# Microcontroller power directly from SMPS DC-DC (SPR01 series) - is it OK?

Is it OK to supply power durectly from 5V SMPS DC-DC converter? Of course I will put several capacitors:

• at the DC-DC output (I'd put 1uF or something),

• 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?

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.

You should be just fine connecting your load circuitry to the DC-DC converter output. Place capacitors across the PWR/GND leads of each load IC chip as you have indicated.

The main caution is to not exceed the specified current rating of the converter output. As a matter of fact it is a good idea to derate the power delivery to 80% or so. This would mean using a 1A converter at 800mA or less load. Things last longer and are generally more reliable when derated. Many data sheets will even have a derating curve that must be followed based based upon the device operating temperature. At too high of temperature you may be limited to using a converter at levels of less than 20% of rated capacity.

If you have no Analogs in your system, you should care about 2 main things:

1- Output capacitor of the SMPS.

To be sure that no possible cumulative Drain pulse of your IC's exceed max output current of your DC-DC converter. Say, your SMPS has max current as 1 A. Also your VDD is 5 V so max output power should more than 5V*1A = 5W. Calculate the sum of Max drain pulse's of your IC's, If they exceed 1 A, you should look for another SMPS which provides more than 1A. And if your total current is around 1 A, you should choose a proper output capactor which can provide you enough current when SMPS can not deliver you that much current.

Also, you should examine the frequencies of your IC's which has high current pulses. You know the equation $d_V/d_t*C$ of the capacitor. You simply ough to use it. If your IC needs 500 mA in 0.1us(switching around 1 MHz) and max ripple you can afford is around 0.2V than equation is simple as this: $0.2V/0.1uS * C = 0.5 A$ so Capacitance value should be more than 25 uF. Of course, you should examine all the IC's like that.

2- Bypass capacitors. Like above, examine all the IC's with big current pulses and their frequencies. And put bypass capacitors to each IC's VDD pins with needed capacitance values to bypass others frequencies. Ceramic capacitors have ESL equivalent series inductor so each ceramic capacitor(XR7 series are suitable for bypassing) is has minimum impadence for spesific frequency. Use the correct ones.

You can also use decoupling method for high drain pulde IC's however, this complicates the circuitry much more and hard to determine ripple factor with so many decouplings. But it is only my opinion, I avoid to use decoupling unless it is so crucially needed to use( generally with one high drain pulse IC's have more than a few amperes).

however, if your cumulative drain pulse is lower than 75% of max current of SMPS, you are fine. USe the recommended output capacitor in the datasheet. So if you examine all the IC's in your system as above pointed, you will be fine I think.

One more think, SMPS's are usually used for mobile(energy efficiency is important) devices. If you have concern about energy efficiency, LDO's are more convinient to use.

I hope I could help.