19-1545; Rev 0; 9/99 KIT ATION EVALU E L B AVAILA Single-Cell Li+ Battery Charger for Current-Limited Supply Features The MAX1679 battery charger for a single lithium-ion (Li+) cell comes in a space-saving 8-pin µMAX package. This simple device, in conjunction with a current-limited wall cube and a PMOS transistor, allows safe and fast charging of a single Li+ cell. ♦ Simple Stand-Alone Application Circuit The MAX1679 initiates charging in one of three ways: battery insertion, charger power-up, or external manipulation of the THERM pin. Charging terminates when the average charging current falls to approximately 1% of the fast-charge current, or when the on-chip counter times out. ♦ 0.75% Overall System Accuracy Key safety features include continuous voltage and temperature monitoring, a programmable charger timeout, and a 5mA precharge current mode to charge near-dead cells. Automatic detection of input power removal shuts down the device, minimizing current drain from the battery. An overall system accuracy of 0.75% ensures that the cell capacity is fully utilized without cycle life degradation. ♦ Top-Off Charging to Achieve Full Battery Capacity ♦ Lowest Power Dissipation ♦ 8-Pin µMAX Package Saves Space ♦ No Inductor Required ♦ Continuous Voltage and Temperature Protection ♦ Safely Precharges Near-Dead Cells ♦ Programmable Safety Timeout ♦ Automatic Power-Down when Power Source is Removed The MAX1679 evaluation kit (MAX1679EVKIT) is available to help reduce design time. Ordering Information PART MAX1679EUA TEMP. RANGE -40°C to +85°C PIN-PACKAGE 8 µMAX Applications Single-Cell Li+ Portable Applications Self-Charging Battery Packs PDAs Cell Phones Typical Operating Circuit Cradle Chargers Pin Configuration CURRENTLIMITED VOLTAGE SOURCE PFET LED GATE IN TOP VIEW BATT CHG BATTERY IN 1 GATE 2 CHG 3 TSEL 4 MAX1679 µMAX 8 BATT 7 THERM 6 GND 5 ADJ TIMER RANGE SELECT MAX1679 TSEL THERM GND ADJ THERMISTOR ________________________________________________________________ Maxim Integrated Products 1 For free samples & the latest literature: http://www.maxim-ic.com, or phone 1-800-998-8800. For small orders, phone 1-800-835-8769. MAX1679 General Description MAX1679 Single-Cell Li+ Battery Charger for Current-Limited Supply ABSOLUTE MAXIMUM RATINGS IN, CHG, GATE to GND ........................................ -0.3V to +26V BATT, TSEL, THERM, ADJ to GND ........................ -0.3V to +6V GATE to IN................................................................-6V to +0.3V THERM, ADJ to BATT...............................................-6V to +0.3V GATE Continuous Current ................................................ -10mA Continuous Power Dissipation (TA = +70°C) 8-Pin µMAX (derate 4.1mW/°C above +70°C) ........... 330mW Operating Temperature Range ...........................-40°C to +85°C Storage Temperature Range ............................ -65°C to +150°C Lead Temperature (soldering, 10sec) ............................ +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 (VIN = V CHG = +10V, VBATT = +4.2V, TSEL = GND, GATE = unconnected, ADJ = unconnected, THERM = 10kΩ to GND, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) MAX UNITS Input Voltage (Note 1) PARAMETER SYMBOL VIN External PMOS FET off CONDITIONS MIN 5 22 V Input Voltage for Removable Battery (Note 1) VIN External PMOS FET off 6 22 V 2.3 V Undervoltage Lockout Trip Point VBATT rising Undervoltage Lockout Trip-Point Hysteresis VBATT falling Fast-Charge Qualification Threshold VBATT rising, transition from precharge to full current 2.425 2.500 2.575 V Fast-Charge