19-2455; Rev 0; 4/02 Li+ Battery-Pack Protector with Integrated Fuse Driver Features ♦ Protects Against Overvoltage The MAX1906 also offers protection against disconnected voltage sense pins. If a disconnected pin is detected, the DISCON output is forced low. The MAX1906 includes a test mode, which determines if the circuit is operating correctly while in an assembled battery pack. ♦ Available in Small 16-Pin QFN Package (5mm x 5mm) The low-cost MAX1906 is available in a thermally enhanced 16-pin QFN package. Applications 2-, 3-, or 4-Series Li+ Battery Packs for Portable Products ♦ ±1% Accurate Protection Thresholds ♦ Integrated 2.1s Fault-Delay Timer ♦ Built-in 1.5A SCR Fuse Driver ♦ Test Mode for Functional Verification in Assembled Pack ♦ 8µA (max) Supply Current ♦ 1µA (max) Standby Current ♦ Protects Against Disconnected B1P–B4P Pins ♦ Protects 2-, 3-, or 4-Series Li+ Battery Packs Ordering Information PART TEMP RANGE MAX1906SEGE -40°C to +85°C MAX1906VEGE -40°C to +85°C MAX1906XEGE -40°C to +85°C B3P DRV MAX1906X B2P 3 TEST 5 PKN B1P BN 15 14 13 I.C. 1 12 B2P DRV 2 11 N.C. TEST 3 10 B1P DISCON 4 9 I.C. 14 12 MAX1906S/V/X 10 8 5 6 7 8 BN 2 DISCON 16 16 OUT OPTIONAL PACK CONTROLLER B4P N.C. 4 OUT PKN 7 N.C. VCC I.C. [B3P] FUSE PACK+ N.C. Pin Configuration I.C. (B4P) Minimal Operating Circuit PIN-PACKAGE CELLS 16 QFN 5mm ✕ 5mm 2 16 QFN 5mm ✕ 5mm 3 16 QFN 5mm ✕ 5mm 4 5mm x 5mm QFN PACK- :MAX1906V, MAX1906X ():MAX1906X ________________________________________________________________ Maxim Integrated Products 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. 1 MAX1906 General Description The MAX1906 protects against overvoltage conditions in lithium-ion/lithium polymer (Li+) battery packs by blowing a three-terminal protection fuse. The IC should be used in conjunction with resettable protection circuits to provide a high level of safety against overcharging Li+ batteries. It can be used with 2-, 3-, or 4-series cell battery packs. The MAX1906 monitors individual cell voltages. If any cell voltage exceeds the overvoltage threshold for greater than 2.1s, the MAX1906 activates an internal SCR. The SCR sinks sufficient current to blow an external protection fuse, permanently disabling the battery pack. Alternatively, the IC can drive the gate of an external MOSFET to blow the fuse. MAX1906 Li+ Battery-Pack Protector with Integrated Fuse Driver ABSOLUTE MAXIMUM RATINGS B4P to BN ...............................................................-0.3V to +24V B3P to BN ...............................................................-0.3V to +18V B2P to BN ...............................................................-0.3V to +12V B4P to B3P, B3P to B2P, B2P to B1P, B1P to BN ....-0.3V to +6V TEST, DRV, DISCON to PKN ....................................-0.3V to +6V OUT to BN ..............................................................-0.3V to +24V BN to PKN ...................................................................-2V to +2V OUT Maximum Current .........................................................2.5A Continuous Power Dissipation (TA = +70°C, per JEDEC JESD51-7) 16-Pin QFN (derate 19mW/°C above +70°C ambient) ....1.5W Operating Temperature Ranges..........................-40°C to +85°C Storage Temperature.........................................-65°C to +150°C Junction Temperature ......................................................+150°C Lead Temperature (soldering, 10s) .................................