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


<zigbee@xxxxxxxxx> wrote in message
news:9fec5662-5499-46e6-a5ee-d36bf6079e86@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
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.

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).

Michael Kellett

www.mkesc.co.uk


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