BQ24450: Charge current too high

Javier Nadal

Part Number: BQ24450

I have created a design following the pre-charger circuit in the data sheet but when I power Vin the current overshoots the expected max value by over 3 times.

The battery is a 12V 5Ah SLA and the system has been calculated for 1A charging current, so the Rsense is 0.25 ohm.

The voltage seems to be regulating well so it would seem that the current limiting loop is not working properly. As soon as I plug it in the current jumps to 3.5A and stays there (at least for a few seconds until I unplug it).

I suspect the part might be faulty, but if there are any other ideas, I'm happy to try them.

Thanks.

over 7 years ago

0 Bill Johns over 7 years ago

TI__Guru* 75656 points

Do you have a scope capture of this condition, would be intersted in Vin vs output current overshoot, Vin at pin 5.
could you send a schematic?

0 Javier Nadal over 7 years ago in reply to Bill Johns

Prodigy 30 points

Hi Bill,

Thanks for your message. I'm afraid I haven't got an oscilloscope to hand to check it.

I have included a schematic and a PDF with the resistors values and the expected characteristics. Vin is straight from a desktop power supply at 18.35V without load. The current shoots straight to about 3.5A when the system is connected. The CHARG and VEXT labels in the schematic are used with a jumper as a switch.

Thanks for your help.

Resistors.pdf

0 Javier Nadal over 7 years ago in reply to Javier Nadal

Prodigy 30 points

I have measured the voltage at the sense resistor and it is almost 1V (which is quite obvious given the current is almost 4A), which would seem to indicate that the current limiting loop is not working properly. I also built a completely new circuit with new components and the behaviour is the same.

0 Bill Johns over 7 years ago in reply to Javier Nadal

TI__Guru* 75656 points

Working this problem off line

Power management

Power management forum

BQ24450: Charge current too high