+300°C 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 (TA = 0°C to +85°C, individual cell voltages = 4.2V unless otherwise noted. Typical values are at TA = +25°C.) MAX UNITS B4P Voltage Range PARAMETER 20 V B3P Voltage Range 15 V B2P Voltage Range 10 V B1P Voltage Range 5 V Overvoltage Detection Threshold SYMBOL VOV_TH Overvoltage Detection Threshold, Test Mode Overvoltage Detection Hysteresis SCR Release Threshold Cell voltage rising 4.4 4.45 4.5 V Cell voltage rising, test mode 2.0 2.225 2.4 V Cell voltage falling Cell voltage falling 3.85 4.0 4.15 V Cell voltage falling 2.3 3.3 4.1 V 1.85 2.1 2.45 s 6 µA tOV ISUP 10 (Note 1) 2.56 (Note 2) 3 Supply Current During Sampling Individual cell voltages = 2.2V Intermediate Cell Quiescent Current (Note 3) OUT Output Sink Current OUT = 2V, current not internally limited OUT Voltage (when SCR Is Triggered) IOUT = 1.5A OUT Leakage Current DRV Output Voltage High mV s 300 Standby Current DRV Output Voltage Low TYP VREL Sampling Interval Supply Current MIN VOV_HYS Standby-Mode Threshold Overvoltage Delay CONDITIONS OUT = 24V VDRVL VDRVH µA 800 0.5 1.0 nA nA 1.5 2.0 A 1.6 2.0 V +1 µA 0.4 V -1 IDRV = 200µA IDRV = 5µA 4.0 4.8 5.5 IDRV = -1mA 2.0 4.8 5.5 V DRV Sink Current IDRV VDRVH = 2.5V 2 mA DRV Source Current IDRV VDRVL = 0V 2 mA Test-Mode Delay tDLY (Note 4) Test-Mode Output Duration tOUT (Note 4) DISCON Output Voltage Low IDISCON = 1mA DISCON Leakage Current VDISCON = 3.3V 2 100 130 -1 _______________________________________________________________________________________ 1.2 ms 160 ms 0.4 V +1 µA Li+ Battery-Pack Protector with Integrated Fuse Driver MAX1906 ELECTRICAL CHARACTERISTICS (continued) (TA = 0°C to +85°C, individual cell voltages = 4.2V unless otherwise noted. Typical values are at TA = +25°C.) PARAMETER SYMBOL Disconnected Pin Test Time CONDITIONS MIN Test time per cell TEST Input High TYP MAX 0.2 ms 50 µs 2.2 Minimum TEST High Duration V TEST Input Low TEST Pulldown to PKN 17 Thermal Impedance, Junction to Case UNITS 0.8 V 33 kΩ 5 °C/W ELECTRICAL CHARACTERISTICS (TA = -40°C to +85°C, individual cell voltages = 4.2V, unless otherwise noted.) MAX UNITS B4P Voltage Range PARAMETER 20 V B3P Voltage Range 15 V B2P Voltage Range 10 V B1P Voltage Range 5 V Overvoltage Detection Threshold SYMBOL VOV_TH Overvoltage Detection Threshold, Test Mode SCR Release Threshold VREL Standby Mode Threshold Overvoltage Delay tOV Supply Current ISUP CONDITIONS MIN TYP Cell voltage rising 4.35 4.55 V Cell voltage rising, test mode 1.95 2.45 V Cell voltage falling 3.80 4.2 V Cell voltage falling 2.25 4.15 V 1.85 2.45 s (Note 2) 8 µA Standby Current Individual cell voltages = 2.2V 1 µA OUT Output Sink Current OUT = 2V, current not internally limited OUT Voltage (when SCR Is Triggered) IOUT = 1.5A 1.0 A 2.2 V DRV Output Voltage Low VDRVL IDRV = 200µA 0.4 V DRV Output Voltage High VDRVH IDRV = 5µA 3.9 5.5 V DRV Output Voltage High VDRVH IDRV = -1mA 2.0 5.5 V DRV Sink Current IDRV VDRVH = 2.5V 2 DRV Source Current IDRV VDRVL = 0V 2 Test-Mode Delay tDLY (Note 4) Test-Mode Output Duration tOUT DISCON Output Voltage Low (Note 4) 95 IDISCON = 1mA TEST Input High mA 1.25 ms 165 ms 0.4 V 2.6 TEST Input Low TEST Pulldown to PKN Note 1: Note 2: Note 3: Note 4: mA 17 V 0.8 V 33 kΩ See the Normal Operating Mode section. The supply current is measured at the top cell and averaged over one sampling interval. Guaranteed by design. See Figure 7. _______________________________________________________________________________________ 3 Typical Operating Characteristics (TA = +25°C, unless otherwise noted.) STANDBY-MODE THRESHOLD vs. TEMPERATURE 3.3 4.445 4.5 4.3 SUPPLY CURRENT (µA) 4.450 EQUAL VOLTAGE APPLIED TO ALL CELL INPUTS (FALLING) 3.1 2.9 2.7 2.3 10 35 60 85 3.9 3.5 -40 -15 10 35 60 85 -40 -15 10 60 85 TEMPERATURE (°C) TEMPERATURE (°C) STANDBY CURRENT vs. TEMPERATURE THERMAL IMPEDANCE, CASE-TO-AMBIENT vs. COPPER AREA TIME-TO-MAX JUNCTION TEMPERATURE vs. POWER DISSIPATION 80 MAX1906 toc04 0.70 0.68 1oz COPPER 10,000 TA = +60°C 1000 0.64 TIME (s) RθCA (°C/W) 0.66 0.25in2 