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National Semiconductor Adds Six New White-LED Drivers to Broad Portable Power Portfolio
Power Management Products Bring White Light to Handhelds Simply and Efficiently
February 11, 2002 - National Semiconductor Corporation (NYSE:NSM) today added six new white-LED drivers to the power management segment of its broad portfolio of chips optimized for use in portable electronics. All six new products are designed for use in handhelds typically powered by batteries, including cell phones, personal digital assistants, digital cameras and similar electronics.
"National's new white-LED drivers will make it easier and simpler for manufacturers to deliver images and text on brilliant full-color handhelds without expanding battery size," said James Schuessler, National's worldwide marketing manager of switched capacitor power converters. "These parts demonstrate National's commitment to both outstanding display technology and innovation in all aspects of power management for portable devices."
The move to color displays in cell phones and other handheld electronics is driving a parallel move to white LEDs for display and keypad lighting. Color displays must be lit with white light for the colors to appear true. When compared to other options like electroluminescent devices and the cold-cathode florescent tubes popular in notebook PCs, white LEDs are more appropriate for lighting handhelds. They're more rugged, more efficient, simpler to drive, smaller, thinner and far less noisy.
"Color displays already dominate the handset landscape in Japan where the popularity of games and iMODE-based web browsing created an early demand for superior information content," Schuessler added. "Leading Japanese cell phone and display makers are currently using National's standard power conversion ICs as white-LED drivers. The six new devices announced today, along with the LM2791 and LM2792 announced last February, are next generation technologies developed for top cell phone makers around the world."
The LM2791, LM2792, LM2794 and LM2795 are constant current drivers optimized to light displays with up to four white LEDs. The LM3354 and LM3355 are constant voltage drivers optimized to drive larger numbers of white LEDs in applications such as keypad backlighting. The LM2703 and LM2704 are magnetic boost converters optimized to drive white LEDs in applications where efficiency is paramount. Together, the eight chips make a complete solution set that addresses all white-LED driver applications and performance requirements found in handheld, portable electronics. Datasheets and other information can be found online at www.national.com/see/wled.
Product characteristics
The LM2794 and LM2795 combine switched capacitor boost technology and matched constant current sources to drive white LEDs. Four current sources matched to (0.5% typical ensure consistent brightness and color among the LEDs, making the LM2794 and LM2795 ideal for driving display LEDs. The typical application of four LEDs driven with 15 mA each is easily realized with the 80 mA available. The drive current is programmable by a single external resistor and both analog and PWM methods of varying the drive current for brightness control are accommodated. Both devices switch at 325 kHz minimum to keep the conducted noise spectrum away from sensitive frequencies within portable RF devices.
The LM3354 and LM3355 also apply switched capacitor DC/DC technology but convert a variable battery voltage to a constant voltage, as opposed to the matched constant currents of the LM279x chips. An innovative regulating buck-boost architecture yields an average efficiency of about 80% over the typical Li-ion discharge range, a much higher efficiency than that delivered by open-loop switched capacitor circuits. The LM335x devices are ideal for driving large numbers of white LEDs in applications where brightness and color uniformity among the LEDs is not a primary concern. A typical application is twelve, 5 mA LEDs backlighting a cell phone keypad. In this application the keypad disperses the light making brightness and color matching a secondary concern and the added cost of individual matched current sources for so many LEDs unnecessary. The LM3354 supplies up to 80 mA while the LM3355 supplies up to 50 mA. Both devices are also applicable as general-purpose regulated DC/DC buck-boost converters. An example application is converting the 2.7V to 4.2V rail supplied by a single-cell Li-ion battery into a regulated 3.3V, 50 mA rail.
The LM2703 and LM2704 are magnetic boost DC/DC converters optimized to drive up to four white LEDs in series and several strings of series LEDs at once. All the LEDs in a particular string are driven with the same constant current resulting in consistent brightness and color among those LEDs. The capability to achieve consistent brightness and color among several white LEDs coupled with the capability to drive large numbers of white LEDs makes the LM270x devices ideal for driving either display LEDs or keypad backlight LEDs or both at once. The LM270x devices drive white LEDs more efficiently than any of the switched capacitor-based drivers. The LM2704 achieves 85% efficiency delivering 20 mA at 20V. The LM2703 has a 300 mA peak current limit, and the LM2704 has a 600 mA peak current limit. Both devices are also applicable as general-purpose magnetic boost DC/DC converters. An example application is the LM2704 boosting the 2.7V to 4.2V rail supplied by a single-cell Li-ion battery into a regulated 5V, 80 mA rail.
Packaging
National's white-LED drivers are offered in a variety of chip-scale packages including National's revolutionary micro SMD and leadless leadframe (LLP) packages. In the case of micro SMD, a package National pioneered, the silicon die is the package giving micro SMD several advantages over traditional plastic leaded packages including reductions in footprint areas on the order of three to nine times, higher I/O density, and better electrical performance. National's LLP packages feature industry-leading thermal capability in dramatically smaller, thinner form factors than traditional power packages. The LM2791 and LM2792 are offered in the LLP-10 package. The LM2794 and LM2795 are offered in the micro SMD-14 package. The LM3354 and LM3355 are offered in the MSOP-10 package. The LM2703 and LM2704 are offered in the SOT23-5 package.
Pricing and Availability
All six new devices are available now. The LM2794 and LM2795 are priced at $1.30. The LM3354 is priced at $1.20. The LM3355 is priced at $0.95. The LM2703 is priced at $0.85. The LM2704 is priced at $0.90. All prices are for a quantity of 1000 units.
For more information on the LM2794, LM2795, LM3354, LM3355, LM2703 and LM2704, visit National's World Wide Web site at: http://www.national.com/pf/LM/LM2794.html, http://www.national.com/pf/LM/LM2795.html, http://www.national.com/pf/LM/LM3354.html, http://www.national.com/pf/LM/LM3355.html, http://www.national.com/pf/LM/LM2703.html, and http://www.national.com/pf/LM/LM2704.html.
To view a high-resolution downloadable photo of these products, visit National's photo gallery at: http://www.national.com/company/pressroom/gallery/power.html.
Powering Innovation
National is powering innovation. First in technology, performance and packaging, we supply best-in-class power management solutions for cell phones, personal digital assistants, digital cameras and other handheld, portable electronics.
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; displays; information appliances; and information infrastructure. With headquarters in Santa Clara, California, National reported sales of $2.1 billion for its last fiscal year and has about 9,800 employees worldwide. Additional company and product information is available on the World Wide Web at www.national.com.
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