MAX1501
Highly Integrated, Linear Battery Charger with
Thermal Regulation for Portable Applications
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The MAX1501 precharges the NiMH/NiCd battery with
10% of the user-programmed fast-charge current at the
start of a charge cycle. Precharge ends and fast
charge begins when the battery voltage exceeds 2.8V.
Set the fast-charge current with a resistor between SETI
and GND (see the Charge-Current Selection section).
The MAX1501 enters constant-voltage mode and
decreases the charge current when the battery voltage
reaches 4.5V.
The thermal-regulation loop limits the MAX1501 die
temperature to the value selected by the TEMP input by
reducing the charge current as necessary (see the
Thermal-Regulation Selection section). This feature pro-
tects the MAX1501 from overheating when supplying
high charge currents, or while operating from high input
voltages.
Set the top-off-current threshold with the three-state
FULLI input (see the Top-Off-Current Selection section).
RLED goes high impedance and GLED asserts low
when the top-off current threshold is reached. The
MAX1501 automatically initiates recharging when the
battery voltage drops below 4V. (The MAX1501Z does
not time out.)
No-Battery Mode
Connect CHGEN and MODE to VL to place the MAX1501
in no-battery mode. An external load can be connected to
BATT in this mode. V
BATT
regulates to 4V in no-battery
mode, regardless of the state of SELV.
The current-control loop, voltage-control loop, and
thermal-control loop all function in no-battery mode.
The loop gain of the voltage-control loop decreases to
ensure stability with no battery present. Connect a 10µF
ceramic capacitor to BATT for stability. RLED and
GLED are both high impedance in no-battery mode.
ACOK
The ACOK output asserts low when V
IN
is present, 4.2V
≤ V
IN
≤ 6.25V, and V
IN
- V
BATT
> 100mV. The ACOK
open-drain output requires an external 100kΩ pullup
resistor to an external supply voltage. The external
supply voltage must be less than 5.5V.
RLED
and
GLED
Indicators
RLED and GLED serve as visual indicators that power
is applied as well as the charge status of a battery.
RLED asserts low when a wall adapter is connected
and a battery is charging, regardless of cell chemistry.
GLED asserts low when power is applied and the bat-
tery is fully charged. Both outputs go high-impedance
in shutdown. Connect the anode of each LED to IN,
and the cathode to RLED or GLED. Table 3 summa-
rizes the behavior of RLED and GLED under normal
operating conditions. Connect pullup resistors to the µP
I/O supply when interfacing RLED and GLED with a
µP’s logic inputs.
Soft-Start
A ten-step, soft-start algorithm activates when entering
fast-charge mode. The charging current ramps up in
10% increments, 20ms per step, to the full charging
current when V
BATT
exceeds 2.8V.
Applications Information
Charge-Current Selection
Program the charging current using an external resistor
between SETI and GND. Set the charge-current resistor
with the following equation:
If V
SETI
= 1.4V, the current-control loop controls the
battery charging. If V
SETI
< 1.4V, either the voltage-
control loop or the thermal-control loop operates.
Measure the charging current by monitoring V
SETI
and
using the following equation:
.
