Re: Driving a led without a series resistor (PWM technique)

"MK" <nospam@xxxxxxxxxx> wrote in message
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<zigbee@xxxxxxxxx> wrote in message
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Hi,

I'm designing a board where the microcontroller is supposed to drive
10 red leds and 4 optocouplers.

I would like to drive the leds and the opto's without a series
resistor using the PWM technique. I made some experiments and all
seems to work fine in the lab.

Under perfect conditions...

1. The supply voltage is 3.3 Volt
2. The microcontroller is an ARM7 by NXP (LPC2387)
3. In DC, the microcontroller's GPIO ports are able to source or sink
20 mA ( I measured that)
4. The leds are driven with a duty cycle of 1/5. The average current
that the leds sink is 5 mA.
5. The light emitted by the leds in these condition (1/5 PWM) is more
than acceptable.

Based on your experience and knowledge, what do you think about this
solution?

Should I sink (the N-MOS will work) or source (I P-MOS will work) the
current?

Thanks in advance for any suggestion,
Enrico


This is bad !

My reading of the LPC2387 data *** says that the short circuit current
on
the standard output pins is between 4 and 50mA. The output pins are not
current sources so the actual current in your leds will vary between 4mA
and
rather less than 50mA depending on chip batch, temperature etc. If you
need
20mA the LPC2387 can't do it reliably - use a proper LED driver (eg Texas
TLC5923).

Then we have the conditions that happen in real life

System startup
Debugging software/system (bug in code or some systems single step)
Fault conditions

Under any of these conditions you could end up with one or more of these
LEDs in the ON condition permanently, with 3.3V and full current.

I wonder how long before the LEDs 'derate' permanently.

Just to save on some resistors and/or drivers, which actually means that
unless
you are doing multiplexing of display, you have added extra complexity to
the
software.

Consider the production size of the design, a few pennies on cheap
components may well save days of development in total (especially
when 'special conditions' are found to be coded round).

--
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