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BQ25792: BQ25792 was charging a dead battery then shut off and now is dead

Paul Saffren
Prodigy 110 points
Part Number: BQ25792
Other Parts Discussed in Thread: TPS25750,

Tool/software:

Our system relies on USB Power Delivery to run, we cannot communicate with the BQ25792 and/or TPS25750 when the charger chip is dead.  The charger chip provides power for the whole system.   We are exclusively a power sink role. 

This morning I came in after running our project overnight to drain the battery for testing.  The battery is a 3S2P configuration.  I connected a suitable USB-C PD supply and turned the system on (enabled current draw from the PD system).  The battery charger is set to charge the battery by default and it started charging and the system came up.  Then after about 10 minutes the system shut off and I cannot restart the system at all.  Looking at the USB-PD input I can see the input voltage is at 15V (which is what we've configured the TPS25750 to deliver) but there is no input current showing. 

There is 15 volts coming out of the TPS25750 chip and it's LDO is working, however the BQ25792 shows no life whatsoever.  I do not see any voltage out of the BC_REGN pin nor any waveforms our of SW1/SW2.  I've also removed the battery and am running solely on USB-C input.  I also tried the barrel connector which we use for optional 12V input and still no joy.   I can only assume the device is dead.   

Is there any way to determine if the device is still working without trying to communicate with it?   I have no way to power the system without the PD chipset working. 

Thanks in advance for any help. 

Paul

  • 0
    TI__Guru**** 166626 points

    HI Paul,

    If the battery is charged, the battery voltage should appear at the SYS pin due to the internal battery FET's diode from BAT to SYS.  Are you using a shipFET to enter ship or shutdown mode?

    If the charger is completely unpowered, can you use an ohm meter to measure VBUS, REGN, PMID, SW1, SW2, SYS and BAT resistance to ground.  If they are all fairly high resistance, then apply power to VBUS and measure the REGN voltage on an oscope.  If V(REGN) pulses 7 times then stopds, there is a fault condition and the charger is auto entering HiZ mode.  

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Prodigy 110 points

    Hi Jeff, 

    Thanks so much for the quick response.  With the board off, completely un-powered with no battery connected I measured the resistances you specified.  Here's what I measured: 

    VSYS  = high impdance

    REGN = 150 ohms

    PMID = 40K ohms

    SW1 & SW2 = 2.4 MegOhms

    SYS = 5.8 MegOhms

    BAT = 2.4 MegOhms

    I also looked at REGN with a scope while inserting the USB-C plug and it stayed at ground, no pulses whatsoever. 

    I'm guessing that REGN is only 150 ohms is the smoking gun. 

    Paul

  • 0 in reply to Paul Saffren
    TI__Guru**** 166626 points

    HI Paul,

    Yes, REGN is damaged.  PMID to ground is also a bit low.  Do you have a 4.7uF >=10V rated ceramic capacitor on REGN that is fairly close to the either the IC REGN pin or GND and connects with no more than 1 via?  

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Prodigy 110 points

    Hi Jeff, 

    I'm looking directly at the PCB layout and the REGN pin has a 4.7 uF 35V cap to ground connected directly with no vias.  The cap is about 100 mils away from the device.   We are also using REGN to power an LED which is connected to the STAT pin.  I calculate about 20 mA are flowing through that LED.  When I looked at the data sheet I see that REGN can source a minimum of 30 mA.  Should I be worried about pulling 20 mA from REGN? 

    Kind regards, 

    Paul

  • 0 in reply to Paul Saffren
    TI__Guru**** 166626 points

    HI Paul,

    20mA from REGN is within spec.  Is the FET between VBUS and PMID when the chip is unpowered?  REGN is powered from PMID.  The only time I have seen REGN damaged is from over voltage at VBUS and therefore PMID, but usually damages VBUS and PMID too.

    Regards,
    Jeff