Optical and temperature feedbacks to control display brightness

An illumination control circuit allows a user to set a desired brightness level and maintains the desired brightness level over temperature and life of a light source. The illumination control circuit uses a dual feedback loop with both optical and thermal feedbacks. The optical feedback loop controls power to the light source during normal operations. The thermal feedback loop overrides the optical feedback loop when the temperature of the light source becomes excessive.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a backlight system, and more particularly relates to using optical and temperature feedbacks to control the brightness of the backlight.

2. Description of the Related Art

Backlight is used in liquid crystal display (LCD) applications to illuminate a screen to make a visible display. The applications include integrated displays and projection type systems, such as a LCD television, a desktop monitor, etc. The backlight can be provided by a light source, such as, for example, a cold cathode fluorescent lamp (CCFL), a hot cathode fluorescent lamp (HCFL), a Zenon lamp, a metal halide lamp, a light emitting diode (LED), and the like. The performance of the light source (e.g., the light output) is sensitive to ambient and lamp temperatures. Furthermore, the characteristics of the light source change with age.

SUMMARY OF THE INVENTION

One embodiment of the present invention is an illumination control circuit which allows a user to set a desired brightness level and maintains the desired brightness level over temperature and life of a light source (e.g., a fluorescent lamp). The illumination control circuit uses an optical sensor (e.g., a visible light sensor) to maintain consistent brightness over lamp life and over extreme temperature conditions. The illumination control circuit further includes a temperature sensor to monitor lamp temperature and prolongs lamp life by reducing power to the fluorescent lamp when the lamp temperature is excessive. In one embodiment, the illumination control circuit optionally monitors ambient light and automatically adjusts lamp power in response to variations for optimal power efficiency.

The brightness (or the light intensity) of the light source (e.g., CCFL) is controlled by controlling a current (i.e., a lamp current) through the CCFL. For example, the brightness of the CCFL is related to an average current provided to the CCFL. Thus, the brightness of the CCFL can be controlled by changing the amplitude of the lamp current (e.g., amplitude modulation) or by changing the duty cycle of the lamp current (e.g., pulse width modulation).

A power conversion circuit (e.g., an inverter) is generally used for driving the CCFL. In one embodiment, the power conversion circuit includes two control loops (e.g., an optical feedback loop and a thermal feedback loop) to control the lamp current. A first control loop senses the visible light produced by the CCFL, compares the detected visible light to a user defined brightness setting, and generates a first brightness control signal during normal lamp operations. A second feedback loop senses the temperature of the CCFL, compares the detected lamp temperature to a predefined temperature limit, and generates a second brightness control signal that overrides the first brightness control signal to reduce the lamp current when the detected lamp temperature is greater than the predefined temperature limit. In one embodiment, both of the control loops use error amplifiers to perform the comparisons between detected levels and respective predetermined levels. The outputs of the error amplifiers are wired-OR to generate a final brightness control signal for the power conversion circuit.

In one embodiment, an illumination control circuit includes an optical or a thermal feedback sensor integrated with control circuitry to provide adjustment capabilities to compensate for temperature variations, to disguise aging, and to improve the response speed of the light source. For example, LCD computer monitors make extensive use of sleep functions for power management. The LCD computer monitors exhibit particular thermal characteristics depending on the sleep mode patterns. The thermal characteristics affect the “turn on” brightness levels of the display. In one embodiment, the illumination control circuit operates in a boost mode to expedite the display to return to a nominal brightness after sleep mode or an extended off period.

In one embodiment, a light sensor (e.g., an LX1970 light sensor from Microsemi Corporation) is coupled to a monitor to sense the perceived brightness of a CCFL used in the backlight or display. For example, the light sensor can be placed in a hole in the back of the display. The light sensor advantageously has immunity to infrared light and can accurately measure perceived brightness when the CCFL is in a warming mode. The output frequency of the CCFL shifts from infrared to the visible light spectrum as the temperature increases during the warming mode.

In one embodiment, the output of the light sensor is used by a boost function controller to temporary increase lamp current to the CCFL to reach a desired brightness level more quickly than using standard nominal lamp current levels. The light sensor monitors the CCFL light output and provides a closed loop feedback method to determine when a boost in the lamp current is desired. In an alternate embodiment, a thermistor is used to monitor the temperature of the CCFL lamp and to determine when boosted lamp current is desired.

