D15V35F5S3 unreliable

Hi All,

My first post here.

I recently bought a D15V35F5S3 step-down regulator to replace a larger LM2940CT-based reg which is used to supply the 5V logic for a Dagu 4ch motor controller board.
Fortunately, I gave it a good test before connecting it to the Dagu board and noticed that, occasionally, after connecting the battery, the output showed (close to) 0V instead of 5V. After further testing, I managed to make this more or less repeatable by (deliberately) not cleanly connecting the battery. If I “jiggle” the connector slightly while plugging it in, I can produce the same result most of the time. I’ve confirmed that around 8V is available at the input when this happens (and also, of course, when it produces 5V).
It’s supplied by a 2S 3000mAh Lipo via 4mm bullet connectors. It’s never been connected to any kind of load other than a DMM and a scope.

Is this perhaps a design feature to detect noise on the input and close down the output as some kind of safeguard? Or is it likely to be simply faulty?



Just found this in the specs for the reg on this site (and it doesn’t appear in the instructions on the site where I bought it):-

“The input voltage, VIN, should be between 4.5 and 24 V. You should ensure that noise on your input does not exceed the 24 V maximum. When power is applied, the input voltage rise time must not be more than a few milliseconds; otherwise, the regulator can latch in an error state that disables its output.”

OK, so this implies that I need to debounce the input to ensure that I don’t disable the output. Or will something like an SPST switch between the battery and the reg achieve that? (Better than my fumbling fingers, I suppose). Or do I just hang a LED on the output to give me a visible indication of 5V?
Any practical experience? I really want to use this board because it’s much smaller than the board it’s replacing but if I apply the power for the motors without 5V to the logic… not optimal, especially with a Lipo connected.

Hello, Paul.

You’re probably right in thinking that a slow input voltage rise time is causing the problem. Adding a switch, as you said, would probably be the easiest way to apply the input voltage more cleanly.

- Kevin

Hi Kevin,

Thanks for the reply. And my apologies for the “unreliable” in the title. I posted that before I read the complete specs on your site. :blush: There is an abbreviated version on my local supplier’s site which leaves out that particular sentence.

It’s made me curious enough to connect my DSO to the input terminals next weekend and capture just what is happening when I plug in a bullet connector. I’ve really only used them before on RC aircraft (where you often get a nice fat spark as the ESC filter capacitors charge). This application is somewhat more “delicate”.