Why use 2 capacitor at output of voltage regulator?

[StupidDum]

Some design use 2 capacitors instead of only 1. I wonder what is the reason for it.

 

[Optikon]

Some design use 2 capacitors instead of only 1. I wonder what is the reason for it.

I'm going to make the assumption you are talking about linear regulators like the standard 78xx series.

Well usually(I stress USUALLY) greater than 1uF is required for stability (this is due to the internal design). Since this is a rather large value, electrolytic types are often used (Tantalum & aluminum being most common). Since these can have rather high ESR, transient response suffers. So to help out with that, one usually adds a smaller ceramic type to provide low output impedance to step changes in load (transient response).

So you usually need the bulk cap for stability and thats it. They will work fine with just that. But if you want to improve transient response, make sure you have a low impedance for step load changes which can easily be provided by the extra ceramic cap. The ceramic cap also help suppress high frequency differential mode noise which, will sail right through the regulator (poor rejection beyond kHz)

There are exceptions. Some newer designs don't need any output caps at all due to the internal design. And if you do need one, some newer electrolytics have such low ESR (and are good out beyond 20MHz) that your transient response will be good enough.

And for reasons I just mentioned, both caps should be placed physically close to the output for best performance. If the regulator drives a long cable, dont bother with the smaller ceramic cap cuz its not doing anything anyways due to cable inductance and will just invite ringing.

Does this help?

 

[StupidDum]

Wow! Im impressed with the your quick response.
Thanks a lot, that really help.

Another question arise, why is the transient response important to a regulator? Isn't the output remain stable for most of the time?

 

[Optikon]

Wow! Im impressed with the your quick response.
Thanks a lot, that really help.

Another question arise, why is the transient response important to a regulator? Isn't the output remain stable for most of the time?

Ideally, the output would remain stable. But in real devices, when the load demands more current quickly, the output voltage may sag and then climb back up to nominal output value. This is due to the fact that the output impedance is not zero but is a function of load current (load regulation spec) and frequency. So this is how it can happen.

Why might this be a bad thing? Well, sometimes its not. It depends on the application and what your circuits can tolerate.

If the regulator is driving LEDs that are switched in and out, it might not be bad because small, quick changes in output voltage will not affect the LED & the light output by any noticable amount.

On the other hand, say the regulator is powering an opamp. A situation might arise where the input to the opamp is higher than its power supply voltage (because it is sagging) and this could damage the opamp are cause phase reversal or a couple other nasty things.

Or, in a more extreme case, the regulator may be powering a relay that is supposed to be on at the time the voltage sags. If the sag is great enough and lasts long enough, the relay may momentarily lose power and disconnect! You can imagine all the possibilities with a failing relay.

Or the regulator might be used as a reference. And anywhere you might need stable voltage, sagging, transient response could be a problem.
I think you get the idea. It depends on your application and if you need to improve your transient response, a capacitor is one low cost way to do it.

 

[StupidDum]

Thanks for your precious information, this really helps.