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BQ24172EVM-706-15V: BQ24172EVM - Series resistor between BTST capacitor and BTST pin

Thomas CN1
Intellectual 300 points
Part Number: BQ24172EVM-706-15V
Other Parts Discussed in Thread: BQ24172

Hi,

What is the purpose of the 0 Ohms resistor R20 connected in series between the BTST pin and bootstrap capacitor in BQ24172EVM? How this resistor value is calculated if required to be used with a value other than 0 Ohms? This resistor is not seen in the typical application schematics given in data sheet.

  • 0
    TI__Guru**** 152906 points

    Hi Thomas,

    In the event you have EMI issues, this resistor can be increased to 5-10 ohms to slow down the high side FET turn on, which might be the cause of the EMI.  Frankly, a small, 0.01-0.1uF capacitor from PVCC to PGND close to the IC generally works better to minimize EMI. 

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Intellectual 300 points

    Hi Jeff,

    Thanks for the reply. 

    We have tried placing a 10 Ohms resistor here and observed a strange behavior.

    The battery voltage is boosted to around 15V and getting transferred to the input side of BQ24172 and holds the input voltage at around 15V when the 10 Ohms resistor is placed in place of R20, resulting in an effect same as the input voltage from AC adapter is being present when the AC adapter is not connected actually. The STAT pin is also being LOW indicating as the AC adapter is present and the battery is charging.

    Is this behavior observed and any workaround available for the same? Are we missing any other changes along with the resistor value change? 

    Regards,

    Thomas CN

  • 0 in reply to Thomas CN1
    TI__Guru**** 152906 points

    Hi Thomas,

    If you are seeing a voltage at the input when no adapter is present, that resistor is causing the charger to boost back.  This can happen due to the forced refresh pulse needed to keep the BTST capacitor charged.  I don't recommend using a resistor.  Is the reason you want to use that resistor because of EMI issues?

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Intellectual 300 points

    Hi Jeff,

    Thanks for the reply. 

    Yes, we were using the 10 Ohms resistor because of EMI considerations. We will go ahead with 0 Ohms resistor here and with 0.01 - 0.1 uF capacitor on PVCC rail as you suggested and finalize based on the practical observations.

    regards,

    Thomas CN