Allowable power supply ripple on 3.3v line for Propellor?

cthomas

Hi all;

I assume this is somehow derivable from the Propellor spec sheet, but it's not leaping out at me.

Can anyone point out how to find it or derive it?

Thanks.

Craig

kwinn

The spec sheet says the propeller can operate from 2.7 to 3.6V so in theory it should be ok as long as the ripple does not put you outside that range. Personally I would stay within the +-5% guideline, so 165mV max at 3.3V.

cthomas

That's probably the right way to look at it. I'm embarrassed I didn't think of it.

Thanks!

Craig

Phil Pilgrim (PhiPi)

Aside from the absolute maximum peak-to-peak mentioned above, a lot depends on your app. If you're doing anything with analog I/O (e.g. sigma-delta ADC or DUTY-mode DAC), you will want to keep ripple as low as possible.

-Phil

jmg

cthomas wrote: »

That's probably the right way to look at it.

Depends what you are trying to do.
For a digital only system, 200mV of ripple would likely be tolerated.

However, for any ADC or Video drive, you will want to go a lot better than that. eg 3mV is ~10 bits equiv.
PLLs also work better with less Vcc ripple.

Peter Jakacki

kwinn wrote: »

The spec sheet says the propeller can operate from 2.7 to 3.6V so in theory it should be ok as long as the ripple does not put you outside that range. Personally I would stay within the +-5% guideline, so 165mV max at 3.3V.

If only it were that simple but that amount of ripple (at what frequency?) would indicate complete lack of regulation and also input thresholds would be changing accordingly. The real question is whether the OP is already experiencing this or is this more general as if it is their first design or something like that. Also as JMG pointed out there is the PLL and oscillator to worry about.

cthomas

Actually, I'm designing an all digital system.
I realized I greatly over designed the full-wave bridge rectified cap filtered aspect of my power supply when I saw the physical size of the capacitor I ordered.
I am driving a 5 volt regulator driving a 3.3 volt regulator so I expect that when I recalculate tomorrow, I'll realize I can get by with a much smaller capacitor after my full wave bridge.
I had to use both because I'm driving high intensity 7 segment LED's with a Maxim 7221 chip that needs 5 v.

As I'm driving through two regulators, each with a respectable ripple rejection, I'll probably be able to say something like I can tolerate around 25mv of ripple and still find I can have a much smaller filter cap after the bridge, but I'll work out numbers tomorrow.

Much thanks to everyone for their thoughts on this. Your ready and helpful advice make's this an excellent forum.

LoopyByteloose

In the early days of microcomputers, everyone was using linear power supplies with huge beer can sized capacitors. Audiophiles still do so when trying to build the perfect Class A system from whatever.

And then, when switchers became available there were immediate problems with design flaws.

All of these have been pretty well resolved, so now it is mostly an issue of just not having a switcher that is harmonically linked to a frequency dependent service.

If you must have the perfect power supply --- batteries and linear regulators prove near zero ripple. That's what a lot of audiophiles have gone to in preamps to avoid the 50/60 cycle hum issues.

If you want small and an a/c mains source - you either have to do some sort of a capacitance multiplier scheme with linear supplies or use a switcher.

It is much easier to design for a specific task oriented target than to try to build a perfect ideal system for all and everything.