Input Voltage from Current-Limited Voltage Source (22V max). Bypass to GND with a 0.1µF
capacitor. The charging current is set by the current limit of the external power supply.
Gate Drive for External PMOS Pass Element. The PMOS device should have a V
Logic-Level Enable Input. Pull low to disable the MAX1736. EN is internally pulled up to V
Charge Time Control. Sets the minimum on-time, minimum off-time, and the IN detection interval.
Cell Voltage Monitor Input, Precharge Current Output, and MAX1736 Power Source. Connect BATT
to the positive terminal of a single Li+ cell. Bypass BATT with a capacitor to GND (1.5µF per amp of
charge current).
Detailed Description
The MAX1736 provides a simple, safe, low-cost method
of charging a single-cell Li+ battery with nearly no heat
generation. Combined with a current-limited voltage
source, the MAX1736 provides precharge, fast-charge,
and top-off-charge capabilities. After constant-current
fast charge, top-off safely finishes charging the battery
by pulse-width modulating charge current. The top-off
on-time is kept below the electrochemical time constant
of the cell. The key advantage of this method is that the
charge circuit is small and generates minimal heat
while providing a safe method of charging to ensure
maximum cell life. Figure 1 shows the MAX1736 func-
tional diagram.
Precharge
To protect Li+ cells from damage that may occur if fast
charged from a dead state, the MAX1736 precharges the
Li+ cell with 6mA at the start of a charging cycle when the
cell voltage is below 2.5V. As soon as the cell voltage
reaches 2.5V, the MAX1736 begins fast charging.
Fast Charge
In fast-charge mode, the MAX1736 turns on the exter-
nal P-channel MOSFET. Charging current is set by the
current limit of the external supply; current is not regu-
lated by the MAX1736. The P-channel MOSFET is
used only as a switch, not as a linear regulator.
Therefore, the circuit’s power dissipation is minimized,
permitting rapid charge cycles with almost no heat
generation. The external power supply should have a
specified current limit that matches the desired fast-
charge current for the Li+ cell.
With the P-channel MOSFET on, V
IN
will be nearly
equal to V
BATT
. To detect that an input supply is con-
nected, the MAX1736 periodically turns the P-channel
MOSFET off and checks the voltage at IN. During fast
charge, this occurs once every input detection interval
(20s with C
CT
= 0.33µF). During pulsed top-off, input
detection occurs more frequently and is continuous
when the MOSFET is off (see Selecting External
Components).
Pulsed Top-Off
When the battery approaches full charge, its instanta-
neous voltage reaches the BATT regulation voltage and
pulsed top-off begins. The MAX1736 uses a hysteretic
algorithm with a minimum on- and off-time. Cell voltage
is sampled with no charging current to minimize errors
due to battery and cell protection resistance.
If the voltage is below the BATT regulation voltage, the
P-channel MOSFET switches on for a minimum on-time.
If, at the end of the minimum on-time, the cell voltage is
still below the BATT regulation voltage, the switch
remains on until the cell voltage reaches the BATT reg-
ulation voltage. At that point, the P-channel MOSFET
then switches off for at least the minimum off-time. The
minimum on-time is set by CT and should be set below
the electrochemical time constant of the cell. A C
CT
value of 0.33µF sets a minimum on-time of 165ms,
which is adequate for most Li+ batteries.
