MAXIM MAX1679

19-1545; Rev 0; 9/99
KIT
ATION
EVALU
E
L
B
AVAILA
Single-Cell Li+ Battery Charger
for Current-Limited Supply
Features
The MAX1679 battery charger for a single lithium-ion (Li+)
cell comes in a space-saving 8-pin µMAX package. 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.
♦ Simple Stand-Alone Application Circuit
The MAX1679 initiates charging in one of three ways:
battery insertion, charger power-up, or external manipulation of the THERM pin. Charging terminates when
the average charging current falls to approximately 1%
of the fast-charge current, or when the on-chip counter
times out.
♦ 0.75% Overall System Accuracy
Key safety features include continuous voltage and
temperature monitoring, a programmable charger timeout, and a 5mA 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.
♦ Top-Off Charging to Achieve Full Battery Capacity
♦ Lowest Power Dissipation
♦ 8-Pin µMAX Package Saves Space
♦ No Inductor Required
♦ Continuous Voltage and Temperature Protection
♦ Safely Precharges Near-Dead Cells
♦ Programmable Safety Timeout
♦ Automatic Power-Down when Power Source is
Removed
The MAX1679 evaluation kit (MAX1679EVKIT) is available to help reduce design time.
Ordering Information
PART
MAX1679EUA
TEMP. RANGE
-40°C to +85°C
PIN-PACKAGE
8 µMAX
Applications
Single-Cell Li+ Portable Applications
Self-Charging Battery Packs
PDAs
Cell Phones
Typical Operating Circuit
Cradle Chargers
Pin Configuration
CURRENTLIMITED
VOLTAGE
SOURCE
PFET
LED
GATE
IN
TOP VIEW
BATT
CHG
BATTERY
IN
1
GATE
2
CHG
3
TSEL
4
MAX1679
µMAX
8
BATT
7
THERM
6
GND
5
ADJ
TIMER
RANGE
SELECT
MAX1679
TSEL
THERM
GND
ADJ
THERMISTOR
________________________________________________________________ Maxim Integrated Products
1
For free samples & the latest literature: http://www.maxim-ic.com, or phone 1-800-998-8800.
For small orders, phone 1-800-835-8769.
MAX1679
General Description
MAX1679
Single-Cell Li+ Battery Charger
for Current-Limited Supply
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, 10sec) ............................ +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.)
MAX
UNITS
Input Voltage (Note 1)
PARAMETER
SYMBOL
VIN
External PMOS FET off
CONDITIONS
MIN
5
22
V
Input Voltage for Removable
Battery (Note 1)
VIN
External PMOS FET off
6
22
V
2.3
V
Undervoltage Lockout Trip Point
VBATT rising
Undervoltage Lockout Trip-Point
Hysteresis
VBATT falling
Fast-Charge Qualification
Threshold
VBATT rising, transition from precharge to full
current
2.425
2.500
2.575
V
Fast-Charge Restart Threshold
VBATT falling, transition from DONE to
PREQUAL state (Figure 2)
3.783
3.89
4.00
V
Precharge Source Current
VBATT = 2V
BATT Regulation Voltage
2.1
TYP
2.2
60
mV
4
5
6
mA
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
Battery Removal Detection
Threshold
VBATT rising
4.875
5.0
5.125
V
%
Battery Removal Detection
Threshold Hysteresis
VBATT falling
125
BATT Input Current (Note 2)
VIN ≤ VBATT -0.3V
0.1
1
µA
mV
BATT Input Current, FastCharge State
IBATT
VBATT = 4V
900
1500
µA
BATT Input Current, Done State
IBATT
VBATT = 4.25V
500
1000
µA
IN Input Current, Fast-Charge
State
IIN
VBATT = 4V, VIN = 4V
30
100
µA
IN Input Current, Done State
IIN
VBATT = 4.25V, VIN = 22V
250
600
µA
Timer Accuracy
See Table 3
-10
+10
%
CHG Output Leakage Current
V CHG = 22V, CHG = high
-1
+1
µA
2
_______________________________________________________________________________________
Single-Cell Li+ Battery Charger
for Current-Limited Supply
(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
MIN
TYP
MAX
UNITS
4
5
6
mA
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
%
CHG Output Sink Current
CONDITIONS
V CHG = 1V, CHG = low
GATE Source/Sink Current
VIN = 10V, VGATE = 8V
75
105
130
µA
GATE Drive Current at Battery
Removal
VBATT = 5.1V, gate driven high
20
40
60
mA
ELECTRICAL CHARACTERISTICS
(VIN = V CHG = +10V, VBATT = +4.2V, TSEL = GND, GATE = unconnected, ADJ = unconnected, THERM = 10kΩ to GND, TA = -40°C
to +85°C, unless otherwise noted.)