In one embodiment, an inverter is used to drive the CCFL. The inverter includes different electrical components, and one of the components with a temperature profile closely matching the temperature profile of the CCFL is used to track the warming and cooling of a LCD display. The component can be used as a reference point for boost control functions when direct access to lamp temperature is difficult.

Providing a boost current to the CCFL during initial activation or reactivation from sleep mode of the display improves the response time of the display. For example, the display brightness may be in the range of 40%–50% of the nominal range immediately after turn on. Using a normal start up current (e.g., 8 mA) at 23 degrees C., the 90% brightness level may be achieved in 26 minutes. Using a 50% boost current (e.g., 12 mA), the 90% brightness level may be achieved in 19 seconds. The boost level can be adjusted as desired to vary the warm-up time of the display. The warm-up time is a function of the display or monitor settling temperature. For example, shorter sleep mode periods mean less warm-up times to reach the 90% brightness level.

In one embodiment, the boost control function can be implemented with low cost and low component count external circuitry. The boost control function enhances the performance of the display monitor for a computer user. For example, the display monitor is improved by reducing the time to reach 90% brightness by 50 to 100 times. The boost control function benefits office or home computing environments where sleep mode status is frequent. Furthermore, as the size of LCD display panels increase in large screen displays, the lamp length and chassis also increase. The larger lamp and chassis leads to system thermal inertia, which slows the warm-up time. The boost control function can be used to speed up the warm-up time.

In one embodiment, a light sensor monitors an output of a CCFL. A boost control circuit compares an output of the light sensor to a desired level. When the output of the light sensor is less than the desired level, the CCFL is operated at a boost mode (e.g., at an increased or boosted lamp current level). As the output of the light sensor reaches the desired level, indicating that the brightness is approaching a desired level, the boosted lamp current is reduced to a preset nominal current level.

In one embodiment, the boost control circuit is part of the optical feedback loop and facilitates a display that is capable of compensating for light output degradation over time. For example, as the lamp output degrades over usage hours, the lamp current level can be increased to provide a consistent light output. LCD televisions and automotive GPS/Telematic displays can offer substantially the same brightness provided on the day of purchase after two years of use.

DETAILED DESCRIPTION OF THE INVENTION

Various embodiments of the present invention will be described hereinafter with reference to the drawings.FIG. 1is a block diagram of a power conversion circuit (or backlight system) with dual feedback loops in accordance with one embodiment of the invention. The backlight system may be advantageously used in automotive applications which are exposed to relatively extreme temperature variations and suffer brightness loss at low ambient temperatures. The backlight system can also be used in other LCD applications, such as computer notebooks, computer monitors, handheld devices, television displays, and the like. The dual feedback loops allow a user to set a desired brightness level for a backlight light source and maintain the desired brightness level over operating temperature and over degradation of the light source efficacy over life. The dual feedback loops also extend the useful life of the light source by maintaining safe operating conditions for the light source.

The power conversion circuit ofFIG. 1generates a substantially alternating current (AC) output voltage (V-OUT) to drive a fluorescent lamp (e.g., a CCFL)106. In one embodiment, an inverter100generates the substantially AC output voltage from a direct current (DC) input voltage. The inverter100includes a controller102which accepts a brightness control input signal (BRITE-IN) and generates switching signals (A, B) to a high voltage circuit104to generate the substantially AC output voltage. A corresponding AC lamp current (I-LAMP) flows through the CCFL106to provide illumination.

In one embodiment, the dual feedback loops control the brightness of the CCFL106and include an optical feedback loop and a lamp temperature feedback loop. The dual feedback loops generate the brightness control input signal to the controller102. The brightness of the CCFL106is a function of the root mean square (RMS) level of the lamp current, ambient temperature of the CCFL106, and life of the CCFL106. For example,FIG. 2illustrates light output of a CCFL with respect to temperature. The lamp brightness is affected by ambient and lamp temperatures. A graph200shows the relationship for a standard pressure CCFL at a nominal operating lamp current of 6 mA.

Lamp brightness decreases as the CCFL106ages (or when the lamp temperature decreases) even though the RMS level of the lamp current remains the same. The dual feedback loops facilitate consistent lamp brightness over lamp life and varying lamp temperature by compensating with adjusted RMS levels of the lamp current. The dual feedback loops further facilitate prolonged lamp life by monitoring the temperature of the CCFL106.

