• State
  • Locked Locked
  • Replies 4 replies
  • Subscribers 63 subscribers
  • Views 138 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

BQ24780S: VCC Overvoltage cycle when charger plugged in with VSYS load

James Torbett
Prodigy 10 points
Part Number: BQ24780S
Other Parts Discussed in Thread: BQ24780,

We have a design using BQ24780 where we are experiencing a strange condition on power-up. During switch on, VCC becomes higher than VIN (24V), causing the device to go into OVP protection and de-assert ACOK. The device then attempts to switch on, causing the spike again. This repeats at several hundred Hz.

The peak of VCC is observed to be around 28V.

Here is the schematic

The charge current is set to 5A, VIN is 24V and the battery is 3S. When a load of around 0.8A or more is present on VSYS when the charger is switched on, it enters this invalid condition. This happens after the AC deglitch delay, eg 1.3 seonds after the adapter is plugged in. No noise or transients are observalbe at VIN, the only place this is observed is at C11 and both sides of R7, and at VCC.

A diode from VCC to VIN (cathode at VIN) solves the issue, but this allows VBATT to be present at the charger port so is undesirable.

What could be causing VCC to go higher than VIN? It happens during the final stages of switch on of ACFET and RBFET. I can respond with oscilloscope traces of the problem in hand if required.

  • 0
    TI__Expert 7516 points

    Hello James,

    I don't see any obvious issues with your schematic. I recommend following the schematic checklist for BQ24780S 2251.BQ24780s_SchematicChecklist.pdf. I have a few questions:

    Does the strange condition only happen when the adapter is present?

    Can you check if this happen when only the battery is present?

    Can you measure VIN at the diode?

    Can you check if VCC is shorted to another pin or component?

     

    Best Regards,

    Christian Moyer

  • 0 in reply to Christian Moyer
    Prodigy 10 points

    Hi Christian

    Thanks for the prompt reply.

    The issue only happens in the following condition: when the adapter is plugged in, and there is a load of approx. >0.5A on VSYS. We notice that it happens in a transient manner in the case where there is no/minimal load on VSYS and it seems to happen as ACFET and RBFET are in the final stages of the switch-on process, perhaps when the bootstrap voltage on ACDRV goes above VCC (I haven't confirmed this yet). 

    In the case where it happens with no load, the first cycle disables ACOK and ACFET/RBFET are switched off. VCC begins to decay back to VIN. Once it passes below 25V, ACDRV is raised again and another VCC excursion happens, although the second time it is smaller and the device doesn't go into OVP shutdown.

    In the case where there is a load on VSYS, the excursion on VCC can exceed 30V, and the device seems to be stuck in the following loop:

    - ACOK asserted
    - ACDRV comes up
    - RBFET and ACFET begin to turn on
    - VCC starts to become higher than VIN (VIN=24V)
    - Once VCC > 26V, ACOK is de-asserted, ACDRV is switched off
    - RBFET and ACFET start to switch off
    - VCC begins to drop back down towards VIN
    - Once VCC < 25V, the process starts again

    Changing values of the capacitor and resistor on VCC seem to affect the slope of the decay on VCC back to VIN, suggesting there is not a lot of current involved. 

    VIN at the anode of the diode is stable at +24V and does not exhibit any spikes, in fact there is a very small (<10mV) dip in VIN when VCC goes high, suggesting maybe a charge-pump is charging or something?

    I will check to see if this still happens when the battery is not present, although I suspect this will not be the case as the existing load on VSYS will not be there.

    VCC is not shorted - we have seen this across multiple commercially-produced prototype boards.

    If the adapter voltage is reduced to around 20V, the problem goes away because the peaks of VCC are below the OVP threshold, however unfortunately in our application 

    When I am back in the lab later I will attach some scope traces.

  • 0 in reply to James Torbett
    TI__Expert 7516 points

    Hello James,

    James Torbett said:
    When I am back in the lab later I will attach some scope traces.

    Yes, that would be helpful. In the meantime, can you add a resistor before the anode side of the VIN diode. I suspect that the diode might have some internal inductance.

    Best Regards,

    Christian.

  • 0 in reply to Christian Moyer
    Prodigy 10 points

    I'll try that. I don't have access to the board for a couple of days because the software verification is being performed, but I will be able to at the end of the week.