LED string lights driven by current source/mirror
The design concept in Figure 1 is a precision current mirror used to drive two strings of five white LEDs.
Figure 1 Current source/mirror driving two LED strings
The op amp has "double" negative feedback (because M2 inverts the signal at the non-inverting input). The voltage drops across R1 and R2 are equal, halving the LED driver current. MOSFET M2 operates with a constant source-to-drain voltage (equal to V GS1 + V GS2 ) while M1 absorbs the mismatch in the voltage drops of the two strings, even in the event of a short circuit of the LEDs.
To protect the LED string from overcurrent due to an open circuit in one of the strings, you can add two diodes and a transistor as shown in Figure 2 and in my previous design idea "Protecting Power LED Strings from Overcurrent" ” is described in.
Figure 2 Anti-open circuit protection
Capacitor C1 and resistors R3 and R4 ensure the stability of the op amp. The voltage drop across R1 and R2 must be appropriately greater than the voltage of the offset op amp to achieve good accuracy in dividing the LED driver current. The current ratio I1:I2 is only equal to the resistance ratio R1:R2.
This circuit is also suitable for driving power LEDs. In this case, you should use MOSFETs with low threshold voltages to limit power dissipation, or you can replace them with a pair of complementary BJTs.
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