BQ24780S: Charger doesn't disconnect battery from system when charging

Leonard Gagnon

Part Number: BQ24780S

Hi, we are using the BQ24780s in a design with a 4 cells battery. Most of our boards works fine, but one of them doesn't .When we connect the AC, the charger connects it to the system, but does not disconnect the battery from the system. We use a 19 V supply with the 16.8V battery. If the battery is around 14 V, BATSRC is at 14 V and BATDRV is at 20 V, everything is normal. When we connect the AC, the BATSRC goes up to 18.4 V and BATDRV to 24.4 V. On a working board, both BATSRC and BATDRV are equal to the battery voltage.

Any idea on what can cause this kind of behaviour?

Regards,
Leonard

over 7 years ago

0 Jing Zou over 7 years ago

TI__Mastermind 20570 points

Hi Leonard,

Based on your description, looks like the IC thinks that it is still in the supplement mode which is the reason keeps BATFET turned on. The internal comparitor monitors ACN and SRN voltage for entering and exiting supplement mode. When ACN falls such that ACN-SRN is 200mV, the BATFET is turned on. Therefore, there is two possible reasons.

1. IC does not detect an input. -->What is the status of ACOK pin?
2. IC detects ACN - SRN is < 200mV. --> please measure the voltage at the IC ACN and SRN pin.