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BQ76942: Pre-Discharge Circuit Does Not Function As expected

Deniz Ilhan
Prodigy 120 points
Part Number: BQ76942

We have the pre-discharge circuit design that follows the reference design except the resistance value being 49.9R, instead of 255R (2x 510R in parallel). I had not probed this line before and we have been keeping our short circuit protection threshold rather high as we have lots of transients in our system during regular operation. Recently I have decided to better understand the pre-discharge phase. Our timeout is 600 msec and stop delta is the default value of 500mV.

When I probed the output of the battery for the first time, I observed the following:

As you can see during the pre-discharge phase of 600 msec, the voltage starts rising, and then chokes at about 5V and stabilizes around 3V.

When I started comparing designs and realized that our pre-discharge resistor is about 5 times smaller then the reference design, I ordered some 499R resistors. When I tested the system with this new value, I did get a consistent rise but it still saturated at about 3V.

Any idea why this might be happening? Here is the P-FET used for this circuit:
https://www.vishay.com/docs/68631/si4455dy.pdf

My next step is to add a second 499R resistor in parallel to get closer to the resistance the reference design has but I'm not convinced this is purely the resistance value. Something else is preventing the voltage rising over 3-4V for some reason. As a last note, our pre-charge circuit never worked properly either and we disabled it from config and relied upon the trickle charge functionality our charger has.

  • 0
    TI__Mastermind 42060 points

    Hello Deniz,

    I am assuming that yellow is battery voltage and blue is PACK+/load voltage, correct?

    This may be due to your load, what is your typical current draw for the device?

    I've seen some cases where it seems that the voltage stopped rising, this was because of voltage division between the resistance on the PDSG resistance and the resistive load current. For example, if you assume a battery voltage of ~43-V and a resistance on the PDSG path of 50-Ohms, the load voltage would clamp to ~3-V if you have a resistive load of ~4.19-Ohms or ~800-mA of load current after the load capacitance charges.

    With a 500-Ohms resistor instead on the PDSG path, the load resistance/current would be ~37.53-Ohms or ~80-mA of load current, where the load would clamp to 3-V after the load capacitance charges.

    So... I do find it somewhat strange that in both cases you tested the voltage settled at ~3-V, unless the load itself seems to settle to these currents.

    Something I do recommend trying out, is to change R23/R24 to ~5-kOhms instead. I do recall people having issues when using ~49.9-kOhms, so it may be worth trying to change these to 5-kOhms. 

    When choosing a PDSG resistance, you would want to choose a value that would allow the load voltage to settle closer to the full battery voltage.

    Best Regards,

    Luis Hernandez Salomon

  • 0 in reply to Luis H. S.
    Prodigy 120 points

    What I am realizing is we have some 12V regulators that turn on as low as at about 3V and they seem to draw enough current that the voltage can not rise above that level. When we removed the connection to those, we were able to show the system turns on as expected. Our next step will be adding some delay circuitry around those so that we will have enough time to charge the bulk capacitors in the system before they get operational. Thank you!