LED Drivers Build Better LCD Backlights

Jun 27, 2007 4:06 PM
By Mark Valentine, Technical Editor, Power Electronics Technology

Maxim Integrated Products and National Semiconductor have developed LED driver families to address the key drawbacks in conventional white-LED backlighting for small LCD viewscreens. These drivers promises to improve battery life and video quality in converged handsets. The drivers also have their own sets of unique features, allowing the power designer to consider novel handset functions in addition to the required display parameters, such as power consumption or color reproduction, when selecting among the components of the two families.

While each device in these families can drive white LEDs, they are primarily designed to drive discrete red, green and blue (RGB) LEDs. The presence of these three colors and the overlap of their respective spectral peaks combine to generate white light. According to sources at Maxim, this white light is richer than what can be generated by a conventional white LED (in which a phosphor coating is optically pumped by a blue LED). Of course, RGB LEDs also have the capability to produce a wide range of colors by mixing various intensities of each color.

For this reason, handset manufacturers are now starting to use RGB LEDs to improve the color gamut of the display, according to National. To fully exploit this range of colors, both the Maxim and National LED drivers have multiple output channels, each with independent current control through an I²C or SPI bus (or, alternatively, autonomous control).

Maxim’s MAX8647 (Figure 1) has six current-sink outputs that are independently controlled linear regulators. According to Maxim, these regulators incorporate a hybrid linear/PWM control scheme (switching at 1 kHz) for LED currents below 6.4 mA in order to maintain accuracy across the full range of LED drive current (0.1 mA to 24 mA).

The flexibility afforded by having six outputs introduces several interesting possibilities. For example, six LEDs can be used to create a relatively bright backlight. Alternatively, three channels can be used to drive the conventional white backlight (generated from RGB LEDs), while the three remaining channels can be used for cosmetic lighting effects, such as driving an external RGB module. According to Maxim, the latter capability is very popular in Asia.

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Figure 1: This block diagram of the MAX8647 RGB LED driver illustrates how the negative charge pump is connected through individual switches to the current regulators to compensate for large forward LED voltages.

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© 2007 Penton Media, Inc.