How to select voltage regulators for dual rail system

Hello, I am creating small sensor device that require 2 voltages and I would like to know what product will best fit my requirements.

I need 2 separate power rails:
A 3.3v power supply with 395ma peak current and a maximum of 100mv ripples peak to peak.
If the regulator use a switch frequencies, the switching frequency should be above 500khz.

The second power supply requires 5 volt and a 10ma peak.

This will be powered by standard alkaline (3 cells) so a voltage range of around 2.7 to 4.8v.

I am trying to keep the combined efficiency to more than 80%.

Thank you for all your help in advance,


Hi, Ben.

For the 5 volts, you could consider the Pololu 5V Boost Regulator NCP1402. For the 3.3 volt rail, we don’t have something that matches your efficiency requirement, but you could consider the Pololu Step-Up/Step-Down Voltage Regulator S8V3A.

- Ryan


I think the only way to get everything you’re looking for with our products would be to use one of our adjustable boost regulators or our new step-up/step-down S7V7F5 regulator to produce 5V, and then use our step-down D15V35F5S3 regulator to get your 3.3V rail from the boosted 5V rail. The efficiency of the step-down regulator should be over 90%, and the boost regulator should have an efficiency close to 90%, which should give you an 3.3V efficiency of close to 80%.

- Ben

Sorry, my suggestions were made under the assumption that both regulators would have the battery as the input, which is a bad assumption. Ben’s solution is better.

- Ryan

Thanks for the great suggestions,

I looked at the 2 regulators that you suggested and for the D15V35F5S3 regulator, I found the switching frequency (700 kHz) and it’s typical quiescent current(15ma).

Do you have the frequency and the typical quiescent current for the S7V7F5 regulator?

Furthermore, the S7V7F5 regulator documentation page states that under little or no load the voltage can be as high as 5.2 V, in my application the range will be between 50ua to 400ma. What should I expect to have for the voltage range?

Looking forward to hearing from you,



We have not characterized those parameters very well (and they depend on the input voltage), but the switching frequency for the S7V7F5 regulator is normally above 2 MHz and drops lower when the output current falls below a few hundred mA, and the output voltage increases slightly when that happens. I would expect you to see around 5.15-5.2 V at the lower end of your range and almost exactly 5 V at the upper end. I measured the quiescent current of the regulator to be around 50-80 uA (microamps).

- Kevin

Thank you Kevin,

The S7V7F5 quiescent current of 50 - 80 microamps is very good compared to the D15V35F5S3 regulator (15ma).

Since my system is battery powered and I must run for a long period of time, I will turn off the sensor and the controller. The system will be in sleep mode most of the time. In sleep mode the controller takes only 50ua but since the D15V35F5S3 has a IQ of 15ma, the battery life will be seriously shorten.

Maybe I should make calculation with the S7V7F5 as the front 5v regulator and a ldo regulator for the 3.3volts. This will lower my efficiency when the controller is not sleeping but overall battery life may be higher.

A 3.3v LDO regulator should give me around 66% efficiency from my 5 volt input voltage rail but the IQ of an ldo regulator is well below the D15V35F5S3.

Am I making a mistake by thinking that the D15V35F5S3 regulator was not designed to run with only a few microamps load?
Unfortunately, I cannot turn off the output(enable pin) since I need the give the controller it’s power during sleep mode.

Thanks for your insight,


As an alternative to the D15V35F5S3, you could also consider our adjustable D24V6ALV regulator, which has a somewhat lower typical quiescent current of 2 mA (but a little lower efficiency than the D15V35F5S3). It’s very possible you could find a linear regulator with a much lower quiescent current, though, so that might be a good idea if you expect to be drawing very little current from the 3.3 V rail most of the time.

- Kevin