Deciding about maximum ripple voltage

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Dear Freaks,

Imagine you need to design a power converter for an aplication. What rules are there to decide the maximum ripple voltage the converter should present?
I've always seen 50mV as a usual value, but I actually can't find an explanation for it. And, 50mV certainly isn't the same safety margem for all output voltages.
If I wanted to power an AVR (think "no external devices" and "no ADC" or analogic stuff), I guess I would use the freq versus Vcc specs or minimum input high voltage, what is more strict. And maybe take into consideration and EMI that could be present in the system's production environment.

Is any "vcc stability" spec in the AVR datasheet that has escaped me?
What do you think about "ripple voltage" and what criteria do you use to set a maximum value for it?

Thanks!!!....

Embedded Dreams
One day, knowledge will replace money.

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What frequency?
switche mode?
what voltage in and out?
what current out?

I think a 50mV ripple at 50Hz is not that problematic as at 10000Hz.
I think of the AVR oscillator jitter...
Also take care of input threshold levels...

Klaus
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Look at: www.megausb.de (German)
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You generallly won't see a specification for ripple. It depends on the application.

For example, if you use a PWM output as a "DAC",. any ripple on the processor Vcc input will appear in the PWM amplitude, also. Ripple there, especially if it is in the filter bandwidth, could be very bad. But, if you were simply building a real-time clock, you could pretty much ignore it.

AVRs, themselves, care very little so long as the supply voltage stays within the specified range for that processor.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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MegaUSBFreak wrote:
What frequency?
switche mode?
what voltage in and out?
what current out?
Is that important? I was looking for understanding, general guidelines, theories, formulas...
MegaUSBFreak wrote:
I think a 50mV ripple at 50Hz is not that problematic as at 10000Hz.
Why?

But I see mega and ka7, the limitation is really only the normal spec limits and application.

Thanks :)!

Embedded Dreams
One day, knowledge will replace money.

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Why?

* Because higher frequencies at same amplitude means higher energy.
* Within every electronic device you have capacitive coupling. Higer frequency means higher current = higher influence
* With higher frequncies you have higher rise/and falltimes.
* every capacitor on VCC causes current on the GND line. This means that the GND Level is not the same on every place of your circuit.

SMPS:
Much higher frequency. peaks in the (current) waveform causing higher overtone frequencies.

Klaus
********************************
Look at: www.megausb.de (German)
********************************