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If my source is 9 or 12v should I first us a linear regulator to step it down to 5v and from the 5v use another linear regulator to step it down to 3.3v? The 3.3v will be going to a micro-controller while the 5v will drive some 5v sensors/relays.
Current is under 500mA total, probably closer to 300. The power source for the loads (relay etc) are coming from a separate power source.
If those sensors/relays are external to your device, there's a fair chance that the 5V supply can get shorted and shut down. If you wish your microcontroller to work through such an event, it'd be necessary to have a separate buck converter for both 5V and 3.3V - if you care about power efficiency. If you don't mind some heat, it'd be OK to use a separate linear regulator for 5V and 3.3V, fed straight from the input source. That way it'd be easier to prevent relay transients from upsetting the microcontroller, too.
Efficiency-wise, when you use linear regulators it doesn't matter how you stack things up: cascaded or independent, all of the current consumed at any regulated voltage has to flow from the input supply. So if you consume 200mA @ 5V and 50mA @ 3.3V, it's still 250mA from the 9-12V supply.
With switching regulators, you have to take each regulator's efficiency at its actual load current and compare the independent-vs-cascaded variants. Of course for the microcontroller to stay ON in spite of control (5V) shorts, you need the regulators to be independent.
I have plenty of 3.3V microcontrollers running from linear regulators fed from 12V - sometimes it's the cheapest option when you have a powerful supply available.
A linear regulator will dissipate power but if the current in the load is small this power loss may be acceptable. If this power loss is not acceptable then use two switching regulators.
Generally having a linear regulator fed from a higher voltage linear regulator is OK but the higher voltage regulator may overheat if too much current is taken by the lower voltage regulator.
