# Light output of LED with various current waveforms (but same average current)

#### treez

Hi

We want to drive 350mA (average) through a Cree XT-E white LED. We want as much light output as possible.

What will be the difference in light output if we drive the LED with…..
..a) 350mA constant DC current (as in the red waveform attached)
..b) 350mA average but it’s a 500Hz triangle wave (as in the green waveform attached.)

The light_Flux vs Current graph at the top of page 27 of the LED datasheet (below) appears to suggest that the maximum “Light flux per Amp” comes at around 100mA.

Do you know what this curve looks like from zero Amps to 100mA?

Datasheet: Cree XT-E Lamp (350mA LED)
https://www.cree.com/led-components/media/documents/XLampXTE.pdf

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#### KlausST

##### Super Moderator
Staff member
Hi,

you might just linearely extrapolate to 0/0.

Thus you might not have much difference in brightness (at least an eye won´t recognize the difference). Even for a luxmeter it should be marginal.

Since there is a resistve component in the LED the power dissipation will be higher at triangle.

Klaus

treez

### treez

points: 2

##### Super Moderator
Staff member
An old article gave a comparison to the effect that subjective brightness is greater when pulsing an led, say at 20mA 50% duty cycle, as compared to 10mA continuous. Power is the same.

This was before 350mA or bright white led's but it should apply today as well.

treez

### treez

points: 2

#### treez

Thanks, i have read that due to "Auger recombination", then when below a certain level of LED current, the light photons just cant all properly escape from the LED silicon, and so at low currents, there is proportionally very much less light output....so in other words, if you view page 27 of the datasheet in the top post (top graph)......the unshown bit below 100mA is a region of very very low light flux output....thats why they dont show it.

Would you agree......if the bit below 100mA was just a plain extrapolation...then they would have shown it...but because it is in fact a region of (proportionally) very poor light output...they dont show it...for marketing reasons. Would you agree?

#### FvM

##### Super Moderator
Staff member
The region below 100 mA isn't shown because it's beyond the normal application of this LED. Other manufacturers show efficiency at lower currents, see e.g. this graph for a 350 mA Osram LED. I added the red line of constant efficiency.

Efficiency is dropping below 100 mA, but not abruptly. Mapping your waveform to this or a similar efficiency curve suggests that you produce most additional losses compared to constant current operation in the range above 350 mA and less below 100 mA. But for an exact calculation, you need to refer to a efficiency curve for pulsed operation (at constant chip temperature) which is rarely given in datasheets. Without it, you don't know which part of efficiency drop at higher currents is caused by temperature increase and which part by instantaneous current.

treez

points: 2