Restart Threshold VBATT falling, transition from DONE to PREQUAL state (Figure 2) 3.783 3.89 4.00 V Precharge Source Current VBATT = 2V BATT Regulation Voltage 2.1 TYP 2.2 60 mV 4 5 6 mA 4.1685 4.2000 4.2315 V BATT Regulation Adjust Range 4.0 4.2 V ADJ Source Impedance 9.8 10 10.2 kΩ ADJ Output Voltage No load on ADJ 1.393 1.400 1.407 V Battery Removal Detection Threshold VBATT rising 4.875 5.0 5.125 V % Battery Removal Detection Threshold Hysteresis VBATT falling 125 BATT Input Current (Note 2) VIN ≤ VBATT -0.3V 0.1 1 µA mV BATT Input Current, FastCharge State IBATT VBATT = 4V 900 1500 µA BATT Input Current, Done State IBATT VBATT = 4.25V 500 1000 µA IN Input Current, Fast-Charge State IIN VBATT = 4V, VIN = 4V 30 100 µA IN Input Current, Done State IIN VBATT = 4.25V, VIN = 22V 250 600 µA Timer Accuracy See Table 3 -10 +10 % CHG Output Leakage Current V CHG = 22V, CHG = high -1 +1 µA 2 _______________________________________________________________________________________ Single-Cell Li+ Battery Charger for Current-Limited Supply (VIN = V CHG = +10V, VBATT = +4.2V, TSEL = GND, GATE = unconnected, ADJ = unconnected, THERM = 10kΩ to GND, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) PARAMETER SYMBOL MIN TYP MAX UNITS 4 5 6 mA THERM Sense Current (for hot qualification) 346 352.9 360 µA THERM Sense Current (for cold qualification) 47.8 48.8 49.8 µA THERM Sense Voltage Trip Point 1.379 1.40 1.421 V % CHG Output Sink Current CONDITIONS V CHG = 1V, CHG = low GATE Source/Sink Current VIN = 10V, VGATE = 8V 75 105 130 µA GATE Drive Current at Battery Removal VBATT = 5.1V, gate driven high 20 40 60 mA ELECTRICAL CHARACTERISTICS (VIN = V CHG = +10V, VBATT = +4.2V, TSEL = GND, GATE = unconnected, ADJ = unconnected, THERM = 10kΩ to GND, TA = -40°C to +85°C, unless otherwise noted.) MIN MAX UNITS Input Voltage (Note 1) PARAMETER SYMBOL VIN External PMOS FET off CONDITIONS 5 22 V Input Voltage for Removable Battery (Note 1) VIN External PMOS FET off 6 22 V Undervoltage Lockout Trip Point VBATT rising 2.05 2.35 V Fast-Charge Qualification Threshold VBATT rising, transition from precharge to full current 2.35 2.65 V Fast-Charge Restart Threshold VBATT falling, transition from DONE to PREQUAL state, Figure 2 3.74 4.04 V Precharge Source Current VBATT = 2V BATT Regulation Voltage BATT Regulation Adjust Range ADJ Source Impedance 3 7 mA 4.137 4.263 V 4.0 4.2 V 9.8 10.2 kΩ ADJ Output Voltage No load on ADJ 1.386 1.414 V Battery Removal Detection Threshold VBATT rising 4.850 5.150 V BATT Input Current (Note 2) VIN ≤ VBATT - 0.3V 1 µA BATT Input Current, FastCharge State IBATT VBATT = 4.0V 1500 µA BATT Input Current, Done State IBATT VBATT = 4.25V 1000 µA VBATT = 4.0V, VIN = 4.0V 100 µA IN Input Current, Fast-Charge State IIN _______________________________________________________________________________________ 3 MAX1679 ELECTRICAL CHARACTERISTICS (continued) MAX1679 Single-Cell Li+ Battery Charger for Current-Limited Supply ELECTRICAL CHARACTERISTICS (continued) (VIN = V CHG = +10V, VBATT = +4.2V, TSEL = GND, GATE = unconnected, ADJ = unconnected, THERM = 10kΩ to GND, TA = -40°C to +85°C, unless otherwise noted.) PARAMETER IN Input Current, Done State SYMBOL IIN CONDITIONS MIN VBATT = 4.25V, VIN = 22V MAX UNITS 700 µA % Timer Accuracy See Table 3 -15 +15 CHG Output Leakage Current V CHG = 22V, CHG = high -1 +1 µA CHG Output Sink Current V CHG = 1.0V, CHG = low 4 6 mA THERM Sense Current (for hot qualification) 342 363 µA THERM Sense Current (for cold qualification) 47.3 50.3 µA THERM Sense Voltage Trip Point 1.358 1.442 V GATE Source/Sink Current VIN = 10V, VGATE = 8V 60 140 µA GATE Drive Current at Battery Removal VBATT = 5.1V, gate driven high 20 90 mA Note 1: The input voltage range is specified with the external PFET off. When charging, the PFET turns on and the input voltage (the output voltage of the constant-current power source) drops to very near the battery voltage. When the PFET is on, VIN may be as low as 2.5V. Note 2: BATT Input Current is the supply current to the device. When VIN is removed, the MAX1679 shuts down and the input current is less than 1µA, even if there is an external RADJ resistor (ADJ to GND). 4 _______________________________________________________________________________________ Single-Cell Li+ Battery Charger for Current-Limited Supply NORMALIZED BATT REGULATION VOLTAGE vs. TEMPERATURE 1.0100 1.0050 1.0000 0.9950 0.9900 -40 1.0010 NORMALIZED TO +25°C 1.0005 1.0000 0.9995 0.9990 0.9985 6.0 5.8 PRECHARGE CURRENT (mA) TIMER PERIOD 1.0150 1.0015 MAX1679 toc02 MAX1679 toc01 NORMALIZED TO +25°C BATT REGULATION VOLTAGE 1.0200 PRECHARGE CURRENT vs. INPUT VOLTAGE -20 0 20 40 60 TEMPERATURE (°C) 80 100 5.6 VBATT = 0V 5.4 5.2 VBATT = 2V 5.0 4.8 VBATT = 4.1V 4.6 4.4 0.9980 0.9975 -40 MAX1679 toc03 TIMER PERIOD vs. AMBIENT TEMPERATURE 4.2 4.0 -20 0 20 40 60 80 100 TEMPERATURE (°C) 5 10 15 VIN (V) 20 25 Pin Description PIN NAME FUNCTION 1 IN Input Voltage from Current-Limited Voltage Source (+22V max). Bypass to GND with a 0.1µF capacitor. The cell charging current is set by the current limit of the external power supply. 2 GATE Gate Drive for External PMOS Pass Element. The PMOS device should have a VGS threshold of 2.5V or less. See Selecting External Components. 3 CHG Charge Status Indication. CHG is an open-drain, current-limited N-channel MOSFET suitable for directly driving an LED. Connect a pull-up resistor to BATT to generate a logic-level signal. See Table 2 for CHG output states. 4 TSEL Maximum Total Charge-Time Selection. See Table 3 for timer options. 5 ADJ Battery Regulation Voltage Adjustment. Bypass to GND with a 1000pF capacitor. Connect a resistor from ADJ to GND to reduce the nominal +4.200V regulation setpoint. See Adjusting the Battery Regulation Voltage. 6 GND Ground. See Layout Guidelines for information on system grounding. Connect the battery’s negative terminal to GND. 7 THERM Thermistor Temperature-Sensor Input. Connect a thermistor from THERM to GND to prequalify the cell temperature for fast-charge. Drive THERM high (> +1.4V) during Done or Fault states to reset the MAX1679 and reinitiate the charging sequence. Replace the thermistor with a 10kΩ resistor if temperature sensing is not required. 8 BATT Cell Voltage Monitor Input, Trickle-Charge Output, and MAX1679 Power Source. Connect BATT to the positive terminal of a single Li+ cell. Bypass BATT with a capacitor to ground (1.5µF per amp of charge current). _______________________________________________________________________________________ 5 MAX1679 Typical Operating Characteristics (VIN = V CHG = +10V, VBATT = +4.2V, THERM = 10kΩ to GND, TA = +25°C, unless otherwise noted.) MAX1679 Single-Cell Li+ Battery Charger for Current-Limited Supply CURRENT-LIMITED VOLTAGE SOURCE (800mA) PFET * REVERSE-CURRENT PROTECTION DIODE IN GATE CHG MAX1679 TRICKLE CHARGE (5mA) 5mA BATT 10kΩ TSEL STATE MACHINE SEE TABLE 3 FOR TIMEOUT OPTIONS 1:3 ADJ 2.2µF 1.4V 1nF TIMER RADJ ** BATT REMOVED 5V COLD QUAL TEST CURRENT 30kHz OSCILLATOR HOT QUAL TEST CURRENT 1.4V THERM NTC THERMISTOR* *OPTIONAL **RADJ = 410kΩ ±1% FOR 4.1V CELL; LEAVE OPEN FOR 4.2V CELL. Figure 1. Functional Diagram 6 _______________________________________________________________________________________ SINGLE Li+ CELL Single-Cell Li+ Battery Charger for Current-Limited Supply CELL NOT INSTALLED (FROM ANY STATE) MAX1679 SHUTDOWN LED: OFF PMOS FET: OFF CHARGING POWER REMOVED (FROM ANY STATE) R OWE ER P D G R CHA APPLIE CELL REMOVAL OR PULL THERM HIGH PREQUALIFICATION LED: 50% DUTY CYCLE, 2Hz PMOS FET: OFF 5mA PRECHARGE: ON VB PULATT < 3.8 L TH 9V O TEM ERM R PER HIGH ATU AND RE O VB K ATT > 2. 