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Re: [avr-chat] high speed LED switching circuit


From: Ned Konz
Subject: Re: [avr-chat] high speed LED switching circuit
Date: Thu, 30 Jun 2005 21:41:28 -0700
User-agent: KMail/1.7.2

On Wednesday 29 June 2005 4:24 pm, Reza Naima wrote:
> I'm looking at building a circuit to drive an IR LED at a pretty fast
> speed to transmit data (~5Mhz).  I tried a simple circuit that used a
> transistor to switch the LED on and off, but it seemed fairly slow.  I'm
> looking at having about 60mW of power (~20mA @ 3v) to drive it.  Any
> suggestions on a high speed switching circuit?

A good small power FET (perhaps the 2N7002) with a good stiff gate drive 
should work OK at that speed
in a simple circuit with a resistor to limit the current.
Of course, you'll have to have a good gate driver; you can't just drive it 
from regular CMOS outputs
and expect it to work well.
Many of the microcontroller outputs are pretty heavy duty;
otherwise, you can take a hex CMOS buffer
and parallel 5/6 of the sections, using the last one to drive the 5 inputs.
The output would drive the gate of the FET.

There's a good discussion here about LED drive circuits:
http://eleit.stlib.gd.cn/jsdt/0406LED/fulltext/Light-emitting%20Diode%20
(LED).htm

For the really high speeds, how about a Fibre Channel LED driver chip?
These are able to drive LEDs at around 270Mbps...
For instance, look at the MAX3967, which you can order right from the Maxim-ic 
web site.
It has PECL inputs, which you'd have to drive via a level translator of some 
sort (perhaps a 10351).
They don't give a price for singles, though; you might be able to get samples.
https://shop.maxim-ic.com/cgi-bin/Maxim.storefront/389466039/Product/View/MAX3967ETG

-- 
Ned Konz
http://bike-nomad.com






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