistor connected between the SNS+ and SNS- pins. The
DS2715 will regulate the charge current to maintain a voltage drop of VFC (or VFC + 0.5 x VHYS-FC in comparator
mode) across the sense resistor during FAST-CHARGE. The sense resistor can therefore be selected by:
Linear Mode: R = |VFC / Desired FAST-CHARGE Current|
Comparator Mode: R = |(VFC + 0.5 x VHYS-FC) / Desired FAST-CHARGE Current|
Timeout Selection
The various charge modes employ different timers to ensure reliable and safe charging. PRECHARGE has a fixed
30 minute limit generated by an internal oscillator. A fault is generated if this time limit is exceeded. FAST-
CHARGE mode normally operates until its dT/dt termination scheme activates. In the event that the dT/dt sensing
does not perform correctly, a safety timeout is required. This timeout is set by an external resistor on the Rt pin to
VSS and provides secondary protection against significant overcharging. As such, the value of the Rt resistor
should be chosen so that the timeout is greater than the FAST-CHARGE time expected in the application, but not
so much greater that its protection is compromised. If the timer expires during FAST-CHARGE, the timer count is
reset and charging proceeds to the TOPOFF charge phase. The Rt resistor also sets the timed charge duration of
TOPOFF mode. The TOPOFF time-out period is fixed at half (a quarter for the DS2715B) of the FAST-CHARGE
time-out period. When the timer expires in TOPOFF, the DS2715 enters the DONE state.
Resistors can be selected to support fast-charge time-out periods of 0.5 to 6 hours and TOPOFF charge time-out
periods of 0.25 to 3 hours (0.125 to 1.5 hours for the DS2715B). The programmed FAST-CHARGE time
approximately follows the equation:
t(minutes) = 1.5 x R(ohms) / 1000
DS2715: NiMH Battery Pack Charge Controller
TEMPERATURE SENSE
Accurate temperature sensing is needed to determine end of charge by dT/dt and to detect over temperature fault
conditions. Connecting an external 10k NTC thermistor between THM and VSS, and a 10k bias resistor between
Div and THM allows the DS2715 to sense temperature. To sense the temperature of the cell pack, locate the
thermistor close to the body of a cell, preferably in the middle of the cell pack. Several recommended 10k
thermistors are shown in Table 3.
Min, Max Temperature Compare
The voltage thresholds of the THM input (VTHM-MIN, VTHM-MAX) are set to allow charging to start if the thermistor
temperature is between 0C and 45C when using the recommended 10k bias resistor and 10k thermistor
circuit. If pre-charging is in progress, and the voltage on THM reaches VTHM-STOP, pre-charging stops and a fault
condition is generated. If the voltage on THM reaches VTHM-STOP during FAST-CHARGE or TOPOFF, charging
stops and the DS2715 enters the DONE state. FAST_CHARGE will complete normally and TOPOFF will begin if
the voltage change on THM exceeds the equivalent TTERM ºC per minute (dT/dt Detect specification).
Table 3. THM THRESHOLDS
TEMPERATURE (C)
THM
THRESHOLD
RATIO
OF VCBIAS
THERMISTOR
RESISTANCE
() Semitec
103AT-2
Fenwal
197-103LAG-A01
173-103LAF-301
MIN 0.73 27.04k
0C 4C
MAX 0.33 4.925k
45C 42C
STOP 0.29 4.085k
50C 47C
Used with a 10k resistor, the Semitec 103AT-2 provides about 0.9% full scale per degree sensitivity. This linearity
is shown in the curve in Figure 4. The left axis is the ratio of the sensed voltage to the divider’s input voltage
(VCbias).
Figure 4. RATIO OF THM PIN TO CBIAS PIN OVER TEMPERATURE
Divider Output
0.30
0.35
0.40
0.45
0.50
0.55
0.60
0.65
0.70
0.75
0 1020304050
Degrees Centigrade
Voltage Ratio
DS2715: NiMH Battery Pack Charge Controller
Application Circuits
Switchmode
Figure 5 shows a typical DS2715 switchmode application circuit for charging a 3-cell battery stack. Connecting the
MODE pin to CBIAS enables the comparator mode of current regulation. The DS2715 regulates the current
through the current sense resistor R7 by switching Q1 on and off as the sense resistor voltage ramps up and down
toward the preset sense voltage thresholds. The .1ohm sense resistor along with the DC ground-referenced sense
threshold level of -(VFC + VHYS-FC/2) sets the average charge current in the example to 1.07A. The sense resistor
should have a proper power rating for the chosen charge current.
