Power Supply

[robmitch]

Hi,
What is the cheapest option for supplying 5v (Zilog mcu @ 20mAmax) AND 3.3(compass sensor @ 20mA max) for my project? I have access to a 10V-12V battery supply, and/or a 6V regulated supply.

I was thinking of getting 2x LM317 (or LM7805) and setting each up individually for 5V and 3.3V (from the 12v Supply)?

Then I thought I could use just one LM317 (or LM7805) to get to 5V, then use a resistor divider to get down to 3.3V? Suppose this wouldnt give a very constant voltage though, with varying loads etc?

What is the cheapest/reliable solution. I suppose I should get rid of the Zilog mcu and use a 5V Atmal Mega...

Look forward to replies,
Cheers,
Rob

 

[robmitch]

Oh,
while I'm at it, what diode do I need for reverse polarity protection (with these currents)?
Cheers,
Rob

 

[geko]

LM2931-5.0 100mA 5V LDO regulator has built in reverse polarity protection so no didode needed and will work with an input voltage of 6 volts.

Cascade a LP2950-3.3 from the LM2931 to get 3.3 volts from that.

There are other Low-Drop-Regulators that you could choose from.

 

[robmitch]

geko,
thanks for that. Are these more efficient than the old 7805 (eg taking down from 12v)?

My preference is to come from the 12v supply, so low dropout is not a major concern.

LM317 is a lot cheaper (from what i can see) than the one you mentioned. is there a cheaper alternative out there?
cheers,
Rob

 

[geko]

Not really, just use a 7805 and a either a 3.3V regulator also off the 12V supply or an LDO 3.3V regulator from the output of the 7805. Put a 1N4001 diode in series with the input to the regulator(s)

If you've aleady got some LM317 use them but I wouldn't go out and buy them to do it, just use a fixed regulator.

 

[robmitch]

pete,
cheers. I didnt think about the need for a LDO from 5V to 3.3V...
rob