General Description
The MAX1898, with an external PNP or PMOS transistor,
forms a complete charger for 1-cell lithium-ion (Li+) bat-
teries. The MAX1898 provides accurate constant-cur-
rent/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 cur-
rent. Additional features include shutdown, optional
charge-cycle restart without input-power cycling, selec-
table charge termination safety timer, and low-current
preconditioning for deeply discharged cells.
The MAX1898 offers the maximum integration and func-
tionality 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 ultra-
thin µMAX package.
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
MAX1898
Linear Charger for Single-Cell Li+ Battery
________________________________________________________________ Maxim Integrated Products 1
Ordering Information
MAX1898
IN
CT
RSTRT
GND
EN/OK
CHG
CS
ISET
BATT
DRV
OFF
ON
LED Li+
BATTERY
SETS CHARGE
CURRENT
CT SETS
CHARGE
TIME
OUTPUT
INPUT
4.5V TO 12V
Typical Operating Circuit
19-2295; Rev 0; 1/02
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.
PART TEMP RANGE PIN-PACKAGE
MAX1898EUB42 -40°C to +85°C 10 µMAX
MAX1898EUB41 -40°C to +85°C 10 µMAX
Pin Configuration
1
2
3
4
5
10
9
8
7
6
CS
DRV
GND
BATTISET
EN/OK
IN
MAX1898
µMAX
TOP VIEW
RSTRTCT
CHG
Applications
Single-Cell Li+ Powered Portables
Self-Charging Battery Packs
PDAs
Cell Phones
Cradle Chargers
MAX1898
Linear Charger for Single-Cell Li+ Battery
2 _______________________________________________________________________________________
ABSOLUTE MAXIMUM RATINGS
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.)
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.
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
PARAMETER CONDITIONS MIN TYP MAX UNITS
Supply Voltage Range 4.5 12 V
Input OK Trip Point, IN Rising
(VIN - VBATT)45 70 100 mV
Input OK Trip Point, IN Falling
(VIN - VBATT)30 55 85 mV
VIN Input OK Trip Point, IN Rising 4.0 4.25 4.5 V
VIN Input OK Trip Point, IN Falling 3.9 4.15 4.4 V
EN/OK Sink Current (When Input
Power Not OK)
VIN = 3.9V, VBATT = 4V,
VEN/OK = 0.8V 100 µA
EN/OK Source Current (When
Input Power OK) VEN/OK = 2V 5 10 20 µA
EN/OK Logic Input High 4.5V ≤ VIN ≤ 12V 2 V
EN/OK Logic Input Low 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 410
EN/OK = GND 4 10
BATT Input Current
VIN = 4V 3 10
µA
VIN = VBATT = 4V (IIN + ICS) 2 10 µA
IN, CS Input Current VIN = 12V, (IIN + ICS), done, RSTRT = open 2 5 mA
VOLTAGE LOOP
MAX1898EUB42 4.168 4.2 4.232
Voltage-Loop Set Point MAX1898EUB41 4.069 4.1 4.131 V
Voltage-Loop Gain ∆VDRV/∆VBATT 51 dB
MAX1898EUB42 2.394 2.520 2.646
VBATT Prequal Voltage Threshold Timers running MAX1898EUB41 2.337 2.460 2.583 V
DRV Sink Current VDRV = VIN - 1V, VBATT = 3.5V 4 mA
DRV Output Resistance 470 Ω
MAX1898
Linear Charger for Single-Cell Li+ Battery
_______________________________________________________________________________________ 3
ELECTRICAL CHARACTERISTICS (continued)
(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 TYP MAX UNITS
Maximum Voltage (VIN - VDRV)V
BATT = 3.5V 3.5 7 V
VBATT Restart Threshold (As a
Percentage of Voltage-Loop Set
Point)
RSTRT = GND 92 95.25 98 %
RSTRT Internal Resistance 100 kΩ
CURRENT LOOP
Internal Current-Sense Resistance IN to CS pin-to-pin resistance 50 105 200 mΩ
RMS Charge Current 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 26 dB
ILOAD falling, RSET = 2.8kΩ15 20 25
CHG Trip Threshold (As a
Percentage of Full Load Current) ILOAD rising, RSET = 2.8kΩ17 22 27 %
TIMER FUNCTION
Timer Source/Sink Current VCT = 0.9V 5 µA
Timer Accuracy CCT = 100nF for 2 hours, 54 minutes nominal -10 +10 %
CHG Output Low Current V
CHG = 1V 4 5 6 mA
CHG Output High Leakage V
CHG = 12V 1 µA
CHG Blink Rate Fault state (50% duty cycle), CT = 100nF 1 1.5 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.)
