BQ25120A: PFM to PWM transition issues

Damien Genouille

Part Number: BQ25120A
Other Parts Discussed in Thread: BQ25120

Hello TI team,

We seem to have the following issue on our design:

While being powered by battery only, when BAT input drops under 3.4V, there is a voltage drop on the BQ25120A output that leads to a reset of ouf MCU. At this point, the BQ25120A output current drawn is ~3mA.

Looking at the SW pin, here is what we can see:

1/Yellow = SYS (after the LC filter on SW)

2/Green = SW pin

It seems that when BAT input drops, the PFM switching stops for ~200µs, causing SYS output to drop, then the swithcing mode goes to PWM, stops again for ~100µs and finaly goes to PFM.

The voltage drop is enough to reset our MCU.

Here is a snapshot of our design:

Our inductance is a Murata LQM18FN2R2M00D (0603 2.2µH 120mA 400mR, datasheet), which has a higher DCR than the recommended inductance listed in the datasheet.

Is there a way to disable the automatic change in switching mode from PFM to PWM?

What can I do to prevent the switching from stopping at this point? Should I change the inductor so that the PFM-to-PWM switching point is shifted? If so, what's the guideline?

Thank you for your help,

Best regards,

Damien

over 6 years ago

0 Gautham over 6 years ago

TI__Genius 12485 points

Damien,

Can you provide the VBAT and PMID pin plots when this happens? Have you tried lowering VBATUVLO setting as a test to see if that was getting tripped?

regards,

gautham

0 Damien Genouille over 6 years ago in reply to Gautham

Intellectual 365 points

Hello Gautham,

Here are VBAT & PMID plots along with SYS:

FYI, VBATUVLO threshold is set to 3.0V. There seems to be a chance for it to be triggered when VBAT drops, however the I think the issue comes from the SYS drop happening shortly before and causing the MCU to reset (acceptable minimum supply voltage from the MCU datasheet is 1.7V).

Do you have any idea why SYS starts dropping at t = ~ -50µs ?

Thank you

0 Gautham over 6 years ago in reply to Damien Genouille

TI__Genius 12485 points

Damien,

I am checking on this one. The transition should be seamless. What is your load step when you do so? (in mA)

Do you see a similar behavior when you have the same load step on the TI EVM?

regards,
Gautham

0 Damien Genouille over 6 years ago in reply to Gautham

Intellectual 365 points

Here is the SYS current measurement:

Zoomed:

The current oscillation seems to occur only after the SYS deregulation, and seems fairly stable before it happens.

Also one more thing I noticed: here is a larger view, showing the repetition of the issue every ~15.5s (VBAT set at 3.38V from bench power supply) :

0 Gautham over 6 years ago in reply to Damien Genouille

TI__Genius 12485 points

Damien,

I am reviewing your data.

0 Gautham over 6 years ago in reply to Gautham

TI__Genius 12485 points

Damien,

I sent you an email related to this. thanks.

0 Damien Genouille over 6 years ago in reply to Gautham

Intellectual 365 points

Gautham,

As mentionned by email, and for anyone having the same issue, we were using the BQ25120 datasheet instead of the BQ25120A.
The register settings for BAT_UVLO have changed a lot between those versions, and we were writing into some reserved bits, causing the BQ to reset all its register, which was messing with our firmware. Updating this has solved our problem.
Thanks for your support,
Best regards

Power management

Power management forum

BQ25120A: PFM to PWM transition issues