5V TEMPERATURE NOT OK OR VBATT < 2.5V E RG HA -C OUT T S FA TIME FAST-CHARGE LED: ON PMOS FET: ON ec RY FAULT LED: 50% DUTY CYCLE, 2Hz PMOS FET: OFF 7s E EV K EO UR T RA E MP TEMPERATURE NOT OK RY CELL VOLTAGE REACHES BATT REGULATION VALUE (e.g., 4.2V) TE FAST-CHARGE QUALIFICATION LED: ON PMOS FET: OFF TOP OFF LED: ON PMOS FET: PULSED TOP-OFF c se 7 E EV RE OK FET O FET N TIM E OFF TIM ≤ E 1% U AT ER TOP-OFF QUALIFICATION LED: ON PMOS FET: OFF MP TE DONE LED: 12% DUTY CYCLE, 0.25Hz PMOS FET: OFF 5mA CANCELLATION CURRENT ENABLED TEMPERATURE NOT OK Figure 2. State Machine Diagram _______________________________________________________________________________________ 7 MAX1679 Single-Cell Li+ Battery Charger for Current-Limited Supply Detailed Description Initiating a Charge Cycle The MAX1679 attempts to initiate fast-charge upon insertion of the battery or application of an external power source (current-limited wall cube). After charge completion, charging restarts when the cell voltage drops below 3.89V or when THERM is pulled above 1.4V. Before a charge cycle can begin, the cell conditions are verified to be within safe limits. The cell voltage must be greater than 2.5V but less than the regulation voltage (default value 4.2V). In addition, the thermistor must indicate an acceptable cell temperature (the default range is +2.5°C to +47.5°C). See the Applications Information section. Li+ cells can be damaged when fast-charged from a completely dead state. Moreover, a fully discharged cell may indicate a dangerously abnormal cell condition. As a built-in safety feature, the MAX1679 precharges the Li+ cell with 5mA at the start of a charge cycle when the cell voltage is below 2.5V. Typically, 5mA is sufficient to bring a fully discharged 1000mAh Li+ cell up to 2.5V in less than 5 minutes. As soon as the cell’s voltage reaches 2.5V and all the other prerequisites are met (see the Fast-Charge section), the MAX1679 begins fast-charging the cell. If the temperature is outside the programmed range, the charger waits. Once all prequalification conditions are met, the charging cycle and timers begin. The MAX1679 continues to monitor these conditions throughout the charging cycle. Fast-Charge Once all cell conditions are determined to be satisfactory, the MAX1679 begins fast-charging the Li+ cell by turning on the external PMOS FET. The cell charging current is set by the current limit of the external power supply; it is not regulated by the MAX1679. The PMOS FET is used as a simple switch, not as a linear regulator. Therefore, the circuit’s power dissipation is minimized, permitting rapid charge cycles with minimal heat generation. The external power supply should have a specified current limit that matches the desired fast-charge current for the Li+ cell. Fast-charge continues until one of the following conditions is reached: 1) cell voltage climbs to the battery regulation voltage (4.2V or as set by ADJ); 2) the fastcharging timer expires (fault condition); or 3) cell temperature rises above +47.5°C or falls below +2.5°C. If the cell temperature moves outside the specified limits, charging is suspended but not terminated. All timers are paused and charging resumes when the temperature returns to the normal range. 