# LED R(d) and V(f) Values

I’m building an IR LED light out of these IR 950nm LEDs:
https://www.digikey.com/product-detail/en/osram-opto-semiconductors-inc/SFH-4725S/475-3012-1-ND/3767470

Using this LED driver which is purposely overkill:
https://www.digikey.com/product-detail/en/stmicroelectronics/LED2001PHR/497-13825-1-ND/4250490

You can calculate passive component values for the LED driver using a GUI called Estudio by STM. The two values I need help calculating and understanding are:

1. The Forward Voltage of the LED. According to Digikey (Vf) is 2.75V. However, a table in the datasheet says the forward voltage for I(f) = 1A and T(p) = 100uS is “typically 2.65V and a max of 3.2V”. So what voltage do I need to apply for the LED to turn on? Somewhere between 2.65V and 3.2V when I(f) =1A for one LED? I’m not sure why they give it for a 100uS pulse, see attached table.

2. The Gui asks me for a Rd value, what is the Rd for this LED?

My approach: The datasheet only gives a graph of the forward current versus the forward voltage for a 100uS pulse. However, I will have my LED on for many minutes at a time if not longer, not a pulse so I’m unclear if I can use this data for calculations but no other information is provided. The datasheet says this LED has a forward current max of 1A. So if I look at the chart for a 100uS pulse where the voltage intercepts 1A of current I get 2.66V. Therefore: 2.66V/1A = 2.66 Ohms. Is that a reasonable number for R(d) and should I expect that the resistance will change dramatically if I have the LED on for a long time (not pulsed). Why do they only provide a 100uS pulse and not a sustained V vs I graph? I attached a picture of the graph I used from the datasheet.

Any help appreciated!    You don't power LEDs by applying some particular voltage, you power them by passing a controlled current through them. Typically you have some stable supply voltage, say 5V for example, you calculate a series resistor for the LED to give the required current.

For example if you wanted 100mA through your LED and you have a 5V supply:
Subtract from the 5V available the voltage that will appear across the LED, which is Vf from the datasheet, so
5V - 2.75V = 2.25V
2.25V is the voltage that will appear across the resistor, so you now need to calculate a resistor value such that with 2.75V across it you will get 100mA.
2.25V / 0.1A = 22.5 Ohms
Typically the exact current is not critical, so you would maybe use 22 Ohms as that is a standard value.

The alternative is to use some kind of current source, which is what any LED driver is at its heart.

I have not studied the data sheet for the LED you are proposing to use, but if it is specifying parameters for 100μs pulses then that suggest the application is for things like IR remote controls, not for illumination. I doubt it would withstand 1A for long before it overheated. You need to know what the continuous current is that it can withstand if you are using the LED for illumination.

As for the calculator for the LED driver, find the Vf for continuous current and use that in the calculation, or use the middle value if there's a range.

You can’t run that LED from a simple resistor, you do require a constant current driver like the one you have linked to. Yes the LED will take 1A continuous there is a graph showing that, providing the temperature of the LED chip is kept below 100 degrees C. So the LEDs will need a heat sink.

You will need to make that driver work correctly, the circuit is basically a buck converter. This needs a PCB and proper layout to be stable and the inductor needs to be chosen so it will not saturate at maximum currents. The design and construction of this is not an easy task, not a beginners project.

The gui you speak of ( you provide no link so I might be wrong ) sounds like it calculates the resistor value for a simple series resistor current limiting circuit which is not suitable for that LED. This is because the forward voltage will change with temperature and will also change over time as the LED ages.

++Karma; // Thank you Mike, that's a much better answer than mine.

Grumpy_Mike:
You can’t run that LED from a simple resistor, you do require a constant current driver like the one you have linked to. Yes the LED will take 1A continuous there is a graph showing that, providing the temperature of the LED chip is kept below 100 degrees C. So the LEDs will need a heat sink.

You will need to make that driver work correctly, the circuit is basically a buck converter. This needs a PCB and proper layout to be stable and the inductor needs to be chosen so it will not saturate at maximum currents. The design and construction of this is not an easy task, not a beginners project.

The gui you speak of ( you provide no link so I might be wrong ) sounds like it calculates the resistor value for a simple series resistor current limiting circuit which is not suitable for that LED. This is because the forward voltage will change with temperature and will also change over time as the LED ages.

Thanks Mike, The GUI calculates inductor, capacitor values, and does a stability analysis. Please see two attached pictures which will give you an idea of the GUI inputs and outputs.

Here's a link to the GUI but because it involves signing up, checking your email, and running a simulation I figured it was far too involved for someone to follow up with: