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19-2278; Rev 0; 4/02
Advanced Li+ Battery-Pack Protectors
General Description
The MAX1894/MAX1924 are lithium-ion/lithium-polymer (Li+) battery-pack protector ICs for 3- or 4-series Li+ battery packs. The MAX1894/MAX1924 enhance the useful operating life of Li+ batteries by monitoring individual cell voltages and preventing over/undervoltage conditions. The MAX1894/MAX1924 also protect the battery pack against charge current, discharge current, and packshort fault conditions. In case of a fault condition, on-board drivers control external P-channel MOSFETs, which disconnect the cells from the pack external terminals. The external protection MOSFETs are connected in a common-source configuration that does not require external pullup resistors. The MAX1894/MAX1924 use only one current-sense resistor to achieve the protection features. All protection thresholds and delays do not require any external components and are trimmed at the factory. If any cell voltage drops below the undervoltage threshold, the MAX1894/MAX1924 disconnect the pack from the load and power down to prevent deep discharge of the pack. The MAX1894/MAX1924 offer a trickle-charge feature, which provides a low-current path to safely charge a deeply discharged pack. The MAX1894/ MAX1924 also have two logic-level inputs, which can be used by a microcontroller to disable the protection MOSFETs and to put the device in shutdown. The MAX1894/MAX1924 have low quiescent current (30A typ) and ultra-low shutdown current (0.8A typ) to prevent deep-cell discharge. The MAX1894X is designed for 4-series battery packs, without hysteresis on the protection thresholds. The MAX1924V and MAX1924X include hysteresis for the 3and 4-series packs, respectively.
Features
o Protect Against Cell Overvoltage Factory Programmable Limits from 4V to 4.4V Accurate to 0.5% o Protect Against Cell Undervoltage Factory Programmable Limits from 2V to 3.2V Accurate to 2.0% o Protect Against Charge, Discharge, and PackShort Current Faults o Automatically Trickle Charges Deeply Discharged Cells o Fully Integrated MOSFET Drivers Do Not Require Pullup Resistors o 0.8A (typ) Shutdown Supply Current Prevents Deep Discharge of Cells o 30A (typ) Operating Supply Current o 28V (max) Input Voltage o Available in Small 16-Pin QSOP Package
MAX1894/MAX1924
Pin Configuration
TOP VIEW
B4P 1 VCC 2 B3P 3 IC3 4 B2P 5 IC2 6 B1P 7 IC1 8 16 SRC 15 DSO 14 CGO 13 TKO
MAX1894 MAX1924
12 SHDN 11 CTL 10 PKN 9 BN
Applications
3- or 4-Series Li+ Battery Packs
QSOP
Ordering Information
PART MAX1894XEEE MAX1924VEEE MAX1924XEEE TEMP RANGE -40C to +85C -40C to +85C -40C to +85C PIN-PACKAGE 16 QSOP 16 QSOP 16 QSOP CELLS 4 3 4 OVERVOLTAGE 4.25V* 4.35V* 4.35V* OVERVOLTAGE THRESHOLD HYSTERESIS No Yes Yes
*Contact factory for alternative threshold voltages. Typical Applications Circuits appear at end of data sheet. ________________________________________________________________ Maxim Integrated Products 1
For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at 1-888-629-4642, or visit Maxim's website at www.maxim-ic.com.
Advanced Li+ Battery-Pack Protectors MAX1894/MAX1924
ABSOLUTE MAXIMUM RATINGS
SRC, IC2, IC3, VCC to BN.......................................-0.3V to +28V IC1 to BN ..................................................................-0.3V to +6V DSO, TKO, CGO to BN.............................-0.3V to (VSRC + 0.3V) B4P to B3P ...............................................................-0.3V to +6V B3P to B2P ...............................................................-0.3V to +6V B2P to B1P ...............................................................-0.3V to +6V B1P to BN .................................................................-0.3V to +6V CTL, SHDN to PKN...................................................-0.3V to +6V PKN to BN ...................................................................-2V to +2V ESD Protection on All Pins ...............................................2000V Continuous Power Dissipation (TA = +70C) 16-Pin QSOP (derate 8.3mW/C above +70C)...........667mW Operating Temperature Range ...........................-40C to +85C Junction Temperature ......................................................+150C Storage Temperature Range .............................-65C to +150C Lead Temperature (soldering, 10s) .................................+300C
Stresses beyond those listed under "Absolute Maximum Ratings" may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability.