As shown inFIG. 1, the optical feedback loop includes a visible light sensor110, an optional gain amplifier112, and a first error amplifier114. The visible light sensor110monitors the actual (or perceived) brightness of the CCFL106and outputs an optical feedback signal indicative of the lamp brightness level. The optional gain amplifier112conditions (e.g., amplifies) the optical feedback signal and presents a modified optical feedback signal to the first error amplifier114. In one embodiment, the modified optical feedback signal is provided to an inverting input of the first error amplifier114. A first reference signal (LAMP BRIGHTNESS SETTING) indicative of a desired lamp intensity is provided to a non-inverting input of the first error amplifier114. The first reference signal can be defined (varied or selected) by a user.

The first error amplifier114outputs a first brightness control signal used to adjust the lamp drive current to achieve the desired lamp intensity. For example, the lamp current is regulated by the optical feedback loop such that the modified optical feedback signal at the inverting input of the first error amplifier114is substantially equal to the first reference signal. The optical feedback loop compensates for aging of the CCFL106and lamp temperature variations during normal operations (e.g., when the lamp temperature is relatively cool). For example, the optical feedback loop may increase the lamp drive current as the CCFL106ages or when the lamp temperature drops.

There is a possibility that an aged lamp in hot ambient temperature may be driven too hard and damaged due to excessive heat. The lamp temperature feedback loop monitors the lamp temperature and overrides the optical feedback loop when the lamp temperature exceeds a predetermined temperature threshold. In one embodiment, the lamp temperature feedback loop includes a lamp temperature sensor108and a second error amplifier116. The lamp temperature sensor108can detect the temperature of the CCFL106directly or derive the lamp temperature by measuring ambient temperature, temperature of a LCD bezel, amount of infrared light produced by the CCFL106, or variations in the operating voltage (or lamp voltage) across the CCFL106. In one embodiment, select components (e.g., switching transistors or transformers) in the inverter100can be monitored to track lamp temperature.

The lamp temperature sensor108outputs a temperature feedback signal indicative of the lamp temperature to an inverting input of the second error amplifier116. A second reference signal (LAMP TEMPERATURE LIMIT) indicative of the predetermined temperature threshold is provided to a non-inverting input of the second error amplifier116. The second error amplifier116outputs a second brightness control signal that overrides the first brightness control signal to reduce the lamp drive current when the lamp temperature exceeds the predetermined temperature threshold. Reducing the lamp drive current helps reduce the lamp temperature, thereby extending the life of the CCFL106.

In one embodiment, the output of the first error amplifier114and the output of the second error amplifier116are wire-ORed (or coupled to ORing diodes) to generate the brightness control input signal to the controller102. For example, a first diode118is coupled between the output of the first error amplifier114and the controller102. A second diode120is coupled between the output of the second error amplifier116and the controller102. The first diode118and the second diode120have commonly connected anodes coupled to the brightness control input of the controller102. The cathode of the first diode118is coupled to the output of the first error amplifier114, and the cathode of the second diode120is coupled to the output of the second error amplifier116. Other configurations or components are possible to implement an equivalent ORing circuit to accomplish the same function.

In the above configuration, the error amplifier with a relatively lower output voltage dominates and determines whether the optical feedback loop or the lamp temperature feedback loop becomes the controlling loop. For example, the second error amplifier116have a substantially higher output voltage during normal operations when the lamp temperature is less than the predetermined temperature threshold and is effectively isolated from the brightness control input by the second diode120. The optical feedback loop controls the brightness control input during normal operations and automatically adjusts the lamp drive current to compensate for aging and temperature variations of the CCFL106. Control of the brightness control input transfers to the lamp temperature feedback loop when the temperature of the CCFL106becomes too high. The temperature of the CCFL106may be excessive due to relatively high external ambient temperature, relatively high lamp drive current, or a combination of both. The lamp temperature feedback loop reduces (or limits) the lamp drive current to maintain the lamp temperature at or below a predetermined threshold. In one embodiment, the first and second error amplifiers114,116have integrating functions to provide stability to the respective feedback loops.

