[sdiy] Power input for 2xD / generic digital module [was: SMD part availability, manufacture]

Seb Francis seb at burnit.co.uk
Thu Nov 13 12:10:22 CET 2008

John Luciani wrote:
> On Wed, Nov 12, 2008 at 7:31 PM, Seb Francis <seb at burnit.co.uk> wrote:
>> For the power supply regulators, I've been looking at low dropout ICs and
>> these seem pretty cool.  Am I really understanding it right that they can
>> work with a output-input of down to like 0.2V.  This would mean only a tiny
>> amount of heat dissipation so no need for any heatsinking.  Are there any
>> disadvantages to these compared to standard linear regulators which need
>> typically 2.5V output-input differential?
> Be careful when calculating the power dissipation. You need to take the ripple
> voltage into account. The trough may be at 0.2V but the peak could be much
> higher. The big LDO advantage is that you can size your filter caps a
> lot smaller
> since your ripple voltage can be 2.5V larger.

Good point!  And anyway I guess it would need a really accurate input 
voltage to ensure minimum power dissipation.

Which brings me to another thought on power.  In making a generic 
digital module, what is the most useful power arrangement for people.  
Do many people have +5V available inside their modulars?

Is it useful to have the option that the whole board can run off only 
+/-12V or +/-15V?

It would need clean +/-12/15V power anyway for the buffers and to be 
regulated down to +5V for the analog side of the CODEC.

But also a power input of +5V would be useful, to be regulated down to 
+3.3V to run the dsPIC and digital side of the CODEC (and hopefully also 
the SRAM if I can track down a suitable and available 3.3V IC).

The same regulator could be used to get +3.3V from +5V or from +12/15V, 
but obviously in the latter case the heat dissipation would be much 
more, which probably restricts me to using a thru-hole TO-220 package.  
Might be that I will use 2 regulators - one for pure digital, and one 
for the digital side of the CODEC.

Any thoughts?


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