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BQ25895 Output Over Current Protection

avraka
Intellectual 855 points
Other Parts Discussed in Thread: BQ25895

Dear TIer.

In my application for powerbank using BQ25895, I have to indicate LED warning when the output on PMID tied to GND.....

I know BQ25895 doesn't hace output current limit function on it.

How can I detect this short condition in boost mode through Status Reg without additional circuitary ?

Or could you recommend right device having OCP function ?

Thanks

  • 0
    TI__Guru**** 166936 points
    If VBUS =PMID will never be more than 6V, you could add an SVS like TPS3839K50 to monitor PMID.
  • 0 in reply to Jeff F
    Intellectual 855 points
    Dear Jeff.
    Thanks for your response,
    It seems that I make you mis-understanding about PMIC short.
    What I said PMID tied to GND mean a kind of abnormal test. If the end of output cable is short to GND, Output current will be increased with following voltage drop at the PMIC ...When I tested with my customers, BQ25895 enters fault mode. In this condition, I want to know the current which lead device fault and register to indicate fault....
    Thanks
  • 0 in reply to avraka
    TI__Guru**** 166936 points

    In OTG boost mode, the typical switching cycle-by-cycle current limit is 7A.  Using an efficiency power balance, you can estimate the load on PMID that caused the cycle by cycle current limit to trip:  eff=Po/Pin=5V*IPMID/(VBAT*7A).  There is no fault register to indicate this.  If battery is discharged below VBAT_DPL, the battery FET is turned off to protect the battery from over discharge.

  • 0 in reply to Jeff F
    Intellectual 855 points
    Dear Jeff.

    Regarding to Boost mode efficiency, BQ25895 datasheet shows almost 90% of boost mode efficiency at the 3A@3.8Vbat load condition.

    I calculated power loss as below.

    Thermal loss = 3A x 5V / 90% ( = 16.7W) – 3A x 5V = 1.7W
    Heat dissipation = 34.8°C/W * 1.7W = 59.2°C , based on Junction to case temperature resistance

    Is my calculation correct?

    If yes, what I have to do reduce case temperature of BQ25895?

    My customer PCB board has not so much PCB area to dissipate temperature properly....

    I'm waiting for your advice..

    Thanks and B/R
  • 0 in reply to avraka
    TI__Guru**** 166936 points

    The calculations appear to be correct.  I suggest you review http://www.ti.com/lit/an/spra953c/spra953c.pdf in order to fully understand how RthetaJC was computed.  I don't think the value will match your application so the value is likely too high.

    Heat sinking by soldering the exposed bottom thermal pad to copper (via'd through to internal and bottom board layers) is the most effective way to remove the heat.  Next step would be to attach a heat sink to the top of the IC using thermal grease.