I am looking to power a PIC with a supply voltage that ranges from 4.5 V to 18 V.
This will be a battery powered device so I would like to conserve power, i.e. linear regulators like the 7805 is less desirable.
Also the application will be producing quite a bit of heat in an enclosed space so no need to add to heat generation with the linear regulator dissipating excess voltage.
After some hunting I suppose I am after a switch mode DC-DC converter, a step down one.
I have been looking at an LM2575 based circuit, but wondering what Vout would be if my supply Vin drops below the minimum 7 V for regulation. The PIC I am working with have an operating range of 2 V to 5.5 V so I am OK with Vout being equal to Vin when Vin < regulation threshold.
So my question is, what happens to Vout when Vin < regulation threshold?
Also are there any additional things/circuits I should be considering?
When Vin goes below your minimum Vin you are going to have your output drop. I have seen many times that people think it will be a small drop and it ends up dropping very quickly when the Vin goes to low.
If you want something that give you 5 out from 4.5 to 18V in, then you should just pick a simple Buck-Boost DC-DC converter. They will be able to handle from 2V to higher than 18 without a problem. By grace of being a buck boost, they can step your voltage either direction and it should make your life quite easy.