Re: High brightness white LEDs damaged by custom switcher


<miso@xxxxxxxxx> wrote in message
news:1178400948.897551.99850@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

Just so I'm reading this correctly, you are not paralleling strings of
leds. That is, you only drive one string of leds. If that is not the
case, I would expect problems.

When I think of the effort it takes to make a bullet-proof DC/DC chip,
I just shake my head at the idea of doing it in software. In a chip,
events take place simultaneously, while a uP is a step at a time using
polling.


The LEDs are in series, so all see the same current, and the voltage
required is about 26 VDC for 7 and 49 VDC for 13. The PIC can respond to
certain events within a few microseconds, by using interrupts. The
difficulty is in generating the interrupt signal outside the PIC. That is
why I plan to put a transistor on the current sense to detect an
overcurrent. I could also add a similar circuit to detect output
overvoltage, and generate the same interrupt.

The first thing to do is disable the PWM output, which can be done in a few
clock cycles. Less than 1 uSec for an 8 MHz clock. Then the A/Ds can be
used to see what caused it, and act accordingly. All three analog inputs
are now read within 1 mSec, but could be within 60 uSec. The A/D can make a
reading in 18 uSec.

As long as the external circuitry has a sufficiently slow response, I don't
see any problem implementing a switcher with a PIC. The real advantage is
that the hardware can be built in a simple, straightforward way, and then
changes can be implemented in PIC code. As requirements change, the same
circuit can be used with little or no change, and the PIC can be reflashed
to the new parameters.

I think the PIC is perfectly suited to this application. It may not be so
for situations where the input voltage may change suddenly, or output loads
are constantly changing. The main problem here seems to have been
identified, and several possible fixes should eliminate it. Extensive
testing should prove that.

Thanks,

Paul


.

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