LDOs that can handle lots of noise?

You have to tell us the frequency range and amplitude of your noise and the frequency range where you need your power to be 'clean.' Pretty much all regulators do just fine at surpressing low frequency noise and get worse and worse with frequency; at some point it's going to necessarily be your bypass capacitor that surpress any remaining noise. The spec you want to look at is the "ripple rejection" on the data sheets for the part in question; something like a 7805 have about 50dB ripple rejection at 100kHz, so you your input has a 1 volt p-p 100kHz sine wave on it, your output will have 3.16mV 100kHz sine wave riding on it. In a digital application, this is pretty much nothing, whereas with some highly sensitive op-amp circuitry this could easily be catastrophic.

---Joel Kolstad

In general, LDO's have poorer high frequency regulation that is poorer than the follower type (high drop) regulators. You should be thinking in terms of filtering the raw supply to remove the noise, and use the regulator just to dump the average excess voltage.

John Popelish wrote in news:qKadnVZ0BYrhDSDeRVn- snipped-for-privacy@adelphia.com:

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filtering

As Google seems to be having fun losing my posts I'm posting this for the 3rd time through my good ol newsgroup server.

Anyways what I left out from my first post was my reason for thinking that filtering the LDO output would be best. The load on the 6V line is probabaly around 2A or so. Thus to smooth that I would think that I would need a very large capacitor. But since I only need 70ma to power the more sensitive device, I was thinking I could use a LDO with a high ripple rejection (I was thinking about the Linear ILC7083 with 85db at

1khz, see: and then filtering the 5v output of the LDO. That way there'd only be a 70ma load on the filter caps, as opposed to over 2A. Does that make sense? And does that chip seem like a good choice, or are there better ones for the job? Price is not a major concern, if it matters, though boardspace most definitely is.

Thanks,

-Mike

Well the raw supply will have maybe 2A or so typically (on average - it will not be even close to steady) being drawn from it - so I thought it might be easier to filter the 5V regulated output as it will be so much smaller and require much smaller caps (as it will be more along the lines of

Hi - I thought I had replied but it doesn't seem to be showing up...

Anyways - I figured if I filtered the raw supply voltage (6V) that the much larger load (about 2A) on the 6V line would pretty much make any reasonably sized filter capacitors pointless.

My thought was that if I were to isolate the supply voltage by using a

5V LDO I could then filter the 5V output voltage much more easily as the load on it would be much less. Is this not so?

By the way, I am looking at the Linear ILC7083

which boasts 85db ripple rejection at 1khz, which I believe is quite good. (though I could be mistaken)

Thanks,

-Mike

M. Noone wrote: (snip)

I wasn't suggesting that you filter the 6 volt line feeding the motors, only the low current 6 volt line feeding the regulator.

The LDO is stable only with a very limited range of loads. I usually end up using a filter made of 1 microfared cap and a 1 ohm resistor in series. I would have to check the data sheet to see if that is appropriate for this one.

I would look into a series inductor between the 6 volt supply line and the input of the regulator and a few hundred microfareds between the regulator input and ground. If you are using surface mount devices, you might look at something like the 1 mHy CDRH127/LDNP-102MC (about $2 from Digikey) and a 470 uf @ 10 v, .15 ohm ESR EEE-FC1A471P (about $09.16 from Digikey). These will remove almost all the high frequency noise at the input of the regulator without lowering the average voltage very much. Then the regulator will provide very clean, regulated 5 volts.