Re: Driving a led without a series resistor (PWM technique)
- From: Mike Silva <snarflemike@xxxxxxxxx>
- Date: Wed, 30 Apr 2008 07:34:33 -0700 (PDT)
On Apr 30, 2:52 am, zig...@xxxxxxxxx wrote:
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
You are forcing your ARM output pins to act as current limiting
devices, and they're not designed for that. They're spec'ed at 4mA
and you're drawing 20! Just because the chip runs cold doesn't mean
you aren't stressing the output drivers. IMO you are guaranteeing
yourself hardware failures down the road. Add the resistors.
.
- References:
- Prev by Date: Re: Anyone out there using Ada ?
- Next by Date: Re: silRTOS Port on ARM9
- Previous by thread: Re: Driving a led without a series resistor (PWM technique)
- Next by thread: Re: Driving a led without a series resistor (PWM technique)
- Index(es):
Relevant Pages
|
