I'm supposed to have two voltages coming out of two buck converters. My starting voltage is 12 V while the buck converters are supposed to output 5 V and 3.3 V respectfully.
Should I make the output voltage of the 5 V the V_in for the 3.3 V buck converter or should both have a V_in of 12 V?
I think the question you mean to ask is, should I wire my 5 V and 3.3 V regulators in a cascade, or normally with a shared input? Image below to make this clear.
Like all things electronics, this isn't a one rule for all applications.
You might use cascade in a situation where you need to guarantee that one supply starts after another (though enable signals or other methods might be preferred), you also might use it if your second regulator needed a lower voltage than the main input. Let's say your 3.3 V regulator had a maximum input of 7V, then you might cascade them as you were going to use the 5V regulator anyway.
Finally, this is often done with a switching regulator cascaded into a linear regulator, so you can get the low noise of a linear, but not burn as much power. Imagine you needed a linear regulator for 3.3, running this directly from the 12V input would have an efficiency of 3.3/12 = 27% or worse. Using a cascade, your efficiency is now (3.3/5) * 92% = 60%.
The cons of using a cascade, are a) you regulate twice, so you burn power twice. In this case, where you have two 92% efficient regulators, your 3.3 is produced at 92% x 92% = 84.6%
Also, you end up with a bigger power requirement for the first regulator, as it must source all the power for its voltage, and all the power to run the cascaded voltage.
All that being said, I would say using cascaded regulators is situational, not the default, and unless there's a specific reason why cascading provides an advantage, it's probably not the option to use.
