AN4374 datasheet(20/25 Pages) STMICROELECTRONICS

20 / 25 page

LED supply voltage

AN4374

20/25

DocID025366 Rev 2

Moreover, the extra-voltage across the red LED driving generators could cause an

erroneous shorted-LED condition detection.

To avoid these drawbacks, two different approaches are possible:

•

A resistor in series can be added to each red LED. In this manner, the voltage excess

drops across the resistor instead of dropping across the current generators. This

solution reduces significantly the power dissipated by the chip (lower Tj). However, the

total power dissipation does not change and a remarkable part of the power wastes on

the series resistor. This affects the efficiency and also raises the cost of the system due

to the need to dissipate the generated heat.

•

Another solution is to split the LED voltage rail: one for blue and green LEDs (VLED)

and one for red LEDs (VLED_RED), which can be derived using a switching regulator.

This solution is the most advantageous in terms of power dissipation. Voltage rails are

tailored to LEDs they drive and the wasted power is significantly reduced as well as the

heat produced.

AN4374 Fiches technique(PDF) 20 Page - STMicroelectronics