From: Harold Hallikainen <harold@mai.hallikainen.org>
To: Microcontroller discussion list - Public. <piclist@mit.edu>
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Date: Sat, 13 Mar 2021 17:52:10 -0800
Subject: {Spam?} Re: [PIC] 1 Controller and 18 Peripheral PICs
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Ah, I did not realize you needed 700 mA per LED. I think the TI part was
good for 40 mA. I once did a strobe light system for motorcycles where I
used an LM317 as a constant current generator then had several series
strings of LEDs with a FET to ground on each string. The PIC drove the FET
to light the appropriate string. These were high power camera flash LEDs.

Is the voltage source and LED drop constant enough that you could just use
a series current limit resistor? Not very efficient, but any linear
regulation method is going to also be inefficient.

In another project (a dental composite curing light), I used a switching
boost converter. The LED took something like 7V at 1.5A. I added a current
sense amplifier and fed the output of that back into the boost converter
feedback pin. The LED current could be varied by adjusting the reference
pin voltage.

Another approach that would be interesting to try is to have + supply go
to an inductor, then to the LED, then to a FET to ground. Place another
LED across the inductor. When the FET is on, the first LED lights as the
inductor current ramps up. When the FET opens, the second LED (the one
across the inductor) lights as the inductor current ramps down. The
inductor saturation current has to be kept high enough and the pulse
duration slow enough to ensure the inductor does not saturate. But, this
would be efficient (no resistive losses) and pretty minimal on the parts
count. You could use a small PIC to drive the FET (maybe even an 8 pin
part).

The internal RC oscillator is definitely good enough for SPI since SPI
provides a clock. Also, I think the internal RC oscillators are generally
good for 1%, which is good for many applications (but not USB). Besides
SPI, you might consider using the UART. That requires only one wire for
one way transmission. You could send packets that address individual PICs.
As with reflections on a clock line, reflections on a UART line could be
an issue. I'd use a separate driver (with series source termination) at
the master to drive the lines to all the other PICs.

Good luck!

Harold
https://w6iwi.org






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