LED balancing circuits

TN0026

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Figure 2.

LED balancing circuit - two string, reference

1.3

LED balancing circuit 3

LED balancing circuit 3 (see Figure 3) balances the current in two or more parallel strings of

series loads where the individual loads vary in voltage, because one is open or one is

shorted. This circuit contains one bipolar transistor and resistor per string, a voltage

reference and a resistor as reference. The current passing through each load is the current

through one transistor.

LED balancing circuit 3 uses LED loads D1 and D2, D3 and D4, D5 and D6, and a power

supply output voltage with a constant output current that provides the circuit input voltage

used.

current through R1 is the voltage V1 divided by resistor R1. Total LED current is

approximately the sum of the current through the emitter resistors R1, R2 and R3.

As with LED balancing circuit 2, the current through each string remains approximately

equal, even if the LED voltage varies or if one is shorted. This is due to the relative constant

voltage of the reference diodes (D7 and D8 in series) which determine the current through

the emitter resistors.

If a LED (for example D1) in one string is open, the current through the string (D1 and D2) is

zero. The base to emitter junction of Q1 behaves like a diode and a small amount of current

voltages drop, and less current flow through the other strings (see Equation 10).