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National voltage regulators drive cell-phone white LEDs

Posted: 14 Feb 2002 ?? ?Print Version ?Bookmark and Share

Keywords:national semiconductor? national? nsc? voltage regulator? voltage regulation?

National Semiconductor Corp. has introduced half a dozen voltage-regulator devices designed to power white LEDs used as backlights for color displays on new-generation cell phones and handheld PCs. The products include buck-boost switching regulators designed to lift the 3.6V produced by a Li-ion cell to the 4.1V or more needed to power an array of white LEDs.

The introductions coincide with dramatic price decreases in white LEDs and with skyrocketing demand for color cell phones in Europe and Japan, said Steve Hunt, National's business unit manager for switched-capacitor converters. At 40 or 50 cents each, white LEDs will become a practical choice for backlighting small color LCD screens, he said.

Color comes on strong

In 2001, perhaps 10 or 12 percent of all cell phones shipped had color, but the number will be closer to 100 percent in three or four years, Hunt speculated. Nokia will offer color displays with its 7650 models, Hunt said, as will Siemens with its S45 GPRS phone.

But seeing the perseverance of CCFL in color Palm PDAs and other pocket PCs, National's competitors in voltage regulators are moving more slowly toward LED-based technology. "Because the cost of white LEDs is dropping, it is becoming a looked-at item in handhelds," said Dave Heacock, business manager of Texas Instruments' battery-management group. "But the LEDs' cost advantage depends on how many you need."

Still, Heacock said that TI is also working on white LED drivers and that it may have an announcement in the second or third quarter. Depending on the required efficiency, Heacock said, piezoelectric-excited CCFL modules will cost between $3.50 and $5without the pen-size fluorescent tube, which lists for $8.

But cost isn't the only issue, said James Schuessler, National's marketing manager for switched-capacitor power converters. Manufacturing for white LEDs is inconsistent, Schuessler said; some devices in an array will be brighter and some dimmer, even with the same, 4.5V forward voltage. Thus, a constant-current regulator is the preferred means of driving these LEDs.

National's 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 up to 20 white LEDs. The LM2703 and LM2704 are magnetic-boost converters optimized to drive white LEDs in applications where efficiency is the key concern. In lots of 1,000 pieces, pricing for all these regulators ranges from 90 cents to $1.30 each.

TI's CCFL drivers depend on new controllers: the UCC3975 flyback, 3976 half-bridge and 3977 push-pull regulator. The devices, intended for laptops and pocket PCs, lift a 3.6V cell phone or 12V computer battery voltage to the 500 or more Vac required to light a CCFL tube.

Stephan Ohr

EE Times





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