ELECTRICAL CHARACTERISTICS
(VSRC = VB4P + 0.1V, each battery cell voltage (VCELL) = 3.6V, VCTL = VSHDN = VPKN, TA = 0C to +85C, unless otherwise noted. Typical values are at TA = +25C.)
PARAMETER SRC Input Current Supply Current (Note 1) Shutdown Supply Current Top Cell Sampling Current (Note 2) Intermediate Cell Input Bias Current (Note 3) VCC Undervoltage Lockout Threshold Charge-Mode Detection Threshold Overvoltage Threshold (Note 4) Overvoltage Threshold Hysteresis Undervoltage Threshold (Note 4) PKN to BN Discharge Current Fault Threshold PKN to BN Charge Current Fault Threshold PKN to BN Discharge Current Fault Threshold, Pack-Short Condition Discharge or Charge Current Fault to DSO, CGO, TKO Transition Delay Time Discharge Current Fault to DSO Transition Time Delay for PackShort Condition VOV_TH VOV_HYT VUV_TH VOD_TH VOC_TH VPS_TH SYMBOL ISUP ISHDN CONDITIONS VSRC - VB_P = 1V No faults Undervoltage without charge source VCELL = 3.6V VCELL = 3.6V Rising edge, hysteresis = 1% falling edge VSRC - VB4P MAX1894X cell voltage rising MAX1924X, MAX1924V cell voltage rising MAX1924X, MAX1924V cell voltage falling Cell voltage falling 2.260 130 -120 25 4.225 4.325 4.250 4.350 200 2.300 145 -100 2.340 160 -80 MIN TYP 20 30 0.8 60 500 4.5 100 4.275 4.375 MAX 40 45 2 UNITS A A A A pA V mV V mV V mV mV
385
405
425
mV
tI-DELAY
2.5
3
3.5
ms
tP-DELAY
400
450
500
s
2
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Advanced Li+ Battery-Pack Protectors
ELECTRICAL CHARACTERISTICS (continued)
(VSRC = VB4P + 0.1V, each battery cell voltage (VCELL) = 3.6V, VCTL = VSHDN = VPKN, TA = 0C to +85C, unless otherwise noted. Typical values are at TA = +25C.)
PARAMETER Charge-Voltage Comparator Threshold for Resetting Discharge-Current Fault Latch Discharge-Voltage Comparator Threshold for Resetting ChargeCurrent Fault Latch DSO, CGO, and TKO Sink Current DSO, CGO, and TKO Source Current Under/Overvoltage to DSO, CGO, TKO Transition Delay SHDN, CTL Input High (Note 6) SHDN, CTL Input Low (Note 6) SHDN, CTL Input Leakage Current SHDN, CTL Delay to Output Change DSO, CGO, TKO Output High (Note 7) DSO, CGO, TKO Output Low (Note 7) VB_P = 4V, VCTL = 3V VB_P = 4V tV-DELAY Rising edge Falling edge PKN = BN 50 -0.1 -14 SYMBOL VSRC - VB4P CONDITIONS MIN 25 TYP MAX 100 UNITS mV
MAX1894/MAX1924
VB4P - VSRC VSRC = 12V, VPIN = 5V (Note 5) VSRC = 12V, VPIN = 10V VCTL = 3V (Note 5)
25 100 3.5 270 2.4 200 5 320
100
mV A mA
370
ms V
1.2 1
V A ns V V
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Advanced Li+ Battery-Pack Protectors MAX1894/MAX1924
ELECTRICAL CHARACTERISTICS
(VSRC = VB4P + 0.1V, each battery cell voltage (VCELL) = 3.6V, VCTL = VSHDN = VPKN, TA = -40C to +85C, unless otherwise noted.)
