[sdiy] switching dc power sources

[Joshua]

A basic voltage buffer is the emitter-follower configuration of a BJT.

https://en.wikipedia.org/wiki/Common_collector

You put your 5V rail at the collector, your reference voltage at the
base, and the emitter will be equal to the reference minus Vbe
(~0.7V).  This is the most common configuration of a linear-regulator.

https://upload.wikimedia.org/wikipedia/commons/9/98/Series_reg.gif

-Josh

On Tue, Oct 27, 2015 at 11:22 AM, Jason Tribbeck <jason at tribbeck.com> wrote:
> 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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>
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