MIN
MAX
UNITS
Input Voltage (Note 1)
PARAMETER
SYMBOL
VIN
External PMOS FET off
CONDITIONS
5
22
V
Input Voltage for Removable
Battery (Note 1)
VIN
External PMOS FET off
6
22
V
Undervoltage Lockout Trip Point
VBATT rising
2.05
2.35
V
Fast-Charge Qualification
Threshold
VBATT rising, transition from precharge to full
current
2.35
2.65
V
Fast-Charge Restart Threshold
VBATT falling, transition from DONE to
PREQUAL state, Figure 2
3.74
4.04
V
Precharge Source Current
VBATT = 2V
BATT Regulation Voltage
BATT Regulation Adjust Range
ADJ Source Impedance
3
7
mA
4.137
4.263
V
4.0
4.2
V
9.8
10.2
kΩ
ADJ Output Voltage
No load on ADJ
1.386
1.414
V
Battery Removal Detection
Threshold
VBATT rising
4.850
5.150
V
BATT Input Current (Note 2)
VIN ≤ VBATT - 0.3V
1
µA
BATT Input Current, FastCharge State
IBATT
VBATT = 4.0V
1500
µA
BATT Input Current, Done State
IBATT
VBATT = 4.25V
1000
µA
VBATT = 4.0V, VIN = 4.0V
100
µA
IN Input Current, Fast-Charge
State
IIN
_______________________________________________________________________________________
3
MAX1679
ELECTRICAL CHARACTERISTICS (continued)
MAX1679
Single-Cell Li+ Battery Charger
for Current-Limited Supply
ELECTRICAL CHARACTERISTICS (continued)
(VIN = V CHG = +10V, VBATT = +4.2V, TSEL = GND, GATE = unconnected, ADJ = unconnected, THERM = 10kΩ to GND, TA = -40°C
to +85°C, unless otherwise noted.)
PARAMETER
IN Input Current, Done State
SYMBOL
IIN
CONDITIONS
MIN
VBATT = 4.25V, VIN = 22V
MAX
UNITS
700
µA
%
Timer Accuracy
See Table 3
-15
+15
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
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/Sink Current
VIN = 10V, VGATE = 8V
60
140
µA
GATE Drive Current at Battery
Removal
VBATT = 5.1V, gate driven high
20
90
mA
Note 1: The input voltage range is specified with the external PFET off. When charging, the PFET turns on and the input voltage
(the output voltage of the constant-current power source) drops to very near the battery voltage. When the PFET is on, VIN
may be as low as 2.5V.
Note 2: BATT Input Current is the supply current to the device. When VIN is removed, the MAX1679 shuts down and the input current
is less than 1µA, even if there is an external RADJ resistor (ADJ to GND).
4
_______________________________________________________________________________________
Single-Cell Li+ Battery Charger
for Current-Limited Supply
NORMALIZED BATT REGULATION VOLTAGE
vs. TEMPERATURE
1.0100
1.0050
1.0000
0.9950
0.9900
-40
1.0010
NORMALIZED TO +25°C
1.0005
1.0000
0.9995
0.9990
0.9985
6.0
5.8
PRECHARGE CURRENT (mA)
TIMER PERIOD
1.0150
1.0015
MAX1679 toc02
MAX1679 toc01
NORMALIZED TO +25°C
BATT REGULATION VOLTAGE
1.0200
PRECHARGE CURRENT
vs. INPUT VOLTAGE
-20
0
20
40
60
TEMPERATURE (°C)
80
100
5.6
VBATT = 0V
5.4
5.2
VBATT = 2V
5.0
4.8
VBATT = 4.1V
4.6
4.4
0.9980
0.9975
-40
MAX1679 toc03
TIMER PERIOD vs.
