Electronic – Discharge risk of an LED fade-in circuit

Something like this could be very efficient (not much more than the LED draw) with a CMOS op-amp. For fairly constant visual brightness (+/-10% current) you could derive the 100mV with a voltage divider from the battery, or use a low current reference such as the TLV431 which would require another 100uA or so but would make the current very constant and accurate (of course the 1.24V output of the TLV431 would have to be divided down to the 100mV).

Q1 is a logic level MOSFET rated for 3V drive. U1 is a CMOS single-supply op-amp with R-R output. Careful layout is required around R2 so that trace resistance does not affect the current sensing.

^{simulate this circuit – Schematic created using CircuitLab}

In operation the op-amp drives the MOSFET gate to maintain a voltage of 100mV at the source so that the current through the LED is 100mV/5\$\Omega\$ or 20mA.

The resistor R1 and capacitor C1 are to prevent oscillation due to the MOSFET gate capacitance.

The minimum voltage across the MOSFET to maintain regulation is Rds(on)\$\times I_{LED}\$ + 0.1V- with a suitable MOSFET with low Rds(on) that won't be much more than 100mV, so it should work down to 3.3V with a 3.2V LED Vf.

The most important thing to understand is that LEDs (if considered simplistically) drop a constant voltage of somewhere between 0.7 - 5 volts. In the datasheet this is shown as the forward voltage as 1.9 for the red and 3.3 for the green and blue leds.

They also don't offer any resistance. So it does not matter what voltage you apply to the LED so long as you limit the current. So if you were to apply 1000V to an LED it would drop say 5 volts across the LED and 995 volts throughout the rest of the circuit (so long as you limit the current and don't blow up the LED).

Your calculation is not quite correct because you did not take the forward voltage of the LEDs into account. So for example, if you want 20 mA through the red LED you need to go:

$$R = \frac{12 - 1.9}{0.02} = 505\Omega$$

But often LEDs are driven by current sources rather than limited by a resistor which is probably what your LED controller is doing but you haven't provided a link for that.