60 0.50in2 100 40 10 0.62 0.04in2 0.60 20 -40 -15 10 35 60 85 1 0.01 0.1 INSTANTANEOUS ON-STATE VOLTAGE vs. CURRENT 10 1.0 1.5 2.0 TEST-MODE TIMING MAX1906 toc07 1.3 TEST PIN VOLTAGE 5V/div 1.0 DRV PIN VOLTAGE 5V/div 110°C 0.7 TJ = 25°C 0.4 DISCON PIN VOLTAGE 5V/div 0.1 0.8 1.0 1.2 1.4 1.6 1.8 2.5 POWER DISSIPATION (W) MAX1906 toc08 1.6 INSTANTANEOUS ON-STATE CURRENT (A) 1 COPPER AREA (in2) TEMPERATURE (°C) 20ms/div INSTANTANEOUS ON-STATE VOLTAGE (V) 4 35 TEMPERATURE (°C) MAX1906 toc06 -15 MAX1906 toc05 -40 4.1 3.7 2.5 4.440 MAX1906 toc03 MAX1906 toc01 3.5 CELL VOLTAGE (V) CELL VOLTAGE (V) 4.455 SUPPLY CURRENT vs. TEMPERATURE MAX1906 toc02 OVERVOLTAGE THRESHOLD vs. TEMPERATURE STANDBY CURRENT (µA) MAX1906 Li+ Battery-Pack Protector with Integrated Fuse Driver _______________________________________________________________________________________ 3.0 Li+ Battery-Pack Protector with Integrated Fuse Driver PIN MAX1906S MAX1906V MAX1906X 1, 9 1, 9 1, 9 6, 11, 13, 15 6, 11, 13, 15 6, 11, 13, 15 NAME DESCRIPTION I.C. Internal Connection. Float pins 1 and 9. N.C. No Connection 2 2 2 DRV MOSFET Driver Output. High when an overvoltage condition is detected. Connect the DRV pin to the gate of an external MOSFET to blow the protection fuse. 3 3 3 TEST Test-Mode Input. Test mode is enabled with a pulse of minimum 50µs duration on the TEST pin. 4 4 4 DISCON 5 5 5 PKN Pack Negative. A sense resistor may be connected between BN and PKN. Anode Output of the SCR. Connect OUT to the fuse’s heater connection (see the Protection Fuse Selection section). Disconnected Pin Output. This is an open-drain output and is high-Z during normal operation. If B4P, B3P, B2P, or B1P is disconnected, this pin is pulled low (see the Disconnected Pin Detection section). 7 7 7 OUT 8 8 8 BN Negative Terminal of Cell 1. Connect BN to the negative terminal of the first series Li+ cell. BN is also chip ground, which is connected to the backside paddle on the QFN package. 10 10 10 B1P Positive Terminal of Cell 1. Connect B1P to the positive terminal of the first series Li+ cell. 12 12 12 B2P Positive Terminal of Cell 2. Connect B2P to the positive terminal of the second series Li+ cell. — 14 14 B3P Positive Terminal of Cell 3. Connect B3P to the positive terminal of the third series Li+ cell. — — 16 B4P Positive Terminal of Cell 4. Connect B4P to the positive terminal of the fourth series Li+ cell. Detailed Description The MAX1906 protects 2-, 3-, or 4-series Li+ battery packs from overcharge by controlling a three-terminal protection fuse. Figures 1 and 2 show two application circuits using the MAX1906. The MAX1906 checks the voltage of each cell at regular intervals. An overcharge condition is detected if any cell voltage exceeds the overvoltage threshold for more than 2.1s. The MAX1906 responds to an overcharge condition by turning on an internal SCR (Figure 1) or an external MOSFET (Figure 2) to blow a three-terminal protection fuse placed in series with the charging path. The MAX1906 checks for disconnected voltage sense pins every time it exits the standby mode or test mode. If a disconnected pin is detected, the DISCON pin is latched low. The MAX1906 also includes a test mode, which determines if the circuit is operating correctly while in an assembled battery pack. A pulse on the TEST pin enables the test mode. Figure 3 shows the cell connections for 2- and 3-series battery packs and Figure 4 shows the functional diagram for the MAX1906. The MAX1906 can be used together with other resettable protection circuits to provide a high level of safety against overcharging Li+ batteries. Figure 5 shows a typical application circuit using the MAX1906 together with the MAX1924. The MAX1924 has a lower overvoltage threshold than the MAX1906. If any