BQ25750: Power path control when using with TPS26750

Hello Nathan,

Thanks for being patient with this. We are swamped at the moment and we will get back to you early next week.

Best Regards,
Ethan Galloway

Hello Nathan,

I'm not too familiar with the TPS26750's POWER_PATH_EN signal. I would recommend using the TPS26750's POWER_PATH_EN signal over the BQ25750 though because I think this will simplify the application. The TPS26750's POWER_PATH_EN function would be similar to the BQ25750's ACFETs.

For the customer's application, do they need the BATFETs of the BQ25750?

Also, do you know what the customer's battery voltage is?

Best Regards,
Ethan Galloway

Hi Ethan,

Thank you for getting back to me on this matter.

The BATFET control was the primary motivation to move from the BQ25756 to the BQ25750.  I had considered using the available GPIO on the TPS26750 to control the common source FETs to disconnect the battery.

We use two batteries, both comprised of the same 3.6V Li-ion cells.  One is a 8S pack and the other is a 4S2P pack.

Since I am building out an evaluation board to compare against two other existing topologies, I decided to evaluate the difference between the two control options.  However I am still interested in your feedback.

Thanks!

Nathan

Hello Nathan,

Thanks.

I think the BQ25750's BATFETs will work well for the 4S2P pack.

The BQ25750's BATFETs might struggle with the 8S pack. You might have to use a dedicated ideal diode controller or the BQ25750's automatic reverse mode to connect and disconnect the battery pack.

We also have an FAQ for the BQ25750's power path switchover here: e2e.ti.com/.../faq-bq25750-what-do-i-need-to-know-about-the-bq25750-power-path-switchover

Best Regards,
Ethan Galloway

Thank you for the feedback and insight, Ethan.

I intended to utilize the auto reverse mode for adapter power removal.  Is there any further documentation or details around the timings of the auto reverse mode?

The BQ25750 documentation also suggests a capacitor between BATDRV and BATSRC to increase the turn on time which should reduce in-rush.  Is this an advisable addition for the 8S pack instance?

Thanks

Nathan

Hello Nathan,

Nathan Weiss said:

Is there any further documentation or details around the timings of the auto reverse mode?

It takes about ≈50µs for auto reverse mode to trigger and start working.

Nathan Weiss said:

The BQ25750 documentation also suggests a capacitor between BATDRV and BATSRC to increase the turn on time which should reduce in-rush.  Is this an advisable addition for the 8S pack instance

If you are using auto-reverse mode, I recommend to depopulate the BATFETs.

Best Regards,
Ethan Galloway

Hi Ethan,

Thanks.

So without the BATFETs populated, power is supplied by a connected adapter and the battery is charged through the buck-boost.  Then when the adapter is removed, the system enters auto-reverse and is powered via the battery through the buck-boost in reverse?

Edit to add:  the device with the 8S pack delivers up to about 20A for 250ms

- Nathan

Hello Nathan,

Thanks for the new information.

Nathan Weiss said:

Then when the adapter is removed, the system enters auto-reverse and is powered via the battery through the buck-boost in reverse?

Yes, this is correct.

Best Regards,
Ethan Galloway

Okay great.  Then to bring this full circle, what advantage am I getting by using the BQ25750 over the BQ25756 if I'm relying on the TPS26750 for Power Path Enable instead of the ACFETs and forgoing the BATFETs?  The only added feature would be the system voltage monitor, but the VAC pin is regulated in reverse mode.  It looks like the only obstacle would be ensuring the DRV_SUP voltage is within its operating window.

Hello Nathan,

Thanks for being patient with this. I will get back to you later this week on this question.

Best Regards,
Ethan Galloway

Thank you, Ethan.  I think my questions have been resolved (for the time being!)