PARAMETER SRC Input Current Supply Current (Note 1) Shutdown Supply Current Charge-Mode Detection Threshold Overvoltage Threshold (Note 4) Undervoltage Threshold (Note 4) PKN to BN Discharge-Current Fault Threshold PKN to BN Charge-Current Fault Threshold PKN to BN Discharge-Current Fault Threshold, Pack Short Condition Discharge- or Charge-Current Fault to DSO, CGO, TKO Transition Delay Time Discharge-Current Fault to DSO Transition Time Delay for Pack Short Condition Charge-Voltage Comparator Threshold for Resetting Discharge-Current Fault Latch Discharge-Voltage Comparator Threshold for Resetting ChargeCurrent Fault Latch DSO, CGO, and TKO Sink Current DSO, CGO, and TKO Source Current Under/Overvoltage to DSO, CGO, TKO Transition Delay SHDN, CTL Input High (Note 6) SHDN, CTL Input Low (Note 6) tV-DELAY Rising edge Falling edge VOV_TH VUV_TH VOD_TH VOC_TH VPS_TH ISUP ISHDN SYMBOL No faults Undervoltage without charge source VSRC - VB4P MAX1894X cell voltage rising MAX1924X, MAX1924V cell voltage rising Cell voltage falling 25 4.21 4.32 2.24 120 -130 CONDITIONS VSRC - VB_P = 1V MIN TYP MAX 40 50 2 100 4.29 4.38 2.36 170 -70 UNITS A A A mV V V mV mV
345
465
mV
tI-DELAY
2
4
ms
tP-DELAY
370
480
s
VSRC - VB4P
25
100
mV
VB4P - VSRC VSRC = 12V, VPIN = 5V (Note 5) VSRC = 12V, VPIN = 10V VCTL = 3V (Note 5)
25
100
mV
100 3.5 270 2.4 1.2 370
A mA ms V V
Note 1: Average current from the top of the battery pack. Measured at VCC. Note 2: Typical supply current for the top cell during the 0.5ms sampling period. Note 3: Input bias current for this measurement is valid when all cell voltages are equal and the measurement is made over a time greater than 3 seconds. Note 4: Each cell voltage is sampled individually and a differential measurement is made (VB4P - VB3P, VB3P - VB2P, VB2P - VB1P, and VB1P - BN). Note 5: VPIN represents VDSO, VCGO, or VTKO. Note 6: Inputs to SHDN and CTL pins are referred to PKN. Note 7: Measurements are with respect to VSRC.
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Advanced Li+ Battery-Pack Protectors
Typical Operating Characteristics
(TA = +25C, unless otherwise noted.)
SHUTDOWN SUPPLY CURRENT vs. TEMPERATURE
MAX1894 toc01
MAX1894/MAX1924
SUPPLY CURRENT vs. TEMPERATURE
MAX1894 toc02
UNDERVOLTAGE THRESHOLD vs. TEMPERATURE
MAX1894 toc03
1.2 1.1 SUPPLY CURRENT (A) 1.0 4-SERIES BATTERY PACK 0.9 0.8 0.7 0.6 0.5 0.4 -40 -15 10 35 60
50 4-SERIES BATTERY PACK 40 SUPPLY CURRENT (A)
2.310
2.305 CELL VOLTAGE (V) B1P B3P 2.300 B4P B2P 2.295 30
20
10
0 85 -40 -15 10 35 60 85 TEMPERATURE (C) TEMPERATURE (C)
2.290 -40 -15 10 35 60 85 TEMPERATURE (C)
OVERVOLTAGE THRESHOLD vs. TEMPERATURE
MAX1924X 4.355 CELL VOLTAGE (V)
MAX1894 toc04
CHARGE-CURRENT FAULT THRESHOLD vs. TEMPERATURE
MAX1894 toc05
CHARGE-CURRENT FAULT RECOVERY THRESHOLD vs. TEMPERATURE
90 80
MAX1894 toc06
4.360
130 120 VBN - VPKN (mV) 110 100 90 80
100
VB4P - VSRC (mV) -15 10 35 60
B1P B3P
70 60 50 40 30 20
4.350 B4P B2P 4.345
4.340 -40 -15 10 35 60 85 TEMPERATURE (C)
70 -40 85 TEMPERATURE (C)
-40
-15
10
35
60
85
TEMPERATURE (C)
DISCHARGE-CURRENT FAULT THRESHOLD vs. TEMPERATURE
MAX1894 toc07
DISCHARGE-CURRENT FAULT RECOVERY THRESHOLD vs. TEMPERATURE
MAX1894 toc08
PACK-SHORT CURRENT FAULT THRESHOLD vs. TEMPERATURE
MAX1894 toc09
155
100 90 80 VB4P - VSRC (mV) 70 60 50 40 30
415
150 VPKN - VBN (mV)
410 VPKN - VBN (mV) -40 -15 10 35 60 85
145
405
140
400
135 -40 -15 10 35 60 85 TEMPERATURE (C)
20 TEMPERATURE (C)
395 -40 -15 10 35 60 85 TEMPERATURE (C)
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Advanced Li+ Battery-Pack Protectors MAX1894/MAX1924
Typical Operating Characteristics (continued)
(TA = +25C, unless otherwise noted.)