cell voltage exceeds 4.35V (typ), the MAX1924 turns off the TKO and CGO MOSFETs and opens the charging path. If the TKO or CGO MOSFET fails and charging continues, the MAX1906 blows the protection fuse and opens the charging path permanently once any cell voltage reaches 4.45V (typ). The MAX1924 also protects the battery pack against undervoltage, charge current, discharge current, and pack-short fault conditions. Refer to the MAX1894/ MAX1924 data sheets for complete details. _______________________________________________________________________________________ 5 MAX1906 Pin Description MAX1906 Li+ Battery-Pack Protector with Integrated Fuse Driver F1 PACK+ SFD-145B VCC 7 B4P OUT R4 10Ω 16 R5 100kΩ 4 MAX1906X 2 PACK CONTROLLER B3P DISCON C4 0.1µF R3 1kΩ 14 C3 0.1µF R2 1kΩ 12 B2P DRV 3 B1P TEST C2 0.1µF R1 1kΩ 10 C1 0.1µF 5 BN PKN 8 RSENSE PACK- Figure 1. Typical Application Circuit for 4-Series Battery Packs—Using the Internal SCR to Blow the Protection Fuse F1 PACK+ SFD-145B VCC 7 OUT B4P R5 100kΩ 4 PACK CONTROLLER DISCON B3P MAX1906X 2 DRV 14 12 C4 0.1µF R3 1kΩ C3 0.1µF R2 1kΩ B2P 3 TEST R4 10Ω 16 B1P 10 C2 0.1µF R1 1kΩ C1 0.1µF 5 PKN BN 8 RSENSE PACK- Figure 2. Typical Application Circuit for 4-Series Battery Packs—Using the External MOSFET to Blow the Protection Fuse 6 _______________________________________________________________________________________ Li+ Battery-Pack Protector with Integrated Fuse Driver IC DISCON DRV MAX1906S TEST B2P B1P 16 14 B3P DISCON R2 10Ω 12 10 IC OUT DRV MAX1906V B2P C2 R1 0.1µF 1kΩ B1P TEST 16 BN R3 10Ω 14 12 10 C3 R2 0.1µF 1kΩ C2 R1 0.1µF 1kΩ C1 0.1µF C1 0.1µF PKN MAX1906 IC OUT 8 BN PKN 8 RSENSE RSENSE Figure 3. Cell Connections for 2- and 3-Series Battery Packs B4P OSCILLATOR LINEAR REGULATOR STATE MACHINE DRV DRIVER FAULT LOGIC PKN COMPARATOR OUT SCR B4P B3P SCR DRIVER MUX BN B2P B1P REF BN TEST LOGIC TEST PKN Figure 4. MAX1906 Functional Diagram _______________________________________________________________________________________ 7 MAX1906 Li+ Battery-Pack Protector with Integrated Fuse Driver OVERDISCHARGE PROTECTION THREE-TERMINAL PROTECTION FUSE R10 10Ω Si4435DY PACK+ TRICKLE CHARGE 16 SRC C6 2.2µF BSS84 SFD-145B 15 OVERCHARGE PROTECTION RTKO 510Ω 14 BN DSO CGO Si4435DY 13 VCC 7 OUT B4P 16 DISCON B3P 14 DISCON 2 C8 0.1µF MAX1906X DRV B2P 12 C7 0.1µF TEST 3 TEST B1P 10 R3 1kΩ R8 1kΩ R7 1kΩ R2 1kΩ R1 1kΩ R6 1kΩ PKN BN MAX1924X 1 C4 1.0µF C5 0.1µF 2 B4P VCC VDD 3 B3P C3 0.1µF 5 SHDN B2P 8 TEST 12 MICROCONTROLLER C2 0.1µF 7 CTL 11 DISCON B1P C1 0.1µF C6 0.1µF 5 CMPSH-3 D1 C9 0.1µF R5 100kΩ 4 R4 51Ω R9 10Ω TKO GND 9 BN PKN RSENSE 0.02 10 PACK- Figure 5. Typical Application Circuit—Using the MAX1906 with a MAX1924 Protection Circuit Modes of Operation Normal Operating Mode The MAX1906 operates in normal mode when at least 1 cell voltage is above the standby-mode threshold. In this mode, the average supply current from the top cell is 8µA (max). The MAX1906 works by sampling cell voltages for 0.8ms and then goes into an idle state for 2.56s to complete a cycle. During the sampling period, the MAX1906 typically consumes 300µA. In the idle state, the MAX1906 typically consumes 3.2µA. Figure 6 shows the device current consumption in different states. Standby Mode When all the cell voltages are below the standby-mode threshold, the MAX1906 goes into the standby mode. In 8 this mode, the device draws 1µA (max) from the top cell. Once any cell voltage goes above the standbymode threshold, the MAX1906 wakes up and goes into the normal mode. Test Mode The test mode is designed to verify the overvoltage detection function in a fully assembled battery pack without blowing the three-terminal protection fuse. Test mode is invoked by a pulse with minimum duration of 50µs on the