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Old 22nd March 2006, 02:26 AM   #1
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Default Ripple rejection caps - multiple?

I was reading through the spec sheet of the LM1085 linear regulator and thought about what they said regarding improving ripple rejection. No surprise, the value of the cap on the bypass pin would have to change depending on the input ripple frequency. I started wondering if using a number of paralleled caps - say 47uF, .22uF, and then .10uF - to "cover" a broader frequency range with the improved ripple rejection would actually work. It may be a dumb thought, but let me know if it is.

Hmm, in looking at the graph it shows 200uF for frequencies below 60Hz, and 25uF for those above 60Hz. Does that mean that if I paralleled these two together I'd get good rejection through the audio band?
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Old 22nd March 2006, 03:14 AM   #2
lndm is offline lndm  Australia
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It's not dumb.

In theory, a capacitor that will work at one frequency, will work at all higher frequencies and what you suggest would not be necessary. But, in practice larger capacitors do not function well at high frequencies, and bypassing with smaller values can help.

IMO, only use quality capacitors (both) when paralleling values, and check your reg for oscillation after adding caps.
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Old 22nd March 2006, 06:43 PM   #3
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Thanks lndm, that's what I suspected. I'll have to see if I can find any more info on the LT1085 that might help in understanding when it would go into oscillation. Unfortunately I don't have a scope! There wouldn't happen to be another way to check for oscillation?
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Old 23rd March 2006, 08:26 AM   #4
AndrewT is offline AndrewT  Scotland
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the cap on the bypass pin is usually of small value, you quoted 25uF to 200uF.

These sort of values can be LESR and use the highest quality to suit the value of your circuit.

There should be no glitches appearing at the bypass pins, I think this is a voltage reference input terminal, therefore there is no need for a ceramic to minimise the glitch on the input voltage.

The input and output pins are a completely different ballgame. Glitches on the rails every time the output current steps from one value to another.
regards Andrew T.
Sent from my desktop computer using a keyboard
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Old 23rd March 2006, 06:58 PM   #5
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Thanks Andrew. I will be posting in the PS forum for thoughts on the design I intend to use. Not rocket science, but I'm sure I'll be able to use the help.
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