General Description
The MAX1879 single-cell lithium-ion (Li+) battery charger
utilizes an efficient pulse-charging architecture to minimize
power dissipation in portable devices. This architecture
combines the efficiency of switch-mode chargers with the
simplicity and low cost of linear chargers. This simple
device, in conjunction with a current-limited wall cube and
a PMOS transistor, allows safe and fast charging of a sin-
gle Li+ cell.
The MAX1879 initiates charging in one of three ways: bat-
tery insertion, charger power-up, or toggling the THERM
pin. Key safety features include continuous voltage and
temperature monitoring, a preset charger time-out, and an
8mA 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.
The MAX1879 is offered in a space-saving 8-pin µMAX
package. An evaluation kit (MAX1879EVKIT) is available
to help reduce design time.
Applications
Single-Cell Li+ Powered Portables
Self-Charging Battery Packs
PDAs
Cell Phones
Cradle Chargers
Features
♦Simple Design Minimizes Heat
♦Low Component Count, No Inductor
♦Battery-Full Indicator
♦0.75% Accurate Battery Regulation
♦1.5µA (max) Battery Current Drain with Wall Cube
Removed
♦Restart Charging at 4.0V
♦Continuous Overvoltage and Overtemperature
Protection
♦Safely Precharges Near-Dead Cells
♦Automatic Power-Down when Power Source is
Removed
♦Charges 1 Cell from as Low as 4.5V
♦Pin-Compatible Upgrade to MAX1679
MAX1879
Simple, Efficient, 1-Cell Li+ Pulse Charger
________________________________________________________________ Maxim Integrated Products 1
19-2061; Rev 0; 5/01
EVALUATION KIT AVAILABLE
Ordering Information
PART TEMP. RANGE PIN-PACKAGE
MAX1879EUA -40°C to +85°C 8-µMAX
BATT
GND
1-CELL
Li+
BATTERY
THERM ADJ
GATE
CURRENT-
LIMITED
AC WALL
ADAPTER
PULSE WIDTH
SELECT INPUT
CHG
LED
THERMISTOR
IN
MAX1879
TSEL
GND
ADJTSEL
1
2
8
7
BATT
THERMGATE
IN
µMAX
TOP VIEW
3
4
6
5
MAX1879
CHG
Typical Operating Circuit
Pin Configuration
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.
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, 10s) ................................ +300°C
MAX1879
Simple, Efficient, 1-Cell Li+ Pulse Charger
2 _______________________________________________________________________________________
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS
(VIN = VCHG = +10V, VBATT = +4.2V, TSEL = GND, GATE = unconnected, ADJ = unconnected, THERM = 10kΩto 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.
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
Input Voltage (Note 1) VIN External PMOS FET off 4.4 22 V
Undervoltage Lockout Trip Point BATT rising 2.1 2.2 2.3 V
Undervoltage Lockout Trip-Point
Hysteresis BATT falling 60 mV
Fast-Charge Qualification
Threshold
BATT rising, transition from precharge to
full current 2.425 2.5 2.575 V
Fast-Charge Restart Threshold BATT falling, transition from Done to
Prequalification state (Note 2) 3.92 4.00 4.08 V
Fast-Charge Reset Threshold
Delay (Note 3) 1 ms
Precharge Source Current VBATT = 2V 5 8 12 mA
BATT Regulation Voltage 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
BATT Removal Detection
Threshold BATT rising 4.875 5 5.125 V
BATT Removal Detection
Threshold Hysteresis BATT falling 125 mV
BATT Input Current (Note 4) VIN ≤ VBATT - 0.3V 0.1 1.5 µA
BATT Input Current, Fast-Charge
State IBATT VBATT = 4.0V 900 1500 µA
BATT Input Current, Done State IBATT VBATT = 4.25V, VIN present 85 200 µA
IN Input Current, Fast-Charge
State IIN VBATT = 4.0V, VIN = 4.0V 50 500 µA
IN Input Current, Done State IIN VBATT = 4.25V, VIN = 22V 700 1200 µA
Timer Accuracy -10 +10 %
CHG Output Leakage Current V CHG = 22V, CHG = high -1 +1 µA
CHG Output Sink Current V CHG = 1.0V, CHG = low 4 5 6 mA
MAX1879
Simple, Efficient, 1-Cell Li+ Pulse Charger
_______________________________________________________________________________________ 3
ELECTRICAL CHARACTERISTICS (continued)
(VIN = VCHG = +10V, VBATT = +4.2V, TSEL = GND, GATE = unconnected, ADJ = unconnected, THERM = 10kΩto GND, TA= 0°C to
+85°C, unless otherwise noted. Typical values are at TA= +25°C.)
