BQ2002E: Determining the resistive divider on the BAT pin

Kevin Braun47

Part Number: BQ2002E
Other Parts Discussed in Thread: BQ2002

Hello,

In the BQ2002E datasheet, it states that Starting A Charge Cycle can be initiated two ways, I am interested in the 2nd method, bringing the BAT pin below Vmcv (2.0V).

I am implementing a 2 NiMH cell with a 5V source supply. I am having trouble restarting a Charge Cycle by using a small FET to pull the BAT pin to ground through the resistive divider (R14 in the SLUU007B example). I have a resistive divider on the BAT pin of 475k and 475k.

I am confused by the example schematic in SLUU007B. If you assume that you are using the "4 CELLS" divider, R9 + R14 (301k + 100k), in order to reach Vmcv(2.0V), BAT+ would need to be 8.0V. How would a 4 Cell NiMH battery ever reach that voltage?

What am I missing here?

...kevin

over 7 years ago

0 Raheem over 7 years ago

TI__Mastermind 21590 points

Hello,
Can you please provide your schematic?
Regards,
Raheem

0 Kevin Braun47 over 7 years ago in reply to Raheem

Intellectual 450 points

Raheem,

Can I provide it to you directly?

Thanks,

kevinb@socketmobile.com

...kevin

0 Raheem over 7 years ago in reply to Kevin Braun47

TI__Mastermind 21590 points

Hello,

I got your schematic. What is the battery voltage when this occurs?

Regards,

Raheem

0 Kevin Braun47 over 7 years ago in reply to Raheem

Intellectual 450 points

Hi Raheem,
I have a 2 cell 1500mAh NiMH AA setup. The charger is capable of either 400mA or 800mA under software control. If charged at 400mA, the battery will never fully charge before the BQ2002E timer times out and goes into trickle charge mode.

What I am trying to do is restart a fast charge cycle after the BQ2002E goes into trickle-charge mode. I think I'm reading in the datasheet, page 4 "Starting A Charge Cycle" Item 2., if the BAT pin falls below Vmcv (2.0V), the BQ2002E will start a new charge cycle. In my case, 2-NiMH cells fully charged would be about 3.15V. With the resistive divider I have, it would be about 1/2 that voltage, so it would never reach 2.0V to begin with.

If I read between the lines, it appears that if BAT reaches Vmcv, it shuts down the BQ2002E. So it's more like an over-voltage protection circuit.

...kevin

0 Raheem over 7 years ago in reply to Kevin Braun47

TI__Mastermind 21590 points

Hello Kevin,
Yes that is correct. The Vmcv acts as an overvoltage protection. The IC can also stop charging if the battery temperature gets outside the safe temperatures. I'll suggest charging the battery at 800mA to avoid the timer expiring if the battery voltage will never go above 2V. You can also try toggling the input voltage with a microcontroller if the LED is low for longer than the timer to restart the charge cycle.

Regards,
Raheem