19-2061; Rev 0; 5/01
ABLE
KIT AVAIL
N
IO
T
A
U
L
EVA
Simple, Efficient, 1-Cell Li+ Pulse Charger
Features
♦ Simple Design Minimizes Heat
The MAX1879 initiates charging in one of three ways: battery 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.
♦ Restart Charging at 4.0V
The MAX1879 is offered in a space-saving 8-pin µMAX
package. An evaluation kit (MAX1879EVKIT) is available
to help reduce design time.
♦ Pin-Compatible Upgrade to MAX1679
♦ Low Component Count, No Inductor
♦ Battery-Full Indicator
♦ 0.75% Accurate Battery Regulation
♦ 1.5µA (max) Battery Current Drain with Wall Cube
Removed
♦ 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
Applications
Ordering Information
Single-Cell Li+ Powered Portables
Self-Charging Battery Packs
PDAs
PART
TEMP. RANGE
PIN-PACKAGE
MAX1879EUA
-40°C to +85°C
8-µMAX
Cell Phones
Cradle Chargers
Typical Operating Circuit
Pin Configuration
TOP VIEW
IN
1
8
BATT
GATE
2
7
THERM
3
6
GND
TSEL 4
5
ADJ
CURRENTLIMITED
AC WALL
ADAPTER
LED
CHG
µMAX
GATE
IN
MAX1879
BATT
CHG
MAX1879
PULSE WIDTH
SELECT INPUT
TSEL
THERM
1-CELL
Li+
BATTERY
GND
ADJ
THERMISTOR
________________________________________________________________ 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
MAX1879
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 single Li+ cell.
MAX1879
Simple, Efficient, 1-Cell Li+ Pulse Charger
ABSOLUTE MAXIMUM RATINGS
IN, CHG, GATE to GND..........................................-0.3V to +26V
BATT, TSEL, THERM, ADJ to GND...........................-0.3V to +6V
GATE to IN................................................................-6V to +0.3V
THERM, ADJ to BATT...............................................-6V to +0.3V
GATE Continuous Current ................................................ -10mA
Continuous Power Dissipation (TA = +70°C)
8-Pin µMAX (derate 4.1mW/°C above +70°C) ............ 330mW
Operating Temperature Range ...........................-40°C to +85°C
Storage Temperature Range ............................ -65°C to +150°C
Lead Temperature (soldering, 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 = V CHG = +10V, VBATT = +4.2V, TSEL = GND, GATE = unconnected, ADJ = unconnected, THERM = 10kΩ to GND, TA = 0°C to
+85°C, unless otherwise noted. Typical values are at TA = +25°C.)
PARAMETER
Input Voltage (Note 1)
SYMBOL
VIN
CONDITIONS
MIN
TYP
MAX
UNITS
22
V
2.3
V
External PMOS FET off
4.4
Undervoltage Lockout Trip Point
BATT rising
2.1
Undervoltage Lockout Trip-Point
Hysteresis
BATT falling
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)
Precharge Source Current
VBATT = 2V
BATT Regulation Voltage
2.2
60
mV
1
ms
5
8
12
mA
4.1685
4.2000
4.2315
V
BATT Regulation Adjust Range
4.0
ADJ Source Impedance
9.8
4.2
V
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
BATT Input Current (Note 4)
VIN ≤ VBATT - 0.3V
0.1
1.5
µA
mV
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
50
500
µA
700
1200
µA
+10
%
IN Input Current, Fast-Charge
State
IIN
VBATT = 4.0V, VIN = 4.0V
IN Input Current, Done State
IIN
VBATT = 4.25V, VIN = 22V
Timer Accuracy
-10
CHG Output Leakage Current
V CHG = 22V, CHG = high
-1
CHG Output Sink Current
V CHG = 1.0V, CHG = low
4
2
5
_______________________________________________________________________________________
+1
µA
6
mA
Simple, Efficient, 1-Cell Li+ Pulse Charger
(VIN = V CHG = +10V, VBATT = +4.2V, TSEL = GND, GATE = unconnected, ADJ = unconnected, THERM = 10kΩ to GND, TA = 0°C to
+85°C, unless otherwise noted. Typical values are at TA = +25°C.)
PARAMETER
SYMBOL
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 = V CHG = +10V, VBATT = +4.2V, TSEL = GND, GATE = unconnected, ADJ = unconnected, THERM = 10kΩ to GN, TA = -40°C to
+85°C, unless otherwise noted.) (Note 5)
PARAMETER
MIN
MAX
UNITS
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
Precharge Source Current
VBATT = 2V
4.5
4.137
12
4.263
Input Voltage (Note 1)
SYMBOL
VIN
CONDITIONS
BATT Regulation Voltage
V
mA
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
600
µA
IN Input Current, Fast-Charge
State
IIN
VBATT = 4.0V, VIN = 4.0V
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
_______________________________________________________________________________________
3
MAX1879
ELECTRICAL CHARACTERISTICS (continued)
MAX1879
Simple, Efficient, 1-Cell Li+ Pulse Charger
ELECTRICAL CHARACTERISTICS (continued)
(V IN = V CHG = +10V, V BATT = +4.2V, TSEL = GND, GATE = unconnected, ADJ = unconnected, THERM = 10kΩ to 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.
