National Semiconductor Optimizes Multiple Output Current-Regulated LED Drivers to Produce Balanced White-Backlight in Color Displays

Business Wire, Feb 13, 2001

Business Editors/High-Tech Writers

SANTA CLARA, Calif.--(BUSINESS WIRE)--Feb. 13, 2001

National Semiconductor Corporation (NYSE:NSM) today introduced a series of constant-current switched capacitor power converters optimized for white LED (Light-Emitting Diode) display lighting. National's charge pumps can power both backlight and frontlight LEDs found in color displays for mobile handsets, PDAs and digital cameras.

Designed to regulate multiple outputs matched to within 1% through each LED, the small, cost-effective ICs eliminate the need for inductors or current limiting resistance. National's LM2791-H/L offers PWM and a 5:1 ratio analog brightness control, while the LM2792-H/L provides zero to full current analog control over brightness. With National's versatile brightness control, the 'coolness factor' of mobile handset displays can be enhanced by dynamically controlling brightness for new effects.

"Color displays using white LEDs already dominate the mobile phone landscape in Japan and it's inevitable that similar displays for the rest of the world will follow," said James Schuessler, product marketing manager for National Semiconductor's Portable Power Group. "Long life and low power are significant advantages over noisy, short-lived electroluminescent backlights or costly, power-draining cold cathode fluorescent tubes. White LEDs are the de-facto choice where color purity and ruggedness are required.

National's white LED drivers extend brightness control over the full range of human perception. Either analog or digital inputs are available by programming the brightness pin through an external DAC or using a digital PWM signal at the shutdown pin. The devices' high switching frequency keeps conducted noise spectrum away from sensitive RF frequencies. Varying the value of the external set resistor to generate a constant current also can be used to vary the brightness or for temperature compensation. The output current linearly tracks the input voltage. The LM2791-H/L and LM2792-H/L are voltage doublers followed by a multiple output current mirror. The LM2792 contains zero offset, allowing the LED brightness to range from fully off to maximum current and giving the widest brightness range possible.

The LM2794 and LM2795, scheduled for release later this year, will offer four independent outputs. All four parts offer a choice of active-high or active-low shutdown control, and brightness offset voltage. All accept inputs from 2.9V to 5.5V and maintain a constant current set by a single external resistor. Typical operating current is just 1500 microamps. Current drops to 2 microamps in shutdown phase.

A 10-pin LLP (Leadless Leadframe Package), measuring just 3mm x 3mm x 0.8mm, houses the LM2791 and LM2792. The LM2794/95 will be offered in a Micro SMD 14-pin package.

Pricing and Availability

The LM2791/92 series is available now and is priced at $1.20 in quantities of 1,000. The LM2794/95 will be available in Q2/01. For more information on these products, visit National's World Wide Web site at http://www.national.com/pf/LM/LM2791.html, http://www.national.com/pf/LM/LM2792.html, or contact National's Design Support Group at 1-800-272-9959.

About National Semiconductor

National Semiconductor is the premier analog company driving the information age. Combining real-world analog and state-of-the-art digital technology, the company is focused on the fast growing markets for wireless handsets; information appliances; information infrastructure; and display, imaging and human interface technologies. With headquarters in Santa Clara, California, National reported sales of $2.1 billion for its last fiscal year and has about 10,500 employees worldwide. Additional company and product information is available on the World Wide Web at www.national.com.