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BQ25792: BQ25792 SYS query

Star Xu
Genius 9645 points
Part Number: BQ25792
Other Parts Discussed in Thread: BQ25798

Hi

The customer design the schematic and then supply voltage to VBUS find the SYS output voltage unstable.

Attached the schematic and waveform for your reference.

3323.BQ25792.pdf1830.waveform.xlsx

If use battery(Vbat＞3.5V) or VBUS + Vbat (Vbat＞3.5V,REG00 default setting) ,SYS output voltage is stable.

Why only supply VBUS the output voltage unstable?

Waiting for your reply.

Thanks

Star

  • 0
    TI__Guru*** 143910 points

    Hi Star,

    The BQ25792 was designed expecting that when the battery is removed either charge would be disabled by CE bit/pin or the removed thermistor would cause a TS fault and disable charge.  With VBUS applied and no battery, the charge current turns on fast, causing a BATOVP fault, which then cause a SYSOVP fault.  The faults then cause the converter to stop.  Adding a large capacitance at BAT (470uF) usually prevents the OVP event. 

    The soon-to-release BQ25798 with same pin out changes the timing of the faults so that the converter does not shut down.

    Regards,

    Jeff