FET are the minimum drain-source breakdown voltage
and the minimum turn-on threshold voltage (V
GS
).
Power dissipation during fast charge is approximately
R
DSON
✕
I
CHG
2
, where I
CHG
is the current limit set by
the input power source. The minimum breakdown volt-
age (BV
DS
) must typically be two times the wall cube’s
open-circuit voltage. An even larger margin may be
necessary if the wall adapter has especially poor tran-
sient response. The MAX1736 can operate with input
voltages up to 22V.
BATT Capacitor
Bypass BATT with at least 1.5µF per amp of charge
current. If the battery is removed while the P-channel
MOSFET is on, a BATT voltage over 5V is quickly
sensed, and the FET is immediately turned off. In appli-
cations where the cell is removable, very large capaci-
tance values may increase transient currents when the
cell is replaced. Therefore, BATT capacitance in
excess of 100µF should be avoided. For best system
performance with large output capacitance, at least
0.47µF of the total capacitance should be low-ESR
ceramic.
C
CT
and C
GATE
Most applications will use the circuit of Figure 3 with
C
GATE
= 0.22µF, R
GATE
= 100kΩ, and C
CT
= 0.33µF.
C
GATE
, R
GATE
, and the internal 100µA pull-up and pull-
down current sources act to slow the switching of the P-
channel MOSFET. This prevents a wall cube with poor
transient response from subjecting V
IN
to excessive
voltage when the P-channel MOSFET turns off, and pre-
vents excessive current into the battery when the P-
channel MOSFET turns on. Excessive voltage at V
IN
can potentially damage the IC, input capacitor, and the
PMOS switch. Excessive current into the battery can
cause errors in the termination process of the MAX1736
(by raising the instantaneous battery voltage) and may
trip the battery’s protection circuitry.
In applications utilizing a wall cube with poor transient
response, increase the value of C
GATE
as needed to
MAX1736
SOT23, Single-Cell Li+ Battery Charger
for Current-Limited Supply
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