This Design Idea circuit regulates the current through one or more LEDs, making it almost independent of supply voltage. Its main advantage is a very small dropout voltage, which can be less than 100 mV. The design could find use on LED strips, where the voltage can vary along the length due to resistive drop, and small voltage changes result in large current changes.
|Figure 1.||LDO constant-current LED driver.|
The voltage drop on the current sensing resistor r is less than 40 mV. The rest of the drop depends on Q3’s parameters.
The nominal LED current here is 7.2 mA at 9 V. Increasing to 20 V causes a current change of +15%, giving a dynamic resistance of about 10 kW.
The value of R1 is suitable for a blue/white LED with a voltage drop in the range of 2.9 V - 3.4 V. To maintain this current level at other forward voltage drops, change the value of R1 proportionally to the voltage drop change.
The current through the LEDs is inversely proportional to the value of r. Current can be roughly changed with this resistor, and finely tuned by varying R1.
To obtain good current stability over temperature, Q1 and Q2 should be in good thermal contact. Ideally, they should be on the same die, but the discretes here show good results when placed close to each other.
The circuit performs well with only one LED. The maximum number of LEDs in the string is only limited by the parameters of the circuit components.
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