TEST pin. The MAX1906 changes the overvoltage threshold from 4.45V to 2.225V in the test mode and samples each of the cell voltages. Individual cell voltages are expected to be above 2.225V during the test mode. If the MAX1906 detects overvoltage condition on all cells during one sampling period, the DRV pin goes _______________________________________________________________________________________ Li+ Battery-Pack Protector with Integrated Fuse Driver MAX1906 STOP SAMPLING AND MONITOR ONLY CELL UNDER MEASUREMENT 300µA ISUP 3.2µA 0.8ms 2.56s 0.8ms 2.1s VOV_TH VREL VB_P 4.8V VDRV NOTE: ALL VALUES ARE TYPICAL. Figure 6. Current Consumption of Chip in Different States VTEST 50µs ISUP 0.8ms IF ALL CELLS ARE TESTED TO BE IN OVERVOLTAGE CONDITION VDRV 130ms VDISCON Figure 7. Timing Diagram for Test Mode high and the DISCON pin is set to its high-impedance state. After 130ms, the DRV pin is pulled low by the MAX1906, exiting the test mode. The time period of 130ms has been chosen not to stress the three-terminal protection fuse if an external MOSFET is used to blow the fuse. The OUT pin is not affected by the test mode. See the timing diagram for the test mode in Figure 7. Entry into test mode is ignored if the MAX1906 has detected an overvoltage condition and has activated the 2.1s delay. Test mode remains disabled until the MAX1906 exits the overvoltage condition. The MAX1906 continues normal operation upon exit from the test mode. _______________________________________________________________________________________ 9 MAX1906 Li+ Battery-Pack Protector with Integrated Fuse Driver SAMPLE MODE EXIT FROM STANDBY MODE OR TEST MODE SAMPLE B_P SAMPLE MODE NO NO ALL CELLS CHECKED? YES B_P > VOV_TH ? CHECK DISCONNECTION OF B_P PIN YES STOP SAMPLING AND START 2.1s TIMER AND MONITOR CELL CONTINUOUSLY WAIT 2.56s NO IS B_P AT LEAST 1.2V ABOVE NEGATIVE TERMINAL NO B_P > VOV_TH CONTINUOUSLY AND TIMER = 2.1s? YES DISCON = L YES DRV = H SCR LATCHED NO B_P < VREL AND THE REST B_P < VOV_TH ? YES NO ALL PINS CHECKED? YES DISCON = H DRV = L Figure 8. Overvoltage Protection Protection Features Overvoltage Detection If any cell voltage exceeds the overvoltage threshold, the MAX1906 stops sampling and monitors the cell voltage continuously. If the overvoltage condition persists for more than 2.1s, the device turns on an internal SCR and also drives the DRV pin high. The internal SCR or the external MOSFET sinks sufficient current to blow the three-terminal protection fuse and permanently open the battery pack’s charge path. See the overvoltage protection flowchart in Figure 8. Also see the Fuse Drive Options section for discussion on current capability for both the internal SCR and external MOSFET. The MAX1906 remains in overvoltage mode until the cell voltage drops to 90% of the overvoltage threshold (VREL) and the rest of cells are below the overvoltage 10 Figure 9. Disconnected Pin Description threshold. The DRV pin then goes low, which turns off an external MOSFET. The internal SCR does not unlatch until power is removed. Disconnected Pin Detection The MAX1906 tests for disconnected voltage sense pins each time it exits the standby or test mode. To check for a disconnection, the MAX1906 applies a 10µA current source to each B_P pin. A disconnected pin is detected if the B_P pin under test falls to within 1.2V of the cell’s negative terminal. The DISCON pin is then pulled low. This condition persists while the MAX1906 is in normal operating mode, and resets only when the MAX1906 enters the standby or test mode. See Figure 9 for the disconnected pin detection flowchart. ______________________________________________________________________________________ Li+ Battery-Pack Protector with Integrated Fuse Driver Fuse Drive Options