Please make sure you check the schematic, as this one may contain errors! I shall not be responsible for any damage to your chip. This post is about the experimentation phase only. Best to start from the spice model provided by Linear and make your own modifications to it.
I took the schematic directly from the LT1618’s datasheet and modified the LTSpice model a bit. I moved the current sensing resistor from the input side (which has advantages as well) to the output side. That way it can be used as a constant current driver. The “SHDN” pin can be connected to V(in) as well, it doesn’t have to be 3.3V. For lack of a model for the 12V LED bar I just used a few other LEDs.
One interesting an annoying thing that showed up during testing is that the switching operation of the chip seems to confuse my power supply’s voltage regulation. More decoupling necessary. Luckily a battery doesn’t care about such things. The brightness of the LED bar was stable although I operated the converter without a current sensing resistor. I just couldn’t find 0.22Ω anywhere. So I will skip the LC output filter I had planned to add.
This circuit here operated a 12V 3W LED bar at 270mA. Input voltage was 4.3V and it was drawing a current of 1.35A. The chip was getting a bit warm to the touch.
It was also really nice to see how it regulates. Set a current for the LED bar and change the input voltage of the circuit. As power should be more or less constant, it automatically adjusted the current it needed.
Here’s a crappy video. I really should get a camera with autofocus that also works in video mode, not just _before_ you start the clip.
Just for the record: the LT1618 is a versatile chip, but I’ve just managed to kill it. And I don’t know how, maybe ESD. The failure mode is especially unpleasant. It starts switching normally for a second (the LED turns on) and then stops. But it just doesn’t shut down, no, it keeps drawing current through the inductor. I’ve looked at it with my old scope. First second or so: heavy switching, after that: several amps of DC current. That creates a lot of heat. I burnt my temperature probe while monitoring the chip for abnormal values. My pinky still hurts. Next time I’ll use the other hand. So now I’ll have to get me a bunch of these chips to keep going. The good thing is that the adapter board has survived ;-)