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12V to 3.3V and 5V

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dssvo

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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!
 
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?
 
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**
 
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