LED Power-Management Strategies For LCD Backlighting

There’s a limit as to how much the input voltage can be multiplied to get to a high output voltage, which is set by the boost converter’s maximum duty cycle. The duty cycle D is defined as the ratio of the switch on-time T_ON to the switching period T (D = T_ON /T), and the input-to-output voltage ratio of the boost converter is:

V_IN / V_OUT = 1 – D

where V_IN = input voltage and V_OUT = output voltage. For example, a maximum duty cycle of 90% is equivalent to a V_IN to V_OUT boost ratio of 10. In other words, from a 3-V supply, the theoretical maximum output is about 30 V.

For a small-size LCD backlight, we may first consider parallel LEDs, where charge-pump drivers offer the easiest design path with small and low-cost solutions. With serial strings of three to 10 LEDs, an inductive boost converter demands more work. But, the end result is slightly better efficiency. There’s not a clear dominance in the market for serial or parallel LED configuration. In the future, we will see LEDs replacing CCFL in backlight applications for larger displays, such as found in car-navigation systems and notebook computers. These larger panels will favor implementation with a serial string of LEDs due to their easier interconnection and the higher efficiency in the driver circuit.