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I am trying to choose a low dropout linear regulator to partially smooth the output of a dual rail SMPS I'm using to power analog circuits. I'm confused about the difference between the specified RMS noise and the PSRR of LDOs. Here is an example from a datasheet I have been looking at. enter image description here

If my SMPS were to have 10 mV @ 1MHz of ripple on the positive output, and I used the device detailed above, should I expect .8uVrms of noise on the output, or would the SMPS ripple be attenuated by 76 dB and appear on the output. The device I'm taking these numbers from is the LT3045, datasheet here.

Also, I understand that the specifications listed in the features section of the datasheet are best-case device parameters, and that a much closer inspection of this LDOs documentation would be required to effectively utilize it. I don't plan on using this particular chip, as I'm just looking for information on how to use these figures found in all LDO datasheets.

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  • \$\begingroup\$ Given many analog circuits have ZERO PSRR (a common emitter gain stage) at DC and at high frequencies, and many opamps have POOR PSRR at high frequencies, I'd assist your LDO by pre-filtering the raw (trash-laden) voltage with an L+C lowpassfilter. 1uH and 1uF provides a 160KHz F3dB. \$\endgroup\$ – analogsystemsrf May 24 at 5:07
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The noise is internal to the LDO and is always on the output whether you power it directly from an SMPS, a battery, or the most perfect noiseless voltage source.

The PSRR is how well the LDO can stop ripple injected into its input from passing straight through to its output. Really, it's how fast the control loop inside the LDO can react to adjust its output to account for changes on the input.

So in your case, you would expect the output to have the SMPS ripple supressed by 76dB with the noise on top of it.

If you powered the LDO directly from a battery, you would only see the noise on the output. No ripple.

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