EDN -- 03.14.96 Avoid pitfalls in dimming and shutting down CCFL backlighting for LCDs

Providing high-efficiency backlighting for LCDs is easier than it used to be thanks to ICs tailored for the purpose, but several elements of the circuit design still require care. Dimming and shutdown are two of them.

Jim Williams, Linear Technology Corp -- EDN, March 14, 1996

Design Feature: March 14, 1996

Many portable computers and instruments now incorporate backlit LCDs. However, applications for such displays include many other types of equipment—from automobiles and gas pumps to retail terminals and medical devices. All backlit LCDs need a source of illumination, and, because so many LCDs go into battery-powered equipment, high-efficiency light sources are extremely important. Although cold-cathode fluorescent lamps (CCFLs) offer the highest efficiency, driving these lamps from battery-powered sources poses many design challenges.

CCFLs operate from high ac voltages. Battery operation with acceptable battery life requires high-efficiency dc/ac inverters. To minimize RF emissions, the inverter output should be sinusoidal. IC manufacturers have responded to the need for CCFL drive circuits by offering a wide variety of products. As with most ICs, successful application of CCFL drive circuits requires equipment designers to understand the IC operation reasonably well and to understand how to select the components that surround the ICs.

Figure 1 You can program the current that drives the ICCFL-control pin of the LT118X ICs from 0 to 50 µA by applying a PWM signal via a pair of resistors and a 2.2-µF capacitor.	Figure 2 If you use a dc reference source, you can program the ICs' control current over a 12- to 50-µA range by using a 50-kOhms potentiometer in series with a 1%-tolerance fixed resistor.

Figure 3 A variable voltage source or a current-source DAC can program the control current with only one additional component—a resistor that isolates the driving source's stray output capacitance from the IC's input.

Two areas of CCFL drive-circuit operation that many equipment designers need to understand are shutdown and intensity (dimming) control; most CCFL applications require both. figures 1 through 5 show various options for Linear Technology's LT118X series parts. Control sources include pulse-width modulation (PWM), potentiometers, and DACs or other voltage sources. In all cases, the resistor values shown in the tables are ideal; use the nearest 1% value. In figures 1, 4, and 5, V_PWM is a duty-cycle-modulated 1-kHz square wave having a base level of 0V, a peak value of 5V, and a duty cycle that varies either from 0 to 90% or 10 to 100%. The LT 1186 floating lamp-drive circuit is not shown. This IC accumulates a serial-bit-stream input and applies the accumulated input to an internal DAC, which provides intensity control.

Figure 4 This circuit uses a PWM signal and a dc reference source to program the LT1183, LT1184, and LT1184F control current. As the duty cycle increases from 0 to 90%, the control current increases from 0 to 50 µA.	Figure 5 The function of this circuit is the same as that of Figure 4, except that, as you increase the duty cycle from 10 to 100%, the control current decreases from 50 µA to 0.

 

Figure 6 Devices in the LT118X series provide a shutdown pin. However, you can also shut down the light source by removing the input voltage (VIN).

In all of the cases that figures 1 through 5 illustrate, the average current into the I_CCFL pin sets the lamp current. Therefore, if using the option in Figure 1, you must control both the amplitude and duty cycle. The remaining examples use the LT118X's reference to eliminate errors resulting from amplitude uncertainty.

Figure 6 shows shutdown options for LT118X series parts. The ICs have a high-impedance shutdown pin. You can also shut off the output by removing V_IN. Switching V_IN requires controlling higher currents but yields somewhat lower shutdown current.

Bright ideas about dimming

Figure 7 A switching regulator, such as the LT1172, gives you several options for intensity control. The circuit shown in (a) uses a potentiometer. In (b), you can use either the upper or lower input. The lower input provides the best transient response. If you use the lower input, the right-hand capacitor is usually smaller than the 1 µF shown.

Figure 7 shows options for dimming control in the LT1172 and similar regulator-based CCFL circuits. The figure illustrates three basic ways to control intensity. The most common approach is to add a potentiometer in series with the feedback termination. When you use this method, make sure that the minimum value (in this case, 562 Ohms) is a 1% resistor. If you use a looser-tolerance resistor, the lamp current at maximum intensity will vary significantly among units.

Intensity control sometimes uses PWM or variable dc. Two interfaces work well. You can control intensity by directly driving the feedback pin with dc or a PWM signal via a diode in series with a 22-k Ohms resistor. A similar method places the 1-µF capacitor outside the feedback loop to obtain the best turn-on transient response and to minimize output overshoot (Reference 1). If you use a PWM signal to control the lamp intensity, the CCFL circuit averages the signal. Thus, at a 0% duty cycle, the PWM source amplitude cannot affect the full-scale lamp current.

Figure 8 With CCFL circuits based on the LT1172 and LT1372, you can shut down the light source by removing the input, pulling VC to ground or, with the LT1372, by applying an appropriate signal to the shutdown pin.

