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19-2295; Rev 0; 1/02
Linear Charger for Single-Cell Li+ Battery
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 MAX package. o Simple, Safe Linear Charger o Uses Low-Cost PNP or PMOS Pass Element o 4.5V to 12V Input Range o Internal Current-Sense Resistor o 0.75% Voltage Accuracy o Programmable Charging Current o Automatic Input Power-Source Detection o LED Charge Status Indicator o Programmable Safety Timer o Current-Sense Monitor Output o Optional/Adjustable Auto-Restart o Small MAX Package
Features
MAX1898
Ordering Information
PART MAX1898EUB42 MAX1898EUB41 TEMP RANGE -40C to +85C -40C to +85C PIN-PACKAGE 10 MAX 10 MAX
Applications
Single-Cell Li+ Powered Portables Self-Charging Battery Packs PDAs Cell Phones Cradle Chargers
Typical Operating Circuit
INPUT 4.5V TO 12V
IN LED
CS
OUTPUT Li+ BATTERY
Pin Configuration
CHG
MAX1898
DRV ON EN/OK OFF BATT ISET
TOP VIEW
IN 1 CHG EN/OK ISET CT 2 3 4 5 10 CS
CT
9
DRV GND BATT RSTRT
MAX1898
8 7 6
CT SETS CHARGE TIME RSTRT GND
SETS CHARGE CURRENT
MAX
________________________________________________________________ Maxim Integrated Products
1
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Linear Charger for Single-Cell Li+ Battery MAX1898
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 = +70C) 10-Pin MAX (derate 5.6mW/C above +70C) ..........444mW 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
(VIN = 8V, VBATT = 4V, EN/OK = open, RSET = 2.8k, RSTRT = GND, TA = 0C to +85C, unless otherwise noted. Typical values are at TA = +25C.)
PARAMETER Supply Voltage Range Input OK Trip Point, IN Rising (VIN - VBATT) 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) EN/OK Source Current (When Input Power OK) EN/OK Logic Input High EN/OK Logic Input Low VIN = 3.9V, VBATT = 4V, VEN/OK = 0.8V VEN/OK = 2V 4.5V VIN 12V 4.5V VIN 12V Done, RSTRT = GND, VBATT = 4.1V for MAX1898EUB42 and 4.0V for MAX1898EUB41 BATT Input Current Done, RSTRT = open, VBATT = 4.1V for MAX1898EUB42 and 4.0V for MAX1898EUB41 EN/OK = GND VIN = 4V IN, CS Input Current VOLTAGE LOOP Voltage-Loop Set Point Voltage-Loop Gain VBATT Prequal Voltage Threshold DRV Sink Current DRV Output Resistance MAX1898EUB42 MAX1898EUB41 VDRV/VBATT Timers running MAX1898EUB42 MAX1898EUB41 2.394 2.337 4 470 4.168 4.069 4.2 4.1 51 2.520 2.460 2.646 2.583 4.232 4.131 V dB V mA VIN = VBATT = 4V (IIN + ICS) VIN = 12V, (IIN + ICS), done, RSTRT = open 40 4 4 3 2 2 CONDITIONS MIN 4.5 45 30 4.0 3.9 100 5 2 0.6 100 10 10 10 10 5 A mA A 10 20 70 55 4.25 4.15 TYP MAX 12 100 85 4.5 4.4 UNITS V mV mV V V A A V V
VDRV = VIN - 1V, VBATT = 3.5V
2
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Linear Charger for Single-Cell Li+ Battery
ELECTRICAL CHARACTERISTICS (continued)
(VIN = 8V, VBATT = 4V, EN/OK = open, RSET = 2.8k, RSTRT = GND, TA = 0C to +85C, unless otherwise noted. Typical values are at TA = +25C.)
