17mm++ single-sided / DD driver w/ low parasitic drain for e-switch lights: AxxDD-SO8+LDO

This driver is a derivative of the AxxDD-SO8 series.

It is intended specifically for multi-cell e-switch applications where the “Zener Mod” would normally be employed. As we’ve discussed in the past, the “Zener Mod” is terrible for momentary lights. It has a giant parasitic drain and can pull your cells down to damaging levels in a matter of days. Therefore it’s inappropriate for things like e-switch MT-G2 bedside table lights.

The solution is also well known, but nobody has gotten around to making and publishing DD or 7135 drivers which feature it… an LDO regulator of the LD2981 / MIC5235 / LT1761 / TS5205 style. I slapped this together very quickly this evening, so it may not be functional or beautiful.

  • All design goals from A17DD-SO8 have been maintained.
  • Many of the LDO regulators which fit this footprint have reverse battery protection, so I have removed the polarity diode. Clearly if you use an LDO without polarity protection you do so at your own peril.
  • This does not eliminate parasitic drain on the cells. It only significantly reduces it. I am not aware of any momentary light which does not have at least a small amount of parasitic drain.




Thanks Wight, again a wonderful job….now to figure out just what the H an LDO is….low down outsider? :slight_smile:

Don’t worry, it’s something you probably already have!

LDO is short for Low Dropout Regulator. As I understand it it’s just a tiny linear regulator with a low dropout voltage.

Mattaus made use of one for the same purpose in his Knucklehead V3 driver. He used the LT1761. IIRC his design included input, output, and reference bypass caps. I’m confident that the reference bypass cap is unnecessary. I think it’s somewhat likely that both input and output capacitors will be needed. 10uF would be the greatest of plenty.

Back in 2012 I made up a nanjg board with an LD2981 instead of the Zener and was told why bother. Did not know about parasitic drain then and since it needed 2 extra caps it did not become my ticket to financial independence or secure my everlasting fame but did succeed in laying the groundwork for pigheaded stubbornness. I’m glad you’re making it work. I tried again last spring using an LFPAK instead of 7135’s but my board surgery was not up to snuff.

I have a box of Knucklehead stuff I haven’t looked at in a while…

One of the things on my to do list over the holidays is a knucklehead sandwich(one side of two boards populated with copper between and a post drilled into the main pad of the Buck IC). The copper is done(has been for months but was waiting on a lamp) and I’m home alone this year, Yippee!

That’s right, you did say you wanted to do that. It’ll be interesting to see both what you come up with and how it does.

I suppose another thing this driver should improve is efficiency in moonlight modes and slow strobes/beacons like an Alpine distress signal, even in clicky flashlights. Normally a Zener modded flashlight in moonlight is dumping a milliamp or three through the Zener any time it’s on. Frankly I just don’t think I care about that though, this driver really is intended to solve the issues with multi-cell e-switch lights.


This is great. I have some regulators I purchased after reading RBD's awesome 7135 experiments thread a long time ago. I wanted them for driving MT-G2's, but then I decided to go Buck as the heat issues with 7135's turned me off. I will be ordering a set of these boards tonight.

Any suggestions on naming? I’m not really 100% happy with the name I’ve given to this driver for now. Maybe just AxxDD-LDO would be better since it seems like continuing the use of the SO8 FETs is a given at this point. What do you guys think?

JTD, Joe the driver. Cause it’s a regulated guy.

Good thinking! I shoehorned an LDO onto one of my 2x MT-G2 Maglites a while ago because as you said, the parasitic drain is simply too high for momentary switch lights. There are a lot of these chips that are inexpensive and only have a 10-30 uA quiescent current drain, which is much much much less than what the zener diode setups use.

Thanks wight!! Really good stuff, once again!

He doesn’t sell the Y3, much less a Y3 w/ MT-G2. Where would he need to make it clear?

Come to think of it, how fast does the Y3 pull down a single cell and/or a multi-cell setup if you leave the tailcap turned on?

EDIT: moved reply over here.

RMM sells two Zener mod drivers - both of them are available only with clicky firmwares. AFAIK nothing RMM sells is actually a victim to what we’re preventing here! I’ve always been under the impression that almost all Zener builds were in clicky lights. In a clicky such high parasitic drain is a non-issue.

I've known this for a long time. The zener mods drain about 0.01-0.03A all the time--enough to drain full cells dead in just a few days.

Roughly speaking, a 3000 mA cell would be dead in around 100 hours with a 0.03A drain on it. In clicky lights it isn't a huge deal (although efficiency will be improved all around, even in a clicky setup--especially in moonlight mode, where the drain from the zener diode might come close to what the LED is using).

Wight, I’ve got the LT1761 in a 3.3V variant, will that work? Richard told me I’m too old to understand this stuff so I’m out to prove him wrong. :stuck_out_tongue:


I’m guessing that’s a bit low on output voltage. The mcu won’t boost input voltage so the pwm signal wont be any higher than the mcu input voltage. Therefore you’d want the LDO regulator to supply enough voltage to easily turn on the FET if it were directly connected.