Drivers And White LEDs Ease Backlighting Designs

March 31, 2003
LEDs offer lower voltage drops, while driver ICs reduce requirements for diode binning.

Fairchild Semiconductor's combinations of low-VF LEDs and low-dropout LED drivers are intended to simplify and lower the cost of backlighting designs for handheld devices.

In its QTLP6XX series, the company developed white and blue surface-mount LEDs with forward-voltage (VF) drops of less than 3.15 V. Thanks to the low-VF values, the LEDs can run directly off a lithium-ion battery and forego the usual boost circuit. Moreover, a narrow VF range (2.75 to 3.15 V) helps maintain the consistency of LED color and brightness in the application.

Meanwhile, the company's FAN56XX driver ICs provide different options for driving two to four white or blue LEDs. Among the drivers with matched-current outputs, some driver ICs also contain a special VF compensation circuit that accommodates unmatched LEDs. The VF compensation circuit varies the drive current to each LED by up to ±3% to achieve uniform brightness among LEDs with different forward-voltage drops. This technique eliminates the need for preselection or binning of LEDs according to VF values. However, binning of LEDs for brightness at a given current is still required. For more on driver features, see "LED Drivers Boost Efficiency And Control" at www.elecdesign.com.

These chips will be offered in SC-70 and 2- by 2-mm MLPs. For more information, see www.fairchildsemi.com/whats_new/LED_nph.html, or contact the company via e-mail at [email protected].

See associated figure.

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