DISCHARGE-CURRENT FAULT TIMING
MAX1894 toc10
CHARGE-CURRENT FAULT TIMING
MAX1894 toc11
PACK-SHORT CURRENT FAULT TIMING
MAX1894 toc12
VPKN - VBN 100mV/div 0
0 VPKN - VBN 100mV/div
VPKN - VBN 200mV/div 0
DSO 5V/div
CGO 5V/div 0 1ms/div 100s/div
DSO 5V/div
0
0
1ms/div
Pin Description
PIN 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 NAME B4P VCC B3P IC3 B2P IC2 B1P IC1 BN PKN CTL SHDN TKO CGO DSO SRC FUNCTION Cell 4 Positive Connection. Short B4P to B3P for MAX1924V. Supply Input. Connect this pin to the top of the battery pack through a diode and a capacitor (see the Typical Application Circuit). Cell 3 Positive Connection Internal Connection. Float this pin. Cell 2 Positive Connection Internal Connection. Float this pin. Cell 1 Positive Connection Internal Connection. Float this pin. Battery Negative. Connection for the cell 1 negative terminal and the top of the current-sense resistor RSENSE. BN is also chip ground. Pack Negative. The sense resistor (RSENSE) is connected between BN and PKN. Control Input. Drive CTL low for normal operation. Drive CTL high to turn off the three external protection MOSFETs. Shutdown. Drive SHDN low for normal operation. Drive SHDN high to put the device into shutdown if no charger is present. Trickle-Charge Driver Output. TKO drives the gate of an external P-channel trickle-charge MOSFET low (on) in normal operation. Fast-Charge Driver Output. CGO drives the gate of an external P-channel fast-charge MOSFET low (on) in normal operation. Discharge Driver Output. DSO drives the gate of an external P-channel discharge control MOSFET low (on) in normal operation. Common Source Connection for MOSFETs. SRC provides the bias for gate drivers DSO, TKO, and CGO.
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Advanced Li+ Battery-Pack Protectors
Detailed Description
The MAX1894/MAX1924 battery-pack protectors supervise the charging and discharging process of Li+ cells. Designed for 3-series (MAX1924V) and 4-series (MAX1894X/MAX1924X) applications, these devices monitor the voltage across each cell to provide protection against undervoltage, overvoltage, and overcurrent damage. Output pins CGO, TKO, and DSO control external MOSFET gates. These MOSFETs, in turn, control the fast-charging, trickle-charging, and discharge processes of the battery pack (Figure 1).
MAX1894/MAX1924
Table 1. Flow Chart Symbol Table
SYMBOL VOC_TH VOD_TH VPS_TH VUV_TH VOV_TH VSRC VRSENSE VOV_HYT OVF UVF DESCRIPTION Charge-Current Fault Threshold (Negative Value) Discharge-Current Fault Threshold Pack-Short Discharge-Current Fault Threshold Undervoltage Threshold Overvoltage Threshold MOSFET Common Source Voltage Sense Resistor Voltage Overvoltage Threshold Hysteresis Overvoltage Sample Counter Undervoltage Sample Counter
Modes of Operation
Shutdown Mode
The MAX1894/MAX1924 go into shutdown mode under two conditions: the SHDN pin is driven high without a charger applied, or a battery cell undervoltage fault is detected, also without a charger applied. In shutdown mode, the device consumes 0.8A (typ) on the VCC pin and all MOSFETs are off. The MAX1894/MAX1924 stay in shutdown mode as long as no charging voltage is applied to the battery pack (VSRC is less than the pack voltage). When the battery pack is connected to a charger (VSRC > VB4P + 0.1V) and the pack voltage is above 4.5V, the device goes into normal operating mode and begins monitoring the pack (see Figure 2).
