Wide voltage supply for PIC microcontroller

Question

I am looking to power a PIC microcontroller with a supply voltage that ranges from 4.5V to 18V.

This will be a battery powered device so I would like to conserve power, ie. 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 7V for regulation. The PIC I am working with have an operating range of 2V to 5.5V 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?

Answer 1

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.

Answer 2

The LM2575 has a drop out voltage of less than 1.5 V. You could stick with the simple buck regulator and set its output voltage to 3 V, assuming you have the adjustable type.

Answer 3

Yes, if you can run at 2.5 V, and your minimum input is 4.5 V, then a buck regulator should be fine.

Look at the datasheet for your buck regulator. The typical "saturation voltage" specification tells you what happens when the input voltage gets too low. Typically it's around 1.0 V -- meaning that if the input voltage to a 5 V regulator drops to 5 V, its output voltage is going to drop to about 4 V.

If you're willing to spend some time tweaking things, Roman Black has some relatively simple buck regulators. Would the 3-transistor Black regulator; or some 2-transistor Black regulator work for you?

On the other hand, if you're going to drive power MOSFETs -- they often require 8 V to turn completely on. You might as well use a boost or boost-buck converter to generate 8 V or so, and then regulate that down to 5V to power your microcontroller.