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TPS74901: PSRR response

Mark Guastaferro
Genius 11095 points
Part Number: TPS74901

Customer is seeing issues with PSRR performance with the TPS74901.  Voltage ripple from switching regulator feeding the LDO is too high at the output.

Conditions: Vin = 4V, Vout = 3.3V, Vbias = 5V (zener), I_out = 2A.  

Presently, there is no CSS cap on the device.  Would adding a CSS cap help PSRR? Data sheet indicates output noise improves, but the impact on PSRR is not clear.

Are there other circuit adjustments that could improve PSRR such as increasing output capacitance? 

  • 0
    TI__Genius 10200 points

    Hi Mark,

    Adding a CSS will typically reduce the LDO's voltage reference noise which can improve PSRR, since any supply ripple that reaches the reference is amplified by the control loop. However, the reference for the TPS749 is supplied by the BIAS pin, so adding a CSS would have more of an impact on Vbias PSRR compared to Vin PSRR.

    An LDO's PSRR is determined by its open loop gain. Any measures that increase this can improve PSRR, including the following:

    • Increasing headroom (Vin - Vout)
    • Reducing load current
    • Including a feedforward capacitor (CFF)

    Here is a visual reference for how much these can impact a typical LDO's PSRR:

    3644.Application Conditions Impact on PSRR and Noise.pdf

    This application note contains more information about LDO PSRR: 

    http://www.ti.com/lit/an/slyt202/slyt202.pdf

    If you do choose to add a feedforward capacitor, this application report discusses some of the pros and cons:

    http://www.ti.com/lit/an/sbva042/sbva042.pdf

    Thanks,

    Gerard