The MAX1906 supports two methods for blowing the external protection fuse: the internal SCR can be directly connected to the fuse’s heater terminal or an external MOSFET can be used to drive the heater. The design procedure for both methods requires matching the drive capabilities in the SCR or the MOSFET with the dissipation required to blow the fuse. The SCR configuration is simple, low cost, and does not require external components. The circuit in Figure 1 is appropriate for fuses that require heater currents up to 2A. Since the voltage drop across the SCR can be up to 2V, care must be taken not to exceed the device’s power ratings. When greater than 1in2 of copper plane is available to conduct heat away from the MAX1906, it can dissipate 1.6A at typically 1.7V indefinitely. When smaller copper planes are used, the time to clear the fuse must be less than the time for the MAX1906 to exceed its absolute maximum thermal ratings. The transient thermal characteristics for the MAX1906 are shown in the Typical Operating Characteristics. Since the thermal resistance varies inversely with the area of the copper plane attached to the device, the time to reach thermal limit also varies with copper area. External MOSFETs should be used with the MAX1906 when the heater current must be greater than 2.0A. MOSFETs with the required thermal characteristics are available from multiple manufacturers (see Table 1). Figure 2 shows the typical application circuit using an external MOSFET. Protection Fuse Selection Protection fuse characteristics can vary considerably from manufacturer to manufacturer. Always review the data sheet carefully when selecting the protection fuse. Table 2 lists the contact information for manufacturers of compatible fuses. There are two methods for opening the protection fuse. The fuse can be blown through the heater or by too much dissipation along the high-current path. The fuse must be selected to accommodate the required operating current without placing stress on the fuse. Once the nominal current-handling characteristics of the fuse are set, determine the amount of drive current and the time required to blow the fuse through the heater terminal. These quantities are also listed in the fuse manufacturer’s data sheet. The fuse blows when sufficient power is dissipated in the heater resistor to melt the fuse’s internal solder joints: PHEATER = VHEATER × IHEATER = (VBATT _ OV − VSWITCH )2 RHEATER VBATT_OV is the battery-pack voltage in the overvoltage condition, which is typically 4.45V per cell. VSWITCH is the voltage drop on the internal SCR or an external MOSFET. RHEATER is the resistance of the heater resistor. The time required to blow the protection fuse, or clearing time, depends upon the power dissipation in the heater resistor and the ambient temperature. Fuse manufacturers typically provide a curve of clearing time vs. voltage, and the clearing time vs. ambient temperature. The greater the power dissipation in the heater resistor, the quicker the fuse blows. Clearing time is also inversely proportional to ambient temperature. The heater resistance for different operating current specifications can range from a few ohms to a few hundred ohms. The resistance should be selected based on the acceptable clearing time and operating temperature range. For a battery pack requiring 4A of operating current, a fuse with a 5A nominal current rating is appropriate. An SFD-145B device made by Sony Chemical Corp. is selected, which has a 22Ω fusible resistor. Based on safety considerations, the clearing time should be no more than 1s or 2s. This is commensurate with the delay time required to detect the fault condition. The power dissipated in the SCR when the fuse is blown is approximately 1.3V ✕ 