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TPS763: Output Impedance of TPS763 when EN is low, Vin is low

JMMN
Guru 60865 points
Part Number: TPS763

Hi,

What is the output impedance of the TPS763 when EN is low or Vin is low (in my application both are tied together).  Would it be the same as the output impedance in dropout?

Thanks,

JMMN

  • 0
    TI__Genius 10200 points

    Hi,

    I assume you are referring to the DC output impedance. The TPS763 output impedance when Vin and EN are 0 V will not be the same as in dropout. In dropout, the internal error amplifier drives the pass FET into the triode region where it acts as a resistor. This resistance, Rdson, is much smaller than the parallel feedback resistor network, so the output impedance is Rdson in parallel with the load resistor. 

    When Vin and EN are 0 V, the LDO's internal circuitry is off and the pass FET is in a high impedance state. As such, the output impedance is determined by the feedback resistors and the load resistor if the output voltage is low. However, the TPS763 has a PMOS pass FET. P channel devices have a parasitic body diode that will conduct current in the reverse direction (OUT to IN) if the output voltage exceeds the input voltage by about 0.7 V. If this is the case, the output impedance will be very small (Ron of the parasitic diode).

    If you wish to know about the high frequency output impedance of an LDO, the following application report has more information in Section 8: Frequency Response.

    www.ti.com/.../slva072.pdf

    Thanks,

    Gerard