[sdiy] switching dc power sources

Jason Tribbeck jason at tribbeck.com
Tue Oct 27 16:22:21 CET 2015


I've been doing work with SIMs, and there's some chips which can be used to
drive SIMs (and also do voltage translation) at 1.8, 3 and 5V.

LTC1555 springs to mind - although I'm not sure if it can drive 20mA (it's
maximum drive is between 50mA and 100mA according to its datasheet).

On Tue, 27 Oct 2015 at 15:05 Nils Pipenbrinck <
n.pipenbrinck at hilbert-space.de> wrote:

> 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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