4
_______________________________________________________________________________________
Simple, Efficient, 1-Cell Li+ Pulse Charger
1.010
1.005
1.000
0.995
0.990
0.985
0.980
-40
NORMALIZED TO +25°C
1.0015
1.0010
1.0005
1.0000
0.9995
0.9990
0.9985
0
20
40
TEMPERATURE (°C)
60
80
8.8
8.6
8.4
8.2
VBATT = 0
8.0
7.8
7.6
0.9980
7.4
0.9975
7.2
VBATT = 2V
7.0
0.9970
-20
9.0
MAX1879 toc03
1.0020
MAX1879 toc02
NORMALIZED TO +25°C
NORMALIZED REGULATION VOLTAGE
NORMALIZED TIMER PERIOD
MAX1879 toc01
1.020
1.015
PRECHARGE CURRENT
vs. INPUT VOLTAGE
NORMALIZED BATTERY REGULATION
VOLTAGE vs. TEMPERATURE
PRECHARGE CURRENT (mA)
NORMALIZED TIMER PERIOD
vs. AMBIENT TEMPERATURE
-40
-20
0
20
40
TEMPERATURE (°C)
60
80
5
10
15
20
25
VIN (V)
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.
3
CHG
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
8
BATT
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.
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).
_______________________________________________________________________________________
5
MAX1879
Typical Operating Characteristics
(VIN = CHG = +10V, VBATT = +4.2V, THERM = 10kΩ to GND, TA = +25°C, unless otherwise noted.)
Simple, Efficient, 1-Cell Li+ Pulse Charger
MAX1879
Functional Diagram
CURRENT-LIMITED
VOLTAGE SOURCE
(800mA)
PFET
REVERSE-CURRENT
PROTECTION DIODE
IN
GATE
CHG
MAX1879
PRECHARGE
CURRENT
(8mA)
5mA
BATT
10kΩ
TSEL
STATE
MACHINE
SEE TABLE 2
FOR PULSE-WIDTH
OPTIONS
1:3
ADJ
2.2µF
1.4V
1nF
TIMER
BATT REMOVED
5V
COLD
QUAL
TEST
CURRENT
30kHz
OSCILLATOR
HOT
QUAL
TEST
CURRENT
1.4V
THERM
NTC THERMISTOR*
*OPTIONAL—REPLACE WITH 10kΩ RESISITOR IF NOT SENSING TEMPERATURE.
**RADJ = 410kΩ ±1% FOR 4.1V CELL; LEAVE OPEN FOR 4.2V CELL.
6
_______________________________________________________________________________________
RADJ **
SINGLE
Li+
CELL
Simple, Efficient, 1-Cell Li+ Pulse Charger
SHUTDOWN
PMOS FET: OFF
LED: OFF
CHARGING POWER REMOVED
(FROM ANY STATE)
CHARGING POWER
APPLIED
VBATT < 4.0V OR PULL THERM HIGH
VBATT > 2.2V
POWER-ON RESET
8.0mA PRECHARGE: ON
PMOS FET: OFF
LED: OFF
BATT NOT INSTALLED OR VBATT < 2.2V
(ASYNCHRONOUSLY, FROM ANY STATE)
PREQUALIFICATION
8.0mA PRECHARGE: ON
PMOS FET: OFF
LED: 50% DUTY CYCLE, 2Hz
VBATT 4.2V
TEMPERATURE NOT OK
OR VBATT < 2.5V
2.5V < VBATT < 4.2V
VBATT < 2.5V
VBATT > 4.0V
DONE
PMOS FET: OFF
LED: OFF
CHARGE
PMOS FET: PULSED
LED: ON
EVERY 9s
TEMPERATURE OK
VBATT < 2.5V
CHARGE QUALIFICATION
PMOS FET: OFF
LED: ON
TEMPERATURE NOT OK
(LED: 50% DUTY CYCLE, 2Hz)
BATT REGULATION VOLTAGE
(e.g., 4.2V) HAS BEEN REACHED
AND FET ON-TIME < 1
FET OFF-TIME
8
TOP-OFF
PMOS FET: PULSED
LED: OFF
TOTAL TIME = 6.25hr
EVERY 9S
TEMPERATURE OK
VBATT < 2.5V
TOP-OFF QUALIFICATION
PMOS FET: OFF
LED: OFF
TEMPERATURE NOT OK
(LED: OFF)
_______________________________________________________________________________________________________
7
MAX1879
Simplified State Diagram
MAX1879
Simple, Efficient, 1-Cell Li+ Pulse Charger
Detailed Description
Initiating a Charge Cycle
The MAX1879 initiates fast-charge upon insertion of a
battery or application of an external power source (current-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 FastCharge 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 conditions 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 minimum 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 bat8
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 completed 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 indicator. 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 integration and functionality in the smallest, most basic
application circuit possible. The only necessary external components are a current-limited wall cube, a
PMOS FET, two small capacitors, and a 10kΩ thermistor/resistor. This simple application circuit appears in
Figure 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 duty cycle < 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
_______________________________________________________________________________________
Simple, Efficient, 1-Cell Li+ Pulse Charger
Adjusting the Battery Regulation Voltage
A typical Li+ cell should be charged at a constant current until it reaches a voltage of about 4.2V, then
charged at this voltage until the current decays below a
predetermined level. The MAX1879 provides a simple
way to reduce this maximum target voltage with a single resistor between ADJ and GND. Internally, ADJ
connects to a precision 1.4V reference through a 10kΩ
resistor. Leave ADJ open for a battery regulation voltage (VBR) of 4.2V; connect a 1% resistor from ADJ to
GND to form a voltage-divider for lower battery regulation voltage (VBR′). Select the external value using:
RADJ =
10kΩ
VBR
−1
VBR ′
Selecting Minimum On-Time
The minimum pulse on-times can be selected by connecting 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 settle 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 circuit is either on or off, allowing the source to be directly
connected to the cell or completely disconnected.
