AAT2556
Battery Charger and Step-Down
Converter for Portable Applications
Application Information
Normal
Set Resistor
Ω
ICHARGE (mA)
Value R2 (k )
Soft Start / Enable
500
400
300
250
200
150
100
50
3.24
4.12
5.36
6.49
8.06
10.7
16.2
31.6
38.3
53.6
78.7
105
The EN_BAT pin is internally pulled down. When
pulled to a logic high level, the battery charger is
enabled. When left open or pulled to a logic low
level, the battery charger is shut down and forced
into the sleep state. Charging will be halted regard-
less of the battery voltage or charging state. When
it is re-enabled, the charge control circuit will auto-
matically reset and resume charging functions with
the appropriate charging mode based on the bat-
tery charge state and measured cell voltage from
the BAT pin.
40
30
20
15
The step-down converter features a soft start that
limits the inrush current and eliminates output volt-
age overshoot during startup. The circuit is
designed to increase the inductor current limit in
discrete steps when the input voltage or enable
input is applied. Typical start up time is 100µs.
Table 1: RSET Values.
1000
100
10
Pulling EN_BUCK to logic low forces the converter
in a low power, non-switching state, and it con-
sumes less than 1µA of quiescent current.
Connecting it to logic high enables the converter
and resumes normal operation.
1
1
10
100
1000
Adapter or USB Power Input
RSET (kΩ)
Constant current charge levels up to 500mA may
be programmed by the user when powered from a
sufficient input power source. The battery charger
will operate from the adapter input over a 4.0V to
6.5V range. The constant current fast charge cur-
rent for the adapter input is set by the RSET resistor
connected between ISET and ground. Refer to
Table 1 for recommended RSET values for a desired
constant current charge level.
Figure 2: Constant Charging Current
vs. Set Resistor Values.
Charge Status Output
The AAT2556 provides battery charge status via a
status pin. This pin is internally connected to an N-
channel open drain MOSFET, which can be used to
drive an external LED. The status pin can indicate
several conditions, as shown in Table 2.
Programming Charge Current
The fast charge constant current charge level is
user programmed with a set resistor placed
between the ISET pin and ground. The accuracy of
the fast charge, as well as the preconditioning trick-
le charge current, is dominated by the tolerance of
the set resistor used. For this reason, a 1% toler-
ance metal film resistor is recommended for the set
resistor function. Fast charge constant current lev-
els from 15mA to 500mA may be set by selecting
the appropriate resistor value from Table 1.
Event Description
Status
No battery charging activity
Battery charging via adapter
or USB port
OFF
ON
Charging completed
OFF
Table 2: LED Status Indicator.
16
2556.2006.09.1.2
AAT [ ADVANCED ANALOG TECHNOLOGY, INC. ]
