19-2295; Rev 0; 1/02 Linear Charger for Single-Cell Li+ Battery Features ♦ Simple, Safe Linear Charger ♦ Uses Low-Cost PNP or PMOS Pass Element ♦ 4.5V to 12V Input Range ♦ Internal Current-Sense Resistor ♦ ±0.75% Voltage Accuracy ♦ Programmable Charging Current ♦ Automatic Input Power-Source Detection ♦ LED Charge Status Indicator ♦ Programmable Safety Timer ♦ Current-Sense Monitor Output ♦ Optional/Adjustable Auto-Restart ♦ Small µMAX Package Ordering Information PART Applications TEMP RANGE PIN-PACKAGE MAX1898EUB42 -40°C to +85°C 10 µMAX MAX1898EUB41 -40°C to +85°C 10 µMAX Single-Cell Li+ Powered Portables Self-Charging Battery Packs Typical Operating Circuit PDAs Cell Phones INPUT 4.5V TO 12V Cradle Chargers IN OUTPUT CS LED Pin Configuration Li+ BATTERY MAX1898 CHG DRV ON TOP VIEW EN/OK BATT CT ISET OFF IN 1 10 CS CHG 2 EN/OK 3 ISET 4 7 BATT CT 5 6 RSTRT MAX1898 9 DRV 8 GND CT SETS CHARGE TIME SETS CHARGE CURRENT RSTRT GND µMAX ________________________________________________________________ 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 MAX1898 General Description The MAX1898, with an external PNP or PMOS transistor, forms a complete charger for 1-cell lithium-ion (Li+) batteries. The MAX1898 provides accurate constant-current/constant-voltage charging. Battery regulation voltage accuracy is ±0.75% for optimum battery performance and cycle life. Charging current is user set, but is sensed internally so no external current-sense resistor is needed. The MAX1898 also provides outputs to monitor charge status, presence-of-input power source, and charge current. Additional features include shutdown, optional charge-cycle restart without input-power cycling, selectable charge termination safety timer, and low-current preconditioning for deeply discharged cells. The MAX1898 offers the maximum integration and functionality in the smallest, most basic application circuit possible. Very few external components are required. The MAX1898 offers two versions to safely charge all Li+ battery chemistries. The battery regulation voltage is 4.2V for the MAX1898EUB42 and 4.1V for the MAX1898EUB41. Both are available in a 10-pin ultrathin µMAX package. MAX1898 Linear Charger for Single-Cell Li+ Battery ABSOLUTE MAXIMUM RATINGS IN, DRV, CS, CHG to GND .....................................-0.3V to +14V BATT, EN, ISET, RSTRT, CT to GND ........................-0.3V to +6V DRV to IN................................................................-14V to +0.3V IN to CS Current ....................................................................±2A DRV Continuous Current ..................................................±10mA Continuous Power Dissipation (TA = +70°C) 10-Pin µMAX (derate 5.6mW/°C above +70°C) ..........444mW Operating Temperature Range ...........................-40°C to +85°C Junction Temperature ......................................................+150°C Storage Temperature Range .............................