Figure 5. TYPICAL Switchmode APPLICATION CIRCUIT FOR A 3-CELL
STACK
A bootstrap subcircuit with C5, R3 and Q3 is used to improve the turnoff time of Q1 to minimize switching losses.
When VCH is pulled low to turn on Q1, C5 is charged through the R6/R3 path. When VCH is left open-circuit to turn
off Q1, the voltage of the VCH node jumps up close to the gate voltage of Q1. This causes the gate of Q3 to be
pushed up due to the voltage across C5. Q3 is then turned on, which helps to speed turn-off of Q1 by discharging
its internal capacitances. The values of the components in this section of the circuit are tuned to work with the
chosen transistors. The values of R6 and R9 should be chosen so that no more than 20mA is sunk by VCH. D4 is
used to clamp the inductive spike that could occur if the batteries are removed during charging. This function can
also be served by an energy absorbing capacitor placed in parallel with the battery stack.
Q2 is used to float the Rt pin when the supply source is disconnected. This puts the DS2715 into SUSPEND mode
and resets the state machine. D1 prevents current from flowing into the charge source. D2 and R2 create the
maintenance charge current path. D1, D2 and R2 should be sized appropriately for the wattage they must dissipate
based on the application parameters. R1 and C3 create a low pass filter to minimize noise transfer as the DS2715
local ground moves in relation to the system or charge source ground.
DS2715: NiMH Battery Pack Charge Controller
The RT resistor (R5) is set to 47kΩ for a timeout of 70 minutes. This would be appropriate for cells with a capacity
of about 1Ah when charged with the 1.07A charge current. The resistor divider with R12 and R13 is configured to
present the voltage equivalent to a single cell on the Vbatt pin.
The value of L1 in the example represents a moderate switching speed of 100-150khz for fast-charge mode. L1
may be adjusted to fit specific application goals as long as the associated change in switching speed does not
exceed the circuit’s ability to maintain proper regulation of the sense resistor voltage. Since TOPOFF and
PRECHARGE modes have a faster switching frequency than FAST-CHARGE, the regulation in these modes must
be considered.
All capacitors should be ceramic surface mount types of good quality where possible. The 47uF capacitor may be
any type that meets the application requirements. A different network for C1 and C2 may be necessary depending
on the types of capacitors used, the layout, and the transient requirements of the application load. All resistors not
previously mentioned are standard surface mount types.
Linear
Figure 6 shows a typical application circuit for charging a 3-cell stack in linear mode. The Mode pin is tied to VSS for
linear operation. A 250mΩ sense resistor (R7) sets the charge current to .484A, which the DS2715 regulates by
controlling the VGS of Q1 through the bias resistor R3. The bias resistor should be chosen so that the current that
VCH is required to sink does not exceed 20mA when VCH is fully turned on. The preferred design target for FAST-
CHARGE conditions is 10mA. The RC network of R3 and C5 set a pole in the control loop to ensure stability.
Figure 6. TYPICAL Linear APPLICATION CIRCUIT FOR A 3-CELL STACK
Vch Vbatt
THM
Div
SNS+
SNS-
Mode
VSS
Rt
Cbias
Vdd
R13
100K
R10
10K
R7 0.25
THM1
10K
R12
200K
C5 4.7 uF
R3
R5 86K
C4
0.1 uF
+6VDC
C2
1 uF
CTG
LOAD
D1
B340A-13
D2
Q1
IRF9Z24
R2 220
470 U1
DS2715
LED1
R4 470
LED1
C1
47 uF
Q2
2N7002
R8
10k
R1
150
C3
.1 uF
A lower charge current is used for linear mode, in addition to a lower supply voltage. This reduces the power
dissipation of Q1 to a manageable level. This dissipation must be closely considered in the application and proper
heatsinking precautions must be taken. Different transistors may be selected for Q1 based on package size and
thermal requirements. An 86kΩ resistor on RT (R5) sets the FAST-CHARGE timeout to about 129 minutes, which
for the given charge rate is appropriate for cells of about .9Ah capacity. The other aspects of the circuit are
equivalent to those of the switchmode circuit.