are returned to the normal low state (see Figure 3). The MAX1924 also includes a hysteresis of 200mV. The overvoltage threshold is preprogrammed and requires no external components. The overvoltage threshold is factory set at 4.25V (typ) for the MAX1894 and 4.35V (typ) for the MAX1924. Contact Maxim for more information on threshold levels between 4V and 4.4V.
Undervoltage Protection
The MAX1894/MAX1924 provide undervoltage protection to avoid overdischarging the cells. With no battery charger present, and an undervoltage fault is detected in four consecutive samples, DSO, CGO, and TKO go high and the device goes into shutdown mode (see Figure 4). If a battery charger is applied to the battery pack and one or more cells are below VUV_TH, then only TKO goes low, allowing trickle-charge current to flow. If no undervoltage is detected in any sample, DSO, CGO, and TKO all go low. The undervoltage threshold is preprogrammed at 2.30V (typ). Contact Maxim for more information on threshold levels between 2V and 3.2V.
Normal Mode
In the normal mode of operation, the MAX1894/MAX1924 are in either a standby mode (29A typ) or sample mode (160A typ). The device enters the standby mode from shutdown mode. The standby mode lasts for 79ms; then the device goes into the sample mode. During sample mode, the MAX1894/MAX1924 check each cell for overvoltage and undervoltage. Sample mode lasts for 0.5ms; then the MAX1894/MAX1924 return to standby mode. During sample mode, the MAX1894/MAX1924 do not introduce cell mismatch. During normal mode operation, the MAX1894/MAX1924 continuously monitor the voltage across RSENSE for charge or discharge current faults, or battery pack-short faults.
Protection Features
Overvoltage Protection
The MAX1894/MAX1924 provide overvoltage protection to avoid overcharging cells. When an overvoltage fault is detected in four consecutive samples, CGO and TKO go high, stopping the charging process. The MAX1894/MAX1924 continue to sample the cell voltages, and if no overvoltage is detected, CGO and TKO
Charge-Current Fault Protection
The MAX1894/MAX1924 protect against excessive charge current by monitoring the voltage developed across RSENSE. RSENSE is connected between BN and PKN. If V RSENSE exceeds the charge-current fault threshold (VOC_TH, typically 100mV) for more than 3ms, the charge current comparator is tripped, setting CGO and TKO high. The charge-current fault condition is latched and is not reset until the MAX1894/MAX1924 detect a reversal in
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Advanced Li+ Battery-Pack Protectors MAX1894/MAX1924
OVERDISCHARGE PROTECTION
R5 10 TRICKLE CHARGE 16 15 OVERCHARGE PROTECTION 14 13 D1 CMPSH-3 2 SRC DSO CGO TKO VCC BN C5 2.2F
PACK +
RTKO 510
1 R4 51 R3 1k C6 0.1F C4 1F 3 C3 0.1F 4 5 C2 0.1F R1 1k C1 0.1F 6
B4P MAX1894X MAX1924X B3P IC3 B2P SHDN IC2 CTL
R2 1k
12 11 MICROCONTROLLER
7 8 9
B1P IC1 BN PKN 10
RSENSE 0.02 PACK -
Figure 1. Typical Applications Circuit with Trickle Charge
the direction of current flow. To reverse the current flow, the charger has to be removed (Figure 5). The sustaining condition for the latch is a 100mV (max) voltage drop across SRC and B4P. Since the charge-current fault threshold between BN and PKN is also 100mV (typ), the RDS_ON of the overcharge protection MOSFET must be greater than the sense resistor in order to ensure a latched state.
than 3ms, the discharge-current comparator is tripped, setting DSO, CGO, and TKO high. Discharge-current fault is latched and is not reset until the MAX1894/MAX1924 detect a reversal in the direction of current flow. To reverse the current flow, a charger must be applied (Figure 6).
