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White LED drivers feature dual-gain architecture

Posted: 31 Mar 2006 ?? ?Print Version ?Bookmark and Share

Keywords:LM27951? LM27952? LM27964? white LED driver? National Semiconductor?

National Semiconductor Corp. introduced three new inductorless white LED drivers for display backlighting in cellphones, PDAs, portable gaming devices and MP3 media players. The LM27951, LM27952 and LM27964 are said to reduce the size of LED lighting solutions.

These white LED drivers feature a dual-gain architecture that enables the regulators to automatically select between one of two gains from input voltage to output voltage. The selection is based on the input voltage of the regulator and the forward voltage of the LED.

For uniform brightness across the display, the drivers offer current matching at just one percent between each LED. The products are adaptive 1.5X/1X switched-capacitor current drivers that include pulse-width modulation brightness control. The LM27964 also includes an I2C-compatible interface to control the brightness of the LEDs, for a total output current of 180mA.

Specs of LM27951 and LM27952
The LM27951 and LM27952 charge-pump-based drivers are capable of driving up to four LEDs with 30mA through each LED. The LED drive current value is programmed by an external resistor. The drivers operate over an input voltage range of 3V to 5.5V and require four ceramic capacitors.

According to National, 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 said to be 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, 750kHz switching frequency optimized for portable applications. The drivers consume less than 1A of supply current when shut down.

LM27964 specs
The LM27964 charge-pump-based driver drives up to six LEDs in parallel along with multiple keypad LEDs, for a total output current up to 180mA.

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 power up to 16 keypad LEDs with a current of 5mA each. The LM27964 has an I2C-compatible 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 2.7V to 5.5V. National said that 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.

The charge-pump-based white LED drivers are available now. The LM27951 and LM27952 are packaged in thermally-enhanced 14-lead LLP packages and are priced at $1.50 each in 1,000-unit quantities. The LM27964 is available in a thermally enhanced 24-lead LLP package and priced at $1.86 each in 1,000-unit quantities. Lead-free package options are also available.




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Related Datasheets
Part Number Description Category
? LM27952 White LED Adaptive 1.5X/1X Switched Capacitor Current Driver from the PowerWise? Family ICs, Interface
? LM27964 White LED Driver System with I2C Compatible Brightness Control from the PowerWise? Family ICs, Interface
? LM27951 White LED Adaptive 1.5X/1X Switched Capacitor Current Driver from the PowerWise? Family ICs, Interface

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