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
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
GATE Source Current VIN = 10V, VGATE = 8V 75 100 125 µA
GATE Sink Current VIN = 10V, VGATE = 8V 19 25 31 µA
GATE Drive Current at Battery
Removal VBATT = 5.1V, gate driven high 18 40 60 mA
ELECTRICAL CHARACTERISTICS
(VIN = VCHG = +10V, VBATT = +4.2V, TSEL = GND, GATE = unconnected, ADJ = unconnected, THERM = 10kΩto GN, TA= -40°C to
+85°C, unless otherwise noted.) (Note 5)
PARAMETER SYMBOL CONDITIONS MIN MAX UNITS
Input Voltage (Note 1) VIN External PMOS FET off 4.6 22 V
Undervoltage Lockout Trip Point BATT rising 2.05 2.35 V
Fast-Charge Qualification
Threshold
BATT rising, transition from precharge to
full current 2.35 2.65 V
Fast-Charge Restart Threshold BATT falling, transition from Done to
Prequalification state (Note 2) 3.88 4.12 V
Precharge Source Current VBATT = 2V 4.5 12 mA
BATT Regulation Voltage 4.137 4.263 V
BATT Regulation Adjust Range 4.0 4.2 V
ADJ Source Impedance 9.8 10.2 kΩ
ADJ Output Voltage No load on ADJ 1.383 1.417 V
BATT Removal Detection
Threshold BATT rising 4.85 5.15 V
BATT Input Current (Note 4) VIN ≤ VBATT - 0.3V 1.5 µA
BATT Input Current, Fast-Charge
State IBATT VBATT = 4.0V 1500 µA
BATT Input Current, Done State IBATT VBATT = 4.25V, VIN present 300 µA
IN Input Current, Fast-Charge
State IIN VBATT = 4.0V, VIN = 4.0V 600 µA
IN Input Current, Done State IIN VBATT = 4.25V, VIN = 22V 1500 µA
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
MAX1879
Simple, Efficient, 1-Cell Li+ Pulse Charger
4 _______________________________________________________________________________________
ELECTRICAL CHARACTERISTICS (continued)
(VIN = V CHG = +10V, VBATT = +4.2V, TSEL = GND, GATE = unconnected, ADJ = unconnected, THERM = 10kΩto GND,
TA= -40°C to +85°C, unless otherwise noted.) (Note 5)
PARAMETER SYMBOL CONDITIONS MIN MAX UNITS
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 Current VIN = 10V, VGATE = 8V 60 140 µA
GATE Sink Current VIN = 10V, VGATE = 8V 15 35 µA
GATE Drive Current at Battery
Removal VBATT = 5.1V, gate driven high 15 90 mA
Note 1: The input voltage range is specified with the external PMOS FET off. When charging, the PMOS FET turns on, and the input
voltage (the output voltage of the current-limited source) drops to very near the battery voltage. When the PFET is on, VIN
may be as low as 2.5V.
Note 2: Restart Threshold tracks battery regulation voltage adjustment.
Note 3: Glitches less than 1ms do not cause a restart.
Note 4: This current is less than 1.5µA, even if an external RADJ resistor is connected from ADJ to GND.
Note 5: Specifications to -40°C are guaranteed by design and not production tested.
MAX1879
Simple, Efficient, 1-Cell Li+ Pulse Charger
_______________________________________________________________________________________ 5
MAX1879 toc01
0.980
0.985
0.995
0.990
1.010
1.015
1.005
1.000
1.020
NORMALIZED TIMER PERIOD
-40 0 20-20 40 60 80
TEMPERATURE (°C)
NORMALIZED TIMER PERIOD
vs. AMBIENT TEMPERATURE
NORMALIZED TO +25°C
0.9970
0.9985
0.9980
0.9975
0.9995
0.9990
1.0015
1.0010
1.0005
1.0000
1.0020
-40 -20 0 20 40 60 80
MAX1879 toc02
TEMPERATURE (°C)
NORMALIZED REGULATION VOLTAGE
NORMALIZED BATTERY REGULATION
VOLTAGE vs. TEMPERATURE
NORMALIZED TO +25°C
7.0
7.6
7.4
7.2
7.8
8.0
8.2
8.4
8.6
8.8
9.0
5 10152025
MAX1879 toc03
VIN (V)
PRECHARGE CURRENT (mA)
PRECHARGE CURRENT
vs. INPUT VOLTAGE
VBATT = 0
VBATT = 2V
Typical Operating Characteristics
(VIN = CHG = +10V, VBATT = +4.2V, THERM = 10kΩto GND, TA= +25°C, unless otherwise noted.)
