LED driving constant current swithcher

[eanema]

Hi All,

I'm trying to build a constant current boost converter to drive a chain of LEDs using an LT1172 switcher. (Datasheet)

This circuit operates by attempting to output what ever voltage the circuit requires to maintain a voltage of 1.24v at the feed back pin. In this case that requirement is only met with and appropriate current through the 68ohm resistor.

I have built the "new circuit" above without the zener and every thing is great. But in prototyping I managed to blow one of my switchers. It happened when I put too many diodes into the LED chain and the circuit attempted to create a voltage too high for the switcher to handle.

Next I got a board printed with three copies of this circuit sharing a common supply (think: red green and blue). After I soldered every thing together I tested each circuit in turn. The first one worked, then the second one, finally the third but when the third one was working, I noticed that the switcher and the inductor for the first two circuit had failed, with the switch open, shunting a lot of power through the inductor to ground.

I replaced the one of the switchers with a spare and every thing was fine again. I *think* that they failed because I operated them with an open circuit for a load. I'd like to prevent this and I'm thinking that a 75v zener in parallel with the LED chain would act as an ~infinite load until the output reaches 75v at which time it will prevent it from going any higher.

Does this sound reasonable?
Are there any other thoughts why this circuit might fail?
Any other possible methods to fix the problem... these switchers are about $10 a piece and I'd like my circuit to be fairly robust.

Thanks for any and all input to come

 

[eanema]

My mistake... I forgot to attach the schematic that I was referring to.

Sorry, here it is

 

[KJ6EAD]

It looks like you've copied component values from the data sheet boost circuit example. These values may not be correct for your intended supply voltage/load/heat sinking.

 

[eanema]

as far as component values go, I had originally used the inductor picker on the coil craft website which gave me 300 uH. I couldn't get this to work properly, so i used the 50uH from the data sheet and it worked perfectly. The capacitor is not a critical component value, because it just controls the ripple.

As for heat sinking, when this circuit is operating normally there is very little heat produced - the package appears to be at room temperature. I don't believe that the circuit is overloaded, and there may be a problem with component values, but I don't think so...

 

[leo_o2]

When you put more LEDs, the feedback loop will increase the output duty cycle and increase the output voltage. Finally, the chip is breakdown at Vsw node. I think a OVP (Over Voltage Protection) detection is needed for Vsw node.

 

[eanema]

Yes, agreed. That's exactly what I was thinking.

Would the zener not function as an overvoltage protection? It seems to me that the circuit would function as if the zener was not present until it reaches the breakdown voltage - say 75v (the switcher is rated to 80v). At this point the zener would conduct shunting all the current through the 68 ohm resistor. The switcher would limit the current to ~20mA (or 1.24v across the resistor). The LEDs would not light, but the switcher would be saved.

Does this seem reasonable?