TN0026 Datasheet, PDF (1/9 Pages) STMicroelectronics – LED balancing circuits

TN0026
Technical note
LED balancing circuits
Introduction
Light Emitting Diodes (LEDs) are often used with an output voltage source from a power
supply. The output voltage increases until the current through the LEDs reaches the value of
the constant current source from the power supply. Maximum output voltage is limited by the
power supply.
LEDs are used individually and when more light is required, LEDs are stacked in series with
the same current passing through each one. The voltage across the string increases with
each LED added. After a number of LEDs are added in series, the power supply may not be
able to handle the higher output voltage required. Also, a change of output voltage could be
a problem for the power supply if the range of LEDs required is high enough to increase the
range of the output voltage and auxiliary circuits.
Various circuits can be used to balance the current in parallel strings of series loads where
the individual loads vary in voltage, because LEDs have been shortened or opened. Without
a circuit, the variation of the parallel loads could cause most, or all of the current to pass
through the path of the load with the lowest voltage and a small amount of current through
the path of the load with the highest voltage. The result would be a variation in light output
and could cause LED failure from excessive current. The objective is to achieve the same
current to pass through each parallel string of series loads even though the voltage across
each load is not the same.
The following technical note is a description of three circuits which can be used to balance
the current through two or more strings of LEDs. The power supply output voltage (LED
circuit input voltage V_in) varies, up to a limit, while providing a constant current to the LED
circuit. All of the circuits will balance the current in LED strings and protect for a shorted or
open LED.
A power limit circuit is also described which limits the power of the LED balancing circuit
when there is a large voltage variation for parallel strings of LEDs.
April 2007
Rev 2
1/9
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