DS2715: NiMH Battery Pack Charge Controller
Cell Stack Size Adjustment
R12 and R13 of the application circuits form a voltage divider such that the voltage of a single cell is present on the
Vbatt pin. This is required for proper operation of the DS2715. Given a 100kΩ resistor for R13, adjust R12 as
follows for the number of cells in the battery pack:
R12 = (Number of Cells -1) * R13
To charge 3-cell stacks, a value of 200kΩ is used for R12 for a 100kΩ R13.
It is important that the voltage seen by the Vbatt pin is relatively error free compared to the actual cell voltages in
the battery pack. Any parasitic resistances in the connections between the battery cells and the resistor divider will
cause errors that will increase with increasing charge current. The error seen by the Vbatt pin is the overall
parasitic error divided by the number of cells. So, for a given parasitic resistance, it is more of a concern for circuits
with a smaller number of cells. If parasitic resistances of a problematic level cannot be avoided, the connection
location of the resistor divider can be manipulated to sense the true battery voltage, or the divider ratio can be
adjusted to account for the sensed voltage error.
Application PCB Layout
Proper layout rules must be followed to ensure a successful application circuit. For all modes of operation, currents
in excess of 1A may flow through the charge and discharge paths. All of these paths should be properly sized to
handle the worst case current flow, whether that be from charging or from powering the load with the battery.
The linear mode of operation in some cases must dissipate large amounts of heat. This is typically accomplished
with either an external heatsink on the regulating transistor, or through the use of PCB copper to spread the
thermal energy that must be removed. Typically, for the TO-220 package transistor used in the application
example, a 1 inch square area of 1oz. copper with the transistor firmly attached will have about a 50°C/W
temperature rise. Utilizing 2oz. or heavier copper can improve this number by 20% or so. If better heatsinking is
needed for the ergonomic or reliability aspects of the application, an add-on heatsink must be used.
Switchmode operation presents its own unique challenges with fast voltage and current transients. Proper
switchmode buck power supply layout should always be observed. Referring to the example circuit and layout of
Figure 7, the loop labeled as Loop1 encompassing C1, C2, Q1 and D1 should be kept as small as possible to
minimize the change in inductance that occurs when Q1 switches to the on state. Loop2 should also be minimized
as much as practical, although it contains DC current components for the most part. The returning ground currents
should be allowed to follow a path on a layer directly under the outgoing path since the high frequency components
will try to follow the path of least impedance. Low ESR and ESL capacitors should be used when possible and for
all capacitors 10uF and smaller. Typical surface mount ceramic types with a X5R or better dielectric are
recommended.
DS2715: NiMH Battery Pack Charge Controller
Figure 7. Switching Circuit with Example Layout
C
1
Q1 L1
D1
D2
R
1
C2 C3
Charge
Source
Side
-
+
-
+
Battery
Side
Control
Backside
Ground Plane
Charge
Source
Side -
+
Control
+
-
Battery
Side
D1
D2
Q1 L1
C3
C1 C2
R1
Loop1 Loop2
Loop Areas
Minimized
Another important layout detail is the connection of the sense resistor. Proper Kelvin connection layout should be
used to ensure the signal quality viewed by the sensing circuit inside the DS2715 is adequate. Figure 8 shows a
recommended connection of the sense lines to the resistor footprint.
Figure 8. Sense Resistor Connection Layout
Package Information
For the latest package outline information and land patterns, go to www.maxim-ic.com/packages.
PACKAGE TYPE PACKAGE CODE DOCUMENT NO.
16 SO — 21-0041
DS2715: NiMH Battery Pack Charge Controller
REVISION HISTORY
REVISION
DATE DESCRIPTION PAGES
CHANGED
011408 Various comprehensive changes. 1, 3, 4, 6–15
031609 Added device version DS2715B 1,4,7,8,10
042809 Updated Ordering Information table. 1
Maxim Integrated 160 Rio Robles, San Jose, CA 95134 USA 1-408-601-1000
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are implied.
Maxim reserves the right to change the circuitry and specifications without notice at any time. The parametric values (min and max limits) shown in the Electrical
Characteristics table are guaranteed. Other parametric values quoted in this data sheet are provided for guidance.
© 2009 Maxim Integrated The Maxim logo and Maxim Integrated are trademarks of Maxim Integrated Products, Inc.
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