AMBIENT TEMPERATURE
4.2
4.0
-20
0
20
40
60
80
100
TEMPERATURE (°C)
5
10
15
VIN (V)
20
25
Pin Description
PIN
NAME
FUNCTION
1
IN
Input Voltage from Current-Limited Voltage Source (+22V max). Bypass to GND with a 0.1µF capacitor. The
cell charging current is set by the current limit of the external power supply.
2
GATE
Gate Drive for External PMOS Pass Element. The PMOS device should have a VGS threshold of 2.5V or
less. See Selecting External Components.
3
CHG
Charge Status Indication. CHG is an open-drain, current-limited N-channel MOSFET suitable for directly driving an LED. Connect a pull-up resistor to BATT to generate a logic-level signal. See Table 2 for CHG output states.
4
TSEL
Maximum Total Charge-Time Selection. See Table 3 for timer options.
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 setpoint. 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 a thermistor from THERM to GND to prequalify the cell temperature for fast-charge. Drive THERM high (> +1.4V) during Done or Fault states to reset the MAX1679 and
reinitiate the charging sequence. Replace the thermistor with a 10kΩ resistor if temperature sensing is not
required.
8
BATT
Cell Voltage Monitor Input, Trickle-Charge Output, and MAX1679 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
MAX1679
Typical Operating Characteristics
(VIN = V CHG = +10V, VBATT = +4.2V, THERM = 10kΩ to GND, TA = +25°C, unless otherwise noted.)
MAX1679
Single-Cell Li+ Battery Charger
for Current-Limited Supply
CURRENT-LIMITED
VOLTAGE SOURCE
(800mA)
PFET
*
REVERSE-CURRENT
PROTECTION DIODE
IN
GATE
CHG
MAX1679
TRICKLE
CHARGE
(5mA)
5mA
BATT
10kΩ
TSEL
STATE
MACHINE
SEE TABLE 3
FOR TIMEOUT
OPTIONS
1:3
ADJ
2.2µF
1.4V
1nF
TIMER
RADJ **
BATT REMOVED
5V
COLD
QUAL
TEST
CURRENT
30kHz
OSCILLATOR
HOT
QUAL
TEST
CURRENT
1.4V
THERM
NTC THERMISTOR*
*OPTIONAL
**RADJ = 410kΩ ±1% FOR 4.1V CELL; LEAVE OPEN FOR 4.2V CELL.
Figure 1. Functional Diagram
6
_______________________________________________________________________________________
SINGLE
Li+
CELL
Single-Cell Li+ Battery Charger
for Current-Limited Supply
CELL NOT INSTALLED
(FROM ANY STATE)
MAX1679
SHUTDOWN
LED: OFF
PMOS FET: OFF
CHARGING POWER REMOVED
(FROM ANY STATE)
R
OWE
ER P D
G
R
CHA APPLIE
CELL REMOVAL OR
PULL THERM HIGH
PREQUALIFICATION
LED: 50% DUTY CYCLE, 2Hz
PMOS FET: OFF
5mA PRECHARGE: ON
VB
PULATT < 3.8
L TH 9V O
TEM
ERM R
PER
HIGH
ATU
AND
RE O
VB
K
ATT
> 2.
5V
TEMPERATURE NOT OK
OR VBATT < 2.5V
E
RG
HA
-C OUT
T
S
FA TIME
FAST-CHARGE
LED: ON
PMOS FET: ON
ec
RY
FAULT
LED: 50% DUTY CYCLE, 2Hz
PMOS FET: OFF
7s
E
EV
K
EO
UR
T
RA
E
MP
TEMPERATURE NOT OK
RY
CELL VOLTAGE REACHES
BATT REGULATION VALUE (e.g., 4.2V)
TE
FAST-CHARGE QUALIFICATION
LED: ON
PMOS FET: OFF
TOP OFF
LED: ON
PMOS FET: PULSED TOP-OFF
c
se
7
E
EV
RE
OK
FET
O
FET N TIM
E
OFF
TIM ≤
E
1%
U
AT
ER
TOP-OFF QUALIFICATION
LED: ON
PMOS FET: OFF
MP
TE
DONE
LED: 12% DUTY CYCLE, 0.25Hz
PMOS FET: OFF
5mA CANCELLATION CURRENT ENABLED
TEMPERATURE NOT OK
Figure 2. State Machine Diagram
_______________________________________________________________________________________
7
MAX1679
Single-Cell Li+ Battery Charger
for Current-Limited Supply
Detailed Description
Initiating a Charge Cycle
The MAX1679 attempts to initiate fast-charge upon
insertion of the battery or application of an external
power source (current-limited wall cube). After charge
completion, charging restarts when the cell voltage
drops below 3.89V or when THERM is pulled above 1.4V.
