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TPS74801: BIAS pin voltage is different before and after enable signal turns high

user4979989
Intellectual 2800 points
Part Number: TPS74801

Hi team,

May I have your help to check below question on TPS74801 BIAS pin voltage?

Customer pulls up BIAS pin to 12V by a 4.22k+4.7k ohm divider which makes BIAS pin equals to 6.3V.

This is already violated our spec in datasheet and I have already asked customer to modify.

But the question I would like to check is, why the BIAS voltage on the scope is different before and after enable signal turns high? (from below waveform, the BIAS from 6.2V to 4V after enable turns high, once enable turns low the BIAS will become 6.2V)

Thanks.

Regards,

Allen

  • 0
    TI__Mastermind 43630 points

    Hi Allen,

    The problem is that the VBIAS pin current could be as high as 2mA:

    The current in the divider path is only 12/(4.7k+4.22k) or 1.3mA. So when you enable the device, the Bias pi current changes the divider. To make it so this does not happen, you can try to swamp out this leakage by lowering the values of resistors, but the leakage may not be acceptable to the end customer. Another approach might be to use a resistor and Zener.

    If you were to use something like the TPS7A24 to drive the VBIAS pin, it would be more accurate with less leakage.

    I hope this answers your question.

    Regards,