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# Power Supply question

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#### pinoy

##### Member level 1
Hi, I have two power source... 12V and 5V... that will supply my buck regulator to 3.3V. My estimated buck input current is arround 5A.. Since this is very big for my 5V/12V connector (Only have 3A rating), I want to distribute from 12V and 5V like 3A from 12V and 2A from 5V or 2.5A from 12V and 2.5A from 5V. How to do this? Because when you connect the 2 supplies (12V and 5V) in parallel with two diodes, obviously 12V will dominate.

add a LDO for 12V source

I don't think it is possible....
If you add a ldo on the 12V to 5V, you will have a drop volatge of 7V iin your LDO, with a current of 2A => 14W !!!!

You could have 2 buck, one on the 12V, and the other on the 5V, and separate your 3.3V in 2 disconnected rails of power suplly.
You could try to connect the 2 rails, but there will be some oscillation between the 2 buck.
There are some components which switch between 2 power sources (ie battery, and line). I don't know if theses components could work in the transition region...

good luck

You can't "mix" voltages, but you can "mix"currents ..

You can build 3A current source (it can be based on the LM350+heatsink and one resitor) and feed it from 12V voltage terminals, that's one "current" ..
The remaining, or second "current" can be drawn from 5V voltage source through a diode ..

Connect both "currents" and that's it ..

Regards,
IanP

Can I step down 12V to 5V using Buck converter? Then I will combine buck converter output of 5V and the 5V input voltage using two diodes. (K is common)

Is the current will now be divided?

After you step-down 12V-to-5V (make this "5V" as close as possible to the other"5V") you can further try to "equalize" currents from both 5V sources by using low-Ω resistors, say, 0.1Ω or even less than that ..

Regards,
IanP

Points: 2