PARAMETER Maximum Voltage (VIN - VDRV) VBATT Restart Threshold (As a Percentage of Voltage-Loop Set Point) RSTRT Internal Resistance CURRENT LOOP Internal Current-Sense Resistance RMS Charge Current Current-Sense Amplifier Gain, ISET to IIN Fast Charge Current-Sense Amplifier Gain, ISET to IIN Prequal Current-Loop Regulation Set Point Current-Loop Gain CHG Trip Threshold (As a Percentage of Full Load Current) TIMER FUNCTION Timer Source/Sink Current Timer Accuracy CHG Output Low Current CHG Output High Leakage CHG Blink Rate VCT = 0.9V CCT = 100nF for 2 hours, 54 minutes nominal V CHG = 1V V CHG = 12V Fault state (50% duty cycle), CT = 100nF 1 1.5 -10 4 5 5 +10 6 1 2 A % mA A Hz ICS = 500mA, VSET = 1.4V ICS = 50mA, VISET = 0.14V, VBATT = 2V RSET = 2.8k, VBATT = 3.5V, closed-loop performance VDRV/VISET ILOAD falling, RSET = 2.8k ILOAD rising, RSET = 2.8k 15 17 0.95 0.73 450 1 1 500 26 20 22 25 27 IN to CS pin-to-pin resistance 50 105 200 1.4 1.05 1.30 550 m A mA/A mA/A mA dB % VBATT = 3.5V RSTRT = GND CONDITIONS MIN 3.5 92 95.25 100 TYP MAX 7 98 UNITS V % k
MAX1898
ELECTRICAL CHARACTERISTICS
(VIN = 8V, VBATT = 4V, EN/OK = open, RSET = 2.8k, RSTRT = GND, TA = -40C to +85C, unless otherwise noted.)
PARAMETER Supply Voltage Range Input OK Trip Point, IN Rising (VIN - VBATT) 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) EN/OK Source Current (When Input Power OK) EN/OK Logic Input High EN/OK Logic Input Low VIN = 3.9V, VBATT = 4V, VEN/OK = 0.8V VEN/OK = 2V 4.5V VIN 12V 4.5V VIN 12V CONDITIONS MIN 4.5 45 30 4.0 3.9 100 5 2 0.6 20 MAX 12 100 85 4.5 4.4 UNITS V mV mV V V A A V V
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Linear Charger for Single-Cell Li+ Battery MAX1898
ELECTRICAL CHARACTERISTICS (continued)
(VIN = 8V, VBATT = 4V, EN/OK = open, RSET = 2.8k, RSTRT = GND, TA = -40C to +85C, unless otherwise noted.)
PARAMETER CONDITIONS Done, RSTRT = GND, VBATT = 4.1V for MAX1898EUB42 and 4.0V for MAX1898EUB41 BATT Input Current Done, RSTRT = open, VBATT = 4.1V for MAX1898EUB42 and 4.0V for MAX1898EUB41 EN/OK = GND VIN = 4V IN, CS Input Current VOLTAGE LOOP Voltage-Loop Set Point VBATT Prequal Voltage Threshold DRV Sink Current Maximum Voltage (VIN - VDRV) VBATT Restart Threshold (As a Percentage of Voltage-Loop Set Point) CURRENT LOOP Internal Current-Sense Resistance RMS Charge Current Current-Sense Amplifier Gain, ISET to IIN Fast Charge Current-Sense Amplifier Gain, ISET to IIN Prequal Current-Loop Regulation Set Point CHG Trip Threshold (As a Percentage of Full Load Current) Timer Accuracy CHG Output Low Current CHG Output High Leakage CHG Blink Rate ICS = 500mA, VSET = 1.4V ICS = 50mA, VISET = 0.14V, VBATT = 2V RSET = 2.8k, VBATT = 3.5V, closed-loop performance ILOAD falling, RSET = 2.8k ILOAD rising, RSET = 2.8k CCT = 100nF for 2 hours, 54 minutes nominal V CHG = 1V V CHG = 12V Fault state (50% duty cycle), CT = 100nF 1 0.93 0.6 440 15 17 -13 3.5 IN to CS pin-to-pin resistance 50 200 1.4 1.07 1.50 560 25 27 +13 7 1 2 m A mA/A mA/A mA % % mA A Hz MAX1898EUB42 MAX1898EUB41 Timers running MAX1898EUB42 MAX1898EUB41 4.137 4.038 2.394 2.337 3.5 3 92 7 98 4.263 4.162 2.646 2.583 V V mA V % VIN = VBATT = 4V (IIN + ICS) VIN = 12V, (IIN + ICS), done, RSTRT = open MIN MAX 100 10 10 10 10 5 A mA A UNITS
VDRV = VIN - 1V, VBATT = 3.5V VBATT = 3.5V RSTRT = GND
Note 1: Specifications to -40C are guaranteed by design and not production tested.
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Linear Charger for Single-Cell Li+ Battery
Typical Operating Characteristics
(VIN = 5V, VBATT = 4V, EN/OK = open, RSET = 2.8k, RSTRT = GND, TA = +25C, unless otherwise noted.)
