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

On 30 Apr, 09:20, "MK" <nos...@xxxxxxxxxx> wrote:
<zig...@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

Hi Michael,

thanks for your answer.

I actually need an average current of 4 mA to turn on the leds (their
package is 0603). Duty cycle is 1/5.

All seems to work fine (in the lab) and the micro is cold.

If the peak current depends on the chip batch I might have problem in
turning on the leds. In this case I might have to enlarge the duty
cycle (let's say 2/5). I'll add a tactile push-button in order for the
user adjust the led's brightness.

Some people designs a small pcb inductor that works in conjunction
with the led parasitic capacitance to limit the peak current.

Enrico

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