- What is the difference in using buck converter vs linear linear regulator
Very minimalistic explanation:
A SMPS (switch mode power supply, e.g. Buck) basically compares the output voltage to a given reference. If the output voltage is above the reference, the regulator basically cuts connection between input and output. If the output voltage is below the reference, input and output are connected. Output capacitance and inductance are used to store energy on the output side and smoothen the output voltage.
benefits: Efficiency and therefore power dissipation (--> heat) because the switches are either closed (no current -> no power dissipation) or open (lowest resistance state -> minimal power dissipation).
downsides: additional parts (usually a voltage devider, inductance, capacitance and maybe a ferrite bead for noise suppresion) and increased price (device itself and additional parts).
Unlike an SMPS a linear regulator doesnt use a transistor as a switch (on/off) but in linear mode (any state between on and off is allowed as well). This leads to increased power dissipation, as you can imagine the transistor as a regulated resistor that is being adjusted for a voltage drop of Vin-Vout.
benefits: cheap; easy; less/no noise due to no switching, might need only a capacitance
downsides: efficiency, especially on high load;
- Would linear regulator (small packages) be a bad idea because it would heat up a lot because there is a big difference in voltage (12-3.3=8.7, 8.7*0.15=1.3W) ?
I would answer this with yes. If you have a look here and consider values like the ones on chapter 6.4 in e.g. this datasheet you will see that the thermal resistance easily goes over 100°C/W (meaning: a temperatue rise of 100 °C for 1W power dissipation). I think having this in a small case wont work, even with a (small, because small package) heat sink and lots of copper area on your PCB determined for cooling (so you won't be able to benefit from the small package at all).
As a rule of thumb I usually use a linear regulator if i need either very low currents (just a few mA at max), very small voltage drop (1..2V) and/or super clean supply voltage for an ADC or other analog parts.
Means in most cases i prefer to use SMPS. These require more parts usually (more caps, resistors, inductance) so its a more expensive and 'complicated' solution.
- Would the frequency of switching, or output voltage ripple (noise) be a big influance on the normal operation of a MCU?
If you design a SMPS based on the devices data sheet there are usually calculations given for the expected ripple noise. These are usually within 1% of the output Voltage what is no problem for digital systems. I have created an excel sheet ot help dimension caps etc, but I dont know how to add an attachment here ...
Also you would probably want to add a 10..100nF cap to each supply input of the MCU and keep the traces from Cap to MCU short to minimize ripple seen by the power pins.
- Conclusion, what is the best way to power it with the input voltages between 6V and 12V?
As you need a big voltage step, more than a few mA and didnt mention any special requirements regarding noise (for analog stuff) I'd go with a SMPS.