Pack-Short Current Fault Protection
The MAX1894/MAX1924 protect against a shorted pack by monitoring the voltage developed across RSENSE. If VRSENSE exceeds the pack-short threshold (V PS_TH, typically 405mV) for more than 450s, the pack-short comparator is tripped, setting CGO, DSO, and TKO high.
Discharge-Current Fault Protection
The MAX1894/MAX1924 protect against excessive discharge-current by monitoring the voltage developed across RSENSE. If VRSENSE exceeds the discharge-current fault threshold (VOD_TH, typically 145mV) for more
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Advanced Li+ Battery-Pack Protectors MAX1894/MAX1924
VUV_TH = 2.3V typ & VOV_TH = 4.25V typ SHUTDOWN DSO, CGO, TKO = H
SHUTDOWN PIN OPERATION NORMAL OPERATION
VSRC > VB4P + 0.1V YES TKO = L
NO
NO
NO
SHDN = H
VCC > 4.5V YES STANDBY = 79ms NO RESET UVF
YES
VCELL < VUV_TH YES LATCH UVF = UVF + 1
SHUTDOWN DSO, CGO, TKO = H
NO
VSRC > VB4P + 0.1V
YES
NO
VCELL > VOV_TH YES LATCH OVF = OVF + 1
DSO, CGO, TKO = H
RESET OVF MAX1924 VOV_HYT = 200mV MAX1894 VOV_HYT = 0 TKO, CGO = H YES
CONTROL PIN OPERATION
OVF = 4 NO
NORMAL OPERATION
RESET VOV_TH = 4.25V typ
NO
UVF = 4 YES
NO
CTL = H
YES
DSO, CGO, TKO = H
VSRC > VB4P + 0.1V YES CGO = DSO = H NO
Figure 2. Undervoltage and Overvoltage Protection Flow Chart
Figure 3. Shutdown and Control Pin Flow Charts
Pack-short current fault is latched and is not reset until the MAX1894/MAX1924 detect a reversal in the direction of current flow. A charger must be applied to reverse the current flow (Figure 7).
Design Procedure
Fast and Trickle-Charge Paths
The MAX1894/MAX1924 offer the designer the flexibility of two charging paths: a fast charging path and a trickle-charge path (see Figure 1). Trickle charging is enabled and TKO is set low when one or more cells are belows VUV_TH.
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Advanced Li+ Battery-Pack Protectors MAX1894/MAX1924
UNDERVOLTAGE CONDITION 320ms
UNDERVOLTAGE THRESHOLD CELL VOLTAGE
SAMPLING
DSO, CGO, TKO
OVERVOLTAGE CONDITION (MAX1894, NO HYSTERESIS) 320ms 80ms
OVERVOLTAGE THRESHOLD CELL VOLTAGE
SAMPLING
TKO, CGO
OVERVOLTAGE CONDITION (MAX1894, WITH HYSTERESIS) 320ms
OVERVOLTAGE THRESHOLD CELL VOLTAGE OVERVOLTAGE THRESHOLD HYSTERESIS
SAMPLING
TKO, CGO
Figure 4. Undervoltage and Overvoltage Timing Diagrams 10 ______________________________________________________________________________________
Advanced Li+ Battery-Pack Protectors MAX1894/MAX1924
NORMAL OPERATION
NORMAL OPERATION
NO
OC FAULT VRSENSE < VOC_TH FOR 3ms
NO
OD FAULT VRSENSE > VOD_TH FOR 3ms
YES
YES
TKO, CGO = H
DSO, TKO, CGO = H
NO
VB4P > VSRC + 0.1V
YES
NO
VSRC > VB4P + 0.1V
YES
Figure 5. Charge-Current Fault
Figure 6. Discharge-Current Fault
Set the nominal values of the trickle charge current by selecting resistor RTKO based on the following equation: RTKO = (VCHRG - VPACK)/ITKO where VCHRG is the charger output voltage, VPACK is the battery-pack voltage, and ITKO is the trickle-charge current. When the trickle-charge option is not used, float CGO and connect TKO to the gate of the overcharge protection MOSFET (see Figure 9). When a charger is applied and the voltage on one or more cells is less than VUV_TH, the MAX1894/MAX1924 modulate the TKO output until all cells exceed VUV_TH.
