Advanced Analog Technology, Inc.

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Page 10 of 16

V 1.0

AAT8636 Series

OPERATION (CONT.)

3. COM2 (Over Discharge Detector)

COM2 monitors the cell voltage at the

DD

V

pin. When

DD

V

drops below the over discharge threshold

1

D

V

during the

discharging process, COM1 detects an over charging condition and after delay time (

1

D

t

) elapses, the

OUT

D

pin falls to a low

level and thus turning off the external discharge control N-channel MOSFET. As soon as this happens, the VN pin will be changed

to high level by an internal PMOS and the AAT8636 series will enter the standby mode.

Connecting the battery pack to a charger is the required condition for AAT8636 series to recover from standby mode. With the

charger connected, the charge current can flow through the parasitic diode of the external discharge control N-channel MOSFET

while

DD

V

is still lower than the over-discharge threshold. Once

DD

V

is charged up to a value higher than

1

D

V, the

OUT

D

pin

goes to a high level. This leads to the “ON” state of both the charge and discharge control MOSFETs, and the charging process

proceeds on as long as the charger is connected. Connecting a charger to the battery pack when

DD

V

is already higher than

1

D

V

will make

OUT

D

instantaneously high

.

4. COM3 (Over Current Detector, Short Circuit Protector)

The over current detector and short circuit protector function normally when both the charge and discharge control N-channel

MOSFETs are “ON”.

In standard operation, voltage on VN pin will be either slightly higher or lower than GND, depending on the impedance of the two

MOSFETs. However, when the VN pin voltage rises to a value between the short circuit detection voltage

short

V

and over current

detection voltage

OC

V

, the AAT8636 series detect an over current condition. As a result, the external discharge control N-channel

MOSFET goes to “OFF” with

OUT

D

pin at the low state. An output delay time for the over current detection is internally fixed. A

quick recovery of VN from a value between

short

V

and

OC

V

to the normal value within the delay time keeps the discharge control

FET staying at the “ON” state.

Short circuit may as well be explained as a “serious” over current condition. When the VN pin voltage rises to a value not only

higher than

OC

V

but also higher than

short

V

, the short circuit protector will shut down the discharge MOSFET immediately to

protect battery pack from being damaged or exploding.

The VN pin has a built-in pull down resistor. Thus once the factors that cause over current or short circuit is removed, the VN pin

will be pulled down to ground level through the internal resistor. Therefore, with VN lower than the over current and short circuit

protection threshold,

OUT

D

will be turned back on and the AAT8636 series can continue normal charge or discharge operation.

When over current or short circuit occurs, the AAT8636 series will not enter a standby mode.