White-LED drivers arrive for color handhelds

White-LED drivers arrive for color handhelds
Santa Clara, Calif. - ICs capable of driving white-light-emitting diodes are on the move. The chips, stimulated by an unexpectedly swift migration from monochrome to color displays in handheld electronic products, are riding an upsurge in demand.

Driver ICs from National Semiconductor Corp. here and Texas Instruments Inc. (Dallas) are among the latest market entrants. National recently added six driver ICs, including two constant-current and two constant-voltage devices, plus two magnetic-boost converters. TI has launched a new constant-current white-LED driver and a liquid-crystal display boost converter. Linear Technology Corp. (Milpitas, Calif.) plans to introduce a new boost converter early in April.

Driving white LEDs is considerably more challenging than driving the green LEDs used in monochrome displays, IC vendors say. "The green LEDs that are widely used in monochrome displays have a significantly lower forward voltage than do white LEDs," said Peter Henry, vice president of the portable-power systems group at National. "Monochrome displays typically light up, then turn off, which prolongs battery life, and they are easier to read in ambient light." With white LEDs, he said, "providing uniform brightness is very important."

"There are a number of ways to drive a white LED, depending on the power source," said Tony Armstrong, product-marketing manager for power products at Linear Technology. "Many makers of handheld products prefer a single-cell lithium-ion source. From a 3- to 4-volt supply, each LED needs a 4.5-V to 5-V switch current and 15 to 25 milliamps. One approach is to use a standard boost converter with an inductor; another is to use a charge pump." Armstrong termed the boost converter "more efficient-in the mid-80 percent range" and the charge pump as preferable for applications where electromagnetic interference is a concern.

Noting that standard boost converters require several external components, Armstrong said Linear Technology's forthcoming LT3465 will be among the first to integrate a Schottky diode.

Meanwhile, National Semiconductor's new LM2794 and LM2795 constant-current drivers are optimized to light displays with up to four white LEDs. They combine switched-capacitor boost technology and four constant-current sources matched to plus/minus 0.5 percent (typical) to ensure consistent brightness and color among the LEDs, which the device can drive easily with 15 milliamps per LED. The drive current is programmable by a single external resistor, and both analog and pulse-width-modulation (PWM) methods can be used to vary the drive current for brightness control.

Both devices switch at 325 kHz minimum to keep the conducted-noise spectrum away from sensitive frequencies within portable RF devices.

National's LM3354, which supplies up to 80 mA, and the LM3355, which supplies up to 50 mA, are constant-voltage devices optimized to drive larger numbers of white LEDs. Both are designed for applications where brightness and color matching among the LEDs are not of sufficient concern to justify the additional cost of individual matched-current sources-for example, when driving a dozen 5-mA LEDs to backlight a cell phone keypad. Both drivers 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.

A regulating buck-boost architecture in National's LM3354 and LM3355 offers an average efficiency that's said to be significantly higher (about 80 percent over the typical lithium-ion discharge range) than what is possible with open-loop, switched-capacitor circuits.

Both devices can also be used as general-purpose regulated dc/dc buck-boost converters, to convert the 2.7-V to 4.2-V rail supplied by a single-cell Li-ion battery into a regulated 3.3-V, 50-mA rail.

Magnetic-boost converters
The LM2703 and LM2704 are magnetic-boost converters optimized to drive up to four white LEDs in series, or several strings of series LEDs simultaneously, in display lighting applications, keypad back-lighting applications or both at once. All LEDs in a string are driven with the same constant current, resulting in consistent brightness and color, the company said. The LM2704 achieves 85 percent efficiency, delivering 20 mA at 20 V. It has a 600-mA peak-current limit, against a 300-mA peak-current limit for the LM2703. Both devices can also be used as general-purpose magnetic-boost dc/dc converters.

National's LM2791 and LM2792 are offered in an LLP-10 leadless lead frame package. The LM2794 and LM2795 come in a micro SMD-14 package. The LM3354 and LM3355 are each available in an MSOP-10 package, and the LM2703 and LM2704 are housed in an SOT23-5.

In 1,000-piece quantities, the LM2794 and LM2795 are priced at $1.30, the LM3354 at $1.20, the LM3355 at 95 cents, the LM2703 at 85 cents and the LM2704 at 90 cents. Additional information is available at www.national.com/see/wled.

Two from TI
Texas Instruments' TPS61042 high-frequency white-LED driver has a 1.8-V to 6-V input range, supplies output voltages up to 28 V and can drive up to eight white LEDs. The LED current can be set with an external sense resistor, and brightness can be controlled by an analog signal or by a 100-Hz to 50-kHz PWM signal applied to a control pin, the company said. The control input also allows the driver to be disabled and the LEDs disconnected to avoid current leakage during shutdown.

TI's TPS61045 adjustable LCD bias-boost converter features a digitally programmable output of up to 28 V for precise contrast control of small to medium-size LCDs without external components. The device's shutdown feature disconnects the load from the input and provides controlled power up/down sequencing of the display. Both converters feature a 1-MHz switching frequency. Each device is packaged in an eight-pin, 3 x 3-mm quad flat pack, no lead (QFN). The TPS61042 is priced at $1.10 and the TPS61045 at $1.25 in 1,000-piece quantities.

Intersil Corp. (Milpitas), Fairchild Semiconductor (South Portland, Maine), Maxim Integrated Products (Sunnyvale, Calif.) and Advanced Analogic Technologies Inc. (Sunnyvale) are also actively pursuing the driver market for white LEDs.

Intersil's EL7513 is a PWM step-up regulator capable of driving four LEDs in series or 12 LEDs in a series/parallel configuration. It can boost up to 18 V, offers plus/minus 4 percent LED diode current accuracy and can achieve 89 percent accuracy when driving a 30-mA load.

The FAN56XX family of LED driver ICs from Fairchild drives two to four LEDs. The chips feature current sources matched within plus/minus 3 percent and achieve up to 97.5 percent efficiency without the need for external current-matched resistors, the company said. Built-in digital, analog and PWM control facilitates brightness adjustment and helps minimize current draw.

Meanwhile, Maxim's MAX1570 combines a high-efficiency, 1x by 1.5x charge pump with low-dropout current regulators in a 4 x 4-mm 16-pin QFN package. Offering 37 percent longer battery life than a 2x charge pump, the 150-mA current-limit device supports up to five white LEDs with 0.3 percent LED current matching and no need for external inductor or ballast resistors.

Advanced Analogic Technologies' AAT3110 switched-capacitor charge pump operates in an output-regulated, voltage-doubling mode and uses a pulse-skipping technique to provide a regulated output ( plus/minus 4 percent) without requiring an external inductor. Power consumption for AnalogicTech's charge pump is 13 microamps, dropping to less than 1 microamps in shutdown mode. A thermal-management circuit provides protection under continuous short-circuit conditions.

Advanced Analogic Technologies Inc.
(408) 737-4600
www.analogictech.com

Fairchild Semiconductor
(800) 341-0392
www.fairchildsemi.com

Intersil Corp.
(408) 945-1323
www.intersil.com

Linear Technology Corp.
(408) 432-1900
www.linear.com

Maxim Integrated Products
(800) 998-8800
www.maxim-ic.com

National Semiconductor Corp.
(800) 272-9959
www.national.com

Texas Instruments Inc.
(800) 477-8924, ext. 4500
www.ti.com