-65°C to +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 (VIN = 8V, VBATT = 4V, EN/OK = open, RSET = 2.8kΩ, RSTRT = GND, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) PARAMETER CONDITIONS MIN MAX UNITS 12 V 70 100 mV 30 55 85 mV 4.0 4.25 4.5 V 3.9 4.15 4.4 V Supply Voltage Range 4.5 Input OK Trip Point, IN Rising (VIN - VBATT) 45 Input OK Trip Point, IN Falling (VIN - VBATT) VIN Input OK Trip Point, IN Rising VIN Input OK Trip Point, IN Falling EN/OK Sink Current (When Input Power Not OK) VIN = 3.9V, VBATT = 4V, VEN/OK = 0.8V EN/OK Source Current (When Input Power OK) VEN/OK = 2V 5 EN/OK Logic Input High 4.5V ≤ VIN ≤ 12V 2 EN/OK Logic Input Low BATT Input Current IN, CS Input Current TYP 100 µA 10 20 µA V 4.5V ≤ VIN ≤ 12V 0.6 V Done, RSTRT = GND, VBATT = 4.1V for MAX1898EUB42 and 4.0V for MAX1898EUB41 40 100 Done, RSTRT = open, VBATT = 4.1V for MAX1898EUB42 and 4.0V for MAX1898EUB41 4 10 EN/OK = GND 4 10 VIN = 4V 3 10 VIN = VBATT = 4V (IIN + ICS) 2 10 µA VIN = 12V, (IIN + ICS), done, RSTRT = open 2 5 mA µA VOLTAGE LOOP Voltage-Loop Set Point 4.168 4.2 4.232 MAX1898EUB41 4.069 4.1 4.131 Voltage-Loop Gain ∆VDRV/∆VBATT VBATT Prequal Voltage Threshold Timers running DRV Sink Current VDRV = VIN - 1V, VBATT = 3.5V DRV Output Resistance 2 MAX1898EUB42 51 dB MAX1898EUB42 2.394 2.520 2.646 MAX1898EUB41 2.337 2.460 2.583 4 V V mA 470 _______________________________________________________________________________________ Ω Linear Charger for Single-Cell Li+ Battery (VIN = 8V, VBATT = 4V, EN/OK = open, RSET = 2.8kΩ, RSTRT = GND, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) PARAMETER CONDITIONS MIN Maximum Voltage (VIN - VDRV) VBATT = 3.5V 3.5 VBATT Restart Threshold (As a Percentage of Voltage-Loop Set Point) RSTRT = GND 92 RSTRT Internal Resistance TYP 95.25 MAX UNITS 7 V 98 % 100 kΩ CURRENT LOOP Internal Current-Sense Resistance IN to CS pin-to-pin resistance 50 105 RMS Charge Current 200 mΩ 1.4 A Current-Sense Amplifier Gain, ISET to IIN Fast Charge ICS = 500mA, VSET = 1.4V 0.95 1 1.05 mA/A Current-Sense Amplifier Gain, ISET to IIN Prequal ICS = 50mA, VISET = 0.14V, VBATT = 2V 0.73 1 1.30 mA/A Current-Loop Regulation Set Point RSET = 2.8kΩ, VBATT = 3.5V, closed-loop performance 450 500 550 mA Current-Loop Gain ∆VDRV/∆VISET CHG Trip Threshold (As a Percentage of Full Load Current) ILOAD falling, RSET = 2.8kΩ 15 20 25 ILOAD rising, RSET = 2.8kΩ 17 22 27 26 dB % TIMER FUNCTION Timer Source/Sink Current VCT = 0.9V Timer Accuracy CCT = 100nF for 2 hours, 54 minutes nominal 5 CHG Output Low Current V CHG = 1V CHG Output High Leakage V CHG = 12V CHG Blink Rate Fault state (50% duty cycle), CT = 100nF µA -10 4 5 1 1.5 +10 % 6 mA 1 µA 2 Hz ELECTRICAL CHARACTERISTICS (VIN = 8V, VBATT = 4V, EN/OK = open, RSET = 2.8kΩ, RSTRT = GND, TA = -40°C to +85°C, unless otherwise noted.) MIN MAX UNITS Supply Voltage Range PARAMETER CONDITIONS 4.5 12 V Input OK Trip Point, IN Rising (VIN - VBATT) 45 100 mV Input OK Trip Point, IN Falling (VIN - VBATT) 30 85 mV VIN Input OK Trip Point, IN Rising 4.0 4.5 V VIN Input OK Trip Point, IN Falling 3.9 4.4 V EN/OK Sink Current (When Input Power Not OK) VIN = 3.9V, VBATT = 4V, VEN/OK = 0.8V EN/OK Source Current (When Input Power OK) VEN/OK = 2V 5 EN/OK Logic Input High 4.5V ≤ VIN ≤12V 2 EN/OK Logic Input Low 4.5V ≤ VIN ≤12V 100 µA 20 µA 0.6 V V _______________________________________________________________________________________ 3 MAX1898 ELECTRICAL CHARACTERISTICS (continued) MAX1898 Linear Charger for Single-Cell Li+ Battery ELECTRICAL CHARACTERISTICS (continued) (VIN = 8V, VBATT = 4V, EN/OK = open, RSET = 2.8kΩ, RSTRT = GND, TA = -40°C to +85°C, unless otherwise noted.) PARAMETER BATT Input Current IN, CS Input Current CONDITIONS MIN MAX Done, RSTRT = GND, VBATT = 4.1V for MAX1898EUB42 and 4.0V for MAX1898EUB41 100 Done, RSTRT = open, VBATT = 4.1V for MAX1898EUB42 and 4.0V for MAX1898EUB41 10 EN/OK = GND 10 UNITS µA VIN = 4V 10 VIN = VBATT = 4V (IIN + ICS) 10 µA VIN = 12V, (IIN + ICS), done, RSTRT = open 5 mA VOLTAGE LOOP Voltage-Loop Set Point MAX1898EUB42 4.137 4.263 MAX1898EUB41 4.038 4.162 MAX1898EUB42 2.394 2.646 MAX1898EUB41 2.337 2.583 V VBATT Prequal Voltage Threshold Timers running DRV Sink Current VDRV = VIN - 1V, VBATT = 3.5V Maximum Voltage (VIN - VDRV) VBATT = 3.5V 3 7 V VBATT Restart Threshold (As a Percentage of Voltage-Loop Set Point) RSTRT = GND 92 98 % IN to CS pin-to-pin resistance 50 200 mΩ 1.4 A 3.5 V mA CURRENT LOOP Internal Current-Sense Resistance RMS Charge Current Current-Sense Amplifier Gain, ISET to IIN Fast Charge ICS = 500mA, VSET = 1.4V 0.93 1.07 mA/A Current-Sense Amplifier Gain, ISET to IIN Prequal ICS = 50mA, VISET = 0.14V, VBATT = 2V 0.6 1.50 mA/A Current-Loop Regulation Set Point RSET = 2.8kΩ, VBATT = 3.5V, closed-loop performance 440 560 mA CHG Trip Threshold (As a Percentage of Full Load Current) ILOAD falling, RSET = 2.8kΩ 15 25 ILOAD rising, RSET = 2.8kΩ 17 27 Timer Accuracy CCT = 100nF for 2 hours, 54 minutes nominal -13 +13 % CHG Output Low Current V CHG = 1V 3.5 7 mA CHG Output High Leakage V CHG = 12V 1 µA CHG Blink Rate Fault state (50% duty cycle), CT = 100nF 2 Hz 1 Note 1: Specifications to -40°C are guaranteed by design and not production tested. 4 _______________________________________________________________________________________ % Linear Charger for Single-Cell Li+ Battery 0.96 0.94 0.92 NORMALIZED TO +25°C 60 1.001 ISET = 2.8kΩ CHARGING CURRENT SET FOR 500mA PRECHARGE CURRENT (mA) 0.98 1.000 0.999 0.998 55 50 45 0.997 0.996 -15 10 35 60 85 40 -15 -40 TEMPERATURE (°C) 10 35 60 5 85 6 7 TEMPERATURE (°C) 1.2 1.0 8 9 10 11 12 VIN (V) NORMALIZED BATTERY CURRENT vs. BATTERY VOLTAGE VOLTAGE-LOOP GAIN MAX1898 toc05 60 MAX1898 toc04 40 GAIN PHASE 0.6 200 150 0.8 GAIN (dB) -40 20 100 0 50 PHASE (deg.) 0.90 NORMALIZED BATTERY CURRENT TIMER PERIOD 1.00 1.002 MAX1898 toc02 NORMALIZED TO +25°C BATT REGULATION VOLTAGE MAX1898 toc01 1.02 PRECHARGE CURRENT vs. INPUT VOLTAGE NORMALIZED BATT REGULATION VOLTAGE vs. TEMPERATURE MAX1898 toc03 NORMALIZED TIMER PERIOD vs. TEMPERATURE 0.4 -20 0.2 0 0 -40 3.9 4.0 4.1 4.2 BATTERY VOLTAGE (V) 4.3 0.01 0.1 1 10 100 -50 1000 FREQUENCY (kHz) _______________________________________________________________________________________ 5 MAX1898 Typical Operating Characteristics (VIN = 5V, VBATT = 4V, EN/OK = open, RSET = 2.8kΩ, RSTRT = GND, TA = +25°C, unless otherwise noted.) Linear Charger for Single-Cell Li+ Battery MAX1898 Pin Description PIN NAME FUNCTION 1 IN Sense Input. Detects power source; positive side of internal current-sense resistor. 2 CHG 3 EN/OK Logic-Level Enable Input and Input Power OK Output. Hold low to disable charger. Float for normal operation, drive with an open-drain device to utilize both pin functions, or connect through 10kΩ for logiccontrolled ON/OFF. See the EN/OK (EN Input, OK Output) section for details. 4 ISET ISET serves two functions. It provides an analog output proportional to the actual charge current. Output current from ISET is 1mA per amp of battery charging current. The charging current is set by connecting a resistor from ISET to ground. See the Current-Limit Mode section. 5 CT Open-Drain LED Driver. See Table 1 for CHG output states. Safety Charge Timer Control. Connect timer capacitor to program safety time-out interval; 100nF for approximately three hours. Connect to GND to disable the timer function. Automatic-Restart Control Pin. When tied to GND, a new charging cycle starts if the cell drops 200mV below the battery regulation threshold. The restart threshold may be lowered by connecting a resistor from RSTRT to ground. See the Reinitiating a Charging Cycle section. Automatic restart is disabled when RSTRT is unconnected or when CT is grounded (timer disabled). 