In one embodiment, the brightness control input signal is a substantially DC control voltage that sets the lamp current. For example, the RMS level of the lamp current may vary with the level of the control voltage. A pull-up resistor122is coupled between the brightness control input of the controller102and a pull-up control voltage (MAX-BRITE) corresponding to a maximum allowable lamp current. The pull-up control voltage dominates when both of the outputs of the respective error amplifiers114,116are relatively high. The output of the first error amplifier114may be relatively high during warm-up or when the CCFL106becomes too old to produce the desired light intensity. The output of the second error amplifier116may be relatively high when the temperature of the CCFL106is relatively cold.

FIG. 3illustrates panel brightness with respect to time as a display panel cycles on and off or exits from sleep mode. Computer applications make extensive use of sleep functions for power management. A graph300shows different warm-up times depending on how much time elapsed since the display panel was turned off or entered the sleep mode and allowed to cool down. For example, initial panel brightness may be only 60–70% of steady-state panel brightness during warm-up after the display panel turns on or exits from sleep mode. The warm-up time takes longer when the display panel has been inactive for a while, in cooler ambient temperatures, or for larger display panels.

In one embodiment, an optical feedback loop or a temperature feedback loop is used to decrease the warm-up time. For example, a controller controlling illumination of the display panel can operate in overdrive or a boost mode to improve response of the display brightness. The boost mode provides a higher lamp drive current than normal operating lamp current to speed up the time to reach sufficient panel brightness (e.g., 90% of steady-state). In one embodiment, the brightness control input signal described above can be used to indicate to the controller when boost mode operation is desired.

FIG. 4illustrates waveforms for panel brightness and a light sensor output with respect to time as a display panel cycles on and off. A graph402shows the panel brightness. A graph400shows the light sensor output which closely tracks the panel brightness. In one embodiment, the light sensor output is produced by a visible light sensor (e.g., part number LX1970 from Microsemi Corporation).

FIG. 5illustrates waveforms for panel brightness and temperatures of select inverter components with respect to time as a display panel cycles on and off. A graph500shows the panel brightness. A graph502shows the temperature profile of a transformer and a graph504shows the temperature profile of a transistor as the panel brightness changes. A graph506shows the temperature profile of a lower lamp and a graph508shows the temperature profile of an upper lamp as the panel brightness changes. As discussed above, a select component (e.g., the transistor or the transformer) can be used in an indirect method to monitor lamp temperature.

FIG. 6illustrates waveforms comparing warm-up times using a standard drive current and a boost current. A graph600shows a relatively slow response time for a lamp when a nominal current (e.g., 8 mA) is used to drive the lamp. A graph602shows an improved response time for the lamp when a boosted current (e.g., 12 mA) is used to drive the lamp during warm-up.

FIG. 7illustrates waveforms comparing percentage of light output with respect to hours of operation for various operating conditions. A graph700shows the light output during life test of a lamp driven by a direct drive inverter running at 1% duty cycle. A graph702shows the light output during life test of a lamp driven by the direct drive inverter running at 150% of the rated lamp current or a typical inverter running at 67% of the rated lamp current. A graph706shows the light output during life test of a lamp driven by a typical inverter running at 100% of the rated lamp current. Finally, a graph708shows the light output during life test of a lamp driven by a typical inverter running at 150% of the rated lamp current. CCFLs degrade at a predictable rate over time. Lamp life specifications are defined as the point at which the display brightness level reduces to 50% of the original level.

FIG. 8illustrates waveforms comparing light outputs with and without optical feedback over the life of a CCFL. A graph802shows the degradation of the light output as the CCFL ages. A graph800shows more consistent brightness over the life of the CCFL by using the optical feedback loop described above. Monitoring the perceived brightness of the CCFL provides a low cost and high performance method to maintain “out of the box” brightness levels as the CCFL ages.

FIG. 9illustrates power savings associated with decreasing brightness based on ambient light environment. A graph900shows increasing power consumption by a CCFL to produce substantially the same perceived intensity for a display panel as the ambient light increases from a dark environment (e.g., on an airplane) to a bright environment (e.g., daylight). Power can be saved by sensing the ambient (or environment) conditions and adjusting the lamp drive current accordingly. In one embodiment, the optical feedback loop described above can be modified to sense ambient light and make adjustments to lamp current for optimal efficiency. For example, operating lamp current can be decreased/increased when ambient light decreases/increases to save power while achieving substantially the same perceived brightness.

