Problem with CHG MOSFET when exiting hibernate mode (BQ7791501)

Martin R.

Part Number: BQ77915

Tool/software:

Hello,

I need help quickly.

I have a battery pack in which I use the Hibernate mode. If the pack is not connected to the charger or application, the connection between PRES and Pack+ is not given and therefore the battery is in Hibernate-Mode.

Now the problem: If the battery is disconnected during the charging process at 4.00~4.05V/cell (85% SOC) and thus goes into hibernate mode, the MOSFETs turn off - it's correct. As soon as the IC leave the Hibernate-Mode by connecting the battery to the charger and return to normal mode, the CHG MOSFET is still locked and only the DSG MOSFET turned on. However, none of the errors from 9.3.10 are present.

The BQ7791501 is used.

It is a stacked schematic and fault comes from the top bq7791501.

I also can not see any peak at exiting from Hibernate mode on CHG Gate.
It is not a single Happening, it happens on all batteries with this schematic.

Do you have any explanation for that?

6 months ago

0 Luis H. S. 6 months ago

TI__Mastermind 42060 points

Hello Martin,

I will get back to you by tomorrow. Sorry for the wait!

Best Regards,

Luis Hernandez Salomon

0 Martin R. 6 months ago in reply to Luis H. S.

Intellectual 335 points

I have done further tests and noticed the following:

4,075V/cell -> exit hibernate: CHG=OFF; DSG=ON
4,025V/cell -> exit hibernate: CHG=ON; DSG=ON
4,06V/cell -> exit hibernate: CHG=OFF; DSG=ON

COV Prot. = 4,25V
COV Recv. = 4,05V

When exiting hibernate mode, the bq77915 goes through something like a boot or initialization.

1. Could it be that a COV= true error is initialized here, which is only reset when the voltage is below 4,05V?

2. How to fix?

0 Martin R. 6 months ago in reply to Martin R.

Intellectual 335 points

It is critically important to receive a prompt answer on this matter.

0 Alexis_H 6 months ago in reply to Martin R.

TI__Genius 17755 points

Hello Martin,

Is there a chance the CTRC pin is in the DISABLED region? Could you show a waveform of the voltages for the CTRC, CTRD, CHG, and DSG Pins when the part exits HIBERNATE mode? When the CTRx pin is in the DISABLED region, the respective FET pin is off, regardless of the state of the protection circuitry.

Best Regards,
Alexis

0 Martin R. 6 months ago in reply to Alexis_H

Intellectual 335 points

As already mentioned, there is a turn-on error at high cell voltage, following the visualization:

CH1 = PRES of U2 (higher bq7791501)
CH2 = CTRC of U1 (lower bq7791501)
CH3 = CTRC of U1 (lower bq7791501)
GND = AGND (Cell1 - ; not in picture)

@4,08V/cell:

CTRC(pink) should turn on to.

@4,03V/cell:

CTRC(pink) turns on.

The lower bq77915 follows the higher bq77915 correctly and the MOSFETs gets controlled correctly.

Hopefully it helps to get to give us an prompt answer.

+1 Alexis_H 6 months ago in reply to Martin R.

TI__Genius 17755 points

Hello Martin,

Thank you for your patience.

Martin R. said:
As already mentioned, there is a turn-on error at high cell voltage

That is correct. Section 9.3.2.1 Operation in OV of the datasheet mentions that 'the device assumes an OV fault after reset and clears automatically after an OV delay if all cell voltages are below the OV threshold minus hysteresis.'

Entering and exiting HIBERNATE mode counts as a reset, and in this case the COV recovery is 4.05V. Here is an old thread that also discusses this: (+) BQ77915: CHG drive output being disabled erroneously? - Power management forum - Power management - TI E2E support forums

Best Regards,
Alexis

0 Martin R. 6 months ago in reply to Alexis_H

Intellectual 335 points

This is unfortunate to hear, as it is an inconvenient experience for the user. That's a pity.

Thanks Alexis for the clarification. Are there plans to eliminate this behavior, or is it recommended to use a different chip?

0 Alexis_H 6 months ago in reply to Martin R.

TI__Genius 17755 points

Hello Martin,

There are no current plans to eliminate this behavior, but we will take this into consideration for the future. Another part that could work for what you need, is the BQ76942. It is a 10S battery monitor that you can OTP to be a standalone device in your system.

Best Regards,
Alexis

0 Martin R. 6 months ago in reply to Alexis_H

Intellectual 335 points

Hello Alexis,

thanks for your answer. Another question has occurred. Why does this behavior exist?

0 Luis H. S. 6 months ago in reply to Martin R.

TI__Mastermind 42060 points

Hello Martin,

The designers that worked on this are unfortunately in different teams or have retired from TI. So we cannot provide any particular reason as to why this behavior exists.

Best Regards,

Luis Hernandez Salomon