Pin Description
PIN NAME FUNCTION
1 IN Input Voltage from Current-Limited Voltage Source (+22V max). Bypass to GND with a 0.1µF capacitor.
2 GATE Gate Drive for External PMOS Pass Element. The PMOS device should have a VGS threshold less than
2.5V. See Selecting External Components.
3CHG
Charge Status Indication. A CHG low indicates that the MAX1879 is charging a battery. CHG goes high
when the on/off duty cycle falls to 1/8, indicating that the battery is charged to approximately 95% of full
capacity. CHG is an open-drain current-limited N-channel MOSFET suitable for directly driving an LED.
Connect a pullup resistor to the logic supply to generate a logic-level signal. CHG blinks at 2Hz during the
Prequalification state (2.2V < VBATT < 2.5V) and during a temperature fault.
4 TSEL Minimum ON/OFF Pulse Width Selection (Table 2)
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 set point. 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 an NTC thermistor (10kΩ at +25°C) from THERM to GND to
allow prequalification of the cell temperature for fast-charge. Replace the thermistor with a 10kΩ resistor if
temperature sensing is not required.
8 BATT
Cell Voltage Monitor Input, Precharge Current Output, and MAX1879 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).
6 _______________________________________________________________________________________
MAX1879
Simple, Efficient, 1-Cell Li+ Pulse Charger
CURRENT-LIMITED
VOLTAGE SOURCE
(800mA)
PRECHARGE
CURRENT
(8mA)
TSEL
5mA
IN GATE
CHG
BATT
1nF
ADJ 2.2µF
RADJ SINGLE
Li+
CELL
10kΩ
SEE TABLE 2
FOR PULSE-WIDTH
OPTIONS
TIMER
THERM
1.4V
HOT
QUAL
TEST
CURRENT
COLD
QUAL
TEST
CURRENT
30kHz
OSCILLATOR
REVERSE-CURRENT
PROTECTION DIODE
**
NTC THERMISTOR*
BATT REMOVED
STATE
MACHINE
1.4V
5V
1:3
*OPTIONAL—REPLACE WITH 10kΩ RESISITOR IF NOT SENSING TEMPERATURE.
**RADJ = 410kΩ ±1% FOR 4.1V CELL; LEAVE OPEN FOR 4.2V CELL.
PFET
MAX1879
Functional Diagram
MAX1879
Simple, Efficient, 1-Cell Li+ Pulse Charger
_______________________________________________________________________________________________________ 7
SHUTDOWN
PMOS FET: OFF
LED: OFF
(FROM ANY STATE)
CHARGING POWER REMOVED CHARGING POWER
APPLIED
DONE
PMOS FET: OFF
LED: OFF
VBATT < 4.0V OR PULL THERM HIGH
BATT NOT INSTALLED OR VBATT < 2.2V
(ASYNCHRONOUSLY, FROM ANY STATE)
VBATT < 2.5V2.5V < VBATT < 4.2V
TEMPERATURE NOT OK
OR VBATT < 2.5V
VBATT > 4.0V
VBATT < 2.5V
VBATT 4.2V
VBATT > 2.2V POWER-ON RESET
8.0mA PRECHARGE: ON
PMOS FET: OFF
LED: OFF
CHARGE
PMOS FET: PULSED
LED: ON
TEMPERATURE OK
EVERY 9s
TOP-OFF
PMOS FET: PULSED
LED: OFF
PREQUALIFICATION
8.0mA PRECHARGE: ON
PMOS FET: OFF
LED: 50% DUTY CYCLE, 2Hz
CHARGE QUALIFICATION
PMOS FET: OFF
LED: ON
TEMPERATURE NOT OK
(LED: 50% DUTY CYCLE, 2Hz)
TEMPERATURE OK
EVERY 9S
TOP-OFF QUALIFICATION
PMOS FET: OFF
LED: OFF
TEMPERATURE NOT OK
(LED: OFF)
VBATT < 2.5V
BATT REGULATION VOLTAGE
(e.g., 4.2V) HAS BEEN REACHED
AND FET ON-TIME
FET OFF-TIME < 1
8
TOTAL TIME = 6.25hr
Simplified State Diagram
MAX1879
Simple, Efficient, 1-Cell Li+ Pulse Charger
8 _______________________________________________________________________________________
Detailed Description
Initiating a Charge Cycle
The MAX1879 initiates fast-charge upon insertion of a
battery or application of an external power source (cur-
rent-limited AC wall adapter). After charge completion,
charging restarts if the cell voltage falls below 4.0V or
when THERM is pulled above 1.4V after the charge
timer expires (approximately 6.25hr).