MAX1898
NORMALIZED TIMER PERIOD vs. TEMPERATURE
MAX1898 toc01
NORMALIZED BATT REGULATION VOLTAGE vs. TEMPERATURE
MAX1898 toc02
PRECHARGE CURRENT vs. INPUT VOLTAGE
ISET = 2.8k CHARGING CURRENT SET FOR 500mA 55
MAX1898 toc03
1.02 1.00 TIMER PERIOD 0.98 0.96 0.94 0.92 0.90
NORMALIZED TO +25C
1.002 1.001 1.000 0.999 0.998 0.997 0.996
NORMALIZED TO +25C
60
BATT REGULATION VOLTAGE
PRECHARGE CURRENT (mA)
50
45
40 -40 -15 10 35 60 85 5 6 7 8 9 10 11 12 TEMPERATURE (C) VIN (V)
-40
-15
10
35
60
85
TEMPERATURE (C)
NORMALIZED BATTERY CURRENT vs. BATTERY VOLTAGE
MAX1898 toc04
VOLTAGE-LOOP GAIN
60
MAX1898 toc05
1.2 NORMALIZED BATTERY CURRENT 1.0 0.8 0.6 0.4 0.2 0 3.9 4.0 4.1 4.2
200
40 PHASE GAIN (dB) 20 GAIN
150 PHASE (deg.)
100
0
50
-20
0
-40 4.3 0.01 0.1 1 10 100 BATTERY VOLTAGE (V) FREQUENCY (kHz)
-50 1000
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Linear Charger for Single-Cell Li+ Battery MAX1898
Pin Description
PIN 1 2 3 NAME IN CHG EN/OK FUNCTION Sense Input. Detects power source; positive side of internal current-sense resistor. Open-Drain LED Driver. See Table 1 for CHG output states. 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 10k for logiccontrolled ON/OFF. See the EN/OK (EN Input, OK Output) section for details. 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. 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). Battery Sense Input. Positive terminal of single Li+ cell. Ground External Transistor Driver. This pin drives the gate/base of an external PMOS/PNP pass transistor. Current-Sense Input. Negative side of internal current-sense resistor. Connect to source/emitter of PMOS/PNP pass transistor.
4
ISET
5
CT
6
RSTRT
7 8 9 10
BATT GND DRV CS
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.
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* 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 0.1F LED IN 4.5V TO 12V 5mA CHG 84 84m
CHARGING FOR 3hr, CCT = 100nF
CT
OSC
/218 CSA 20m CS TIMEOUT CHG LOGIC 470 0.1F
ISET RSET (2.8k FOR 500mA CHARGE CURRENT) ICHARGE < 20% 280mV ISET (FALLING) 305mV ISET (RISING) HV DRV
DRV
REF 1.4V CONTROL LOGIC
CA PREQUAL 2.5V BATT
VA 1.4V DONE 3V 100k VL REG SHUTDOWN RESTART BATT
69.275k 10F
1.4V 34.637k
IN EN/OK UVLO DONE 2.653k
SHUTDOWN SHUTDOWN
RSTRT
GND
Figure 1. Functional Diagram _______________________________________________________________________________________ 7
Linear Charger for Single-Cell Li+ Battery MAX1898
Reinitiating a Charging Cycle
4.2 BATTERY VOLTAGE 1.4 1.2 1.0 3.8 AMP HOURS BATTERY CURRENT 3.6 0.8 0.6 0.4 3.4 POWER DISSIPATION 3.2 0 30 60 90 120 150 TIME (MINUTES) 0.2 0 180 AMPS, WATTS, AMP HOURS
4.0 BATTERY VOLTAGE (V)
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.
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 100k 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
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 = ICHG x RSET 1000
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 OFF ON 100k
IN
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 100k) to a logic supply provides a logic-level output. Table 1 outlines the status of CHG for various charger states.
