BQ25895: PMID drop off on unplugging causes CM5 to shutdown

Muhammad Shah

Part Number: BQ25895
Other Parts Discussed in Thread: BQ25638, TPS61088

Tool/software:

Hi!

Ive designed a custom CM5 carrier board and it seems to work well except on unplugging VBUS power in the Pi which is powered via 5V PMID shuts down. I think this is due to a short drop off as it switches to battery but is there a simple way to fix this issue? I was thinking of adding more capacitance on the PMID pin or maybe a super capacitor but im not sure what the consequences of that will otherwise be. There isnt really board space otherwise to power via an external boost converter from the SYS output so I'm stuck in that regard.

Any help would be appreciated!

Kind regards,

Muhammad

3 months ago

0 Ning T 3 months ago

TI__Guru 59111 points

Hi Muhammad,

Please refer to below.

https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/712374/bq25895-output-voltage-pmid-40ms-drop-when-poweroff-vbus/2626902#2626902

Thanks,

Ning

0 Muhammad Shah 3 months ago in reply to Ning T

Prodigy 120 points

Thanks, yes came across this earlier but is there no way to reduce the voltage drop off i.e. with a suitable sized (super)capacitor? I have attached my schematics:

The drop to battery voltage of 3.6V or lower is not adequate and still causes Pi to shutdown...

0 Ning T 3 months ago in reply to Muhammad Shah

TI__Guru 59111 points

Hi,

Referring to 8.2.5 Boost Mode Operation from Battery, The boost operation can be enabled if the conditions are valid:

1. BAT above BATLOWV

2. VBUS less than BAT+VSLEEP (in sleep mode)

3. Boost mode operation is enabled (OTG pin HIGH and OTG_CONFIG bit =1)

4. Voltage at TS (thermistor) pin is within range configured by Boost Mode Temperature Monitor as configured by BHOT and BCOLD bits

5. After 30 ms delay from boost mode enable

Condition 2. VBUS less than BAT+VSLEEP and 5. After 30 ms delay from boost mode enable are unavoidable. Please feel free to try adding capacitance and see.

Thanks,

Ning

0 Muhammad Shah 2 months ago in reply to Ning T

Prodigy 120 points

Dear Ning,

Thanks for the assistance, unfortunately these didn’t solve my issues at all. I’ll probably switch to an external boost converter but before that wanted to check, does the BQ25638 suffer from the same drop off or is it able to sustain 5V on PMID despite unplugging?

0 Juan Ospina 2 months ago in reply to Muhammad Shah

TI__Mastermind 18375 points

Hi Muhammad,

This behavior is still present on the BQ25638. A capacitor to bridge the 30mS gap would have to be large enough to sustain the voltage despite any load on the rail including the any device's quiescent current that may pull the capacitor down. An external boost might be the most robust solution for this voltage dip.

Best Regards,

Juan Ospina

0 Muhammad Shah 2 months ago in reply to Juan Ospina

Prodigy 120 points

Dear Juan,

Thanks for clarifying. I will probably go with the external boost then. Which converter would you suggest for up to a fixed 5V 3A or possibly even 5V 5A? I’ve previously used the TPS61088 but it has a relatively large footprint and is also a bit overkill.

Thanks!

0 Juan Ospina 2 months ago in reply to Muhammad Shah

TI__Mastermind 18375 points

Hi Muhammad,

My expertise is only with battery chargers. I recommend creating a new thread with a discrete Boost part number as the Boost team may be better able to assist you.

Best Regards,

Juan Ospina

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BQ25895: PMID drop off on unplugging causes CM5 to shutdown