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questions about PSRR and output noise of LDO

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chaojixin

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Hi, dear all:
the figure is captured from a datasheet of LDO. And I find that most of LDO datasheet report the PSRR of 10~1M Hz, and the noise of 100~100k Hz. why these frequency range? the higher frequency is no need to care about? or the PSRR and noise is attenuated too much at higher frequency?

thanks in advance!

LDO.JPG
 

chaojixin,
Noise on the input of the LDO that is not rejected will be present at the output and the connected circuits. If these are fast and sensitive anaolog circuits it will matter. Precision analog require normally a high PSRR and special care on the power supply generation. DC/DC converter are worst then LDO in PSRR, or create their own noise. That is why sometimes you find a combination of LDO and DC/DC conversion (you may loo at: **broken link removed** ).
 

The voltage regulator is a simple feedback control system. Analog feedback control systems have to satisfy certain criteria in order to remain stable. One such criteria is that the open loop transfer function has to have some phase margin in order to remain stable. At higher frequencies (above 1 MHz) there is too much phase shift in the op amp to maintain that phase margin, so they have to roll off the system gain. When the system gain hits 0 dB at some higher frequency, there is no longer any PSRR.

That is why you put a ceramic capacitor on the output--to reject higher frequency noise.
 

thank you! I thought it is due to application environment before...quite unexpected it is because hard to design
biff44 said:
The voltage regulator is a simple feedback control system. Analog feedback control systems have to satisfy certain criteria in order to remain stable. One such criteria is that the open loop transfer function has to have some phase margin in order to remain stable. At higher frequencies (above 1 MHz) there is too much phase shift in the op amp to maintain that phase margin, so they have to roll off the system gain. When the system gain hits 0 dB at some higher frequency, there is no longer any PSRR.

That is why you put a ceramic capacitor on the output--to reject higher frequency noise.
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