LED DRIVER CIRCUITS The design in Figure 3 represents a constant current chopper driver that provides a dc current with 10% ripple to an LED string used to edge light an LCD. The pass switching device is a P-channel FET that provides the current to the LED string and, in conjunction with the inductor, sense resistor, and boost voltage, establishes the chopping current and frequency.
The dc-dc boost stage is a closed-loop boost supply that provides sufficient voltage to drive current to the LED string with at least 2 V of headroom. The part of the diagram designated Section A shows a comparator and associated resistors that form a positive hysteresis circuit. It compares the voltage across the sense resistor to a known reference. Section B in the diagram shows another comparator and associated resistors that buffer the Section A output to ensure proper hysteresis and provide drive to the pass device.
Section C in the diagram supplies LED on/off and dimming control. The +ENABLE input turns the backlight on or off, and +PW pulse-width modulates the chopper driver on and off for dimming. Implementation can be quite compact (Fig. 4).
THERMAL CONSIDERATIONS The ambient temperature in which the LCD operates is a key consideration for the backlight-driver designer. CCFL starting or strike voltage is inversely proportional to temperature. Figure 5a shows a typical relationship between CCFL strike voltage and temperature, and Figure 5b illustrates CCFL brightness variation with increasing lamp current.
The time a CCFL needs to reach specified brightness is also inversely proportional to temperature. Mission-critical applications that require rapid brightness increase may need the inverter to provide a higher boost current for a short time to enhance CCFL warm-up and to accelerate the time to required brightness. However, as helpful as higher CCFL current is to lamp warm-up, sustained high current can saturate the lamps. It may also produce an actual decrease in brightness along with elevated lamp temperature and accompanying decreased lamp life. Rated lamp current for most CCFLs is between 3 and 8 mA rms.
LED backlights are less sensitive to low temperatures. Slight changes in LED electrical characteristics and turn-on time at lower temperatures don’t demand any special driver design considerations.
High application temperatures also affect driver design. In fact, above all other variables, high temperature has the most significant impact on CCFL driver function and reliability.
Copper and core losses in the transformers for CCFL drivers can be significant heat sources. Transformers typically operate at temperatures as much as 30ºC above the local environment. Copper and core losses can be minimized by tailoring the driver design to the CCFL, which sustains voltage and current.
High application temperatures are also important to LED backlights. However, the focus here is on the temperature of the LED itself, not the driver components. Recent advances in LED technology, packaging, and materials have generated dramatic increases in LED brightness. The challenge for LED backlights is to get the heat out of the LED device itself and then out of the display assembly. The key design point is to keep the LED junction temperature below 100ºC to ensure reliability.
DIMMING LCD applications requiring a wide range of brightness are ever increasing. The driver must be able to deliver high brightness for daylight vision and low brightness for night vision. Brightness control across this wide requirement range must be smooth and free of flicker.
Analog dimming of CCFL backlights, wherein the driver output current is modulated to change lamp brightness, provides coarse dimming to about 30% of full brightness—not enough dynamic range for most application requirements. Furthermore, analog dimming can stress the oscillator transistors and reduce inverter reliability.
PWM dimming brings significantly better dimming control. In this type of dimming, the CCFL or LED is pulsed on and off at a fixed frequency, and the duty cycle is modulated to provide variable brightness. Typically, CCFL backlights are modulated at frequencies between 100 and 500 Hz. Low-level brightness control of CCFL backlights with four or more lamps can be enhanced by selective enable techniques, in which the lamps are sequenced off as brightness is reduced.
Also, the best way to dim LED backlights is via PWM dimming. Much wider dimming ratios can be achieved with LED backlights because the basic switching time of an LED is measured in nanoseconds compared to milliseconds for a CCFL.
We are a professional CCFL (Cold Cathode Lamp) factory, we can proivde all kinds of LCD replacement or repair Backlight tube. Out-side diameter from 1.80-4.0mm, length from 10-600mm.
sissy -July 10, 2009
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