TN0026 datasheet(3/9 Pages) STMICROELECTRONICS

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TN0026

LED balancing circuits

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Figure 1.

LED balancing circuit 1- two string resistor divider

1.2

LED balancing circuit 2

LED balancing circuit 2 (see Figure 2) contains two bipolar transistors, eight resistors and

four diodes. The current passing through each load is primarily the current through one

transistor.

LED balancing circuit 2 uses LED loads D1 and D2, D3 and D4, and a power supply output

voltage with a constant output current that provides the circuit input voltage (VIN). As each

string contains the same number of LEDs, any number of LEDs can be used.

The voltage at V1 is the input voltage, VIN, minus the load voltage VD1 plus VD2. The

voltage at V2 is the voltage drop across diodes D7 and D8. The voltage at V3 is the voltage

at V2 minus Vbe2. The current through R7 is the voltage V3 divided by the resistor R7.

Conversely, the current through R4 is the voltage at V6 divided by R4. The total LED current

is approximately the sum of the current through R7 and R4.

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 two diodes in series

(D7 and D8) which determines the current through the emitter resistor (R7).

If LED (for example D1) in one string is open, the base current for Q2 is zero, Q2 is switched

off, the current through R7, which normally flows through D3 and D4, is zero and the current

through D1 and D2 is zero. A small amount of current, (Vin -VD3-VD4-VD5-VD6)/R2, flows

through D3 and D4.

TN0026 Datasheet(PDF) 3 Page - STMicroelectronics