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BQ25895: Schottky diode between SW and PMID

anishp
Prodigy 60 points
Part Number: BQ25895

Hi,

The datasheet for the BQ25895 shows 2 application notes, one without the schottky diode between SW and PMID (Fig 47) and one with it (Fig 60). What is the purpose of having this schottky diode? Is it only required for high current applications and can be left out for lower current use cases? (That seems to be the only difference I see between the the two applications) Our application would use <1A from the boost.

Secondly, for a separate application where we do not intend on using the boost mode, is it okay to leave PMID floating? Do I need to add the capacitors on this line? If so, whats the minimum capacitance needed? Is it okay to leave the diode out in this case?

Also, I noticed that there is a different part - BQ25895M which is pin compatible with the BQ25895. They both look to be the exact same parts. What is the difference between the two?

Thanks!

-Anish

  • 0
    TI__Guru**** 166626 points

    Hi Anish,

    The schottky diode is only necessary if you want maximum output current in boost mode.

    PMID is the input to the buck converter and still requires at least a 10 uF capacitor.  I also recommend a small 0.1uF decoupling capacitor close to the IC.

    The main difference between the M and non-M is the default VBATREG=4.35V for the M version.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Prodigy 60 points

    Hi Jeff,

    Thanks for the prompt reply! Did you mean VBUS is the input to the buck converter? I thought PMID was the output of the boost? To clarify,  you're recommending the 10uF and 0.1uF on the unused PMID line?

    Thanks,

    Anish

  • 0 in reply to anishp
    TI__Guru**** 166626 points

    Hi Anish,

    VBUS is the input to the chip.  There is reverse blocking FET and input current measuring FET between VBUS and PMID. To be clear, that means that the voltage applied to VBUS will be seen at PMID in buck charge mode. The buck side FET connects to PMID so it is the input to the buck converter and also the output of the boost converter, when the buck converter runs in reverse as boost converter.

    Regards, 

    Jeff