I need to send "messages" over an over-current driven IR-LED. I'm going to realize the over-current part through a logic-level MOSFET IRLD110 (http://www.vishay.com/docs/91309/sihld110.pdf). The IR-LED has a maximum peak-current of 1.5A.
The LED also needs to be pulsed with 56KHz. I don't want to bother the Arduino with this, so I'm going to use a TS555 timer circuit. If I pulse the LED with 56KHz, The LED will be on, respectively off for about 18 microseconds per peak. Will the MOSFET do this? A complete cycle (on delay time, rise time, off delay time and fall time) is 47ns, so I guess the MOSFET won't have a problem with the frequency? I never done something like this with a MOSFET.
And another question while we're at it; since the LED will be pulsed, so only half the time switched on, can I choose the resistor half the wattage it should actually be?
Be careful when over current driving an IR LED. They need to be pulsed in such a way that the average current doesn't exceed the continuous forward current rating of the LED. For example, if the peak current rating is 1A and continuous rating is 100mA, then you can pulse the LED at 1A with maximum 10% duty cycle (not 50%). How to pulse high powered infrared Led
dlloyd, thanks for this hint. Unfortunately, I can't really choose neither the protocol, nor the LED. But the question is, on which time can I average this? Over the message? Over the time from the start of a message to the start of the next message?
Unfortunately, I can't really choose neither the protocol, nor the LED. But the question is, on which time can I average this? Over the message? Over the time from the start of a message to the start of the next message?
This can be averaged any time as long as the max. pulse width limitation isn't exceeded. Its normally done for each and every pulse. You can get the max current ratings and max pulse duration at peak current hopefully from the datasheet.
See page 3 here for absolute maximum ratings on some typical IR LEDs. I'm familiar with the OP293, which has a free air continuous forward current of 100 mA and peak forward current (25µs pulse width) of 2.0 A.
If being pulsed at 2.0A, 56KHz, 50% duty cycle, it would be on 9µs and off 9µs. Although below the 25µs max pulse width, it averages to 1.0A continuous - this exceeds the 100mA continuous limitation.
Therefore, Max duty cycle % = (Continuous Current/Required Pulse Current) x 100
So for the OP293 with 2A impulse, it would be 5% duty cycle or 0.9µs maximum on time with 17.1µs off time.