RSENSE Selection
All current faults are detected using a current-sense resistor connected between BN and PKN. The value of this resistor sets the fault current levels. Charge-current fault is given by: IOC _ TH = VOC _ TH RSENSE = 100mV RSENSE
Discharge-current fault is given by: IOD _ TH = VOC _ TH RSENSE = 145mV RSENSE
Protection FET Drivers
All three external MOSFETs have their source pins connected to the SRC pin. When a MOSFET is turned off, FET drivers pull the gate to the SRC voltage. Additional external pullup resistors are not needed. When the MOSFET is turned on, the VGS is limited to -14V by a clamp circuit built in the drivers. This allows use of MOSFETs with maximum VGS of -20V. All three drivers have the same circuitry and drive capability. The quiescent current in normal operation is less than 3A per driver. Pack-short current fault is given by: IPS _ TH = VPS _ TH RSENSE = 405mV RSENSE
Select RSENSE to obtain the desired fault current levels. For example, a 20m RSENSE sets the charge current fault at 5A. Choose an RSENSE that can withstand the dissipation during normal operation and current fault conditions. For example, pack-short current is given by:
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11
Advanced Li+ Battery-Pack Protectors MAX1894/MAX1924
Table 2. State Table
STATE Undervoltage Overvoltage Charge Current Fault Discharge Current Fault Pack Short Current Fault Forced Shutdown by External P CTL Deep Discharge (VCC < 4.5V) Normal Operation CTL STATE L L L L L L X X H L L SHDN STATE L L L L L L H H L L L CHARGER APPLIED Yes No X Yes No No No Yes X Yes X GOES INTO SHUTDOWN MODE No Yes No No No No Yes No No No No TKO L H H H H H H H H L L CGO H H H H H H H H H H L DSO H H L L H H H H H H L
X: Don't care.
IPS =
VCELL x NS N RDSON _ DSO + RDSON _ CGO + RSENSE + RCELL x S NP
The power dissipation in the MOSFETs is given by: P = I2 RDSON The MOSFET should be chosen to withstand power dissipation during normal operation and all current fault conditions. Additional MOSFETs can be added in parallel to help these requirements. Table 3 lists some suitable MOSFETs in a small SO-8 package.
where NS is the number of cells in series, NP is number of cells in parallel, and V CELL is the cell voltage. Dissipation during pack-short current fault condition is given by: PPS = (IPS )2 x RSENSE The RSENSE chosen should be able to withstand PPS dissipation. Verify power dissipation in normal operation and other current fault conditions as well.
Decoupling Considerations
The MAX1894/MAX1924 must have a reliable VCC bias to function properly. A severe overload, such as a short circuit at the pack terminals, can collapse the batterypack voltage below the V CC undervoltage lockout threshold. The use of a diode-capacitor peak detector on the VCC input ensures continued operation during voltage transients on the battery (Figure 1). Since the MAX1894/MAX1924 typically consume only 30A, D1 and C6 can be small, low-cost components. A 30V Schottky diode with a few mA current capability and a 0.1F capacitor are sufficient. The MAX1894/MAX1924 continuously monitor the differential voltage between the B4P and SRC inputs to detect the application of a charger. RC filters with similar time constants must be added to both inputs to ensure the differential voltage is not corrupted by noise.
Choosing External MOSFETs
The external P-channel MOSFETs act as switches to enable or disable charging and discharging of batteries. Different P-channel MOSFETs may be selected depending on the charge and discharge currents anticipated. In most applications, the requirements for fast-charge and discharge MOSFETs are similar and the same type of MOSFETs can be used. The trickle-charge MOSFET can be a small-signal type to minimize cost. The MAX1894/MAX1924 MOSFET drivers have a VGS clamp of -14V typical and MOSFETs with maximum VGS of -20V can be used. MOSFETs must have a V DS greater than the maximum pack voltage.
