12V to 3.3V and 5V

Hi,

I am making a circuit board where two voltages are needed:

• 3.3V, to power 3 microcontrollers. Probably around 1.2A
• 5V, to power a host USB port and one IC. Probably around 1A

The input voltage is 12V unregulated, from a lithium-ion battery.

Do you have any recommendations on circuits to use for power regulation?

I have looked at the National Semiconductor SIMPLE SWITCHER LMZ12003. Space-wise, I have just enough room to fit two circuits using these (one for 3.3V and one for 5V) on my board. Does that sound reasonable? I have also considered just using the LMZ12003 for 3.3V and then using a boost regulator (LT1935) to get the 5V needed for USB - is that a better approach?

Thanks for your feedback!

That idea about going down to 3.3 then back up to 5 sounds wrong.

use lm7805 to change 12v to 5v.or use lm317 or lm2941 to change to whatever you want!

The first thing to do is find out how much current you really need on each of the rails. When I see 3 micro-controllers, I think more like 300mA not 1.2 amps.

In any case, that switcher looks like it is up to the task for your application. At 12vin and 5vo @ 1 amp, it is 92ish % efficient and will need to dissipate about 0.5 watts. At 12vin and 3.3vo @ 1.2 amps, it is about 87% efficient and will need to dissipate about 0.5 watts.

aishiqi
You don't want to use a 12v to 5v linear regulator here as it'll need to dissipate about 7 watts. It'd need a large heat sink and maybe even a fan.

The current requirements sound way too high to me; I'm thinking 600ma at 5V (500ma at the USB jack), and less than 200mA at 3.3V.

How about a switcher to go from 12>5, and a linear reg from 5>3.3?

If you look at figure 13 in https://www.electro-tech-online.com/custompdfs/2011/01/LM25576.pdf that is a way of getting two voltages out of a buck regulator. I've used that to get 3.8 V and 5V. Only the lower voltage is regulated, but you could use a low dropout 5 V regulator if needed. That wouldn't need much of heat sink as it would only have to drop 0.25 V or so at full current.

Thanks for all your responses!

I decided to go with the LMZ12003 for the 3.3V, and then compromised in favor of cost to use a smaller, 5 times cheaper Micrel switching regulator to get 5V.

I think VMP3201-3.3V and VMP3201-5V are what you need. You can browse the datasheet from **broken link removed**