0.75A or 1W. To ensure that the junction temperature in the MAX1906 never exceeds 150°C at 60°C ambient temperature, the required thermal resistance must be: RθCA + RθJC < (TMAX - TA ) / (Pd) < (150°C - 60°C) / (1W) < 90°C / W where RθJC is the thermal impedance from junction to case, and RθCA is the thermal impedance from case to ambient. RθJC is fixed, and is about 5°C/W for the 16-lead 5mm ✕ 5mm QFN package. RθCA varies with copper area, and is shown in the Typical Operating Characteristics. Even though a combined thermal resistance of 90°C/W is achievable with less than 0.04in2 of copper area, it is advisable to include some margin to reduce the rise in device temperature. Using 0.25in2 copper area is conservative, and is available in most designs. ______________________________________________________________________________________ 11 MAX1906 Design Procedure MAX1906 Li+ Battery-Pack Protector with Integrated Fuse Driver RC Filters On Cell Inputs The MAX1906 has an unused pin placed between each of the cell connections. These extra pins minimize the risk of a solder short between pins during the assembly process. 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 the Typical Application Circuits). The MAX1906 is powered from the top cell during the sampling period. The 300µA typical sampling current, multiplied by a 10Ω series resistor can move the overvoltage trip point on the top cell by 3mV. The intermediate cell quiescent current is typically 500pA. A 1kΩ resistor in series with any cell except the top one alters the overvoltage trip point by typically 0.5mV. It is recommended to use a resistor of 10Ω in series with the top cell and 1kΩ resistors in series with the rest of the cells to achieve the desired overvoltage threshold tolerance while limiting the potential short-circuit current. The MAX1906 has internal ESD diodes on each B_P pin for ESD protection up to 2kV. When higher ESD ratings are needed, capacitors (typically 0.1µF) can be added across adjacent B_P pins (see the Typical Application Circuits). The RC filters improve the device immunity to ESD. Layout Guidelines Good layout is important to minimize the effects of noise on the system and ensure accurate voltage measurements. Use appropriate trace widths for the highcurrent paths and keep traces short to minimize parasitic inductance and capacitance. Provide adequate space and board area for the sense resistor to dissipate heat. Place RC filters close to B1P–B4P pins. If some amount of heat sinking is needed to use the internal SCR, connect the exposed backside paddle to as large a copper area as practical. Chip Information TRANSISTOR COUNT: 4027 PROCESS: BiCMOS Table 1. MOSFET Suppliers SUPPLIER Fairchild International Rectifier Vishay Siliconix USA PHONE 408-721-2181 310-322-3331 408-988-8000 FACTORY FAX 408-721-1635 310-322-3332 408-567-8979 WEBSITE www.fairchildsemi.com www.irf.com www.vishay.com Table 2. Recommended Fuse Manufacturers MANUFACTURER Sony Chemicals Corp. Uchihashi Estec Co., Ltd 12 PHONE +81-3-3279-0448 +81-6-6962-6661 FAX +81-3-5255-8448 +81-6-6962-6669 WEBSITE www.sccj.co.jp/html_e/ www.uchihashi.co.jp/ ______________________________________________________________________________________ Li+ Battery-Pack Protector with Integrated Fuse Driver ______________________________________________________________________________________ 13 MAX1906 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.) MAX1906 Li+ Battery-Pack Protector with Integrated Fuse Driver Package Information (continued) (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.) 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. 14 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 © 2002 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.