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BQ25895EVM Switchover to OTG mode voltage dip.

Chris Jordan
Prodigy 40 points

Other Parts Discussed in Thread: BQ25895EVM-664

We have the EVM working well, running from a 5V power supply and charging a 10AH cell.   The product is primarily mains powered with the battery for backup. The load is connected to the PMID and GND terminals and typical power consumption is around 1 Amp.

Our problem is the 30mS delay in the boost converter starting when the mains power is removed. The voltage on the PMID output is 5V when on mains power and 5.1V when on battery power, but unfortunately dips to about 3V6 on the mains to battery changeover for around 30mS.  This is sometimes enough (particularly if the battery is not fully charged) to cause a reset on the attached load.

Is there any way around this?

Thankyou... Chris Jordan

  • 0
    TI__Mastermind 25800 points
    Hey Chris,

    Could you provide a schematic of your circuit to see how your system and our chip are being used together?


    Thanks,
    Joel H
  • 0 in reply to Joel Hernandez II
    Prodigy 40 points

    Hi Joel

    We are using an unmodified Eval board (BQ25895EVM-664).  Circuit diagram attached.  At the moment it is powered from a bench power supply giving 5.0V at the VBUS/GND (J1) terminals of the board. The load is attached to PMID/GND (J2) terminals and draws approx. one amp. continuously.  The battery (1 cell 10AH) is connected to J4 as you would expect. The charging current is set to 690mA.

    Hope this clarifies the setup... Chris

    BQ25985EVM_page14.pdf

  • 0 in reply to Chris Jordan
    TI__Guru**** 180516 points
    The IC needs this 30ms to switch the logic from charge to boost. The IC was designed originally for power bank applications where this 30ms delay was not a concern. How much power do you need during this changeover? Can you increase the size the capacitance at PMID to hold up the PMID voltage for 30ms? If not, can you add a small boost, low cost boost converter to maintain just under 5.1V (or the minimum voltage your system can handle) for 30ms?
  • 0 in reply to Jeff F
    Prodigy 40 points
    Hi Jeff, I rather suspected that was the case. The load is about 1A and can tolerate a supply dip to 4V, a rough calculation suggests that a 30,000uF capacitor should hold it up long enough. It's a bit crude but we have room and it's a cheap solution, I will give it a try next week.
    I also quite like the idea of the 'in line' boost converter, but I'm never quite sure how they behave when Vin = Vout, ie. after the supply dip?
    Thanks for the advice... Chris
  • 0 in reply to Chris Jordan
    TI__Guru**** 180516 points
    30000uF on PMID might be a problem for boost startup. We have not tested with that much capacitance. Regarding the external boost converter, if it is a nonsynch boost, meaning there is output diode instead of a FET, there should be no problem because the diode would block any reverse current. Alternatively, if you set the boost output to say 4.1V, you could design a simple circuit to turn off the boost when its output rises to 5.1V.