Figure 8 shows methods for shutting down switching-regulator-based CCFL circuits. In LT1172 circuits, pulling the V_C pin to ground puts the circuit into micropower shutdown. In this mode, 50 µA flows into the LT1172 V_IN pin, and the circuit draws essentially zero current from the main supply (the push-pull dc/dc converter's transformer center tap). Turning off V_IN eliminates the LT1172's 50 µA drain. Other regulators, such as the LT1372, have separate shutdown pins.Avoiding problems with potentiometers used for CCFL dimming requires some thought. In particular, issues such as resistance-ratio tolerances can upset poorly thought-out designs. Remember that you can usually control resistor ratios more closely than you can control absolute values (figures 9 and 10). For this reason, it is sometimes better to use a potentiometer as a voltage divider than as a rheostat.

Figure 9 This discussion of variable-resistor characteristics uses the following definitions: (a) Ratio tolerance=RA/RB vs setting. (b) Resistance tolerance is the tolerance on the resistance between points A and B. (c) End-resistance tolerance defines the range of resistances with the wiper shorted to terminal A. (d) Taper—usually linear or logarithmic—refers to the resistance change vs equal increments in the mechanical setting.

Usually, the key issue in potentiometer-based dimming is preventing overdrive. For this reason, you should generally design for maximum intensity with the potentiometer "shorted." Check the end resistance and its tolerance. The end resistance is often insignificant and is controlled more closely than is the maximum resistance or the resistance ratio. Other important specifications are the maximum allowable wiper current and the potentiometer's taper. Although few CCFL dimmers draw wiper current, you should understand how much wiper current a circuit can draw and understand how the circuit behaves if the wiper opens.

To simplify setting the light output, match the potentiometer's taper (linear or logarithmic) to the lamp's light output-vs-current characteristic. Choosing a potentiometer with the wrong taper can result in a circuit whose entire useful light-output range covers a small fraction of the potentiometer travel. Always determine how the circuit behaves if any terminal opens. Opens do happen, and you need to make sure that an open can't cause a harmful condition, such as excessive lamp current.

Figure 10 Electronic potentiometers usually have 255 resistors in a tapped switching arrangement. Some units include nonvolatile memory to retain settings when power is removed, much as electromechanical potentiometers do.

To eliminate potentiometer problems that stem from the devices' mechanical nature, you can use ICs consisting of monolithic resistors tapped by MOS switches. Some of these all-electronic potentiometer equivalents include nonvolatile memory that retains the setting when you remove power, just as real potentiometers do. However, potentiometer-like ICs are not a cure-all. For backlight dimming, these ICs' most serious problem is high end resistance—typically 200 ohms. Because of this characteristic, designers usually use electronic potentiometers as voltage dividers in dimming applications.

Figure 11 shows a simple closed-loop pulse-width modulator, which generates precision variable-width pulses that you can use to test PWM-based intensity-control schemes. The crystal-controlled 1-kHz input clocks the IC₁/Q₁ ramp generator via the differentiator/ CMOS inverter network and the LTC201 reset switch. IC₁'s output drives a CMOS inverter to provide the output. The output is resistively sampled, averaged, and presented to IC₂'s negative input. IC₂ compares this signal with a variable voltage from the 10-turn potentiometer.

Figure 11 In this calibrated pulse-width test circuit, IC2 controls and stabilizes the operating point of a pulse-width modulator built around IC1.

IC₂'s output biases the pulse-width modulator, closing a loop around the modulator. The CMOS inverter's purely ohmic output structure combines with IC₂'s ratiometric operation (that is, both of IC₂'s input signals derive from the 5V supply) to hold the pulse width constant. Variations in time, temperature, and supply voltage have essentially no effect on the pulse width; only the 10-turn potentiometer's setting has an effect. You calibrate the pulse width by adjusting the 2-kOhms trimming potentiometer while monitoring the pulse width with a counter. The Schottky diodes protect the output from latch-up caused by cable-induced ESD or mistakes that occur in testing.

Although the pulse width is insensitive to power-supply variations, the CCFL circuit can't distinguish between supply-voltage variations and duty-cycle shifts because it averages the pulse generator's output. Therefore, you should trim the pulse generator's power supply to 5V±0.01V to simulate a "design-centered" logic supply under normal operating conditions. Do not parallel additional logic inverters to lower the output impedance because, in practice, a single inverter drives the CCFL dimming port, and the pulse generator must mimic the actual driving source as closely as possible.

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Author's Biography

Jim Williams, staff scientist at Linear Technology Corp, Milpitas, CA, specializes in analog-circuit and instrumentation design. He has served in similar capacities at National Semiconductor, Arthur D Little, and the Instrumentation Laboratory at the Massachusetts Institute of Technology. A former student at Wayne State University, Detroit, Williams enjoys art, collecting antique scientific instruments, and restoring old Tektronix oscilloscopes.

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Reference
1. Williams, Jim, "A Fourth Generation of LCD Backlight Technology," Linear Technology Corp, Milpitas, CA. Application Note AN-65, October 1995.

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