MAX1679
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 nom-
inal 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 fast-
charge; 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 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
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 inten-
tionally 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 (V
GS
), and current
handling and power-dissipation qualities. The minimum
breakdown voltage (BV
DS
) 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.
Thermistor
The intent of THERM is to inhibit fast-charging the cell
when it is too cold or too hot (+2.5°C ≤ T
OK
≤ 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 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 (140-103LAG-
RB1) 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 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.
Single-Cell Li+ Battery Charger
for Current-Limited Supply
10 ______________________________________________________________________________________
3.75ADJ
TOTAL
CHARGE
TIME LIMIT
(hours)
2.8
6.25GND
140
BATT
TSEL
CONNECTION
MIN
ON/OFF TIME
(IN TOP-OFF)
(ms)
70
280
75
Off
55
FAST-
CHARGE
TIME LIMIT
(minutes)
Table 3. Timer Option (TSEL) Definitions
1C
<1C
1.5C
RECOMMENDED
CHARGE RATE
1/128
ON/OFF DUTY
CYCLE
FOR DONE
INDICATION
1/256
1/64