8 Pulsed Top-Off Charge In the most common case, where fast-charge is terminated because the battery regulation voltage has been reached (that is, the cell is nearly fully charged), the MAX1679 will top off the cell. The MAX1679 uses a hysteretic algorithm with minimum on-times and minimum off-times (Table 3). The cell voltage is sampled every 2ms. If the cell voltage (at BATT) is less than the battery regulation voltage, the external PMOS FET turns on or remains on. If the cell voltage is greater than, or equal to, the battery regulation voltage, the FET turns off or remains off until the next sample. By also measuring the cell voltage when the PMOS FET is off, the MAX1679 eliminates voltage errors caused by charging current flowing through the series resistance of protection switches or fuse links that may be in the charging path. At the beginning of this top-off state, the current stays on for many consecutive cycles between single off periods. As the cell continues to charge, the percentage of time spent in the “current-on” mode decreases. Toward the end of top off, the current stays off for many cycles between single “on” pulses. During these final pulses, the instantaneous cell voltage may exceed the battery regulation voltage by several hundred millivolts, but the duration of these pulses is several orders of magnitude shorter than the intrinsic chemical time constant of Li+ cells. This does not harm the cell. Cell top off is completed either when the duty ratio of “on” cycles to “off” cycles falls below 1/64 to 1/256 as set by TSEL (see Table 3), or when the charging timer expires. Switched 5mA Cancellation Current When the charge cycle is complete, the MAX1679 replaces current drawn by the BATT pin (to sense battery removal) with a 5mA (nominal) switched current. This current is turned on and off by an on-chip comparator as needed to maintain the battery regulation voltage. The cell is maintained in this manner as long as the battery is inserted and power is connected. This cancellation current is turned off when the MAX1679 is shut down. Note that BATT draws less than 1µA from the battery when power is removed. Charge Status with CHG CHG indicates the cell’s charging status. An LED can be connected directly from IN to CHG for a visible indicator. Alternatively, a pull-up resistor (typically 100kΩ) from a logic supply to CHG provides a logic-level output. Table 2 relates the status of the CHG to the condition of the charger. _______________________________________________________________________________________ Single-Cell Li+ Battery Charger for Current-Limited Supply The MAX1679 was designed to offer the maximum integration and functionality in the smallest, most basic application circuit possible. The only necessary external components are a current-limited wall cube, a PMOS FET, two small capacitors, and a 10kΩ thermistor/resistor. This simple application circuit appears in Figure 3. Optionally (as shown in Figure 4), an LED can be added as a charge-state indicator, a resistor (RADJ) can be used to trim down the maximum charge voltage from 4.2V, and/or a reverse-current-protection diode can be added in line at the source. If the input is shorted, the MAX1679 will not allow current to flow from BATT back through IN to the source. However, the body diode inherent in the enhancement- mode FET would still allow the cell to discharge rapidly. To prevent this, add a power Schottky diode between the source and IN as in Figure 4. Adjusting the Battery Regulation Voltage A typical Li+ cell should be charged at a constant current until it reaches a voltage of about 4.2V, then charged at this voltage until the current decays below a predetermined level. The MAX1679 provides a simple way to reduce this maximum target voltage with a single resistor between ADJ and GND. Internally, ADJ connects to a precision 1.4V reference through a 10kΩ resistor. Leave ADJ open for a battery regulation voltage (VBR) of 4.2V; connect a 1% resistor from ADJ to GND to form a voltage divider for lower battery regulation voltage (VBR¢ ). Select the external value using: Table 2. CHG Output States RADJ = CHG CONDITION No battery or no charger, or cell voltage < 2.2V High impedance Fast-charge or pulsed topoff charge in progress Low (LED on) Fast-charge timer expiration or initial prequalification state (VBATT < 2.5V or initial temperature fault) 2Hz, 50% duty factor (LED flashing) Charge cycle complete LED blinking 0.5sec on (low), 3.5sec off (high impedance) A 1% tolerance resistor at ADJ degrades system accuracy by only a fraction of a percent. For example, an RADJ of 410kΩ ±1% reduces the battery regulation voltage by 2.4% (VBR¢ = 4.1V from equation above, and PMOS FET FAIRCHILD FDC638P -4.5A, -20V 0.07Ω AT VGS = -2.5V CURRENT-LIMITED WALL CUBE (800mA, 6V max) 10kΩ VBR −1 VBR '¢ (VBR¢ - VBR) / VBR = (4.1 - 4.2) / 4.2 = -2.4%). Therefore, the additional system error is simply the RADJ tolerance multiplied by the percent change in the battery regulation voltage, or (1%)(2.4%) = 0.024%. CURRENT LIMITED WALL CUBE (800mA, 6V max) GATE GATE IN IN 5mA LED BATT MAX1679 CHG PMOSFET FAIRCHILD FDC638P -4.5A, -20V 0.07Ω AT VGS = -2.5V SCHOTTKY DIODE 30V, 1A ZETEX ZHCS1000 TSEL* ADJ THERM CHG SINGLE Li+ CELL 0.001µF 0.001µF 10k 2.2µF *SEE TABLE 3 FOR TSEL POSITION AND ASSOCIATED TIMER SETTINGS. Figure 3. Simple Application Circuit MAX1679 RADJ 410k 1%** BATT TSEL* ADJ THERM 2.2µF NTC THERMISTOR FENWAL 140-103LAG-RBI (10kΩ AT 25°C) SINGLE Li+ CELL *SEE TABLE 3 FOR TSEL POSITION AND ASSOCIATED TIMER SETTINGS. ** RADJ SETS BATTERY REGULATION VOLTAGE TO 4.10V; LEAVE OPEN FOR 4.2V. Figure 4. Application Circuit Including LED, Thermistor, and Reverse-Current Protection Diode _______________________________________________________________________________________ 9 MAX1679 Applications Information MAX1679 Single-Cell Li+ Battery Charger for Current-Limited Supply Table 3. Timer Option (TSEL) Definitions TSEL CONNECTION RECOMMENDED CHARGE RATE FASTCHARGE TIME LIMIT (minutes) BATT 1.5C 55 ADJ 1C 75 GND <1C Off 6.25 Selecting Maximum Charge Time As a safety feature, fast-charging and pulsed top-off charging will stop if their respective counters time out. The MAX1679 offers a choice of three timeout periods set by TSEL (Table 3). In Table 3, C represents the nominal capacity of the battery cell in ampere hours. Both timers begin upon entering the fast-charge state. The fast-charge timer is disabled upon leaving fastcharge; the total charge timer continues through top-off but is disabled in the done state. Selecting External Components Power Supply One reason the MAX1679 Li+ cell-charging solution is so compact and simple is that the charging current is set by the external power source, not by the MAX1679 charging circuit. The PMOS FET in this application circuit is either on or off, allowing the source to be directly connected to the cell or completely disconnected. Therefore, it is very important to choose a power supply with current limiting. In most applications, this will