How to implement small LCD backlighting--LED driver considerations

Color LCD displays require a white backlight for proper viewing in any lighting environment. This backlight subsystem consists of an array of high brightness white LEDs, a diffuser to distribute the light and a backlight driver to convert the available power into regulated constant current to drive the LEDs. A 1-1.5" display may have 2-4 LEDs while a 3.5" display could easily have 6-10 LEDs.

For LEDs, the light output is proportional to current and since LEDs have a very steep I-V curve it is important that the current through the LEDs be closely matched to assure even lighting since LEDs are typically distributed across one edge of the LCD display. In addition software control is necessary so the user can adjust the brightness and compensate for ambient light conditions. The color point of the LED can shift depending on the current through the LED so it is common to set the LED current to a fixed value and pulse width modulate the LEDs to reduce the average light output. There are a number of factors that need to be considered when incorporating a small color LCD display into a design to achieve the right balance of cost, performance and battery lifetime in a handheld product.

Battery powered products require optimized LED driver architectures that deal with the simultaneous challenges of limited space, need for high efficiency, and a varying battery voltage that may be either greater or less than the forward voltage of LEDs. Two common topologies are used, a charge pump architecture/constant current source architecture where the LEDs are in parallel, and an inductive boost architecture where the LEDs are connected in series. Both approaches have tradeoffs that need to be considered, a boost architecture ensures that all the LEDs have the same current flowing through it but requires an inductor for energy conversion while the charge pump uses small capacitors for energy transfer, but all the LEDs are in parallel so tight current matching is critical for even backlighting. Examples of both approaches are illustrated in Figure 1.

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