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BQ76952: Host controlled pre-discharge

Baptiste Bainier
Prodigy 135 points
Part Number: BQ76952


Hi, 

We are trying to use the pre-discharge of the chip. Everything works as expected with both voltage or timeout conditions to exit pre-discharge. 

Our problem is that, due to our application, we can't use the voltage condition to exit pre-discharge, because our pre-dicharge end delta is higher than 2.5V.

We don't want to use the timeout either, in case the charge is drawing too much current and the bus doesn't rise to an acceptable voltage (which would lead to unacceptable current rush).

Ideally, we would like to start pre-discharge normally, and check (with the host) the delta between Stack and LD, and then send a command that manually ends pre-discharge (to enable the discharge FET).

Currently we use the timeout condition, but abort predischarge with FETOFF command before timeout if the conditions are not met. This is not ideal in case we have to abort and the host MCU is busy with a higher priority task. 

This workaround is not very clean and I'm wondering if there is a better and safer way to achieve the same result ?

Best regards

  • 0
    TI__Guru* 81035 points

    Hi,

    This is a challenging case. It seems your present approach may be best. Other options that come to mind also have their disadvantages:

    • Maybe the LD voltage measurement can be scaled by adjusting LD Gain in calibration to allow you to detect the needed Predischarge Stop Delta. However, this would affect the LD voltage measurement in general, so you would need to carefully review any other protections or measurements using the LD pin voltage to understand the impact.
    • The PDSG FET can be manually controlled using the FET Test mode subcommands (see Table 5-7 of the TRM), but while using FET Test mode (by setting FET_EN to 0) the device will not be able to automatically disable FETs in response to a protection event. This still also has the same drawback in that it heavily relies on the MCU.

    Best regards,

    Matt