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TPS7A21: why PSRR is related to internal OPA's bandwidth

justifice zhou
Intellectual 400 points
Part Number: TPS7A21

Hi,

I'm reading the article of Masashi Nogawa.

LDO noise examined in detail (ti.com)

In page 3 it says:

But in reality an error amplifier tends to have a very wide bandwidth, so the LDO device has very good power-supply ripple rejection (PSRR), which is another key performance parameter of high-performance LDOs.

Why wider bandwidth can result in higher PSRR of LDO?

  • 0
    TI__Mastermind 37895 points

    Hello,

    This is because at frequencies within the bandwidth of the LDO, the LDO can actually keep the output more firm at the target output voltage when noise couples from the input. Outside of the LDO bandwidth, the LDO error amp does not have any control over the output ripple, and the noise attenuation becomes dependent on the output capacitor. 

    Regards,

    Nick