[sdiy] switching dc power sources

[Nils Pipenbrinck]

Not strictly synth related but...


In one of my projects I want a low impedance power supply that can
either source 3V or 1.8V. The load is kind of weak, it's a CPLD IO bank
that I'm going to power, so 20mA would fit my bill, 50mA would be better.

The voltages itself are available as well, with required current source
capabilities.

At first I thought: Just generate the voltages from my microcontroller
with a resistor divider and run that through an opamp voltage follower
to generate the current, but no: My load requires decoupling capacitors
and opamps don't like capacitive loads. Those who can cost a fortune.

Then I thought: Screw it, I'll just take a ordinary adjustable 3
terminal LDO and switch one of the voltage defining resistors. That does
not work either because LDOs can only source current, not sink any. If I
switch from 3V to 1.8V the output voltage will just stay high until I've
consumed enough energy to drop it down.


So, any idea how to buffer a reference voltage in a way that is fine
with capacitive loads? I searched long and hard on the internet but
haven't found anything cheap with low part count.

Probably also good to know:

 - I have an additional voltage-source of 5V available.
 - input/reference voltage won't be modulated. It's either 3V or 1.8V.
 - I don't need super fast switching. It is okay for the output voltage
to take a quater second to reach it's final voltage.


Best,
  Nils