The definitive MIDI controller | This is not rocket science

Tiny 3.3V switching regulator boards

The protosynth front panel is shaping up nicely, and I’ve been looking for ways to make the design more modular and reusable. I tried first a fully modular design made of small sub-boards (with some 4 knobs or a couple of buttons+leds each) hooked to a master controller board, but it felt too fragile and would have required a lot of tricky wiring to finish. So instead of making the whole panel modular, I’m now going for a more conservative design consisting of just one large board, yet there’s still some room for reuse…

The logic level of the front panel board will be 3.3V, just like the rest of the processors on the synth. But since the front panel is at the end of a longish cable, and because it’ll likely boast plentiful leds that actually consume a measurable amount of power, I’d rather make use of the main 9V power input and regulate it down to the correct level separately. Having a dedicated regulator for the front panel board and its logic chips will hopefully result in less noise making its way to the DSPs and codecs on the main board.

There could easily be some 20-40 leds total on the panel, in addition to one or more microcontrollers, tens of potentiometers and encoders, and a bunch of logic ICs to probe all the controls. A linear regulator might be able to deal with this just fine, but since the panel will be quite crowded (the knobs are large), there won’t be much space for a heat sink. The voltage drop across the regulator would be some 5.7V, and thus the total power consumption for the regulator alone can be in the range of several watts. That’s a lot of heat…

So this time a linear regulator most likely won’t do, and switching regulators are the way to go. And for sure this won’t be the only time that they are useful. It would be very convenient to have a tiny regulator component that you could just drop in when needed, without having to worry every time about excess heat or careful routing.

I drew these tiny switching regulator boards loosely based on the LM22675 reference design. The result is a small “chip”, roughly 25x20mm in size, with the corner pins 900 mil apart. It leaves plenty of space even on a breadboard. And it’s light enough to be mounted by just soldering the pins to the main board.

3.3V regulator

The regulator is spec’d for 1.0 A, but since I used some leftover diodes, the practical maximum current output is some 0.8A at 3.3V. And the thing won’t even heat up noticeably above ambient temperature with such a load! The efficiency is a nice 90%.

The pinout is extermely simple: input, output, and enable. And the enable pin can be left floating, as it has an internal pull-up. Pulling it down will immediately cut the power.

The boards are intended to be soldered directly on top of a main board. The pins will separate the two boards by a bit more than a millimeter, so some small smd components, such as the 0603 SMD capacitors and resistor I mostly use, will easily fit in between them.

Here is the pinout:


And finally, the schematic.

3.3V switching regulator schematic

I’ve built a few extra ones, send me email if you find interest in them.

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