[sdiy] switching dc power sources

[Roman Sowa]

AD8565 opamp
continuous output current 35mA, can drive up to 1uF at unity gain 
without compensation as datasheet says.

Roman

W dniu 2015-10-27 o 16:02, Nils Pipenbrinck pisze:
> 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
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