PARAMETER CONDITIONS MIN MAX UNITS
Supply Voltage Range 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 100 µA
EN/OK Source Current (When Input
Power OK) VEN/OK = 2V 5 20 µA
EN/OK Logic Input High 4.5V ≤ VIN ≤12V 2 V
EN/OK Logic Input Low 4.5V ≤ VIN ≤12V 0.6 V
MAX1898
Linear Charger for Single-Cell Li+ Battery
4 _______________________________________________________________________________________
Note 1: Specifications to -40°C are guaranteed by design and not production tested.
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 CONDITIONS MIN MAX UNITS
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
BATT Input Current
VIN = 4V 10
µA
VIN = VBATT = 4V (IIN + ICS)10µA
IN, CS Input Current VIN = 12V, (IIN + ICS), done, RSTRT = open 5 mA
VOLTAGE LOOP
MAX1898EUB42 4.137 4.263
Voltage-Loop Set Point MAX1898EUB41 4.038 4.162 V
MAX1898EUB42 2.394 2.646
VBATT Prequal Voltage Threshold Timers running MAX1898EUB41 2.337 2.583 V
DRV Sink Current VDRV = VIN - 1V, VBATT = 3.5V 3.5 mA
Maximum Voltage (VIN - VDRV)V
BATT = 3.5V 3 7 V
VBATT Restart Threshold (As a
Percentage of Voltage-Loop Set
Point)
RSTRT = GND 92 98 %
CURRENT LOOP
Internal Current-Sense Resistance IN to CS pin-to-pin resistance 50 200 mΩ
RMS Charge Current 1.4 A
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
ILOAD falling, RSET = 2.8kΩ15 25
CHG Trip Threshold (As a
Percentage of Full Load Current) ILOAD rising, RSET = 2.8kΩ17 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 1 2 Hz
MAX1898
Linear Charger for Single-Cell Li+ Battery
_______________________________________________________________________________________ 5
0.90
0.94
0.92
0.98
0.96
1.00
1.02
-40 85
NORMALIZED TIMER PERIOD
vs. TEMPERATURE
MAX1898 toc01
TEMPERATURE (°C)
TIMER PERIOD
10-15 35 60
NORMALIZED TO +25°C
0.996
0.998
0.997
1.000
0.999
1.001
1.002
-40 85
NORMALIZED BATT REGULATION
VOLTAGE vs. TEMPERATURE
MAX1898 toc02
TEMPERATURE (°C)
BATT REGULATION VOLTAGE
10-15 35 60
NORMALIZED TO +25°C
40
45
50
55
60
PRECHARGE CURRENT
vs. INPUT VOLTAGE
MAX1898 toc03
VIN (V)
PRECHARGE CURRENT (mA)
5896 7 10 11 12
ISET = 2.8kΩ
CHARGING CURRENT SET
FOR 500mA
0
0.4
0.2
0.8
0.6
1.0
1.2
3.9 4.14.0 4.2 4.3
NORMALIZED BATTERY CURRENT
vs. BATTERY VOLTAGE
MAX1898 toc04
BATTERY VOLTAGE (V)
NORMALIZED BATTERY CURRENT
-40
-20
20
0
40
60
-50
0
100
50
150
200
0.01 10.1 10 100 1000
VOLTAGE-LOOP GAIN
MAX1898 toc05
FREQUENCY (kHz)
GAIN (dB)
PHASE (deg.)
