High-Efficiency Negative Charge-Pump Circuit for WLED Backlights

Positive charge pumps, also known as inductor-less DC/DC converters, are very common in white LED drivers. They are less expensive and simpler to use, but they usually achieve a lower efficiency than inductor-based boost circuits. In this paper, we describe a novel negative charge-pump design for a white LED driver that can automatically select the 1X/1.5X mode. Unlike a conventional positive charge-pump circuit, the negative charge-pump circuit is integrated with current sources having an ultra-low dropout voltage, and the current source dropout is typically 80 mV. The negative charge-pump does not require an additional series-voltageregulated transistor to adjust the output voltage, which can extend the operating time of the 1X mode and dramatically improve the efficiency of the lithium-ion battery. In addition, the negative charge pump does not require a substrate selection circuit, which reduces the circuit complexity. The proposed negative charge pump is realized in a 0.5-μm 5-V BiCMOS process.