Optic Driving - Reply
I thought that I'd chirp up on this one...not something I intend to do
You need 4 basic pieces of info:-
1. Your 'illuminated' time - from software.
2. Your 'not illuminated' time - from software.
3. The max. average LED forward current - from the device data sheet.
4. The max. peak (surge) forward current - from the device data sheet.
You may not exceed either of the two LED current ratings.
You first need to ascertain the Mark:Space ratio under which the LEDs
will be operating. This is calculated as Ton/Tcycle where Tcycle = Ton +
Toff. However, you state that you will be running the LEDs at a 50% M:S
ratio and so, to a first approximation, you may operate them at a peak
current of twice the average value. Note that I am assuming that the
current wave-shape is a simple rectangle.
However, as always, it is not as simple as that!!
You must also ensure that the 'on' time dissipation does not exceed the
maximum allowable for the device but this is usually only a problem when
the 'on' time is 'long', say above about 1ms, depending on the device.
You say that your Ton = 128us so, I suspect, you do not need to worry
about this aspect.
Some manufacturers provide a set of 'Pulse Handling' curves in their
data sheets - if you have one of these then it is easy to find the
maximum allowable peak current. Obviously more LED amps means better
range and/or signal but, as a final thought, I would be tempted to limit
the peak current in the LED to a maximum of about 70-80% of the
calculated-as-above value to ensure it has a long and healthy life.
>Looking at my new optical system, my processor will take around 128uS to
>read 4 channels. If I turn on my set of 4 IR LEDS for 128uS, and reading in
>sets of 4 giving a 50% mark-space ratio, how do I go about calculating how
>much current I can dump through the LED's without cooking them. In other
>words what do I need to know andtake into consideration.
Hope that this helps.