LED current regulator has low dropout
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 100mV. 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.
LDO constant-current LED driver
The voltage drop on the current sensing resistor r is less than 40mV. The rest of the drop depends on Q3’s parameters.
The nominal LED current here is 7.2 mA at 9V. Increasing to 20V causes a current change of +15%, giving a dynamic resistance of about 10kO.
The value of R1 is suitable for a blue/white LED with a voltage drop in the range of 2.9V - 3.4V. To maintain this current level at other forward voltage drops, change the value of R1 proportionally to the voltage drop change.
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.
- The skinny on LED strips
- Protect power-LED strings from overcurrent
- Overcome the challenges of driving parallel LED strings
- Circuit achieves constant current over wide range of terminal voltages
Peter Demchenko studied math at the University of Vilnius and has worked in software development.