From: email@example.com (Paul Mathews)
Subject: Re: Driving the HSDL4230 AIGaAs Infrared LED
Date: 25 Oct 2002 07:16:51 -0700
NNTP-Posting-Date: 25 Oct 2002 14:16:52 GMT
"Markus" wrote in message news:<firstname.lastname@example.org>...
> I want to drive the HSDL4230 between 4 an 5 Mhz at its max rated current of
> 500 ma.
> What component would be the best choice, a high speed mosfet ? or something
> Have seen it done with a bunch of TLL buffers in parallel, but apart from
> price tag, i didn't like that method much
This is normally done using a simple bipolar transistor current
source. Connect the LED cathode to NPN collector, LED anode to a
well-bypassed positive voltage source (e.g., 12V), and connect the NPN
emitter to a resistor Re. You then drive the NPN base with a voltage
source, for example, a CMOS buffer output. The value of Re can be
calculated as follows:
Re = (Vin - Vbe) / I
where Vin is the drive voltage, e.g. 5V for 5V CMOS logic, Vbe is the
base-emitter forward voltage on the NPN (use 1.0V for this power
application), and I is the desired LED current. For 5V drive and
Re = (5 - 1) / 0.5A = 8 Ohms
This circuit will deliver more current at higher temperatures, due to
tempco of Vbe. However, LEDs generally deliver less light output at
higher temperatures, so this provides some amount of compensation.
You may also need some small amount of base resistance to stabilize
the circuit (prevent ringing on edges). Use Rb = 47 Ohms.
Choose a transistor that has high current gain at 500mA and good
speed. Have a look at the Zetex line if you're trying to keep size
down. They have SOT23 transistors with current gain of 350 at 500mA!