6 RSTRT 7 BATT Battery Sense Input. Positive terminal of single Li+ cell. 8 GND Ground 9 DRV External Transistor Driver. This pin drives the gate/base of an external PMOS/PNP pass transistor. 10 CS Current-Sense Input. Negative side of internal current-sense resistor. Connect to source/emitter of PMOS/PNP pass transistor. Detailed Description The MAX1898 forms a complete charger for 1-cell Li+ batteries. It includes precision voltage control (±0.75%) to optimize both cell performance and cycle life. Factory-set 4.2V (MAX1898EUB42) and 4.1V (MAX1898EUB41) versions charge all common Li+ chemistries. Externally selectable charge current is sensed internally, eliminating the need for a currentsense resistor. The charger also supplies outputs that indicate charge status (CHG), the presence of input power (EN/OK), and charge-current magnitude (ISET). Other features include a shutdown control input (EN/OK), selectable threshold for charge-cycle restart without cycling input power (RSTRT), and a selectable charge termination safety timer. See Figure 1. The MAX1898 initiates fast charge when any of these conditions are met (see Figure 6): • An external power source is connected AND the cell voltage is above 2.5V. • The cell voltage falls to the restart threshold, 4.0V in the MAX1898EUB42, or 3.9V for the MAX1898EUB41. • The IC is reset by driving the EN/OK pin low and then high again. 6 • The prequalification cycle ends with the cell reaching 2.5V. Li+ cells can be damaged when fast-charged from a completely dead state. A fully discharged cell may indicate an abnormal cell condition. As a built-in safety feature, the MAX1898 prequalifies the cell with 10 percent of the user-programmed fast-charge current at the start of a charge cycle. When the cell voltage rises to 2.5V, the MAX1898 begins fast-charging. Current-Limit Mode The MAX1898 regulates charging current by linearly controlling an external PMOS or PNP transistor. The maximum charging current is programmed by an external RSET resistor connected from ISET to GND. Select the RSET value based on the following formula: I FASTCHG= 1400V RSET where: IFASTCHG is in amps and RSET is in ohms. The ISET pin may also be used to monitor the actual charging current at any time. The output current from _______________________________________________________________________________________ Linear Charger for Single-Cell Li+ Battery MAX1898 DC IN 0.1µF LED IN 4.5V TO 12V 5mA CHG 84Ω CHARGING FOR 3hr, CCT = 100nF CT 84mΩ ÷218 OSC CSA 20mΩ CS CHG LOGIC TIMEOUT 0.1µF 470Ω ISET RSET (2.8kΩ FOR 500mA CHARGE CURRENT) HV DRV ICHARGE < 20% DRV 280mV ISET (FALLING) 305mV ISET (RISING) REF 1.4V CA BATT PREQUAL CONTROL LOGIC 2.5V VA 69.275kΩ 1.4V 10µF DONE 3V 1.4V SHUTDOWN VL REG 100kΩ 34.637kΩ RESTART BATT IN EN/OK DONE UVLO SHUTDOWN 2.653kΩ RSTRT SHUTDOWN GND Figure 1. Functional Diagram _______________________________________________________________________________________ 7 Linear Charger for Single-Cell Li+ Battery 1.4 BATTERY VOLTAGE 1.2 BATTERY VOLTAGE (V) 4.0 1.0 3.8 AMP HOURS 0.8 BATTERY CURRENT 3.6 0.6 0.4 AMPS, WATTS, AMP HOURS MAX1898 Reinitiating a Charging Cycle 4.2 3.4 0.2 POWER DISSIPATION 