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Original question:

[FAQ] BQ76952: Do the DCHG and DDSG signals on the BQ769x2 follow the CHG and DSG FET driver pin states?

BQ76952: Does DDSG and DCHG also reflect the FET_CONTROL subcommand?

René Lalla
Prodigy 20 points
Part Number: BQ76952

Tool/software:

Hey community,

I am using an external driver for the main switch (FETs) for my application and want to control it via the DDSG and DCHG signals. I am not using the internal high-side NFET drivers. My question is when does the DDSG/DCHG signals an active state? 
There is the subcommand FET_CONTROL which can be used to enable or disable each FET individually. When using the FET_CONTROL to disable the discharge FET, will this also set the DDSG signal to active? 

The documentation says the following about FET_CONTROL: "An 8-bit field is sent with bits 3:0 matching those in 0x7F FET Status(). When a bit is set using this subcommand, the corresponding FET is blocked from being enabled. This subcommand should not be used if the DDSG or DCHG pins are being used in DDSG or DCHG mode." 

What would be the best way to achieve the behaviour of manually controlling the CHG and DSG behaviour (by also keeping the body diode protection)? 

According to https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/963339/bq76952-need-clarification-on-ddsg-and-dchg-operation one side affect would be, that the by turning off the DSG FET, the CHG FET would also be off for up to 250ms. Is this the only side affect? 

Another question is related to the body diode protection. While using the internal NFET drivers, BQ is able to use the body diode protection. Is this also the case when I am using the DDSG and DCHG mode to drive the external main switch?

Thank you!

  • 0
    TI__Mastermind 18005 points

    Hello Rene,

    Whether or not the DDSG signal will be active will depend on what the pin is configured to. If they are configured to DDSG and DCHG functionality, they should be active when the CHG/DSG drive is disabled. Section 6.7 DDSG and DCHG Pin Operation in the BQ76952 Technical Reference Manual can tell you more about the different functionalities available.

    When using FET_CONTROL() to disable the discharge FET, the DDSG signal will become active. The forum post you found should explain why it is not recommended.

    Section 5.2.2 High-Side NFET Drivers can tell you more about the different ways to control the internal NFETs to your requirements. There is a ‘Partially Autonomous’ and ‘Manual Control’ mode that may be what you are looking for.

    Yes, even if you are using the DDSG and DCHG mode to drive the external main switch, you should be able to use the body diode protection.

    Best Regards,
    Alexis

  • 0 in reply to Alexis_H
    Prodigy 20 points

    Hi Alexis,

    thanks for your response. 

    So my DDSG / DCHG are configured in the DDSG and DCHG mode. I am using an external driver for the main switch, because this switch is controlled also by an STM32, for security reasons. This is why I am not using the internal NFET drivers. 

    My biggest concern is the body diode protection. The documentation says when using CFETOFF or DFETOFF, which disables CHG or DSG, the body diode protection is disabled. But I am using these two pins as temperature pins, anyway. 

    Before using the security approach to also drive the main switch via the STM32, I was using the internal NFET drivers. Now that I am not able to use them anymore, I found the DDSG signal mode as the only alternative - to use its signal to drive the external main switch. 

    Is it correct to assume when using the FET_CONTROL to disable e.g. the DSG, that the CHG will also be disabled for up to 250ms but after that acts normal again - and be active? 

  • +1 in reply to René Lalla
    TI__Mastermind 18005 points

    Hello Rene,

    Yes, when using FET_CONTROL to disable, e.g. the DSG, the CHG FET will turn off up to 250ms but after that, it will be active again.

    Best Regards,
    Alexis