BQ25120: SYS output gets set to 1.1V and we are not sure how.

Kelly Hathaway

Part Number: BQ25120

Hi,

We are having an issue with our product using this battery IC. Somehow we get into a state were the output voltage is set to 1.1 V (below our CPU operating voltage) and our product seems bricked until we open it up and disconnect the battery to reset the BQ25120 and bring the SYS output back to the desired 1.8V. So we are wondering is there a fault or some event this part can experience which would cause the SYS output to drop to 1.1V?

We are never writing to the control register that would change the voltage from 1.8V so we are scratching our heads at this point. Any help would be appreciated.

Thanks,

over 8 years ago

0 Raheem over 8 years ago

TI__Mastermind 21590 points

Hello Kelly,
Can you please post your schematic. Also what is the load on SYS?
-Raheem

0 Kelly Hathaway over 8 years ago in reply to Raheem

Prodigy 210 points

This is all I can share of our schematic. If a TI FAE could reach out to us we could share more. The battery negative terminal is connected to ground.

Thanks,

0 Raheem over 8 years ago in reply to Kelly Hathaway

TI__Mastermind 21590 points

Kelly,
Can you please change the capacitor on SYS to 10uF and check if you still have this issue. If you do, please post scope shots of VIN, VBAT, PMID and SYS. Also what is the load on SYS?
-Raheem

0 Kelly Hathaway over 8 years ago in reply to Raheem

Prodigy 210 points

Raheem,

The load is always less than 20mA based on our design. The challenge is this issue is rare and we do not know how to repeat the problem.

Tell me, do you have any suspicions about the output capacitance being too low? Any ideas or thoughts could help.

Kelly

0 Raheem over 8 years ago in reply to Kelly Hathaway

TI__Mastermind 21590 points

Kelly,

The recommended capacitor value on SYS is 10uF.

-Raheem

0 Kelly Hathaway over 8 years ago in reply to Raheem

Prodigy 210 points

Raheem,

We added 4.7uF in parallel to on the output SYS (~9.4uF now). I do not suspect this is the reason at all for our issues, but we have added it on one device we are testing.

We think we may be having an I2C bus error with another device were the the command gets written to the BQ25120 IC to set the SYS output to 1.1V. Our MCU cannot operate at this low voltage and the GPIO pins go to Hi-Z. Because the CD pin gets pulled low the watchdog is not resetting the voltage back to 1.8V. We are able to reproduce this condition by writing to the register and forcing 1.1 V on SYS output. We are searching for the possible error on the I2C bus.

In the meantime I am going to try pulling the CD pin up to the SYS output and so it defaults to be high and hopefully trip the watchdog.

Kelly

0 Raheem over 8 years ago in reply to Kelly Hathaway

TI__Mastermind 21590 points

Kelly,
Any update on this? Did pulling CD up help with the SYS voltage?
Regards,
Raheem

0 Kelly Hathaway over 8 years ago in reply to Raheem

Prodigy 210 points

Yes, we pulled CD high with a 100k Ohm resister and wrote the register for SYS 1.1 V. As we thought, the watchdog reset the voltage to 1.8 V at about 50 seconds. We are pretty sure we are just having I2C issues.

Thanks,

Kelly

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BQ25120: SYS output gets set to 1.1V and we are not sure how.