I'm searching for a way to drive a couple LEDs with a single-cell LiPo battery, such that the LEDs will be cut off completely when the LiPo voltage drops below, say, 3.3V. PCB space, component count and price are critical, as the project is low-volume DIY stuff. Anything else but under-voltage protection is not required, the lipos will be charged when disconnected from the device, and the batteries are not assumed to have any built-in protection circuit. The LiPo choice is fixed at one cell with low capacity (around 150-350mAh), LED forward voltages will vary, but range from 2.0V to perhaps 2.8V, and I will use multiple of those LEDs (of the same type) in parallel so the total current might get as high as 200mA.
I'm pretty sure there are several good options for this since it seems like a common problem, but I found multiple ways and am unsure what is the best way to do this.
I've researched simple under-voltage detection systems to cut off the LEDs once voltage drops below a threshold, plus a latch since the voltage might rise again after load drops, combined with a constant current driver afterwards. But I'm not sure about startup behaviour, I fear it might be triggered once the battery is connected, and the complexity is quite high. There are two general ways I found to do it. First, a comparator with integrated latch and voltage reference, however I haven't found an IC that includes both, just references that they exist. My hope is to connect the output to the latch enable to lock the result. Second, to detect undervoltage with just transistors, with a latch afterwards (either transistors or IC). Unfortunately that's a lot of components and I'm not sure that works as expected
However I also reserched using voltage regulators with adjustable voltage (like the LM137 series) for constant current drivers and found that I might be able to use the voltage drop to cut of the LEDs without a separate undervoltage detection circuit. This would be possible using a LDO with adjustable voltage (not sure if these exists so you can create constant current drivers like with the LM137 series), with a resistor to drop the LiPo voltage down so that the difference between the LED voltage and the desired cutoff voltage coincides with the voltage drop. However, I haven't found a good choice of IC, since the LM137 voltage drop is too high to power LEDs with a 1S LiPo. Also I am unsure how strong the LiPo voltage difference is once the load is removed - will the voltage regulator start oscillating? I have found a few comments referring to this but I don't know if this is recommended.
What is the recommended way to do this, if there is one? Are any of the ways I mentioned not recommended? I don't need certification or anything but I don't want people who rebuilt this to start a fire because the LED driver keeps draining the LiPo to a point where it gets dangerous. It should be obvious that the battery is drained, the LEDs off. Any input is appreciated, thanks!