From: "Harry Dellamano"
Subject: Re: current source for array of infrared leds
Date: Mon, 2 Dec 2002 13:35:33 -0800
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"Spehro Pefhany" wrote in message
> On Mon, 2 Dec 2002 08:53:18 -0800, the renowned "Harry Dellamano"
> > Now the question is, how do we set the voltage to control LED current
> >wide ranges of temperatures for constant light intensities?
> There are at least two requirements to call this a temperature
> compensated system:
> 1) Light intensity at nominal die temperature (most important)
> 2) Light intensity at one other die temperature
> You are not free to set the voltage to achieve nominally zero tempco
> between the two points (if it is possible), as getting the nominal
> light level right is paramount. You obviously need two degrees of
> freedom to be able to set the two independently.
> Best regards,
> Spehro Pefhany
Let's say light intensity at normal temp (25C) is like "Voltage Offset" and
light intensity at elevated temps (80C) is like "Voltage Gain" in our
voltage mode system. The offset is set at 25C by setting the supply's output
to the correct voltage (+/-0.25%) for the desired LED intensity (current).
The LED's negative voltage tempco is about right to get increased current at
80C for constant intensity. It's only a first order approximation and you
also have the voltage source's tempco. For closer intensity tracking a
thermistor, either PTC or NTC, can be added to the voltage feedback network
to set the gain. The units I've done, no thermistor was necessary.
Bottom line, there is a large intensity drop-off, to the eye, at 80C with
CC mode of operation and undetectable drop-off with CV mode.