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BQ24195: Body Diode Between Vbus and PMID

Russell Wetherley1
Expert 4255 points
Part Number: BQ24195

Hello E2E,

On the block diagram on page 11, between Vbus and PMID there would be a conducting body diode if Vbus is greater than let's say 6V, but the part is rated for 17Vin. How does PMID work if we have a voltage of greater than 6V on Vbus?  Is there a condition of operation for PMID across Vbus input?

Thanks!

Russell

  • 0
    TI__Mastermind 25800 points

    Hey Russel,

    I am having a bit of difficulty understanding your question.

    The body diode of the Q1 FET is always there, regardless of the voltage applied on VBUS. When a VBUS voltage is applied, the immediate voltage on PMID is a diode drop below VBUS. Once a valid source is verified by internal logic, the Q1 FET turns fully on, and the PMID voltage is then a function of the Rdson of the FET and IBUS.

    Now, whether that's at 6V or 17V is somewhat inconsequential. To your comment about rating, the absolute max rating of the individual VBUS and PMID pins is 22V, but the recommended operating max is 17V to see normal behavior. 

    And from a condition of operation standpoint, Q1 turns fully if the part is not in HIZ mode, and VBUS is above the UVLO threshold and below the OVP threshold.

    Regards,
    Joel H

  • 0 in reply to Joel Hernandez II
    TI__Expert 4255 points
    Hi Joel,
    Thanks for the quick response. In that last sentence, do you mean "Q1 turns fully ON if...?"

    So at startup or when VBUS is applied, PMID will be equal to VBUS - Vbody_diode, correct?

    After startup, Q1 will will fully turn on, and there will be an IR drop across the fully turned on FET. But then how does PMID regulate to 5.1V? Is it only regulating to this when VBUS is below 5.1V?

    Russell
  • 0 in reply to Russell Wetherley1
    TI__Mastermind 25800 points
    Hey Russell,

    That is correct, fully ON.

    At the very very beginning, PMID = VBUS - V_Q1body. However, once our internal comparators verify the voltage is within the voltage range, Q1 should turn fully on; then as you said, PMID = VBUS - I*Rdson_Q1.

    In terms of regulation of PMID, this only occurs when operating in OTG (boost) mode. This is when only the battery is connected and we are driving the internal converter essentially in reverse to power PMID. This means the battery (VBAT) is the input to the boost converter and PMID is it's regulated output.


    Regards,
    Joel H