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TPS74801: Vbias Question

Part Number: TPS74801

Hi Expert,

Customer used TPS74801 with 3.3Vin and other 5Vbias power from 12V using resistor divider. However, there is no output and measured Vbias is only 0.6V.  They have tried to use 5Vsupply directly for BIAS and it works ok. Would you please have a look and suggest what might goes wrong? 



  • Hi Expert,

    May I know what's the BIAS internal impedance? It seems it's current limit at BIAS cause once customer decrease the resistor divider, it works. 



  • Hi Allan,

    I'm reviewing your question and will get back to you within 2 days with further comments.


    - Stephen

  • Hi Alan,

    The BIAS pin current is 1 mA typical, 2 mA maximum according to the Electrical Characteristics table in the datasheet.  You will need to buffer the voltage going into the BIAS pin or connect it to a voltage converter.  The BIAS pin is an actual power rail that is required to operate internal circuitry; it is not a high impedance node like an EN pin or PG pin.  The BIAS pin is usually the positive power rail to the internal OP-AMP.

    The BIAS pin current in the datasheet is measured at Vbias = 5V.  If you wish to model this as an impedance to RTN, the effective impedance in this test condition is 5V / 1mA = 5 kiloohms.


    - Stephen