BQ25708: ADC Never Finishes

Hi Ray,

Can you find out the operating conditions when this occurs, like input power voltage, battery voltage, charge current if charging?

Regards,

Jeff

Hi Jeff,

Good Day. Please see below the response of our customer to your reply. Thank you very much.

What I can say now is this:

Our system includes two BQ25708 and two removable batteries.

So there is an option, when only one battery is inserted that the host microcontroller is powered and the second BQ25708 is not.

In this case, when inserting the second battery host controller realizes there is a battery and starts communicating with the charger and the battery.

In this turn, it also starts the BQ25708-ADC which then fails to finish the conversion.

Today I will check these things:

I want to check the sequence of communication to the newfound BQ25708 in the above-described case, I will send you this.

I never noticed this behavior when no battery was inserted and then one was inserted, I want to find out what is different then.

These steps reliably reproduce the issue:

• Start our system by inserting one battery. This also starts host-uC which in turn initializes the charger and battery
• Insert the second battery which is also initialized along with the charger that is also found because it is now supplied
• Remove the second battery
• Insert the second battery again
• Now in the following initialization sequence, ADC does not finish

It seems that there is a remaining charge at the capacitors for Vsys that keeps the BQ25708 alive, also enabling communication to the host uC.

When shorting this capacitor after removing the battery the issue does not occur when inserting the battery again.

So it seems that a remaining supply from the Vsys side prohibits a clear reset of the device when a battery is connected.

Next, I will try to set Reg 0x32 ChargeOption3 bit RESET_REG once I see a battery removal

I will report the results. Any hints from your side are greatly welcome.

Best Regards,

Ray Vincent

Not sure if this is related. I found a similar thread. 

https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/1174705/bq25703a-adc-operation-does-not-complete

My suggestion is to try new part like BQ25720. 

Regards,

Tiger

Hi Tiger,

Good Day. Please see below the response of our customer to your reply. Thank you very much.

Different from that I have CMP_DEG set to 0b01, but PP_ICRIT is 0.

I tried to change my software so that it follows the same sequence at init after power up and after finding a newly inserted battery because I never had issues right after power up. This seems to work better but I need to do more experiments.

Also ADC not finishing did not occur every time, there were situations when everything was fine.

Regarding the recommendation for BQ25720:

What are the benefits of this one over BQ25708?

Is it hardware and software compatible with BQ25708?

If not is there a migration guide?

Best Regards,

Ray Vincent

Hi Ray,

BQ25720 is pin-to-pin compatible with BQ25708, with enhanced functions and improvements. Software update is needed since it has more registers and bits. 

The difference and comparison can be viewed here. 

https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/1032726/faq-which-buck-boost-charge-controller-should-i-select-for-my-charging-application-in-bq25700x-bq25710-bq25720-and-bq25730-families

Regards,

Tiger

Hi Tiger,

Good Day. Another query from the customer. Please see below. Thank you very much.

We are currently investigating BQ25720. There are two important questions we want to ask:

• Is there a known issue with BQ25708 as described in this case, so there is a risk of ADC not finishing? Ie the observed behavior cannot be fixed by configuration.
• Is that fixed with BQ25720?

One more question on BQ25708:

We have here one device that repeatably powers SM-bus (ie I can talk to the device from host µC) when a battery is connected, but no input (Vbus), and another one that repeatably does not. Datasheet is not quite clear on that, it talks about Vcc that is never specified. Still, the other devices I have worked with in previous revisions of our board all had SM-bus powered with battery only.

Which one is the „normal“ or reliable behavior and how is that with BQ25720? Here datasheet says SM-bus should work with battery only. Right?

Best Regards,

Ray Vincent