EN/OK
MAX1898
GND
Figure 3. Driving EN/OK with Open-Drain Logic to Retain ON/OFF Control with Power OK Indication
Table 1. CHG Output States
CONDITION No battery or no charger input. The cell voltage is less than 2.5V and charge current is regulated to 10% of the selected fastcharge current (prequalification state). Cell voltage is greater than 2.5V (fast-charge state). Charge cycle complete--charge current is less then 20% of the selected fast charge current or safety timer expires (3 hours for CCT = 100nF). Charge cycle fault--the cell voltage is less than 2.5V and the prequalification timer (45min for CCT = 100nF) has expired. CHG High impedance (LED off) Low impedance (LED on) Low impedance (LED on) High impedance (LED off) Blinking, LED on 50% duty factor, 1.5Hz
8
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Linear Charger for Single-Cell Li+ Battery
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. 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 3A. When input power is present and charging is complete (the charge timer expires), battery drain is then typically 40A. This current may be reduced to 4A by disabling the restart feature.
MAX1898
5V TO 12V
220nF
CS DRV 1 IN BATT CHARGE LED INDICATOR 2 CHG INPUT POWER OK OUTPUT OR ENABLE INPUT 3 6 EN/OK ISET RSTRT GND 8
10 9
7 10F Li+ CELL
MAX1898
CT
5 CCT, 100nF CAPACITOR SETS CHARGE TIME TO 3hr 4 OUTPUT VOLTAGE PROPORTIONAL TO CHARGE CURRENT RSET SETS CHARGE CURRENT
Applications Information
The MAX1898 basic application circuit is shown in Figure 4.
5V TO 12V 100nF
CS DRV 1 IN BATT CHARGE LED INDICATOR 2 CHG INPUT POWER OK OUTPUT OR ENABLE INPUT 3 6 EN/OK ISET RSTRT GND 8
10 9
Figure 5. Application Circuit with PNP Pass Transistor
CCT [nF] = 34.33 x t CHG[hours]
10F Li+ CELL
7
MAX1898
CT
5 CCT, 100nF CAPACITOR SETS CHARGE TIME TO 3hr 4 OUTPUT VOLTAGE PROPORTIONAL TO CHARGE CURRENT RSET SETS CHARGE CURRENT
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.
Figure 4. Basic Application Circuit
An LED can be added as a charge-state indicator. A 10F 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.
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
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:
_______________________________________________________________________________________
Linear Charger for Single-Cell Li+ Battery MAX1898
ASYNCHRONOUS FROM ANYWHERE VIN < 4.5V OR VBATT > VIN
VBATT < 2.5V
SHUTDOWN CHARGER = OFF LED = OFF
TIMEOUT/4 EXPIRES AND VBATT < 2.5V FAULT LED BLINKING AT 1.5Hz
PREQUALIFICATION 10% CHARGE CURRENT LED = ON
VIN < 4.5V OR VBATT > VIN
VBATT > 2.5V, 3hr TIMER STARTS RSTRT = GND AND VBATT < 4.0V (MAX1898EUB42) VBATT < 3.9V (MAX1898EUB41) TIMEOUT = 3hr
FAST CHARGE 100% CHARGER CURRENT LED = ON ICHARGE > 22% OF ISET TOP OFF LED = OFF
ICHARGE < 20% OF ISET TIMEOUT = 3hr DONE (CHARGE COMPLETE) CHARGER = 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 4A. With the restart function enabled, battery drain current after charge completion is 40A. * 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. * If desired, lower the restart-threshold voltage by connecting a resistor from RSTRT to GND. Select the
resistor using the following equations: For MAX1898EUB42: 69.275k RRSTRT [k] = - 37.301k VRSTRT - 1 1.4V for VRSTRT values between 3V and 4V. For MAX1898EUB41: 66.822k RRSTRT [k] = - 37.301k VRSTRT - 1 1.4V for VRSTRT values between 3V and 3.9V. Note that if CT is grounded (charge timer disabled), the restart function is also disabled.
POWER DISSIPATION (W) 3 3 PACKAGE SOT223 SOT223 -- hFE = 300min hFE = 160min, 1W rating suitable for 0.5A charge or less COMMENTS
Table 2. External Transistors
EXTERNAL TRANSISTORS PMOS Fairchild NDT454P PNP Zetex FZT968 PNP FCX717 RESISTANCE RATING 50m 0.17V, VCE(sat) 0.32V, VCE(sat) VOLTAGE LIMIT (V) -30 -15
-12
1
SOT89
10
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Linear Charger for Single-Cell Li+ Battery
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: 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 100m to 200m 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.
MAX1898
Chip Information
TRANSISTOR COUNT: 1816 PROCESS: BiCMOS
______________________________________________________________________________________
11
Linear Charger for Single-Cell Li+ Battery MAX1898
Package Information
10LUMAX.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.
12 ____________________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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