GAIN
PHASE
Typical Operating Characteristics
(VIN = 5V, VBATT = 4V, EN/OK = open, RSET = 2.8kΩ, RSTRT = GND, TA= +25°C, unless otherwise noted.)
MAX1898
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 current-
sense 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.
•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 indi-
cate an abnormal cell condition. As a built-in safety fea-
ture, 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 exter-
nal RSET resistor connected from ISET to GND. Select
the RSET value based on the following formula:
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
IV
R
FASTCHG
SET
=1400
Linear Charger for Single-Cell Li+ Battery
6 _______________________________________________________________________________________
Pin Description
PIN NAME FUNCTION
1 IN Sense Input. Detects power source; positive side of internal current-sense resistor.
2CHG Open-Drain LED Driver. See Table 1 for CHG output states.
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 logic-
controlled 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.
5CT
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.
6 RSTRT
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).
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.
MAX1898
Linear Charger for Single-Cell Li+ Battery
_______________________________________________________________________________________ 7
LED
CHG
0.1µF
IN
4.5V TO 12V
OSC ÷218
CT
CHARGING FOR
3hr,
CCT = 100nF
RSET
(2.8kΩ FOR
500mA CHARGE
CURRENT)
ISET
TIMEOUT CHG
LOGIC
5mA
CONTROL
LOGIC
280mV ISET (FALLING)
305mV ISET (RISING)
CA
VA
ICHARGE < 20%
REF
1.4V
1.4V
1.4V
RESTART
HV
DRV
PREQUAL
2.5V
DONE
UVLO
EN/OK
BATT
IN
SHUTDOWN
VL REG
3V
100kΩ
SHUTDOWN
GND
SHUTDOWN
DONE
69.275kΩ
34.637kΩ
2.653kΩ
RSTRT
10µF
BATT
DRV
CS
0.1µF
470Ω
84mΩ84Ω
20mΩ
DC IN
CSA
Figure 1. Functional Diagram
MAX1898
the ISET pin is 1mA per amp of charging current. The
output voltage at ISET is proportional to charging cur-
rent as follows:
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.
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 (typi-
cally 100kΩ) to a logic supply provides a logic-level
output. Table 1 outlines the status of CHG for various
charger states.
Reinitiating a Charging Cycle
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 low-
ered 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 addi-
tion 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 100kΩinternal 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
VIR
ISET CHG SET
=×
1000
Linear Charger for Single-Cell Li+ Battery
8 _______________________________________________________________________________________
CONDITION CHG
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 fast-
charge 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
Table 1. CHG Output States
OFF
MAX1898
ON
POWER OK IN
GND
EN/OK
3V
100kΩ
OPEN-DRAIN
LOGIC
Figure 3. Driving EN/OK with Open-Drain Logic to Retain
ON/OFF Control with Power OK Indication
3.2
3.4
3.8
3.6
4.0
4.2
0
0.2
0.4
0.6
1.0
0.8
1.2
1.4
06030 90 120 150 180
TIME (MINUTES)
BATTERY VOLTAGE (V)
AMPS, WATTS, AMP HOURS
BATTERY VOLTAGE
AMP HOURS
BATTERY CURRENT
POWER DISSIPATION
Figure 2. Li+ Charging Profile
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 com-
pletely 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.
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 exter-
nal 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-con-
trol 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 rec-
ommended timer settings.
The charge safety timer may also be completely dis-
abled by connecting CT to ground. Note: If the charge
timer is disabled, both the restart function and prequali-
fication fault indication are also disabled. Only the indi-
cation (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 bat-
tery 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
C nF t hours
CT CHG
[]
=×
[]
34 33.
