Extremely Low Drop-Out Current Limiter/LED Driver

First off, the approach I used here has only one key advantage: it allows for an extremely low minimum voltage drop (~0.12 V) across the driving transistor and current sense resistor, which results in very high efficiency (~97 %) when running at close to or below the nominal voltage of the load device and allows the use of a low-voltage power source. If you plan on stepping down the supply voltage by several volts, then I recommend a dedicated linear regulator or switching converter, but I found it useful for driving a high-wattage LED that ran at just under the voltage of the Li-ion battery I was powering it with, which is something a standard linear regulator could not do. The attached video shows well-regulated LED brightness (indicative of current) across a wide range of input voltages. It would be possible to decrease the minimum voltage drop even further by using a resistive divider to step down the reference voltage, but this would introduce a bit more error.

Links for some of what I used:

U1: TLV2341

D1: 1N5818

D2: G26285

C1

For this circuit, you will need an op-amp of a suitable supply voltage tolerance for your power source, as well as a MOSFET with a Vth small enough to be fully turned on by a high signal from the op-amp. In this case, I would expect to get an ON voltage of at least 2 V from op-amp, so I chose a MOSFET with Vth = 1 V to give it a good margin of error. Generally, MOSFETs don't perform well if operated too close to Vth. D1 sets the reference voltage, to which the voltage across R2 is compared. This is not the most stable voltage reference, and that is one disadvantage to this design, but it has the advantage that the sense resistor voltage can be minute, making the design vastly more efficient and capable of operating at low voltage than any common linear regulator IC. The smaller R2 is, the higher the current limit. C1 is important to prevent oscillation, and for it, I would recommend a ceramic capacitor. The exact value of C1 is not important. D2 just happens to be what I was powering with it - it could be any load component. Please note that if you use the same op-amp as I did, I would recommend operating it in low-bias mode (connect pin 8 to the positive lead of your power source. On a somewhat tangential note, one common source of rechargeable Li-Ion batteries is from disposable vape pens, which can often be seen littering the ground in any location where people go to smoke. I recommend wearing gloves if you open one, as the smell of the vape juice is most vile, in my humble opinion, and it lingers on anything it spills on.

The parts listed in the schematic will work fine for powering a single LED from a 3.7 V battery at roughly 0.5 A, but the input voltage can't go above 8 V because of the op-amp. For a 12 V battery, I would recommend a LM741. The two most important MOSFET parameters are Vth and power dissipation. A higher Vth MOSFET can be chosen if using a higher voltage power source, but for lower voltage power sources, Vth should be as low as reasonably possible. The rated power dissipation of the MOSFET should be greater than the maximum voltage difference you expect to see between your power source and the load times the current. Be careful not to think that the rated MOSFET current is the amount you'll be able to push through this device; that current rating only applies when the MOSFET is fully turned on, not when it is being used as a regulator. Chose your R2 resistor based on the current you want to put through it.

R2 Resistance = 0.12/I

R2 Power = I^2*R

Some notes on construction:

• C1 should go directly from the gate of the MOSFET to ground in as short of a path as possible.
• If your set current is close to the maximum current of your load device, use a slightly higher-value R2 to start out with to make sure it doesn't go up in smoke (the previously mentioned current calculation does not account for variation in components). You can always swap for a smaller resistor later.
• An optional slow-turn-on feature can be added by placing a 10 kOhm or lower resistor between the op-amp output and C1, adding a 1-2 MOhm bleeder resistor in parallel with C1, and increasing or decreasing C1 for the desired time delay. This will slow the regulation, so only use this if powering it with a battery or other very low-transient source.
• Touch something metal to ground yourself before handling the MOSFET.
• If using the TLV2341, don't forget to set it to low-bias mode by connecting pin 8 to the positive voltage rail.
• High power LEDs need to be heatsinked if driving them at high current
• If you expect the MOSFET to dissipate more than 1 W at at the maximum input voltage, it needs a heatsink too
• It's fine to test the circuit without any heatsinks, just don't leave it on for too long

Note: if you don't have means to adjust the input voltage, I recommend powering it up with a 10-20 Ohm current limiting resistor in series with the supply to begin with just to ensure that the circuit is operating correctly before hooking it up directly.

1. Connect your meter to measure the current into the circuit.
2. Power the circuit up with the lowest voltage the op-amp can handle (also needs to be high enough to turn on the transistor after going through the op-amp).
3. Measure the current - it should be about the same as hooking the load device directly to the supply. (I'm assuming this voltage would produce a current in the load device smaller than the set current).
4. Increase the voltage to the nominal voltage of your device.
5. Measure the current - this should be your set current. If it is different, try swapping out R1. A larger resistance will set the reference voltage lower, causing the op-amp to compensate by decreasing the current. A smaller resistance will do the opposite.
6. If the previous step was successful, increase your voltage to the maximum voltage which the circuit will ever be powered by. For me, I tested 7 V, since that is only 1 V below the maximum voltage my op-amp could handle. Also keep the MOSFET's power dissipation rating in mind. More voltage drop -> more power dissipated in the MOSFET. You can generally dissipate around 1 W in a high power transistor without needing a heatsink, but above that, it would be wise to use one.
7. Measure the current. If it is the set current or very close, then your circuit works. Good job!
8. Fine-tune the current by changing either R1 or R2 if needed. Making either resistor smaller will increase current and vice-versa.

For this step, I soldered the circuit on a piece of prototyping board, added a switch and a lithium ion battery, and stuffed it into an old toothpick box to make a portable mini-blacklight. For any permanent assembly with continuous operation where significant power dissipation occurs, I recommend a heatsink for both the LED and the MOSFET; especially the LED. I didn't use one because I set the current to only about 0.5 A, the input voltage is very close to the driving voltage, and the LED does not need to operate continuously.