# How do I calculate the efficiency of a linear constant current LED driver?

I'm trying to select a driver design for a LiPo battery powered 3W LED light. I'm looking at switching options, but see that there are discrete linear options such as this one. How do I determine efficiency vs a switching design from the data sheet?

If I'm only looking to power a single LED (implying a Vf that's slightly below that of a LiPo battery), does a Linear CCR become an effective option?

• That one needs 1.8V (above the LED voltage) to work. Commented Dec 12, 2017 at 5:04
• Define Vf vs If and Vbat min Commented Dec 12, 2017 at 5:33

This is a straightforward calculation, since a linear regulator works by acting as a resistor--just the right resistor. The power it dissipates is the voltage across it times the current through it. If you put this in series with a 3 volt LED across a 10 volt supply, there will be 7 volts across the regulator.

In fact, since the current through the regulator is the same as the current through the LED, the currents will cancel out in any ratio you calculate, and so you can just use the ratio of the voltages to get your answer. In this example, 3v/10v or 30% of the total power goes to the LED, while 7/10 or 70% is burned in the regulator.

• +1 The maximum efficiency is Vout/Vin. There may also be an additional term for the power used by the regulator itself, if it isn't dumped into the load. Commented Dec 12, 2017 at 14:40
• I like to also put it into LTSpice, so I can play with it and verify my numbers. I play with discrete regulators that are not completely inline and so the simplification above is not accurate (currents don't cancel out), although the numbers are only a few % off, max. The calculations for my discrete circuit applied to your situation ranges from 74 percent efficiency at 4.1v, going up to to 94 percent efficiency when your LiPo goes down to 3.3v, my numbers for your "ballpark" double-check. So, actually not too bad, and that's because we're not skimming that much voltage off the top. Commented Oct 15, 2018 at 17:38

Efficiency = Power out/in

Inefficiency is waste power out of total input.

If your current flows thru LED, FET and Rsense then only the total waste voltage (*I) drop counts as inefficiency and thus you choose those parts to minimize this as I said before in your previous question.. Make it 50mV max for R sense and FET RdsOn @3V

A buck regulator stores current so that the DCR losses are low so the efficiency can be high.