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Original question:

BQ76942: MOSFET Driver

BQ76942: PCHG Drive

Sean Chan1
Prodigy 150 points
Part Number: BQ76942
Other Parts Discussed in Thread: BQSTUDIO

Hi Matt,

I am trying to turn on PCHG mosfet drive, but fail.

The setting is as below:

1, I turn on all fets. AFE_ALL_FETS_ON();

2, I turn off the PDSG fet and CHG fet . AFE_FET_Control(0x06);

The CHG fet can be turn off, but the PCHG fet is kept off.

How to turn on PCHG fet?

Thank you for your help.

  • 0
    TI__Guru 54715 points

    Hi Sean,

    You might want to be sure the precharge path will switch using the PCHGTEST command while FET_EN is low.  That way you know the circuit is functional.

    With FET_EN set high, the FETs will come on when conditions are suitable.  Sleep is commonly enabled with the charge path off during sleep. The precharge voltage is set to 0 by default to disable the feature.

    When the charge path is on, the voltage of the cells will determine whether the main charge path is enabled (CHG) or the precharge path (PCHG).  See the BQ76942 technical reference manual section Settings:FET:Precharge Start Voltage, and 13.3.6.4 Settings:FET:Precharge Stop Voltage.

    So to see PCHG switch with the All FETs commands:

    Enable FET_EN

    Set Precharge Start Voltage and Precharge Stop Voltage above the cell voltage (Or the cell voltage below the Precharge Start Voltage level.

    Disable sleep

    Send ALL_FETS_OFF and ALL_FETS_ON alternately to observe switching.

  • 0 in reply to WM5295
    Prodigy 150 points

    Hi Matt,

    Why the pchg mosfet driver only turn on for  only several seconds then automatically turn off?

  • 0 in reply to Sean Chan1
    Prodigy 150 points

    Hi Matt,

    I use the PCHG driver  to drive a high side PMOS fet. My setting is as below:

    If I want to enable this driver, I will set config update mode and set the start voltage 4180mV the end voltage 4300mV. And exit the config update mode. Then turn on all fets and set fet control = 0x06.

    The pchg fet driver will be turn on just for several seconds and automatically turn off. I don't know why.

    If I want to disable this driver, I will set config update mode and set the start voltage 0mV the end voltage 0mV. And exit the config update mode. Then turn on all fets and set fet control = 0x0A.

    Please help me to figure out my mistake. I don't know where the wrong setting lives.

    Thank you very much.

  • 0 in reply to Sean Chan1
    TI__Guru 54715 points

    Hi Sean,

    The PCHG will turn on only a few seconds and turn off if the part goes to sleep with the default charge settings.  If there is no current it will go to sleep and if the FET settings are default the charge path is turned off in SLEEP.  

    The config update mode will clear the sleep disable command, so the part will revert to sleep enable mode.  You can likely see the FET toggle in the registers pane of BQStudio. on:

    off:

    The part controls the charge path with the FET commands, but the FET used in the charge path is selected by the voltage.  If the voltage is above the Precharge Stop voltage the charge FET is used, if the voltage is below the Precharge Start voltage the precharge FET is used.

  • 0 in reply to WM5295
    Prodigy 150 points

    Many thanks for you. It does help me a lot.

    So, only if I turn off the sleep enable mode, it will work, right?

  • +1 in reply to Sean Chan1
    TI__Guru 54715 points

    Hi Sean,

    Yes, it should work.

    Remember, you can only turn on FETs when the part finds no protection restriction.  For the charge path PCHG vs CHG is selected by voltage.  For the discharge path PDSG vs DSG is selected by time.