Abstract:
A computer system that dynamically optimizes the power efficiency of a display backlighting system based on the current load is described. Specifically, the computer system may be optimized to operate in a battery mode or an AC line driven mode.

Description:
FIELD OF THE INVENTION 
   The present invention pertains to the field of computer design. More particularly, the present invention relates to a method for optimizing power in a computer having a liquid crystal display. 
   BACKGROUND OF THE INVENTION 
   A liquid crystal display (LCD) is one means for providing an interface between a computer and a computer user. For instance, notebook or laptop computer systems typically have a LCD panel. Most computer LCD panels are illuminated with built-in fluorescent tubes. The tubes may be placed above, beside, or behind the LCD. A cold cathode fluorescent lamp (CCFL) is a common fluorescent tube for providing a uniform white light inside a LCD panel. The LCD often uses a diffusion layer to redirect and scatter the CCFL light evenly. Because the CCFL operates at high AC voltages, the CCFL typically requires a power converter circuit to convert an input DC voltage to an AC voltage. 
     FIG. 1  depicts a diagram of a backlight LCD system. Inverter  10  receives a DC voltage as input and generates an AC voltage. CCFL  20  is coupled to inverter  10  and uses the AC voltage generated by inverter  10  to generate white light. 
   The output load of the inverter  10  is set to a level of energy that the inverter  10  must generate in order to adequately power the CCFL  20  to satisfy the system display&#39;s brightness setting. The output load may be defined by intensity depending on how much current the CCFL  20  requires for operation. For example, the computer system may operate in a low intensity state when the computer display is set to a non-visible brightness level, a medium intensity state when powered by a battery, and high intensity state when powered by an AC line power outlet. The computer is considered to be in a “battery mode” when the system is powered only by a battery. The computer is considered to be in “AC mode” when the system is powered by an AC line power outlet. 
   Inverter efficiency is the ratio of the generated inverter output electric power divided by the input electric power. Inverters of a computer system are typically designed to have optimal power conversion efficiency when operated near the maximum output load range. Thus, inverters are often most efficient when the CCFL is in a high intensity light output state. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a prior art backlit LCD system; 
       FIG. 2  is an embodiment of a system that optimizes the load efficiency; 
       FIG. 3  is an embodiment of a flowchart for setting the operating mode; 
       FIG. 4  is a graph of the load efficiency of an embodiment of a mobile display backlighting system; and 
       FIG. 5  is an embodiment of a backlight power module. 
   

   DETAILED DESCRIPTION 
   In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the invention. However, it will be understood by those skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known methods, procedures, components and circuits have not been described in detail so as not to obscure the present invention. 
   In an electronic system, the brightness setting of the system when powered by an AC source may be different than the brightness setting of the system when powered by a battery. Inverters are typically designed to generate the maximum efficiency near the maximum load that the inverters must drive. The inverter maximum load is achieved during high intensity light output from the backlight. Thus, in a system powered by only a battery, the system is often operating at less than maximum efficiency. Improving the load efficiency during medium intensity would help to improve the average battery operating time of the system. The electronic system may be a notebook computer, a cell phone, a desktop monitor, or a digital television. 
   For one embodiment of the invention,  FIG. 2  depicts a backlight power module  230  that improves the load efficiency when the system is operated in a medium intensity state. Power module  230  is coupled to a DC source  210 , a CCFL  240 , and a circuit  220  to determine output load or load power demand for the CCFL  240 . The circuit  220  may comprise combinational logic with ordinary transistors. The power module  230  may comprise an inverter to generate an AC signal to power the CCFL  240 . 
   When the system is in battery mode, DC source  210  may be a battery. The battery supplies a DC voltage to the power module  230 . Alternatively, if the system is powered by an AC power outlet, the DC source may be a downstream regulated DC source derived from the AC line power. The circuit  220  provides a second input to inverter  230  and determines the load power demand of the CCFL  240 . 
   For this embodiment of the invention, circuit  220  senses whether an AC power source is charging the battery in the system. If the circuit  220  determines that the system is running off of an AC source rather than the battery alone, the circuit  220  communicates to the power module  230  that the system is operating in a high intensity state. The power module  230  then generates an AC signal to drive the CCFL  240  based upon a high intensity load. 
   Otherwise, if the circuit  220  determines that the system is running in a medium intensity state, the power module  230  generates an AC signal to drive the CCFL  240  based upon a medium intensity load. Power module  230  may comprise an inverter to generate the AC signal. One embodiment of power module  230  will be further described below in  FIG. 5 . 
   For another embodiment of the invention, the circuit  220  for determining the required CCFL  240  load is performed through system software instead of hardware. Computer systems often have a range of brightness settings that may be controlled using an operating system (OS) sensor control. For example, the OS sensor control may be used to interpret the brightness setting made through a potentiometer, a keyboard entry, or an analog brightness adjustor. The system software then determines the range of brightness settings to transmit to the power module  230 . The power module  230  takes the brightness setting and generates an AC signal to enable the CCFL  240  to create a certain amount of white light. 
   In AC mode, the display panel may operate at a higher brightness setting to improve the visibility of the image for the user. However, if the system detects that the AC power source has been withdrawn, the system may limit the upper brightness setting. As a result of the system being placed in battery mode, the load capacity may shift from high intensity to medium intensity. The circuit  220  detects and communicates the mode change to the power module  230 , which then makes the appropriate adjustment to improve load efficiency. 