FIGS. 10A and 10Brespectively illustrate a block diagram and wavelength sensitivity for one embodiment of a light sensor1000used to monitor visible light output of a CCFL or ambient light. CCFLs emit less visible light and more infrared light under relatively cold operating temperatures (e.g., during warm-up). The light sensor1000advantageously monitors mostly the visible portion of the light. In one embodiment, the light sensor (e.g., the LX1970 from Microsemi Corporation)1000includes a PIN diode array1002with an accurate, linear, and very repeatable current transfer function. The light sensor1000outputs a current sink1004and a current source1006with current levels that vary with sensed ambient light. The complementary current outputs of the light sensor1000can be easily scaled and converted to a voltage signal by connecting one or more resistors to either or both outputs. Referring toFIG. 10B, a graph1008shows the frequency response of the light sensor1000which approximates the frequency (or spectral) response of human eyes shown by graph1010.

FIG. 11is a schematic illustration of one embodiment of an automatic brightness control circuit that senses lamp light and generates a control signal for adjusting the operating current of the lamp. For example, the automatic brightness control circuit can vary the control signal until the sensed lamp light corresponds to a desired level indicated by a user input (e.g., DIM INPUT). Alternately, the automatic brightness control circuit can indicate when boost mode operation is desired to improve response speed of the lamp. The automatic brightness control circuit includes a visible light (or photo) sensor1100and an error gain amplifier1110. In one embodiment, the visible light sensor1100and the error gain amplifier1110are both powered by a substantially DC supply voltage (e.g., +5 VDC). The visible light sensor1100monitors the lamp light and outputs a feedback current that is proportional to the level of the lamp light.

In one embodiment, the feedback current is provided to a preliminary low pass filter comprising a first capacitor1102coupled between the output of the visible light sensor1100and ground and a resistor divider1104,1106coupled between the supply voltage and ground. The filtered (or converted) feedback current is provided to an inverting input of an integrating amplifier. For example, the output of the visible light sensor1100is coupled to an inverting input of the error gain amplifier1110via a series integrating resistor1108. An integrating capacitor1112is coupled between the inverting input of the error gain amplifier1110and an output of the error gain amplifier1110.

In one embodiment, a desired intensity (or dimming) level is indicated by presenting a reference level (DIM INPUT) at a non-inverting input of the integrating amplifier. The reference level can be variable or defined by a user. The reference level can be scaled by a series resistor1116coupled between the reference level and the non-inverting input of the error amplifier1110and a resistor divider1114,1118coupled to the non-inverting input of the error amplifier1110. The output of the error amplifier1110can be further filtered by a series resistor1120with a resistor1122and capacitor1124coupled in parallel at the output of the automatic brightness control circuit to generate the control signal for adjusting the operating lamp current.

FIG. 12is a graph illustrating panel brightness and temperatures of select inverter components with respect to time using the automatic brightness control circuit to monitor lamp intensity as a display panel cycles on and off. A graph1200shows the panel brightness modified by the automatic brightness control circuit. A graph1202shows the associated temperature profile for a transformer and a graph1204shows the associated temperature profile for a transistor in the inverter. Finally, a graph1206shows the upper lamp temperature profile. In comparison with similar graphs shown inFIG. 5, the corresponding graphs inFIG. 12show faster transitions in reaching the desired panel brightness after turn on or exiting sleep mode by using the automatic brightness control circuit.

FIG. 13illustrates one embodiment of a LCD monitor1300with light detectors1306,1312which are interfaced to a lamp inverter1304for closed loop illumination control. One or more visible light detectors1312may be located proximate to one or more backlight lamps to monitor lamp intensity. The visible light detectors1312enhance warm-up and maintain constant backlight intensity over lamp life and operating temperature. An additional visible light detector1306may be located in a corner of the LCD monitor1300for monitoring ambient light. The additional visible light detector1306facilitates automatic adjustment of backlight intensity based on environment lighting. The lamp inverter1304with one or more low profile transformers1302can be located in a corner of the LCD monitor1300. In one embodiment, the LCD monitor1300further includes embedded stereo speakers1308and a Class-D audio amplifier1310.

Although described above in connection with CCFLs, it should be understood that a similar apparatus and method can be used to drive light emitting diodes, hot cathode fluorescent lamps, Zenon lamps, metal halide lamps, neon lamps, and the like