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The LM27951 and LM27952 provide high efficiency without the use of an inductor by operating the charge pump with dual-gain architecture. Maximum efficiency is achieved over the input voltage range by actively selecting between a gain of 3/2 and a gain of one, based on the LED forward voltage requirements.
The LM27951 and LM27952 use constant frequency pre-regulation to minimize conducted noise on the input. Each has a fixed, 750 kHz switching frequency optimized for portable applications. To save energy, the drivers consume lessthan 1 uA of supply current when shut down.
Key Technical Specifications of National's LM27964 White LED Driver
The LM27964 charge-pump-based driver drives up to six LEDs in parallelalong with multiple keypad LEDs, for a total output current up to 180 mA. Regulated internal current sources deliver excellent current matching to allLEDs.
The LED driver current sources of the LM27964 are split into two independently controlled groups. The primary group of four LEDs can be used to backlight the main phone display, and the second group of two LEDs can be used to backlight a secondary display. A single keypad LED driver can powerup to 16 keypad LEDs with a current of 5 mA each. The LM27964 has an I2 Ccompatible interface that allows the user to independently control the brightness on each bank of LEDs.
The LM27964 operates from an extended Li-Ion input voltage range of 2.7Vto 5.5V. The device provides excellent efficiency without the use of an inductor by operating the charge pump in a gain of 3/2, or in pass-mode. The proper gain for maintaining current regulation is chosen based on LED forward voltage, so that efficiency is maximized over the input voltage range.