MAX1898
Linear Charger for Single-Cell Li+ Battery
_______________________________________________________________________________________ 9
MAX1898
LED
CS
DRV
BATT
GND
CT
6
3
2
1
ISET
IN
4
8
5
7
9
10
EN/OK
RSTRT
CHG
10µF
100nF
Li+ CELL
CCT, 100nF CAPACITOR
SETS CHARGE TIME
TO 3hr
OUTPUT VOLTAGE
PROPORTIONAL
TO CHARGE CURRENT
RSET SETS
CHARGE
CURRENT
INPUT POWER
OK OUTPUT OR
ENABLE INPUT
CHARGE
INDICATOR
5V TO 12V
Figure 4. Basic Application Circuit
MAX1898
LED
CS
DRV
BATT
GND
CT
6
3
2
1
ISET
IN
4
8
5
7
9
10
EN/OK
RSTRT
CHG
10µF
220nF
Li+ CELL
CCT, 100nF CAPACITOR
SETS CHARGE TIME
TO 3hr
OUTPUT VOLTAGE
PROPORTIONAL
TO CHARGE CURRENT
RSET SETS
CHARGE
CURRENT
INPUT POWER
OK OUTPUT OR
ENABLE INPUT
CHARGE
INDICATOR
5V TO 12V
Figure 5. Application Circuit with PNP Pass Transistor
MAX1898
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 func-
tion is disabled, battery drain after charge comple-
tion is reduced to 4µA. With the restart function
enabled, battery drain current after charge comple-
tion is 40µA.
•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 con-
necting a resistor from RSTRT to GND. Select the
resistor using the following equations:
For MAX1898EUB42:
for VRSTRT values between 3V and 4V.
For MAX1898EUB41:
for VRSTRT values between 3V and 3.9V.
Note that if CT is grounded (charge timer disabled), the
restart function is also disabled.
Rk k
V
V
k
RSTRT RSTRT
Ω
[]
=Ω
−
−Ω
66 822
14 1
37 301
.
.
.
Rk k
V
V
k
RSTRT RSTRT
Ω
[]
=Ω
−
−Ω
69 275
14 1
37 301
.
.
.
Linear Charger for Single-Cell Li+ Battery
10 ______________________________________________________________________________________
VBATT < 2.5V
TIMEOUT/4
EXPIRES AND
VBATT < 2.5V
RSTRT = GND AND
VBATT < 4.0V (MAX1898EUB42)
VBATT < 3.9V (MAX1898EUB41)
PREQUALIFICATION
10% CHARGE CURRENT
LED = ON
FAST CHARGE
100% CHARGER CURRENT
LED = ON
TOP OFF
LED = OFF
SHUTDOWN
CHARGER = OFF
LED = OFF
FAULT
LED BLINKING AT 1.5Hz
DONE
(CHARGE COMPLETE)
CHARGER = OFF
LED = OFF
ASYNCHRONOUS FROM
ANYWHERE
VIN < 4.5V OR
VBATT > VIN
VIN < 4.5V OR
VBATT > VIN
VBATT > 2.5V,
3hr TIMER STARTS
TIMEOUT = 3hr
ICHARGE < 20%
OF ISET
DOTTED LINE INDICATES THE STATES THAT
ARE NOT AVAILABLE WHEN CT = 0 (TIMER DISABLED)
ICHARGE > 22%
OF ISET
TIMEOUT = 3hr
Figure 6. Charger State Diagram
EXTERNAL
TRANSISTORS
RESISTANCE
RATING
VOLTAGE LIMIT
(V)
POWER
DISSIPATION (W) PACKAGE COMMENTS
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
Table 2. External Transistors
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 tran-
sistor. The worst-case power dissipation will be:
PDISS(MAX) = IFASTCHG (VIN - 2.5V)
The pass transistor and heatsinking (typically an expand-
ed 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
100mΩto 200mΩRDS(ON)) performs more than ade-
quately. 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
MAX1898
Linear Charger for Single-Cell Li+ Battery
______________________________________________________________________________________ 11
MAX1898
Linear Charger for Single-Cell Li+ Battery
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.
Package Information
10LUMAX.EPS
