Design of energy-efficient portable display lighting solution

The increasing functionality and power requirements in handheld applications continue to challenge the power of portable batteries. By better understanding the energy consumption of the application, designers can provide energy-efficient solutions that extend battery life and provide a better end-user experience.

Saving Energy in Portable Displays As handheld displays increase in size, resolution, and brightness, display backlighting presents significant energy requirements. For a reasonable display brightness, a typical small-size display module requires 3 to 10 LEDs. For the energy consumption of backlit displays, an efficient power supply technology to drive LEDs is the switched capacitor. This technology connects the input voltage to a multiplying switched capacitor, which produces a regulated voltage output. During open-loop operation, the output voltage is equal to the input voltage multiplied by the gain. Closed-loop operation produces a fixed output that should be slightly higher than the forward voltage of the LED and other voltage drops.

The voltage difference between the output voltage and the forward voltage is the margin, which is monitored to ensure the current passing. The switched capacitor technology consumes the least energy when maintaining regulation by changing the gain according to the LED forward voltage and load requirements over the widest input voltage range, which maintains the gain for the highest efficiency. The dual gain boost mode (1x and 3/2x for LM2755 and LM2756, 2x and 3/2x for LM2757) can maximize efficiency over a wide input voltage range, thereby extending battery life.

Consumer trends continue to show the need for smaller portable devices, which makes printed circuit board area more precious. Another additional benefit provided by the switched capacitor topology is the inductor-free solution, which saves solution size and materials. The LM2755, LM2756, and LM2757 are examples of switched capacitor boost technology that can drive up to 10 LEDs (each device can drive up to 30 mA of diode current). This smaller solution also allows the driver to be placed locally, which can reduce EMI compared to centralized drive methods.

The programmable nature of white LEDs is important in order to control the brightness of a display. For example, when an end user is having a conversation on a mobile phone, they are no longer interacting with the display, so the display needs to have the option to dim the screen. Both the LM2755 and LM2756 contain an I2C compatible interface that can control the brightness of the display based on the operation of the phone.