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BQ76952: Do DDSG DCHG pins work with predischarge mode?

Mathew Morrow
Prodigy 35 points
Part Number: BQ76952
Other Parts Discussed in Thread: BQSTUDIO,

For our application we are not using the built in high side CHG DSG pins to drive FETS. We are instead hoping to use the TTL DDSG and DCHG pins to drive an isolated high side gate driver circuit to handle +100amp 48V fast switching FETs.

DCHG and DDSG pins configured to their respective modes using REG1=3.3V. They work as expected when predischarge is disabled and FETs turn On/Off when faults are present and cleared.

The issue we are seeing is that the DDSG pin turns on immediately and does not follow the configured predischarge timeout or voltage stop delta. 

Checking all register with BQStudio and the predischarge settings are correctly configured. I am testing with a large capacitive load that takes about 1 seconds to charge with the precharge circuit when DDSG removed.

Do the DDSG DCHG pins exactly follow the DSG and CHG pins?

Or, do the DDSG and DCHG pins act as a fault indicator pin for DSG and CHG ie. the DDSG pin will go high when nothing is blocking the DSG pin despite the precharge circuit still needs to activate?

  • 0
    TI__Guru* 81035 points

    Hi Mathew,

    DDSG and DCHG will follow the DSG and CHG pins. Section 8 of this application report might be helpful if you have not already seen it: https://www.ti.com/lit/an/sluaa84a/sluaa84a.pdf

    Something seems wrong if your DDSG pin is turning on immediately without a delay. How are you enabling the discharge - is this through a command?

    Regards,

    Matt

  • 0 in reply to Matt Sunna
    Prodigy 35 points

    Hey Matt,

    Thank you for your response.

    After reading the low-side FET app note it appears that the DDSG and DCHG pins do not follow the DSH and CHG pins.

    "The DDSG and DCHG outputs from the BQ769x2 combine the pre and main FET drive signals. If the designer wants precharge and or predischarge features, options include controlling the precharge or predischarge path from the host and using logic to inhibit the main FET driver when the presignal is applied"

    "PDSG and PCHG are included in the DDSG and DCHG timing, if the user wants precharge and predischarge controlled by the BQ76952 some work must be done to separate the FET controls."

    This is unfortunate and is an odd design decision for the DDSG/DCHG pins. Very much narrows down the full functional applications to the on chip high side drivers which are not very powerful.

  • 0 in reply to Mathew Morrow
    TI__Guru* 81035 points

    Hi Mathew,

    Yes, you're correct about the behavior - it's been a while since I've looked at this. 

    Matt