Therefore, it is very important to choose a power supply
Table 2. TSEL Minimum On-Time Settings
A 1% tolerance resistor at ADJ degrades system accuracy by only a fraction of a percent. For example, an
RADJ of 410kΩ ±1% reduces the battery regulation voltage by 2.4% (VBR′ = 4.1V from equation above, and
(VBR′ - VBR) / VBR = (4.1 - 4.2) / 4.2 = -2.4%).
PMOS FET
FAIRCHILD FDC638P
-4.5A, -20V
0.07Ω AT VGS = -2.5V
CURRENT-LIMITED
WALL CUBE
(800mA, 6V max)
Therefore, the additional system error is simply the RADJ
tolerance multiplied by the percent change in the battery regulation voltage, or (1%)(2.4%) = 0.024%.
TSEL
CONNECTION
MINIMUM ON-TIME IN TOP-OFF
(ms)
BATT
34
ADJ
69
GND
137
PMOS FET
FAIRCHILD FDC638P
-4.5A, -20V
0.07Ω AT VGS = -2.5V
CURRENT-LIMITED SCHOTTKY DIODE
30V, 1A
WALL CUBE
ZETEX ZHCS1000
(800mA, 6V max)
GATE
GATE
IN
IN
5mA
LED
BATT
MAX1879
MAX1879
CHG
CHG
TSEL*
ADJ
THERM
0.001µF
10kΩ
0.001µF
2.2µF
*SEE TABLE 2 FOR TSEL POSITION AND ASSOCIATED TIMER SETTINGS.
Figure 1. Simple Application Circuit
SINGLE
Li+
CELL
RADJ
410kΩ
1%**
BATT
TSEL*
ADJ THERM
2.2µF
NTC THERMISTOR
FENWAL
140-103LAG-RBI
(10kΩ AT 25°C)
SINGLE
Li+
CELL
*SEE TABLE 2 FOR TSEL POSITION AND ASSOCIATED TIMER SETTINGS.
** RADJ SETS BATTERY REGULATION VOLTAGE TO 4.10V; LEAVE OPEN
FOR 4.2V.
Figure 2. Application Circuit Including LED and Thermistor
_______________________________________________________________________________________
9
MAX1879
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 enhancementmode 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.
MAX1879
Simple, Efficient, 1-Cell Li+ Pulse Charger
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 particular about the power FET it drives. Specifications to consider when choosing an appropriate FET are the
minimum drain-source breakdown voltage, the minimum
turn-on threshold voltage (VGS), and current-handling
and power-dissipation qualities. The minimum breakdown voltage (BVDS) must exceed the open-circuit voltage of the wall cube by at least 25%. Note that this
open-circuit voltage may be twice as high as the specified
output voltage, depending on the 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 qualification. The thermistor should be 10kΩ at +25°C and
have a negative temperature coefficient (NTC); the
THERM pin expects 3.97kΩ at +47.5°C and 28.7kΩ at
+2.5°C. Connect the thermistor between THERM and
GND. If no temperature qualification is desired, replace
the thermistor with a 10kΩ resistor. Thermistors by
Philips (22322-640-63103), Cornerstone Sensors
(T101D103-CA), and Fenwal Electronics (140-103LAGRB1) work well.
Bypass Capacitors
Bypass the ADJ pin with a 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 identify momentary cell removal events and may increase
transient currents when the cell is replaced. Therefore,
values in excess of 100µF should be avoided in those
cases. For best system performance, at least 0.47µF of
the total capacitance should be low-ESR ceramic.
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 system is insensitive to ground drops. However, Maxim recommends establishing good grounding areas and large
traces for high-current paths.
Chip Information
TRANSISTOR COUNT: 4692
SUBSTRATE CONNECTED TO GND
10
______________________________________________________________________________________
Simple, Efficient, 1-Cell Li+ Pulse Charger
8LUMAXD.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.
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.
MAX1879
Package Information
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