MAX8856
USB/AC Adapter, Li+ Linear Battery Charger
with Integrated 50m
Ω
Battery Switch in TDFN
_______________________________________________________________________________________ 9
V
L
Internal Voltage Regulator
The MAX8856 linear charger contains an internal linear
regulator to supply the power for the IC. Bypass V
L
to
GND with a 0.1µF ceramic capacitor. V
L
is regulated to
3.3V whenever the input voltage is above the battery
voltage and can source up to 500µA for external loads.
CHG
Charge Indicator Output
CHG is an open-drain output that indicates charger sta-
tus and can be used with an LED. CHG goes low during
charging. CHG goes high impedance when V
BAT
equals
4.2V and the charging current drops below 50mA, or
when the MAX8856 is in suspend mode. When the
MAX8856 is used in conjunction with a microprocessor
(µP), connect a pullup resistor between CHG and the
logic I/O voltage to indicate charge status to the µP.
Soft-Start
To prevent input transients, the rate of change of the
charge current is limited when the charger is turned on
or changes its current compliance. It takes approxi-
mately 1ms for the charger to go from 0mA to the maxi-
mum fast-charge current.
THM Input
The MAX8856 monitors the battery temperature with an
external NTC thermistor that is in close thermal contact
with the battery. Select a thermistor resistance that is
47kΩ at +25°C and has a beta of 3400 Kelvins. The IC
compares the voltage at THM to an internal reference
voltage generated from V
L
and suspends charging
when it is greater than 0.542 x V
L
or less than 0.225 x
V
L
. Connect THM to GND to disable the temperature
control function.
GATE Input and SYS Output
The MAX8856 contains a SYS output that delivers up to
1A
RMS
at 3.5V to 4.2V to power an external system.
Bypass SYS to GND with a 4.7µF or larger ceramic
capacitor. When V
BAT
exceeds 3.5V or when the
MAX8856 is in suspend mode, and the GATE signal is
low, the MAX8856 internally connects SYS to BAT
through a 50mΩ switch. When the GATE signal is dri-
ven HIGH, the SYS to BAT switch is held open, regard-
less of operating condition. See Tables 1 and 2 for
more information.
When charging a battery, the load on SYS is serviced first
and the remaining available current goes to charge the
battery. SYS is connected to BAT when V
IN
is not valid.
POK
The MAX8856 contains an open-drain POK output that
goes low when a valid input source is detected at IN. A
valid input source is one whose voltage is between 4V
and 5.8V and exceeds the battery voltage by 250mV.
After a valid input has been established, charging is
sustained with inputs as low as 3.5V as long as the
input voltage remains above the battery voltage by at
least 55mV. POK is high impedance otherwise.
Applications Information
Charge-Current Selection
For USB applications, the charging current is internally
limited to 100mA or 500mA. For wall-cube applications
requiring a different current requirement, set the charg-
ing current with an external resistor from SETI to GND
(R
SETI
). Calculate R
SETI
as follows:
R
SETI
= 8000 x 2.1V/(I
BAT
+ I
SYS
)
where EN1 = high and EN2 = low.
The SETI input also enables the user to monitor the
charging current. Under fast-charge operation, the
SETI voltage regulates to 1.4V (EN1 low and EN2 high)
or 2.1V (EN1 high and EN2 low). As the charging cur-
rent decreases, V
SETI
decreases. This is due to either
the thermal regulation control or voltage regulation con-
trol (4.2V) of the MAX8856. V
SETI
is calculated using
the following equation:
V
SETI
= (I
BAT
+ I
SYS
) x R
SETI
/8000
MAX8856
