First of all, although I couldn't find any similar question(s), excuse me if one has been asked before.
I want to make an LED driver for 2 different LED strips. One needs about 45 VDC and the other needs about 54 VDC. The manufacturer recommends using 150 mA constant-current driver.
So I'll make a single driver operating from 18-32 VDC for driving both. Note that only one of them will be driven at a time, not both at the same time.
Anyway, I have an LM2577 on hand and I want to use it as a constant current driver despite it is a voltage regulator. Of course there are many ICs on the market for this purpose, but I want to use this IC if possible. So here's my concept design:
simulate this circuit – Schematic created using CircuitLab
Since the IC will keep the FB pin at 1.23VDC, I think the strip will be driven with a constant current of 1.23V / 8R2 = 0.15A.
Will this work? Is there anything wrong with this? Any disadvantages or potential faults? Needs any improvements?
Best Answer
This will work with some modifications. First, the inductor needs to be increased to 680uH according to the datasheet, or else system may be unstable. Output cap should be rated for 100V. If you get flicker, you may need to increase size of output cap.
You need to select compensation resistor value of about 1K based on your load, min input voltage and max output voltage. Feedback cap will need to be computed according to the datasheet.
Also, since Vf of LEDs varies from one lot to another you need to verify if 54V is typical, or max. If typical, than you need to determine max. Even with typical value, you are closing in on the maximum operating voltage of 60V for the IC, as LEDs will require minimum of 54V plus 1.23V across Rsense plus 0.5V across diode. Also keep in mind that Vf of LEDs increases with increased temp, so you may get very close to the operating limit of the IC. If you exceed it, this may result in damage to the internal switch, or flicker if the output voltage does not get high enough.
Good luck and enjoy.