BQ25756EEVM: BQ25756EEVM

Thanks.

I have a design completed that uses a

bidirectional power switch, TPS26633, TUSB320LA, BQ25756 and a BQ7427. There is a lot going on. Is there someone who might review this?

Dan

Hello Dante,

I can review your schematic for the BQ25756.

The USB-PD team can review the schematic for the TPS26633 and TUSB320LA.

The battery gauge team, BMS-BGP, can review the schematic for the BQ7427.

Can you ask a new question for the review of each part's schematic? That will keep the threads cleaner.

Best Regards,
Ethan Galloway

Hi Ethan,

So where should I post the schematic?

To this thread?

Do I need to post it to other forum's as well?

Dan

Hi Ethan,

Before I submit the schematic I have a question. My design has an input power switch (2 FETs) followed by an eFuse (TPS2663) and then a battery charger (BQ25756E).

The eFuse can handle -60V to +67V on its input. 

I need to prevent any negative voltage surges from getting to the battery charger and beyond.

Do you have a recommendation for how I can achieve that? (Maybe a TVS on the eFuse output/battery charger input)

Also, keep in mind the power flow is bidirectional, meaning that my board can sink power or source power.

Dan

Hello Dante,

I think a TVS diode or a schottky diode on the input might be your best choice for preventing negative voltage surges. Do you think that will work for your schematic?

Best Regards,
Ethan Galloway

Hi Ethan,

For a 1 cell design, where should I set the FB?

On the eval board is set it for 3.5V. In reality I think a 1 cell battery can be

anywhere from 3.5V to 4.2V.

Thanks,

Dan

Hi Ethan,

I think I figured out the last question, but I am having trouble getting reverse mode to work.

Can you tell me what I need to do to get it to work?

I disconnected my VIN supply, the battery is attached and I enabled reverse mode via EN_REV.

Thanks,

Dan

Hello Dante,

Thanks for being patient with the delay. I was out of office Thursday and Friday.

What's the battery voltage when you are using reverse mode and what is the reverse voltage set to? Are you seeing any switching from the charger?

Best Regards,
Ethan Galloway

It looks like the UVLO has a min of 4.2V which is not suitable for single cell designs.

I thought this part could do that?

If I put a power supply in place of the battery at set it to 5V, the reverse mode works.

In addition the DRV_SUP has to be set to >4V.

Hello Dante,

Reverse mode should be able to work when the battery voltage is above 3V. You can find the checklist for reverse mode to work in section of 8.3.6. I've posted a screenshot of the datasheet below.

What's your reverse voltage set too? I think your application should work for this.

Best Regards,
Ethan Galloway

Hi Ethan,

Here is what I found in the datasheet.

This is verified on the bench. If the battery is less than this reverse mode does not work

Also, the DRV_SUP must be >4V.

Hello Dante,

I'll get back to you tomorrow on this.

Best Regards,
Ethan Galloway

Hello Dante,

I've been really busy and I'll get back to you before the end of the week. Thanks for being patient with this.

Best Regards,
Ethan Galloway

ok, please do, I need to close this out.

Thanks,

Hi Ethan,

Based upon work I did on the bench and specs in the datasheet can you please confirm.

DRV_SUP must be > 4V

UVLO must be > 4.2V

This makes usage with a single cell Li battery problematic unless DRV_SUP and UVLO are powered from something other than

the battery, this is because the battery voltage can range from 3.5V to 4.2V

Notice DRV_SUP

Maybe you can explain what this means for reverse mode:

(Vvac_ok)

Ethan,

I just discovered when I select reverse mode on the eval kit it will not work unless the battery voltage is

over 4.2V or so.

For this test I had DRV_SUP = 5V and ACUV = 5V.

So, the reverse voltage will only turn on if the voltage at the battery terminals is 4.2V or higher.

Dan

Hello Dante,

UVLO and VVAC_OK only apply to the converter operating in forward mode.

In the lab, I'm able to measure reverse operating at a minimum voltage of 3.8V. Are you using the BQ25756E EVM for these tests?

Best Regards,
Ethan Galloway

Hi Ethan,

Thanks for the response.

Yes in reverse mode maybe 3.8V on the battery will work, unfortunately that is not good enough for a 1 cell Li battery.

That type of battery can get down to 3.5V or so and then reverse mode will not work.

I found I can boot-strap the reverse mode operation by using a diode to OR in 5V onto the battery line for 100ms or so and then disconnect it.

Comments?

Dan

Hi Ethan, I am getting a lot of pressure from my customer to complete the design and resolve open issues.

Can we continue this on an email?

Dan

I requested an email address from you to continue this discussion.

The most important thing right now is to understand why the datasheet says the battery needs to be

greater than 3V (pg 23 and many other places in the datasheet to engage reverse mode), yet on the bench

I need to set the battery voltage to >3.8V to get the eval board into reverse mode.

Is the datasheet incorrect??

Please respond ASAP.....things are getting critical.

Dan

Hello Dante,

Dante Fasano said:

Is the datasheet incorrect??

Yes, the datasheet is incorrect and we are working to correct this on the datasheet. I should have been more clear on this in my response.

Dante Fasano said:

I found I can boot-strap the reverse mode operation by using a diode to OR in 5V onto the battery line for 100ms or so and then disconnect it.

Comments?

That should work. Keep in mind that the voltage difference between the SRN and SRP pins should not be more than 0.3V.

Dante Fasano said:

Can we continue this on an email?

Yes, I'll msg you the details.

Best Regards,
Ethan Galloway