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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!

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  • \$\begingroup\$ A reference and a comparator with a healthy dose of hysteresis instead of a latch, wouldn't that work? No oscillation or reset (because of rising no-load voltage) when you choose the hysteresis well. \$\endgroup\$ – ocrdu Sep 10 '20 at 19:46
  • \$\begingroup\$ Hm makes sense, that would work, thanks. Will have to take another dive to find any cheap options for that (comparator with integrated voltage reference? What to choose for an external one? etc). Everything I found so far would be over 1$, without even the power transistor and main voltage regulator \$\endgroup\$ – Seneral Sep 10 '20 at 20:59
  • \$\begingroup\$ Guess a simple LED driver IC like this will be the best solution... you just need a little glue circuit which sets enable = gnd when the battery is too low (or high wenn its good enough). ti.com/lit/ds/symlink/… \$\endgroup\$ – schnedan Sep 16 '20 at 22:08
  • \$\begingroup\$ Thanks, although that seems overkill for what I'd need it for - a constant current driver with integrated enable pin. The undervoltage detection is still a problem. For now I've opted for a LD1117 to try if the 1V voltage drop is enough to turn off the leds reliably. Unfortunately that makes the cutoff voltage dependant on LED color - with 2V forward voltage I hope to get a cut off at 3.0V (0% battery). At this point, if I can solve it with such a cheap, simple setup, I'd go for that for now. Will report how it works. \$\endgroup\$ – Seneral Sep 18 '20 at 2:46
  • \$\begingroup\$ So as I said I ordered the LD1117 which has a dropout voltage of 1V min, and drove 5 LEDs at 100.8mA with a forward voltage of 2.0V using a simple circuit (LD1117, 12.4Ohm Resistor, 5 LEDs). It happily allowed discharging the battery down to around 2.87V (measured after the fact), just barely dimming the LEDs. But they certainly don't cut off. I don't know what the voltage on the battery was during the test, but I don't think it would be above 3.0V. So what happens here? Is the LD1117 oscillating (resulting in a dim) or are some parts not to spec? Certainly not safe to use... \$\endgroup\$ – Seneral Sep 26 '20 at 14:54
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Most small LiPo cells sold for electronic hobbyists (not RC flyers) have a built-in protection circuit like this one from Adafruit. It's a small PCB under that yellow mylar tape at the end of the battery. It already has an undervoltage cutoff function to protect the battery, so you don't need to duplicate that function. It does not oscillate, and only reactivates when a charging voltage (well above the cutoff voltage) is applied to the wires. You should inform DIY builders of your design to only use LiPo's that include built-in protection.

With that out of the way, you can use any LED driver that meets your specs.

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  • \$\begingroup\$ The RC LiPos are my target, because that's the only source of cheap lipos in many different sizes you can get everywhere in the world. Relying on adafruit is a bit annoying, because their shipping can get pretty expensive around the world. Looking at the typical 1S RC Lipos, they do seem to have some kind of PCB, but I don't want them to be discharged down to 2.4V until the emergency cutoff triggers. I want to have some kind of backup to ensure the LiPos last, so I'll at least try to get a proper protection on that board. \$\endgroup\$ – Seneral Sep 10 '20 at 18:43

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