Appliance Design - November 2007 - (Page 26)

DISPLAYS & INDICATORS Fig. 4. Backlighting for this medium-size LCD panel includes switched power and an LDO driving eight 10-inseries strings of LEDs. 10 LEDs/string, 8 strings = 80 LEDs 55 mA/string the brightness of the LCD screen by controlling the total current passing through this LED group. A low-dropout voltage (LDO) regulator is commonly used to drive an LED group (See Fig. 2). It accepts the 12-VDC input required by a CCFL driver, and delivers an output current set by the external current-sense resistor, R1. The LDO shown (U1) can support a PWM dimmer while maintaining constant output current. U1 delivers output currents as high as 350 mA directly, and with the addition of an external bipolar-junction transistor, can deliver output currents as high as 2 A. Another LDO from the same series as U1 (the MAX16804) integrates a PWM circuit on the CMOS chip. It simplifies circuit design by directly supporting analog dimmer signals. The biggest advantages of using an LDO to drive the LED group are simplicity and the absence of EMI. Another advantage of the LDO — it is easily fabricated along with the LED light group, and assembled directly within the LCD screen. Its main problem is lower efficiency, because the difference between the input and driving voltages appears directly across the LDO. When this difference is large, the efficiency is low, and the LDO becomes very hot. Even worse, one must choose LEDs with matched forward conducting voltages to minimize the effect of unequal current distribution in the parallel chains of series LEDs. Replacing the LDO with switched power is the most direct method for improving the driving efficiency of the LED group. (See Fig. 3.) The main advantage of switched power is better efficiency. The input voltage of U1 (MAX16819) is 4.5 V to 28 V, which makes the LED backlighting module compatible with the original CCFL input voltage. One can also power U1 directly from a battery or AC adapter, which minimizes power switching links and improves the system efficiency. This scheme also supports PWM dimming directly. Like the LDO approach, however, one cannot eliminate the effect of unequal current distribution among the series strings, even by driving the LED group with switched power. Thus, current distribution among series strings of LEDs cannot be made equal by controlling the current for the whole LED group, even by driving with the same voltage and choosing LEDs with similar electrical parameters (which would incur a higher cost for the LEDs). The series-LED string that conducts with the highest voltage would probably have insufficient current, and the string with the lowest voltage would probably conduct an excessive current that oversteps the nominal value. The result would then be unequal string illuminations and a short lifetime for some of the LEDs. To avoid hidden trouble, one should configure the LED group to exploit the best features of both LDOs and switching regulators. One can obtain better efficiency by powering the whole LED group with a switching supply, and then place a series LDO in each branch to ensure equal currents in the branches. The 8-channel LED driver MAX16807, for example, equalizes the branch currents (to the level set by a single external resistor, 55 mA maximum) by including an LDO in the loop for each branch. These LDOs automatically adjust the load voltage for each branch. (See Fig. 4). The system processor controls each branch and the PWM dimmer’s duty cycle via a serial I2C bus interface. The output voltage can be as high as 36 V. Every loop can have as many as 10 white LEDs in series, so each group can include about 80 LEDs. Summary LED backlighting is becoming more and more popular, as the price of high-brightness LEDs decreases and their light efficiency increases. The LED-backlighting drivers for large-size and medium-size LCD panels are also making progress in response to market pressures. One consequence of these gradual changes is a trend toward replacing CCFL backlights with LED backlights. LED-driver circuits depend on the changing character of LEDs, so new circuits must continually emerge to take maximum advantage of the new LED devices. < For more information, enter . . . . . . . . . . . . . . . . . 305 Or email: info2@maxim-ic.com 26 applianceDESIGN November 2007 www.applianceDESIGN.com http://www.appliancedesign.com

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Appliance Design - November 2007 Contents Editorial Shipments/Forecasts News Watch Today’s LEDs are Brighter and More Colorful than Previous Versions, Allowing Designers More Opportunities to Improve and Differentiate Their Products. When Converting from CCFL to LED Backlights in LCD Panels, New Driving Methods are Needed. Electrowetting Technology Allows Displays to be Designed in an Extensive Range of Colors. Noise Problems are Not Always Easy to Find or Fix, but Acoustic Labs Can Help Designers Ferret Out the “Good” Noise from the “Bad.” A New Sanitization Protocol for Clothes Washers Combines the Best of Three Well-Established Provisions to Ensure Efficiency, Quality and Safety. Stricter EMC Requirements Raise the Bar for Appliances with Electronic Controls and Require Testing to New Levels. Despite Added Cost and Complexity, Gas-Modulating Systems have Migrated from Large-Scale Boiler Applications to Smaller Residential and Commercial Furnaces and Appliances. A New Treatment Technology Hardens the Surface of Light Alloys such as Aluminum and Magnesium and Expands Their Potential Applications. DesignMart Advertiser’s Index Association Report: AHAM

Appliance Design - November 2007

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