# Is it safe to limit current and increase voltage above VF for LED

I am designing high power LED Driver. I want to know that if I limit output current as required by a specific LED and my voltage is significantly greater than VF will it be safe for the LED extended operation time.

For example for http://www.cree.com/led-components/products/components/xlamp-cxa1512 LED module if my voltage is 60V and I limit the current thought my driver to 1A.

Will this be a durable design? if not why?

Thanks

• The voltage across the diode in forward direction is $V_f$ (by definition). So what voltage do you want to have significantly higher than $V_f$?
– Curd
Commented Oct 20, 2017 at 7:18
• What is the design of your LED driver? Commented Oct 20, 2017 at 7:25
• @Curd i want my led driver to give output of 60V and limit current to 700mA as required by LED. Commented Oct 20, 2017 at 9:03
• @AltAir I am using linear.com/product/LT3761 for my design. Commented Oct 20, 2017 at 9:03
• @Mudaser: you are not free to choose both current and voltage. Just like you can't say "I want a mass of 5kg water, but have volume limited to 4 liters". There is a relation between both. If you determine one the other one is fixed too.
– Curd
Commented Oct 20, 2017 at 9:32

If your circuit delivers limited current, then the LED itself will 'regulate' the output voltage to whatever Vf it needs at that current and temperature.

In the datasheet you linked to, the forward voltage at any given current is given as typical. You supply the 700mA, and the LED will clamp that to 18.2V, maybe more, maybe less.

If the supply voltage to your current limiter is well above Vf, then your current limiter needs to be designed to handle the voltage difference across it, when passing the required LED current.

If it's a linear device, then it needs to be able to handle the heat dissipation.

If it's a switch mode (SMPS) device, then it doesn't need to handle so much heat, but does need the right inductor value and timings to deliver sufficiently constant current at the input voltage you provide.

The data sheet gives a typical forward voltage. It would have been nice if they had also given an absolute maximum, just in case you were designing a buck SMPS, or if not caring about efficiency, a linear regulator. In those two cases, your supply voltage has to be above the highest expected LED Vf. With a boost SMPS, no such problem arises.

• Look under the documentation tab... actually they have an incredible amount of stuff in there.. including very extensive over-current driving info. Commented Oct 20, 2017 at 7:16
• @Neil_UK thanks for the elaboration. Please further explain the point "If the supply voltage to your current limiter is well above Vf, then your current limiter needs to be designed to handle the voltage difference across it, when passing the required LED current." You are talking about the output voltage or the LED driver right? I am using LT3761A in my design. Commented Oct 20, 2017 at 9:04

The Linear LT3761 may not be the correct choice. If you have a stable voltage input to the driver then a buck driver would be a better choice. Less components, less cost, less real estate.

In general a buck is the least expensive, most efficient, and simplest constant current driver. It requires the Vin be a little greater than Vout. The closer Vin is to Vout, the more efficient it will be.

There is no problem powering your driver with 60v as the only requirement for a buck driver is the input be greater than output voltage.

The Linear Tech part you chose has an input range from 4.5v to 60v. If you are going with the 18V Cob then a 24v input would suffice. If you go with the 36v CoB then you would need an input of 40+ volts. 60v would work for either the only impact would be the efficiency.

If your input voltage must be less than 18 or 36v then a Boost driver would be best. If the input voltage will vary greatly (e.g. solar panel) then a buck/boost would be required.

This would be my choice. This circuit is optimized for 48Vin, 40Vout @ 700mA. Changing Riadj, Rfs, and the inductor easily optimize it to your needs. TI's WebBench can give you the values.