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Advanced Li+ Battery-Pack Protectors MAX1894/MAX1924
Table 3. MOSFET Selection
P-CHANNEL MOSFETS IRF7404 IRF7406 Si4431 Si4947 (dual) MAXIMUM DRAIN CURRENT (A) 6.7 5.8 5.8 3.5 EA
NO VRSENSE > VPS_TH FOR 450s NORMAL OPERATION
Protecting and Filtering Cell Inputs
Resistors in series with each B_P pin are recommended to limit the current in case there is a short between adjacent B_P pins (see Figure 1). The intermediate cell input bias current is typically 0.5nA. A 1k resistor in series with any intermediate cell moves the overvoltage trip point by typically 0.5mV, which is insignificant compared to the 25mV tolerance in the overvoltage threshold. The top cell input bias current during sampling period is typically 60A. To reduce the voltage change on the top cell input due to sampling current, a filter resistance of 10 to 50 should be added in series with the top cell. To attain the desired filter characteristics, the capacitance across the two top cell input pins should be 1F. The MAX1894/MAX1924 have internal ESD diodes on each B_P pin for ESD protection up to 2kV. When higher ESD ratings are needed, capacitors (typically 0.1F) can be added across adjacent B_P pins (see Figure 1). The RC filters improve the device immunity to ESD and filter the noise spikes on B1P-B4P to prevent the MAX1894/MAX1924 from being triggered and latched prematurely by noise spikes.
YES
SET OD = 1 DSO, CGO, TKO = H
NO
VSRC > VB4P + 0.1V
YES
Figure 7. Pack-Short Current Fault
Layout Considerations
Good layout is important to minimize the effects of noise on the system and to ensure accurate voltage and current measurements. Use the appropriate trace widths for the high-current paths and keep traces short to minimize parasitic inductance and capacitance. Minimize current-sense resistor trace lengths and make use of Kelvin connections to the resistor. Provide adequate space and board area for the external MOSFETs and sense resistor to dissipate the heat required. Place RC filters close to B1P-B4P pins.
Control Pins SHDN and CTL
SHDN and CTL allow external logic or microprocessors to control the MAX1894/MAX1924 gate drivers. Drive CTL high to turn off the three protection MOSFETs: DSO, CGO, and TKO. Drive SHDN high to force the MAX1894/ MAX1924 into shutdown mode (with no charger applied). SHDN and CTL do not affect the state machine. Toggling these two pins does not change the state or reset any fault conditions. If external control circuitry or a microprocessor is not used, connect SHDN and CTL to PKN.
Chip Information
TRANSISTOR COUNT: 4259
______________________________________________________________________________________
13
Advanced Li+ Battery-Pack Protectors MAX1894/MAX1924
SHDN
CTL
B4P B3P SRC RDY CLOCK HV DRIVER OV FAULT CMP OVER AND UNDER THRESHOLDS HYSTERESIS BN CMP +145mV PKN CHARGE CURRENT FAULT CMP -100mV DISCHARGE-CURRENT FAULT PACK-SHORT CURRENT FAULT MAX1894 MAX1924 SRC HV DRIVER BN TKO STATE MACHINE UV FAULT BN SRC FAULT LOGIC HV DRIVER CGO DSO
CELL SELECTOR B2P B1P
REF
SET/RESET TIMER
BN
CMP +405mV
VCC CMP -50mV DSCHG S LATCH R B4P SRC CMP +50mV CHG UV FAULT SHDN RDY Q ON 3.5V REG BIAS
Figure 8. Simplified Functional Diagram
14
______________________________________________________________________________________
Advanced Li+ Battery-Pack Protectors MAX1894/MAX1924
OVERDISCHARGE PROTECTION
R5 10 16 15 OVERCHARGE PROTECTION 14 13 D1 CMPSH-3 2 1 R4 51 C4 1F R3 1k C3 0.1F R2 1k C6 0.1F MAX1894X MAX1924X SRC DSO CGO TKO VCC B4P BN C5 2.2F
PACK +
3 4 5
B3P IC3 B2P IC2 B1P IC1 BN
C2 0.1F R1 1k
6 7 8 9
SHDN CTL
12 11 MICROCONTROLLER
C1 0.1F
RSENSE 0.02 PACK -
PKN 10
Figure 9. Typical Applications Circuit without Trickle Charge
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15
Advanced Li+ Battery-Pack Protectors MAX1894/MAX1924
Package Information
(The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information, go to www.maxim-ic.com/packages.)
QSOP.EPS
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.
16 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 (c) 2002 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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