Tiny Quadcopter (Part 3)

I though the regulator on board the micro-controller would not be enough for the range extender, but it can actually output 300mA, so I think it is. Here's a bunch of reference information on the board, which may be useful in any case. The regulator is an AP7343. It's drop-out voltage is only 310mV, but that still means I'd need 3.6V or so to still have regulated power, which is more than the LIPO will have at the end of it's discharge.

At least when I plug in the USB, it can power the radio for testing.

What can I do otherwise?

The radio and the micro-controller can run on a voltage as low as 2V. I have adjustable regulators on hand, but they don't help much, because they have much higher drop out voltages.

I have 2 Pololu S7V8F3s somewhere. They would be ideal if I could find them - they are pretty much designed for my purpose. What I actually found is an S7V8A, which I guess will do... Although I have the challenge of adjusting it before soldering it to anything.

I think it would be good to get a low drop out adjustable regulator for future projects. The TS9011 regulators I have quite a few of were meant for this, but obviously I didn't give it enough thought. NCP718 might be alright, apart from the tiny package. MIC5219 has a bigger package, but it's also 3x as expensive, and has a more limited input range (12V vs 24V).

It's annoying to buy something.

You can make an adjustable LDO regulator from an NPN transistor with a low Vce(sat) and an adjustable shunt regulator. The minimum voltage you can regulate is Vbe + Vref.

I have many TS432A with Vref=1.25V, so the minimum output voltage is ~2V.

I have a couple of NPN transistors:

  • 2SD1805 has Vce(sat) = 0.5V, so I would go for 2.5V output. The voltage difference then, at worst, is 4.2-2.5 = 1.7V, current is 200mA, so power is 340mW. The package can dissipate 1W without any sort of heatsink, so that certainly works.
  • MMBTA05L has Vce(sat) = 0.25V, so output would be 2.75V. Voltage difference = 4.2 - 2.75 = 1.45V, power is 290mW. This is more than this device can dissipate: the maximum is 225mW on roughly a square inch of copper. Two of them would do it easily though.

So I guess it really has to be the 2SD1805. Mounting a SOT-23 on an inch share of copper each is quite complicated.

I definitely can get better transistors than those mentioned above. The PBSS8110X seems worth adding to my collection. Perhaps I'll experiment with this a bit.

This is turning into the drawn out process that battery power supplies always seem to result in for me.

While I'm here I might as well link an interesting thing I've found in my travels: This uses a charge pump, N-channel mosfet and shunt regulator to create an 'ultra' low drop-out regulator. It made me realise I have no 555 timer chips! It's quite a high component count, however interesting it looks.

It's still interesting though - because it's quite easy to make programmable: replace the shunt regulator with an op-amp, and then control the output voltage with the negative input.

But for this project, I've worked out I can just use 4 NiMH batteries, or perhaps just a USB battery bank, and the 3.3V regulator I already have.

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