be a small “wall cube” switching converter with an output voltage limit of about 5V or 6V, which is advertised as “current-limited” or “constant current.” PMOS Switch The PMOS FET is used to switch the current-limited source on and off into the Li+ cell. Because of the intentionally slow switching times and limited slew rate, the MAX1679 is not particular about the power FET it drives. Specifications to consider when choosing an appropriate FET are the minimum drain-source breakdown voltage, the minimum turn-on threshold voltage (VGS), and current handling and power-dissipation qualities. The minimum breakdown voltage (BVDS) must exceed the open-circuit voltage of the wall cube by at least 25%. Note that this open-circuit voltage may be twice as high as the specified output voltage, depending on the converter type. 10 TOTAL CHARGE TIME LIMIT (hours) MIN ON/OFF TIME (IN TOP-OFF) (ms) ON/OFF DUTY CYCLE FOR DONE INDICATION 2.8 70 1/256 3.75 140 1/128 280 1/64 Thermistor The intent of THERM is to inhibit fast-charging the cell when it is too cold or too hot (+2.5°C ≤ TOK ≤ 47.5°C), using an external thermistor. THERM time multiplexes two sense currents to test for both hot and cold qualification. The thermistor should be 10kΩ at +25°C and have a negative temperature coefficient (NTC); the THERM pin expects 3.97kΩ at +47.5°C and 28.7kΩ at +2.5°C. Connect the thermistor between THERM and GND. If no temperature qualification is desired, replace the thermistor with a 10kΩ resistor. Thermistors by Philips (22322-640-63103), Cornerstone Sensors (T101D103-CA), and Fenwal Electronics (140-103LAGRB1) work well. Bypass Capacitors Bypass the ADJ pin with a 0.001µF ceramic capacitor. Bypass BATT with a capacitor with a value of at least 1.5µF per amp of charge current. The MAX1679 has a built-in protection feature that prevents BATT from rising above 5.5V. The device recognizes a rapid rise at BATT, indicating that the cell is being removed with the FET on. A capacitor from BATT to GND that’s too small does not give the MAX1679 adequate time to shut off the FET. BATT may then rise above 6V (towards the open-circuit source voltage), violating the absolute maximum rating and damaging the device. In applications where the cell is removable, very large capacitance values make it increasingly difficult to identify momentary cell removal events and may increase transient currents when the cell is replaced. Therefore, values in excess of 100µF should be avoided in those cases. For best system performance, at least 0.47µF of the total capacitance should be low-ESR ceramic. ______________________________________________________________________________________ Single-Cell Li+ Battery Charger for Current-Limited Supply TRANSISTOR COUNT: 4692 SUBSTRATE CONNECTED TO GND ______________________________________________________________________________________ 11 MAX1679 Chip Information Layout Guidelines The MAX1679 controls the GATE slew rate. The layout is not as sensitive to noise as a high-frequency switching regulator. In addition, since the cell voltage is sensed both during and between high-current pulses, the system is insensitive to ground drops. However, Maxim recommends establishing good grounding areas and large traces for high-current paths. Single-Cell Li+ Battery Charger for Current-Limited Supply 8LUMAXD.EPS MAX1679 Package Information Note: The MAX1679 does not have an exposed pad. 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. 12 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 © 1999 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.