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Put LC filter before or after LDO?

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Put LC filter before or after LDO?

Before LDO, this reduces higher frequency "noise" before it enters the LDO.

Regulators typically have a weaker rejection response in the PSRR at higher frequencies, usually meaning a rising response vs frequency to injected "noise" at the input.


For the simple linear PSRR it would not make a difference if the filter is before or after the LDO. However for the high frequency part there can also be some demodulation effect: RF signal can be demodulated to a change in the DC voltage.  Another advantage of the filter in front of the LDO is that the regulator also compensates for drop at the filter and the resistance of the filter is thus less relevant.

To do what? -- what noise is being filtered?  What's it coming from?

To what extent? -- how much filtering is actually required?  What's the load?  Also, why drop 0.3V, is that really so important?  If so, could the 3.3V supply be turned down to suit both?  What are the tolerances?

The filter is probably fine, but I would go with an electrolytic or tantalum, or add ESR, to dampen it.  Ceramics have very low ESR, ensuring a high impedance peak where the LC resonate.


What nobody has addressed yet is a major concern: regulator stability. A capacitor is needed on the output of almost every linear regulator to provide a pole for stability. For a regulator like this, the pole is formed by RL*COUT.

When you add an LC on the output you create a double pole at the LC resonant frequency. If this happens to be before the unity gain frequency of the regulator, you get a 180 degree phase shift while you still have gain, leading to oscillation.


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