3.2 0 30 60 90 120 150 0 180 TIME (MINUTES) Figure 2. Li+ Charging Profile the ISET pin is 1mA per amp of charging current. The output voltage at ISET is proportional to charging current as follows: VISET = The MAX1898 can be configured so that a charging cycle automatically restarts when the cell voltage drops 200mV below the battery regulation voltage (connect RSTRT to GND). The restart threshold can also be lowered by connecting an external resistor between RSTRT and GND. If automatic charging restart is not desired, this feature may be disabled by floating the RSTRT pin. With restart disabled, charging can only be reinitiated by pulling EN/OK low, or by removing and reapplying the input power source. ICHG × RSET 1000 EN/OK (EN Input, OK Output) The EN/OK pin serves two functions. It primarily serves as a logic input (active high) to enable the charger. In addition to ON/OFF control, EN/OK also indicates when input power is connected. When input power is applied to IN (VIN > VBATT and VIN > 4.25V) EN/OK pulls high (to 3V) with a 100kΩ internal pullup resistance. Consequently, EN/OK may be output to a logic input, to indicate AC adapter presence, but at the same time may be driven by open-drain logic to turn the charger ON and OFF. An open-drain logic-low input will over-ride the internal pullup resistor and turn off the charger (see Figure 3). When the open drain goes high impedance, EN/OK will go high if The voltage at ISET is nominally 1.4V at the selected fast-charge current, and falls with charging current as the cell becomes fully charged. See Figure 2 for typical Li+ charging profile. POWER OK 3V OPEN-DRAIN LOGIC CHG Status Output CHG is an open-drain output that monitors the cell’s charging status. CHG has a 5mA current limit so an LED can be connected directly from IN to CHG for a visible indication. Alternatively, a pullup resistor (typically 100kΩ) to a logic supply provides a logic-level output. Table 1 outlines the status of CHG for various charger states. IN OFF 100kΩ EN/OK MAX1898 ON GND Figure 3. Driving EN/OK with Open-Drain Logic to Retain ON/OFF Control with Power OK Indication Table 1. CHG Output States CHG CONDITION 8 No battery or no charger input. High impedance (LED off) The cell voltage is less than 2.5V and charge current is regulated to 10% of the selected fastcharge current (prequalification state). Low impedance (LED on) Cell voltage is greater than 2.5V (fast-charge state). Low impedance (LED on) Charge cycle complete—charge current is less then 20% of the selected fast charge current or safety timer expires (3 hours for CCT = 100nF). High impedance (LED off) Charge cycle fault—the cell voltage is less than 2.5V and the prequalification timer (45min for CCT = 100nF) has expired. Blinking, LED on 50% duty factor, 1.5Hz _______________________________________________________________________________________ Linear Charger for Single-Cell Li+ Battery Battery Drain Current The MAX1898 uses advanced CMOS circuitry to completely monitor battery state, while drawing minimal current from the cell itself. When the input voltage is less than the battery, battery drain is typically 3µA. When input power is present and charging is complete (the charge timer expires), battery drain is then typically 40µA. This current may be reduced to 4µA by disabling the restart feature. Applications Information The MAX1898 basic application circuit is shown in Figure 4. 