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BQ76952: Question about BQ76952

Jimmy Huang
Genius 3525 points
Part Number: BQ76952

Hello：

I have a question for you. The recommended circuits of BQ76952 include the following:

D36 is used in the figure. Our current test situation is: when the temperature of D36 rises, the leakage current increases, which will pull down the voltage of the DSG, thus reducing the driving voltage of the MOS tube, so that its internal resistance will also increase, which further leads to serious heating. Such a positive cycle will affect the use of the whole board.

Therefore, I would like to ask you the following questions:

1. What is the function of this diode?

2. Can this diode be replaced by 1N4148 and other switching diodes, because the temperature of these diodes will not affect the leakage current?

3. Can this diode not be installed?

  • 0
    TI__Mastermind 42060 points

    Hello Jimmy,

    1. What is the function of this diode?
    This diode is part of the reverse-polarity charger protection circuitry. D36 is placed to carry the current during a reverse-charger condition instead of the DSG pin. There is more in-depth explanation on this in our Multiple FETs with the BQ769x2 application note under Section 2 Reverse Charge Circuit and Section 3 Reverse Charge Component Selection.

    2. Can this diode be replaced by 1N4148 and other switching diodes, because the temperature of these diodes will not affect the leakage current?
    Yes, it could be replaced with other diodes, the Schottky was to minimize the power loss.

    3. Can this diode not be installed?
    Yes, this is not required to be installed and can be left unplaced.

    Best Regards,

    Luis Hernandez Salomon

  • 0 in reply to Luis H. S.
    Genius 3525 points

    Hello Luis:

    Thanks for your fast reply!

    And I read from the document, and know that the function of this diode. It's protect the DSG to avoid violating the ABS MAX limit. And which is VSS-0.3V. Schottoky has less voltage drop, and for a normal switch diode, it's about 0.5V. And you told me that this doide can been replaced by a 1N4148, or even no needed. So which means it's safe to the DSG pin even under special condition?

  • 0 in reply to Jimmy Huang
    TI__Mastermind 42060 points

    Hello Jimmy,

    You are correct! This seems to be one of the reasons the author chose to use a Schottky diode instead. I forgot to think about this. Is the leakage current a big concern? If using a normal diode I am concerned that the current would begin flowing through internal paths and exceed the ABS max of the DSG pin. The paths can be seen in Figure 3-1. Reverse Charge Current Paths of the app note I shared previously. You can see that if D3 is not placed or if it is not a Schottky, it could be possible for current to flow through Vss and internal diodes out of the DSG pin.

    Is the leakage a big concern? This should be at most a few micro-amps.

    Having a diode here is only recommended if you believe there is a risk of having a reverse-charger condition, if there is no risk, these do not to be placed at all. The diode will carry the reverse-charger current instead of the pin. 

    Best Regards,

    Luis Hernandez Salomon

  • 0 in reply to Luis H. S.
    Genius 3525 points

    Hello Luis：

        I'm not sure if the leakage was a big concern or not. I think it's better to be well protected if the user connect a charger reversed. In which condition, a diode should be placed to protect the DSG pin. But a schottky diode can make some driver issue, so we have to add a other kind of diode.

        We need the revere protect function, but we don't want the driver issue.

  • 0 in reply to Jimmy Huang
    TI__Expert 8526 points

    Hi Jimmy,

    Unfortunately, all of our engineers will be out for the holidays.  Our apologies for the delay, and we will get back to you following the vacation!

    Best,

    Andria

  • 0 in reply to Jimmy Huang
    TI__Mastermind 42060 points

    Hello Jimmy,

    The Schottky should not cause driver issues, the driver would not be affected by it.

    Could you specify what is the concern there?

    Best Regards,

    Luis Hernandez Salomon