Ripple Rejection

[Brian Hoskins]

Hi everyone,

I've been assigned with the task of designing a project to suit a particular specification. I won't bother going into the details of the project, but part of the specifications says that the power supply must be regulated and have less than 100mV ripple.

With that in mind I've been looking at the datasheet for a few linear voltage regulators. They quote a Ripple Rejection ratio which is expressed in dB. From the specification I know that a maximum of 100mV ripple is permitted on the output side of the voltage regulator, and I hoped to be able to use information in the datasheet to calculate how much ripple would be permitted on the input side of the regulator such that it's rejection capabilities would leave less than 100mV ripple on the output side. The Ripple Rejection ratio is expressed in dBs and I'm at a loss as to how I use this figure to my advantage to be honest.
If I knew how much ripple I was permitted on the input side of the regulator, I could work out the size of the resevoir capacitors on the output of the bridge rectifier, to make sure the ripple applied to the input of the regulator results in less than 100mV ripple on it's output. Does that make sense?

I've done a bit of googling but I haven't found it too helpful to be honest. I'm not expecting you guys to google for me - if you have some suggestions to mind then I'd be happy if you could put them forward for me, and other than that I'll see if I can find any examples of power supply design where ripple limits are taken into consideration.

Brian

 

[Nigel Goodwin]

Assuming you're using 78xx regulators, the ripple will be well below 100mV - build it and measure it! - ensure you do so with the entire load connected and working, otherwise you're just wasting your time.

 

[eng1]

With that in mind I've been looking at the datasheet for a few linear voltage regulators. They quote a Ripple Rejection ratio which is expressed in dB. From the specification I know that a maximum of 100mV ripple is permitted on the output side of the voltage regulator, and I hoped to be able to use information in the datasheet to calculate how much ripple would be permitted on the input side of the regulator such that it's rejection capabilities would leave less than 100mV ripple on the output side. The Ripple Rejection ratio is expressed in dBs and I'm at a loss as to how I use this figure to my advantage to be honest.

The basic equation for the PSRR in dB is:
PSRR = 20 log (Vri / Vro)
Vri = Ripple Input Voltage
Vro = Ripple Output Voltage

If PSRR = 80 dB, for example, this means that the input ripple voltage is attenuated by 10^(80/20) = 10^4 = 10000

 

[Brian Hoskins]

So there'd have to be a HUGE amount of ripple on the input side of the regulator in order to see 100mV ripple on the output side.

Ok, that's helpful thanks both.

Brian

 

[Torben]

So there'd have to be a HUGE amount of ripple on the input side of the regulator in order to see 100mV ripple on the output side.

Yep, but it does depend on how much current you're pulling through it and what the input caps look like. What are your voltage/current requirements?


Torben

 

[Roff]

And keep in mind that the valleys of your input ripple must not go below the headroom requirements of the regulator (about 3V for a 78XX).

 

[Papabravo]

Also note that ripple at the output may not be related to ripple on the input. Some voltage regulators have stability problems with some types of loads. If the output does become unstable then that will introduce a new class of problems. Be especially careful with LDO (Low Dropout) regulators. Your mileage may vary, so read the datasheet very carefully.