# Selection of LED Driver

I have a display whose backlight section has 7 LEDs in each of its 4 strings. So, total LEDs are 28.

The total current consumption of the backlight as mentioned in the datasheet is 200mA, typ.

My question is, whether this LED Driver would be suitable?

Asking this because, eventhough the above LED Driver satifies my current rating requirement, in the circuit application images on page 1 of the datasheet, only 1 string of LEDs is connected?

What should I do if I have 4 strings? Can I short all the 4?

• You also need to specify the input voltage and LED string voltage (21v?). But in general the number of parallel strings is irrelevant so long as you design for the correct current since to the driver two low current strings in parallel look similar to one larger current string. Commented Oct 30, 2023 at 12:38

You can drive your 4 parallel strings with this driver.

If there is no way of balancing the current drawn by each string at a given voltage you could get current and brightness variations between strings. However, if the 4 strings were previously driven by another driver they may already have provision for current balancing.

The normal balancing method is to provide a series resistor for each string that drops maybe around 1 volt in each case.
In your case with 4 strings x 50 mA per string = 200 mA
A resistor of R = V/I = 1V/50 mA ~= 20 Ohms would be a good start.
Lower may work well enough.
Higher works better, but wastes more power.

• Thank you for the answer. But, just to clarify, that 200mA is a typical current for all the 4 strings, 28 LEDs together. I had heard that That is a fundamentally bad ideas because small differences in Vf can lead to big differences in the light output of the different strings. What are your thoughts on this? Commented Oct 30, 2023 at 17:23
• @Timerace Yes - I understood that that was the total load. I have slightly edited my answer to clarify that. || My resistor suggestion is intended to help the 4 strings match. The larger the resistors the more balance you get but the brightness drops accordingly. There are not too hard ways of more closely balancing strings BUT what I said above is usually acceptable. Commented Nov 6, 2023 at 11:20
• @Timerace As Russell said, you can add a resistor in series with each LED series. This helps balancing a bit. Do this only if there is no balancing component on the LED series already. See if there are extra components beside the LED, repeated on each series. Commented Nov 6, 2023 at 11:41
• @Timerace If the LED typical voltage is equal or close than input voltage, you can connect the LED series directly with resistor on each series if there is no balancing component yet. It's possible that the display has already a current regulator or PWM regulator to drive the LED's. Check if the current for the LED's comes directly from the power supply of the display or from some dedicated on board circuit. If the LED's are powered at a voltage slightly lower than their typical voltage, then there is little risk of overheating or heavy imbalance. Commented Nov 6, 2023 at 11:49