100nF 5V TO 12V CS DRV 10 BATT CT EN/OK ISET 6 RSTRT GND 8 Li+ CELL CCT, 100nF CAPACITOR SETS CHARGE TIME TO 3hr CHG 3 DRV 10 9 1 IN BATT 7 10µF CHARGE LED INDICATOR MAX1898 CT CCT, 100nF CAPACITOR SETS CHARGE TIME TO 3hr 2 CHG INPUT POWER OK OUTPUT OR ENABLE INPUT 3 EN/OK ISET 6 RSTRT GND 8 Li+ CELL 5 4 OUTPUT VOLTAGE PROPORTIONAL TO CHARGE CURRENT RSET SETS CHARGE CURRENT CCT [nF] = 34.33 × t CHG[hours] 5 2 INPUT POWER OK OUTPUT OR ENABLE INPUT CS 7 10µF MAX1898 220nF 5V TO 12V Figure 5. Application Circuit with PNP Pass Transistor 9 1 IN CHARGE LED INDICATOR MAX1898 power is present at IN. If there is no (or insufficient) voltage at IN, EN/OK will remain low and the charger will remain off. 4 OUTPUT VOLTAGE PROPORTIONAL TO CHARGE CURRENT RSET SETS CHARGE CURRENT Figure 4. Basic Application Circuit An LED can be added as a charge-state indicator. A 10µF bypass capacitor must be added to the BATT pin. A Schottky diode should be added between the external transistor drain/collector and BATT, to avoid battery discharge in the event of an input source short circuit. Figure 5 shows a charger circuit using an external PNP pass transistor. Selecting Maximum Charge Time The maximum charge time can be set with an external capacitor connected from CT to GND. Select the timing capacitor using: where: tCHG = nominal charge time limit in hours. The maximum charge timer is intended as a safety timer. It is normally not intended to be part of the charge-control loop. When charging a Li+ cell at a 1C rate, normal charge time is approximately 1.5 hours, but this may have wide variations that depend on temperature and cell type. In most cases, 3 hours is recommended for the maximum charge time limit when fast-charging at 1C so that normal-charging is never interrupted by the charge timer. Consult the battery manufacturer for details on recommended timer settings. The charge safety timer may also be completely disabled by connecting CT to ground. Note: If the charge timer is disabled, both the restart function and prequalification fault indication are also disabled. Only the indication (blinking LED) is disabled, the prequalification function itself is not modified. Controlling Automatic Restart The MAX1898 features automatic restart that resumes charging when the cell voltage drops to a predetermined level. In most timed chargers, once charging times out, subsequent loading of the battery can discharge the battery and charging will not resume unless the charger is restarted by an outside signal. By automatically resuming charging when the battery voltage drops, the MAX1898 ensures that the cell does not remain partially charged _______________________________________________________________________________________ 9 MAX1898 Linear Charger for Single-Cell Li+ Battery SHUTDOWN CHARGER = OFF LED = OFF VBATT < 2.5V TIMEOUT/4 EXPIRES AND VBATT < 2.5V PREQUALIFICATION 10% CHARGE CURRENT LED = ON ASYNCHRONOUS FROM ANYWHERE VIN < 4.5V OR VBATT > VIN VIN < 4.5V OR VBATT > VIN VBATT > 2.5V, 3hr TIMER STARTS FAULT LED BLINKING AT 1.5Hz RSTRT = GND AND VBATT < 4.0V (MAX1898EUB42) VBATT < 3.9V (MAX1898EUB41) FAST CHARGE 100% CHARGER CURRENT LED = ON TIMEOUT = 3hr ICHARGE < 20% OF ISET ICHARGE > 22% OF ISET DONE (CHARGE COMPLETE) CHARGER = OFF LED = OFF TIMEOUT = 3hr TOP OFF LED = OFF DOTTED LINE INDICATES THE STATES THAT ARE NOT AVAILABLE WHEN CT = 0 (TIMER DISABLED) Figure 6. Charger State Diagram after use when charger power is available. The restart feature can be configured in