Is it OK to supply power durectly from 5V SMPS DC-DC converter? Of course I will put several capacitors:
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at the DC-DC output (I'd put 1uF or something),
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at the controller power input(s) (I'd put 100nF or something at each input).
The schematic has no analog circuits. Only RS-485 interface (based on ADM2587) and several digital inputs and outputs.
How likely that I will face with some problems due to low power quality?
Best Answer
Running a microcontroller directly from the output of a switching power supply is done routinely.
However, the real answer can only come from the power supply and microcontroller datasheets. The micro datasheet tells you the operating voltage that it requires. The power supply datasheet tells you the worst case range of voltages that it will put out. If the latter is completely within the former, then you're OK
Mostly. The micro can also be susceptible to fast changes in its power voltage, even if the actual voltage is always within spec. This is not something you're likely to find in the datasheet. However, in reality, you are very unlikely to bump into this issue as long as you've done the usual things to deal with power supply transients. This includes observing the input and output requirements of the switcher, and putting bypass caps (like 1 µF ceramic) on all power pins of the micro physically close to the micro.
Even strict "3.3 V" micros usually allow 3.0-3.6 V power. Switcher chips with the proper circuitry around them will usually ripple around 100 mVpp or less. This leaves decent margin. With the right ceramic caps on the output of the switcher, then good bypassing right at the micro, something would have to be very off for it not to work properly.
I've done this a number of times, even when the micro had analog inputs. What I usually do in that case is put a L-C filter in front of the AVdd supply (ferrite "chip inductor" and 20 µF or so ceramic cap), then use a separate reference chip to make the voltage reference for the A/D. At least in PICs, you can use 3.0 V reference for the A/D while Vdd is run directly from the switcher, and AVdd separately low pass filtered from the switcher. Again, I've done this a number of times with no problems.