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Negative charge pumps achieve inductor-like efficiency for WLED backlights

Posted: 24 Feb 2009 ?? ?Print Version ?Bookmark and Share

Keywords:backlight WLED? pump charge? efficiency inductor?

Offering a small footprint and high light output, white LEDs (WLEDs) provide an ideal backlight solution for small color displays in cellphones and other portable devices. WLEDs, however, do present one difficulty in devices powered from a single-cell Li-ion battery. The operating voltage from most Li+ cells is 3V to 4.2V, while a WLED�s forward voltage is typically 3.5V to 3.8V (at 20mA). Consequently, the voltage output at the lower end of a Li+ battery�s operating range is not enough to bias WLEDs.

Designers conventionally use boost circuits with capacitor charge pumps or inductors to provide the necessary forward bias for WLEDs. Charge pumps are less expensive and simpler to use, but until now, they have also been less efficient than inductor-based boost circuits.

This application note presents a negative-charge-pump design that achieves the efficiency of inductor-based designs while still retaining the simplicity and low cost of an inductorless design.

View the PDF document for more information.

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