the following ways: • Floating the RSTRT pin disables restart. Once the battery is charged and the charge timer expires, charging can only be started by cycling input power to IN, or by toggling EN/OK. When the restart function is disabled, battery drain after charge completion is reduced to 4µA. With the restart function enabled, battery drain current after charge completion is 40µA. resistor using the following equations: For MAX1898EUB42: 69.275kΩ RRSTRT [kΩ] = − 37.301kΩ VRSTRT − 1 1.4V for VRSTRT values between 3V and 4V. For MAX1898EUB41: • Ground the RSTRT pin to enable the default restart threshold of 4V for the MAX1898EUB42 and 3.9V for the MAX1898EUB41. Once the battery is charged and the charge timer expires, charging restarts when the cell voltage drops to the restart-threshold voltage. 66.822kΩ RRSTRT [kΩ] = − 37.301kΩ VRSTRT − 1 1.4V • If desired, lower the restart-threshold voltage by connecting a resistor from RSTRT to GND. Select the for VRSTRT values between 3V and 3.9V. Note that if CT is grounded (charge timer disabled), the restart function is also disabled. Table 2. External Transistors EXTERNAL TRANSISTORS RESISTANCE RATING VOLTAGE LIMIT (V) POWER DISSIPATION (W) PACKAGE PMOS Fairchild NDT454P 50mΩ -30 3 SOT223 — PNP Zetex FZT968 0.17V, VCE(sat) -15 3 SOT223 hFE = 300min PNP FCX717 0.32V, VCE(sat) -12 1 SOT89 hFE = 160min, 1W rating suitable for 0.5A charge or less 10 ______________________________________________________________________________________ COMMENTS Linear Charger for Single-Cell Li+ Battery PDISS(MAX) = IFASTCHG (VIN - 2.5V) The pass transistor and heatsinking (typically an expanded PC board plane) must be rated for PDISS(MAX). The pass device will most commonly be a P-channel MOSFET. Its operating voltage must exceed the expected input voltage, but MOSFET on-resistance is important only in low-dropout designs where VIN is only slightly higher than the battery regulation voltage. In most charger designs, a fairly low-cost FET (around 100mΩ to 200mΩ RDS(ON)) performs more than adequately. Lower resistance FETs perform well also, but are not worth the added cost in this case. A PNP bipolar transistor may also be used as a pass device. DRV can sink up to 4mA, so reasonable high current gain (hFE or Beta) is required to allow DRV to drive the base of a bipolar pass transistor. For a 500mA fast-charge current, an hFE of 125 is required. Although their current gain is high, Darlington PNP transistors are not recommended due to stability limitations. If VIN is less than VBATT, the body diode inherent in the external enhancement-mode FET allows current to flow from the cell back to the source. To prevent this, add a Schottky diode between the drain and BATT, as shown in Figure 4. Chip Information TRANSISTOR COUNT: 1816 PROCESS: BiCMOS ______________________________________________________________________________________ 11 MAX1898 Selecting External Components The MAX1898 drives an external transistor to control charging current. The most important specifications for the pass transistor (MOSFET or bipolar) are current rating and package power dissipation. Since the MAX1898 operates as a linear charger, heat will dissipate in the external transistor. The worst-case power dissipation will be: Linear Charger for Single-Cell Li+ Battery 10LUMAX.EPS MAX1898 Package Information 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 © 2002 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.