Step-up/Step-down Regulator Circuitry (S18V20F5)


I am looking at incorporating the S18V20F5 regulator board into my product design, which has its own power supply protection IC/circuit, and wanted to know if there are any other protections implemented on your regulator board other than the reverse polarity protection to 30V? Ie. overvoltage? overcurrent? overtemperature? short-circuit?

Also how much total ceramic bypass capacitance is on the input, and output of the board?

Thanks in advance,

Hello, Chris.

As mentioned in the Overview section of the regulator’s product page, the regulator on the S18V20F5 features over-current protection, thermal shutdown, and under-voltage lockout.

There is a 47uF electrolytic and 30uF of ceramic capacitors on the input and a 220uF electrolytic and 50uF of ceramic capacitors on the output.


Thanks for the reply Claire, I glazed over the overview page too quickly!

I want to use these boards to power Arduino Unos and Mega2560s, have you guys tested whether these regulator boards power these Arduinos reliably?

For example whether output ripple voltage of the regulator boards causes any havoc with the Arduino. Are the output capacitors sized so as to eliminate output ripple voltage? I will be drawing ~60mA so I believe the chip goes into ‘pulse-skipping’ mode at this low load and output ripple increases.

I am specifically looking at the S18V20ALV (set to 5V) now in regards to the above.

Also do the capacitor values you provided apply to the S18V20ALV board as well?

Thanks again

There is always some amount of ripple with a switching regulator, and exactly how much depends on many things in your setup like the input voltage and load, but I do not expect there to be any issues powering 5V on an Arduno Uno or Mega from that regulator. The input and output capacitance on the S18V20ALV is the same as it is for the fixed 5V version.


Thanks again Claire

In regards to the reverse voltage protection to 30V of this board, I assume this is achieved by using back to back MOSFETS or similar, therefore the 30V is the breakdown voltage of the body diode of the MOSFET that is preventing the reverse current?

When a reverse input voltage is applied to the board does the IC shutdown and turn off the aforementioned MOSFETs, and again it’s just the body diode of one MOSFET preventing reverse current to 30V?

I’m designing a protection circuit for an automotive application which this board will be used in.


There is one MOSFET, not two. It doesn’t really make sense to say the body diode is what is blocking the current since the whole MOSFET is in the current path and the whole MOSFET is blocking it.

You might find this video on reverse voltage protection circuits helpful. We use a circuit similar to the one with a MOSFET that he describes at the end.


Thanks Claire, good video.

Another question, the reverse voltage protection is to -30V. Is there any leeway in the MOSFETs specifications with this, in that could it potentially be a little higher for very breif periods and still not conduct?

I have an automotive circuit with TVS diodes that are ‘transparent’ to the circuit at 24V (for example during 24V jump starting etc.), but they clamp at +/-32V (the lowest I can get it). The MOSFET will protect the circuit up to -30V during negative voltages, but if there is a >-30V transient then the circuit will be exposed to some negative voltage, as the TVS won’t clamp until -32V.

If the MOSFET can not stop any breif negative transients as low as -32V, is it possible for me to remove the MOSFET and solder in a replacement, or could that potentially affect other things on the board. If so I may need to implement another MOSFET externally.


The MOSFET on there is good to 30V; you can put a 40V one on there to get protection past -30V. But do you really need reverse-voltage and transient protection that low? What’s the scenario in which you get -30V transients? You should also be able to arrange your TVS diodes to clamp at a lower voltage for reverse protection. A single-directional TVS should give you the +32V clamp and basically clamp to around -1V in the other direction. That would protect your circuit if you have some fuse or similar protection there that would blow before the TVS.