MAX1758
Stand-Alone, Switch-Mode
Li+ Battery Charger with Internal 28V Switch
14 ______________________________________________________________________________________
capacitor. This capacitor (between BST and LX) is
charged through a diode from VL when LX is low. An
internal N-channel MOSFET turns on momentarily after
the high-side switch turns off, pulling LX to PGND to
ensure that the bootstrap capacitor charges. The high-
side MOSFET gate is driven from BST, supplying suffi-
cient voltage to fully drive the MOSFET gate even when
its source is near the input voltage.
Timers
The MAX1758 includes safety timers to terminate
charging and to ensure that faulty batteries are not
charged indefinitely. TIMER1 and TIMER2 set the time-
out periods.
TIMER1 controls the maximum prequalification time,
maximum full-charge time, and the top-off time. TIMER2
controls the maximum fast-charge time. The timers are
set by external capacitors. The typical times of 7.5 min-
utes for prequalification, 90 minutes for full charge, 45
minutes for top-off, and 90 minutes for fast charge are
set by using a 1nF capacitor on TIMER1 and TIMER2
(Figure 1).
Charge Monitoring Outputs
FASTCHG, FULLCHG, and FAULT are open-drain out-
puts that can be used as LED drivers. FASTCHG indi-
cates the battery is being fast charged. FULLCHG
indicates the charger has completed the fast-charge
cycle (approximately 85% charge) and is operating in
voltage mode. The FASTCHG and FULLCHG outputs
can be tied together to indicate charging or done
(Figure 2). FAULT indicates the charger has detected a
charging fault and that charging has terminated. The
charger can be brought out of the FAULT condition
only by removing and reapplying the input power, or by
pulling SHDN low.
Thermistor
The intent of THM is to inhibit charging when the bat-
tery is too cold or too hot (+2.5°C ≤ T
OK
≤ +47.5°C),
using an external thermistor. THM time multiplexes two
sense currents to test for both hot and cold qualifica-
tion. The thermistor should be 10kΩ at +25°C and have
a negative temperature coefficient (NTC); the THM pin
expects 3.97kΩ at +47.5°C and 28.7kΩ at +2.5°C.
Connect the thermistor between THM and GND. If no
temperature qualification is desired, replace the ther-
mistor with a 10kΩ resistor. Thermistors by Philips/BC
components (2322-640-63103), Cornerstone Sensors
(T101D103-CA), and Fenwall Electronics (140-103LAG-
RB1) work well. The battery temperature is measured at
a 1.12Hz rate (C
TIMER1
= C
TIMER2
= 1nF). Charging
pauses briefly to allow accurate measurement.
If the temperature goes out of limits while charging is in
progress, charging will be suspended until the temper-
ature returns to within the limits. While charging is sus-
pended, the timers will also be suspended but will
continue counting from where they left off when charg-
ing resumes.
Shutdown
When SHDN is pulled low, the MAX1758 enters the
shutdown mode and charging is stopped. In shutdown,
the internal resistive voltage-divider is removed from
BATT to reduce the current drain on the battery to less
than 5µA. The high-side power MOSFET switch is off.
However, the internal linear regulator (VLO) and the ref-
erence (REF) remain on. Status outputs FASTCHG,
FULLCHG, and FAULT are high impedance. When exit-
ing the shutdown mode, the MAX1758 goes to the
power-on reset state, which resets the timers and
begins a new charge cycle.
Source Undervoltage Shutdown (Dropout)
If the voltage on DCIN drops within 100mV of the volt-
age on BATT, the charger turns off. This prevents bat-
tery discharge by the charger during low input voltage
conditions.
Design Procedure
Setting the Battery Regulation Voltage
VADJ sets the per-cell voltage limit. To set the VADJ
voltage, use a voltage-divider from REF to VADJ. A
GND-to-V
REF
change at VADJ results in a ±5% change
in the battery limit voltage. Since the full VADJ range
results in only a 10% change on the battery regulation
voltage, the resistor-divider’s accuracy need not be as
high as the output-voltage accuracy. Using 1% resis-
tors for the voltage dividers results in no more than
0.1% degradation in output-voltage accuracy. VADJ is
internally buffered so that high-value resistors can be
used. Set V
VADJ
by choosing a value less than 100kΩ
for R5 (Figure 1) from V
ADJ
to GND. The per-cell bat-
tery termination voltage is a function of the battery
chemistry and construction; thus, consult the battery
manufacturer to determine this voltage. Once the per-
CELL CELL COUNT (N)
GND 1
Float 2
REF 3
Table 2. Cell-Count Programming Table
VL 4