Before a charge cycle can begin, 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 (default range
+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 dangerous abnormal cell condition.
As a built-in safety feature, the MAX1879 precharges
the Li+ cell with 8mA at the start of a charge cycle if the
cell voltage is below 2.5V. As soon as the cell reaches
2.5V and the safety conditions are met (see the Fast-
Charge section), fast-charging begins.
If the temperature moves outside the programmed
range, the charger pauses. When the temperature
returns to the safe charging range, the charging cycle
continues, and the charge timer resumes counting from
the point at which the temperature fault occurred. The
timer is not reset. The MAX1879 monitors these condi-
tions throughout the charging cycle.
Fast-Charge
Once cell conditions are determined to be satisfactory,
the MAX1879 fast-charges the Li+ cell by pulling GATE
low, turning on the external PMOS FET. Cell charging
current is set by the current limit of the external power
supply; it is not regulated by the MAX1879. The PMOS
FET operates only as a switch, not as a linear regulator.
Therefore, power dissipation is minimized, permitting
rapid charge cycles with minimal heat generation. The
external power supply must have a specified current
limit that matches the desired fast-charge current for
the Li+ cell.
The MAX1879 uses a hysteretic algorithm with mini-
mum on-times and minimum off-times to top-off the cell.
The cell voltage is sampled every 2ms. If VBATT 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 and remains off for the minimum off-time. By
measuring the cell voltage when the PMOS FET is off
as well as when it is on, the MAX1879 eliminates bat-
tery-voltage sensing errors due to charging current
flowing through the series resistance of cell protection
switches or fuse links in the charging path. Fast-charge
continues until the on/off duty cycle falls to 1/8, then the
CHG LED turns off, indicating that the battery is
charged to approximately 95% of full capacity.
Pulsed Top-Off Charge
At the beginning of the Top-Off state, the current stays
on for a 1/8 duty cycle. As the cell continues to charge,
the percentage of time spent in the “current-on” mode
decreases. Towards 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, but the
duration of these pulses is several orders of magnitude
shorter than the intrinsic chemical time constant of the
Li+ cell. This does not harm the cell. Cell top-off is com-
pleted when the charging timer expires (approximately
6.25hr).
Charge Status with
CHG
CHG indicates the cell’s charging status. An LED can
be connected directly from IN to CHG for a visible indi-
cator. Alternatively, a pullup resistor (100kΩ) from a
logic supply to CHG provides a logic-level output.
Table 1 relates the LED status to the charger condition.
Applications Information
The MAX1879 was designed to offer the maximum inte-
gration and functionality in the smallest, most basic
application circuit possible. The only necessary exter-
nal components are a current-limited wall cube, a
PMOS FET, two small capacitors, and a 10kΩthermis-
tor/resistor. This simple application circuit appears in
Figure 1. Optionally (as shown in Figure 2), an LED can
be added as a charge-state indicator, a resistor (RADJ)
can be used to trim down the maximum charge voltage
Table 1. CHG Output States
CONDITION CHG (LED)
No battery or no charger, or
cell voltage < 2.2V High impedance (LED off)
Fast-charge in progress Low (LED on)
Top- Off: on/off d uty cycl e < 1/8 High impedance (LED off)
Done (charge complete) High impedance (LED off)
Temperature fault LED blinks at 2Hz
Prequalification
(2.2V < VBATT < 2.5V) LED blinks at 2Hz
from 4.2V, and/or a reverse-current-protection diode
can be added in line at the source.
If the input is shorted, the MAX1879 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 cur-
rent until it reaches a voltage of about 4.2V, then
charged at this voltage until the current decays below a
predetermined level. The MAX1879 provides a simple
way to reduce this maximum target voltage with a sin-
gle 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 volt-
age (VBR) of 4.2V; connect a 1% resistor from ADJ to
GND to form a voltage-divider for lower battery regula-
tion voltage (VBR′). Select the external value using:
A 1% tolerance resistor at ADJ degrades system accu-
racy by only a fraction of a percent. For example, an
RADJ of 410kΩ±1% reduces the battery regulation volt-
age by 2.4% (VBR′= 4.1V from equation above, and
(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 bat-
tery regulation voltage, or (1%)(2.4%) = 0.024%.
