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BQ25895: connect Vbus and PMID together

Howard Zou
Genius 12365 points
Part Number: BQ25895

Hi,

my customer is wondering how to use the OTG when they have only one USB port for both device(charged by adaptor) and host(charge other devices).

1. Since they have only one mechanical port, they have to connect Vbus and PMID together. Is it okay to do that?

2. If they make the boost circuit for OTG outside the device and 5V output connected to Vbus, it will make BQ25895 start to charge again. So they have to use the OTG circuit inside the device, right?

3. Q1 diode is from Vbus to PMID, so it can only block the voltage from PMID to Vbus, but not Vbus to PMID, right? So why we need to block the voltage from PMID to Vbus?

  • 0
    TI__Genius 10240 points
    Hi Howard,

    1. The Vbus and PMID can't be connected together. The Q1 FET serves as the reverse blocking FET for safety and the input current sensing.

    2. External circuit to generate the 5V connected to the VBUS can be used. The charging can be disabled by either setting REG03[4]=0 or pulling /CE high, but the 5V still powers the system. If you want to isolate the external 5V from the charger, the REG00[7] can be set to enable HIZ mode.

    3. You are correct. As I commented on 1, Q1 FET serves as the reverse blocking FET for safety to prevent the current from the battery to the Vbus if Vbus is shorted to GND.
  • 0 in reply to ezhao
    TI__Genius 12365 points
    1. The charging can be disabled by either setting REG03[4]=0 or pulling /CE high, but the 5V still powers the system.
    So by either setting REG03[4]=0 or pulling /CE high, the BATFET will be turned off but the buck converter is still working, right?
    So how about REG09[BATFET_DIS]=1, is it the same function?

    2. In HiZ mode, the buck converter will stop working, but BATFET will be on, right?
  • 0 in reply to Howard Zou
    TI__Mastermind 25800 points
    Hey Howard,

    You are correct for both of these points.

    1) BATFET_DIS will disable charging and also prevent current from discharging to SYS when the adapter is not present.

    2) Yes, the BATFET by default should still be on.


    Regards,
    Joel H
  • 0 in reply to Joel Hernandez II
    TI__Genius 12365 points
    Joel,
    so setting REG03[4]=0 or pulling /CE high will only block the current from SYS to BAT, not the other way around.
    BATFET_DIS will block the current in either way, right?
    May I know how BATFET is consist? Is it 2 MOSFETs connected together back to back?
  • 0 in reply to Howard Zou
    TI__Genius 10240 points
    Howard,

    You are correct that disable charging only not allow the current form SYS to BAT, not BAT to SYS; disabling BATFET will prevent the current flowing of either direction. You can consider the BATFET equivalent to 2 back-to-back MOSFET, so there is no body diode overall.