Hi

I have a board that I need to supply with a variable supply of 5 - 15V. It needs to be a separate 'bolt on' circuit supplied with 15V, and a 5 - 15V output. However this output voltage needs to be controlled with a 0-10V analogue input. I have found lots of examples of using variable resistors to alter the output voltage, but how can i encorporate using a 0-10V analogue signal into this?

Regards

Oliver King

 

If you're only going to supply it with 15v, then you will need a buck-boost DC-DC converter in order to output a range from 5 to 15v. You would not be able to simply use a regulator like an LM317, or even a low-dropout linear regulator.

You say you need 5v to 15v out, but at what current?

 

I need a max current of 2Amps. I dont know what a buck boost DC DC Converter is, could you expand on that a bit please.

 

If i supplied it with 24V, would that make it simpler?

 

Yes, supplying it from 24v would only require a "buck-type" DC-DC converter.

You would not want to use a simple linear regulator such as an LM338 due to the very high power dissipation that would result.

Are you in the USA? If so, you might find that a pre-manufactured DC-DC converter to be an easy option. Otherwise, you will need to build a circuit.

 

Digikey has this DC-DC module available for under $24:
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=555-1153-ND

Manufacturer's datasheet link: http://www.lineagepower.com/oem/pdf/lynx_24v_sip_ds.pdf

Better than 90% efficiency at your input voltage and load current. You would not be able to even approach that using a linear regulator.

 

If you'd rather build than buy, it should be possible.

I picked a switching regulator more or less at random from the Linear Tech catalog:

http://cds.linear.com/docs/Datasheet/1765fd.pdf

The variable version of this part simply requires that you send 1.2V into a feedback pin when the output voltage is at its nominal level. Normally that's done with a resistive divider, but it should be possible to do it with an op-amp circuit that combines the 0-10V input with the 5-15V output (hedging slightly because it might not be stable). But I expect it would be OK.

 

Thank you for your help guys. Im not in the US, but i can get hold of a DC-DC module.
I would rather build a circuit than buy pre manufactured, so using the feedback pin on the switching regulator sounds good. However I am new to electronics, so an example circuit using op-amps to make it controllable with 0-10V would be very helpful.