Before a charge cycle can begin, the 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 (the
default range is +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 dangerously abnormal cell condition. As a built-in safety feature, the MAX1679
precharges the Li+ cell with 5mA at the start of a
charge cycle when the cell voltage is below 2.5V.
Typically, 5mA is sufficient to bring a fully discharged
1000mAh Li+ cell up to 2.5V in less than 5 minutes. As
soon as the cell’s voltage reaches 2.5V and all the
other prerequisites are met (see the Fast-Charge section), the MAX1679 begins fast-charging the cell.
If the temperature is outside the programmed range,
the charger waits. Once all prequalification conditions
are met, the charging cycle and timers begin. The
MAX1679 continues to monitor these conditions
throughout the charging cycle.
Fast-Charge
Once all cell conditions are determined to be satisfactory, the MAX1679 begins fast-charging the Li+ cell by
turning on the external PMOS FET. The cell charging
current is set by the current limit of the external
power supply; it is not regulated by the MAX1679. The
PMOS FET is used as a simple switch, not as a linear
regulator. Therefore, the circuit’s power dissipation is
minimized, permitting rapid charge cycles with minimal
heat generation. The external power supply should
have a specified current limit that matches the desired
fast-charge current for the Li+ cell.
Fast-charge continues until one of the following conditions is reached: 1) cell voltage climbs to the battery
regulation voltage (4.2V or as set by ADJ); 2) the fastcharging timer expires (fault condition); or 3) cell temperature rises above +47.5°C or falls below +2.5°C. If
the cell temperature moves outside the specified limits,
charging is suspended but not terminated. All timers are
paused and charging resumes when the temperature
returns to the normal range.
8
Pulsed Top-Off Charge
In the most common case, where fast-charge is terminated because the battery regulation voltage has been
reached (that is, the cell is nearly fully charged), the
MAX1679 will top off the cell. The MAX1679 uses a hysteretic algorithm with minimum on-times and minimum
off-times (Table 3). The cell voltage is sampled every
2ms. If the cell voltage (at BATT) 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 or
remains off until the next sample. By also measuring
the cell voltage when the PMOS FET is off, the
MAX1679 eliminates voltage errors caused by charging
current flowing through the series resistance of protection
switches or fuse links that may be in the charging path.
At the beginning of this top-off state, the current stays
on for many consecutive cycles between single off periods. As the cell continues to charge, the percentage of
time spent in the “current-on” mode decreases. Toward
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 by several hundred millivolts, but the
duration of these pulses is several orders of magnitude
shorter than the intrinsic chemical time constant of Li+
cells. This does not harm the cell. Cell top off is completed either when the duty ratio of “on” cycles to “off”
cycles falls below 1/64 to 1/256 as set by TSEL (see
Table 3), or when the charging timer expires.
Switched 5mA Cancellation Current
When the charge cycle is complete, the MAX1679
replaces current drawn by the BATT pin (to sense battery removal) with a 5mA (nominal) switched current.
This current is turned on and off by an on-chip comparator as needed to maintain the battery regulation
voltage. The cell is maintained in this manner as long
as the battery is inserted and power is connected. This
cancellation current is turned off when the MAX1679 is
shut down. Note that BATT draws less than 1µA from
the battery when power is removed.
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 pull-up resistor (typically 100kΩ)
from a logic supply to CHG provides a logic-level output.
Table 2 relates the status of the CHG to the condition of
the charger.
_______________________________________________________________________________________
Single-Cell Li+ Battery Charger
for Current-Limited Supply
The MAX1679 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 3. Optionally (as shown in Figure 4), an LED can
be added as a charge-state indicator, a resistor (RADJ)
can be used to trim down the maximum charge voltage
from 4.2V, and/or a reverse-current-protection diode
can be added in line at the source.
