Advanced Analog Technology, Inc.
AAT8636 Series
OPERATION
1. COM1 (Over-Charge Detector)
COM1 is the monitor of the voltage at the V
DD
pin. COM1 senses an over charging condition and if this condition continues
longer than the delay time ( t
C1
), when the voltage at V
DD
exceeds the over charge detector threshold V
C1
. Under this condition,
C
OUT
pin falls to a low level and thus turns off the external charge control N-channel MOSFET. An internal level shifter is
incorporated in the buffer driver at the C
OUT
pin to drive the low level of C
OUT
to be equal to the VN pin voltage. Furthermore,
the high level of C
OUT
is set to be equivalent to V
DD
.
The over charge delay time t
C1
of over charging is determined by the external capacitor C
3
connected between the GND and
C
ext
pin. The delay time t
C1
can be estimated as the following:
C
3
x
(
V
DD
−
0.7
)
0.48 x 10
−
6
t
C1
=
However, if the duration of the over charge condition is not long enough to cover the time delay window, C
OUT
will not change to
the low level and the charging process continues.
2. Required Conditions to Reset COM1 from Over Charging
After the MOSFET, the charge control, is turned off, COM1 can be reset under two conditions. As resetting COM1means C
OUT
returns to high level again and the charging path is reconnected to restart charging process, one of the conditions to reset COM1
is when
V
DD
falls to lower than “ V
C1
−
V
HYS
” as a result of battery cell internal discharge. Disconnecting the battery pack from
the charger can also reset COM1 even when
V
DD
is still within the hysteresis window ( V
C1
−
V
HYS
≤
V
DD
<
V
C1
).
After the battery pack is disconnected from the charger, connecting a system load to the battery pack allows current to flow
through the parasitic diode of external charge control MOSFET. This discharging current will bring V
DD
down eventually and
once
V
DD
drops below V
C1
, C
OUT
will change to high level
.
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–
–
Advanced Analog Technology, Inc
.
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Page 9 of 16
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AAT [ ADVANCED ANALOG TECHNOLOGY, INC. ]
