I want to drive multiple strings of XTE LEDs (total of ~36 LEDs). The relevent (-ish) specs are as follows:
- I_f,max = 1.5 A
- Desired I_f = 0.5 A
- V_f = 2.94 V (0.5A @ 85C)
According to some calculations, I found that the larger the voltage dropped across the resistor the more stable the LED current is with shifts in supply voltage. Voltage drop ratio between resistor and all LEDS in a single string determines some part of current stability. Is this generally true when looking at voltage source deviations? I have looked around and haven't found something that explicitly confirms this.
But I don't want to use a large resistor as too much is dissipated in the resistor. For simplicity, I want to choose a small [resistance] resistor and use a power supply with tight voltage tolerance (1%, according to some Meanwell HLG specs). Will the right PSU be enough to maintain a fairly consistent voltage and thus consistent current?
Here is my follow-up concern, will thermal runaway be a large concern because of the small resistors? Say I use 2 strings x 18 LEDs = total of 36 LED. This is using a constant Voltage source, not a CC source.
How big of a voltage difference should I expect between each string of LEDs due to dissimilarities at the per LED basis?
Would splitting up into smaller series strings like a 4 x 8 LEDs while still using the same ratio of (Voltage drop over Resistor)/(Voltage drop over all LEDs in a string). Is thermal runaway better/worse/no different in this scenario?
Best Answer
The LT3517 may be used with 30 Vin and a string of 9 LEDs. For 36 LEDs four strings with an LT3517 for each string may do.