Selecting Minimum On-Time
The minimum pulse on-times can be selected by con-
necting TSEL as indicated in Table 2. A short pulse
time may be preferred to minimize voltage overshoot at
the battery and prevent pack overvoltage protection
circuitry from prematurely activating. A long minimum
on-time may be needed when using AC adapters with
high turn-on overshoot to allow charging current to set-
tle before the end of the pulse.
Selecting External Components
Power Supply
One reason the MAX1879 Li+ cell-charging solution is
so compact and simple is that the charging current is
set by the external power source, not by the MAX1879
charging circuit. The PMOS FET in this application cir-
cuit 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
R10k
V
V1
ADJ BR
BR
=
′−
Ω
MAX1879
Simple, Efficient, 1-Cell Li+ Pulse Charger
_______________________________________________________________________________________ 9
THERM
ADJ
0.001µF
IN
CHG
GATE
PMOS FET
FAIRCHILD FDC638P
-4.5A, -20V
0.07Ω AT VGS = -2.5V
CURRENT-LIMITED
WALL CUBE
(800mA, 6V max)
BATT
TSEL*
10kΩ
SINGLE
Li+
CELL
2.2µF
*SEE TABLE 2 FOR TSEL POSITION AND ASSOCIATED TIMER SETTINGS.
MAX1879
NTC THERMISTOR
FENWAL
140-103LAG-RBI
(10kΩ AT 25°C)
THERM
IN
CHG
GATE
PMOS FET
FAIRCHILD FDC638P
-4.5A, -20V
0.07Ω AT VGS = -2.5V
SCHOTTKY DIODE
30V, 1A
ZETEX ZHCS1000
CURRENT-LIMITED
WALL CUBE
(800mA, 6V max)
BATT
TSEL*
RADJ
410kΩ
1%**
SINGLE
Li+
CELL
2.2µF
0.001µF
5mA
LED
*SEE TABLE 2 FOR TSEL POSITION AND ASSOCIATED TIMER SETTINGS.
** RADJ SETS BATTERY REGULATION VOLTAGE TO 4.10V; LEAVE OPEN
FOR 4.2V.
MAX1879
ADJ
Figure 1. Simple Application Circuit Figure 2. Application Circuit Including LED and Thermistor
Table 2. TSEL Minimum On-Time Settings
TSEL
CONNECTION
MINIMUM ON-TIME IN TOP-OFF
(ms)
BATT 34
ADJ 69
GND 137
MAX1879
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 specified as
“current-limited” or “constant current.”
PMOS Switch
The PMOS FET switches the current-limited source on
and off. Because of the intentionally slow switching
times and limited slew rate, the MAX1879 is not particu-
lar about the power FET it drives. Specifications to con-
sider 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 break-
down voltage (BVDS) must exceed the open-circuit volt-
age 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 AC adapter type.
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 qualifi-
cation. The thermistor should be 10kΩat +25°C and
have a negative temperature coefficient (NTC); the
THERM pin expects 3.97kΩat +47.5°C and 28.7kΩat
+2.5°C. Connect the thermistor between THERM and
GND. If no temperature qualification is desired, replace
the thermistor with a 10kΩresistor. Thermistors by
Philips (22322-640-63103), Cornerstone Sensors
(T101D103-CA), and Fenwal Electronics (140-103LAG-
RB1) work well.
Bypass Capacitors
Bypass the ADJ pin with a 2000pF ceramic capacitor.
Bypass BATT with a capacitor with a value of at least
1.5µF per amp of charge current. The MAX1879 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 MAX1879 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 iden-
tify 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.
Layout Guidelines
The MAX1879 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 sys-
tem is insensitive to ground drops. However, Maxim rec-
ommends establishing good grounding areas and large
traces for high-current paths.
Chip Information
TRANSISTOR COUNT: 4692
SUBSTRATE CONNECTED TO GND
Simple, Efficient, 1-Cell Li+ Pulse Charger
10 ______________________________________________________________________________________
MAX1879
Simple, Efficient, 1-Cell Li+ Pulse Charger
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.
Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 ____________________ 11
© 2001 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.
Package Information
8LUMAXD.EPS