If the input is shorted, the MAX1679 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 current until it reaches a voltage of about 4.2V, then
charged at this voltage until the current decays below a
predetermined level. The MAX1679 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:
Table 2. CHG Output States
RADJ =
CHG
CONDITION
No battery or no charger, or
cell voltage < 2.2V
High impedance
Fast-charge or pulsed topoff charge in progress
Low (LED on)
Fast-charge timer expiration
or initial prequalification state
(VBATT < 2.5V or initial temperature fault)
2Hz, 50% duty factor (LED
flashing)
Charge cycle complete
LED blinking 0.5sec on (low),
3.5sec off (high impedance)
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
PMOS FET
FAIRCHILD FDC638P
-4.5A, -20V
0.07Ω AT VGS = -2.5V
CURRENT-LIMITED
WALL CUBE
(800mA, 6V max)
10kΩ
VBR
−1
VBR '¢
(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 battery regulation voltage, or (1%)(2.4%) = 0.024%.
CURRENT LIMITED
WALL CUBE
(800mA, 6V max)
GATE
GATE
IN
IN
5mA
LED
BATT
MAX1679
CHG
PMOSFET
FAIRCHILD FDC638P
-4.5A, -20V
0.07Ω AT VGS = -2.5V
SCHOTTKY DIODE
30V, 1A
ZETEX ZHCS1000
TSEL*
ADJ
THERM
CHG
SINGLE
Li+
CELL
0.001µF
0.001µF
10k
2.2µF
*SEE TABLE 3 FOR TSEL POSITION AND ASSOCIATED TIMER SETTINGS.
Figure 3. Simple Application Circuit
MAX1679
RADJ
410k
1%**
BATT
TSEL*
ADJ THERM
2.2µF
NTC THERMISTOR
FENWAL
140-103LAG-RBI
(10kΩ AT 25°C)
SINGLE
Li+
CELL
*SEE TABLE 3 FOR TSEL POSITION AND ASSOCIATED TIMER SETTINGS.
** RADJ SETS BATTERY REGULATION VOLTAGE TO 4.10V; LEAVE OPEN
FOR 4.2V.
Figure 4. Application Circuit Including LED, Thermistor, and
Reverse-Current Protection Diode
_______________________________________________________________________________________
9
MAX1679
Applications Information
MAX1679
Single-Cell Li+ Battery Charger
for Current-Limited Supply
Table 3. Timer Option (TSEL) Definitions
TSEL
CONNECTION
RECOMMENDED
CHARGE RATE
FASTCHARGE
TIME LIMIT
(minutes)
BATT
1.5C
55
ADJ
1C
75
GND
<1C
Off
6.25
Selecting Maximum Charge Time
As a safety feature, fast-charging and pulsed top-off
charging will stop if their respective counters time out.
The MAX1679 offers a choice of three timeout periods
set by TSEL (Table 3). In Table 3, C represents the nominal capacity of the battery cell in ampere hours.
Both timers begin upon entering the fast-charge state.
The fast-charge timer is disabled upon leaving fastcharge; the total charge timer continues through top-off
but is disabled in the done state.
Selecting External Components
Power Supply
One reason the MAX1679 Li+ cell-charging solution is
so compact and simple is that the charging current is
set by the external power source, not by the MAX1679
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
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 advertised as
“current-limited” or “constant current.”
PMOS Switch
The PMOS FET is used to switch the current-limited
source on and off into the Li+ cell. Because of the intentionally slow switching times and limited slew rate, the
MAX1679 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 converter type.
10
TOTAL
CHARGE
TIME LIMIT
(hours)
MIN
ON/OFF TIME
(IN TOP-OFF)
(ms)
ON/OFF DUTY
CYCLE
FOR DONE
INDICATION
2.8
70
1/256
3.75
140
1/128
280
1/64
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 0.001µF ceramic capacitor.
Bypass BATT with a capacitor with a value of at least
1.5µF per amp of charge current. The MAX1679 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 MAX1679 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.
______________________________________________________________________________________
Single-Cell Li+ Battery Charger
for Current-Limited Supply
TRANSISTOR COUNT: 4692
SUBSTRATE CONNECTED TO GND
______________________________________________________________________________________
11
MAX1679
Chip Information
Layout Guidelines
The MAX1679 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.
Single-Cell Li+ Battery Charger
for Current-Limited Supply
8LUMAXD.EPS
MAX1679
Package Information
Note: The MAX1679 does not have an exposed pad.
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
© 1999 Maxim Integrated Products
Printed USA
is a registered trademark of Maxim Integrated Products.