   For yet another embodiment of the invention, the circuit  220  may incorporate an ambient light sensor that senses the available light in which the computer system is operating. The ambient light sensor may be located within the body of a notebook computer or external to a computer system. Depending on the amount of light detected by the backlight power module  230  through the environment, the circuit  220  may switch the power module  230  to a different brightness level to reduce power consumption. 
   For yet another embodiment of the invention, the power module  230  may auto-detect the load power demand for the CCFL. The load power demand for the CCFL may depend upon the system display&#39;s brightness setting. 
   For yet another embodiment of the invention, power module  230  may comprise an input that allows the power module  230  to override the brightness setting as provided by circuit  220 . In this embodiment of the invention, the user may determine whether to set the system to maximize efficiency in medium intensity or high intensity regardless of whether the system is actually in battery mode or AC mode. 
   A flowchart that describes the operations of system that maximizes the load efficiency based on the current load is depicted in  FIG. 3 . After system powerup in operation  310 , operation  320  determines whether the system is enabled to auto-detect the load power demand of the CCFL  240 . For example, if the user overrides the power module  230 , the auto-detect is no longer enabled. In this embodiment of the invention, if the auto-detect mode is not enabled, the system is operated in the maximum load operating mode or AC mode in operation  330 . 
   However, if the auto-detect mode is enabled, the circuit  220  determines the load power demand for the lamp in operation  340 . The system then determines in operation  350  whether the load power demand detected is greater than a specified limit. If the load demand is greater than this predefined limit, the system is placed in the maximum load operating mode or AC mode in operation  330 . Otherwise, if the load demand is less than the predefined limit, the system is placed in the mid-load range operating mode or battery mode in operation  360 . The brightness level of the system display in the mid-range operating mode is reduced compared to the brightness setting of a system in the maximum load operating mode. If at any time, the circuit  220  detects a change in load power demand, the circuit  220  may switch the power module  230  to a different operating mode. 
     FIG. 4  depicts a graph of one embodiment of the available power module  230  setting options. X-axis  410  represents normalized power regions and y-axis  420  is the corresponding load efficiency. Curve  430  is a load efficiency curve of a power module that is set to AC mode and therefore maximizes the load efficiency for a high intensity state. Curve  440  is a load efficiency curve of the power module that is set to battery mode to maximize the load efficiency for a medium intensity state. 
   X-axis  410  is a function of time. Voltage and current measurements of inverters are taken over a period of time because the values vary over time. As a result, normalized power values depend upon voltage and current measurements over a given period of time. For the embodiment of the invention represented in  FIG. 4 , region  1  of x-axis  410  constitutes a low intensity state; region  2  constitutes a medium intensity state; and region  3  constitutes a high intensity state. 
   Curve  440  offers an approximately 10–40% improvement over curve  430  in load efficiency during medium intensity. However, the load efficiency of curve  440  may have a depreciated load efficiency with respect to curve  430  during high intensity. 
   Power module  230  may have an inverter or a plurality of inverters that are capable of generating output power signals that have a load efficiency of curve  430  and curve  440 . Power module  230  may be switched to either curve depending on the detected load power demand of circuit  220 . If the load power demand is greater than a predefined limit, then the power module may be set for operation based upon curve  430 . On the other hand, if the load power demand is less than a predefined limit, then the power module may be set for operation based upon curve  440 . Switching the power module  230  to optimize the load efficiency may allow a computer system powered by a battery to have a longer operating time because power consumption is reduced. 
   For another embodiment of the invention, an option may be available to further optimize the load efficiency during a low intensity state. The system may offer a power module setting to optimize the low intensity load efficiency. 
     FIG. 5  depicts an embodiment of a power module  230 . The power module  230  comprises an AC converter  510  coupled to a transformer  520  and a controller  530 . The transformer  520  and the controller  530  are coupled to the limit reference and comparator  540 . The AC converter  510  receives a DC voltage input from a DC source such as a battery. The AC converter  510  may convert the DC input into an AC output by switching the DC power source across an inductor. 
   The controller  530  turns the AC converter  510  on or off depending on the feedback the controller  530  receives from the output of the transformer  520  via the limit reference and comparator  540 . The signal from the controller  530  to the AC converter  510  may be analog or digital. If the AC converter  510  is enabled by the controller  530 , the AC converter provides a low voltage AC signal to the transformer  520 . The transformer  520  then generates a high voltage output signal to power the CCFL  240 . The controller  530  compares the transformer  520  output signal with an internal limit reference or references through the limit reference and comparator  540 . The controller  530  is responsible for managing the energy sent to the transformer  520  by the AC converter  510 . 
   The CCFL  240  acts as an open circuit, drawing no current, before the CCFL  240  starts generating light. Voltage of a sufficient level may be applied by the transformer  520  to ignite the CCFL  240 . Once the light emission begins, the lamp no longer appears as an open circuit, but as a relatively low impedance circuit. When the output reaches a certain voltage level, the AC converter  510  is reduced or turned off turned off to prevent failure of the CCFL  240 , to limit the lamp&#39;s power consumption, and to reduce light emission levels. 
   The limit reference and comparator  540  may detect the point at which the CCFL  240  ignition occurs by monitoring the current flow from the transformer  520 . The limit reference and comparator  540  may measure the transformer  520  power applied to the CCFL  240  and serve as a reference for the controller  530 . 
   The limit reference and comparator  540  also receives an input to select an appropriate load efficiency curve. This input may be a function of the brightness setting of the system display. Depending on the load efficiency curve selected, the AC converter  510  may be turned on by controller  530  and used to pass an AC voltage source to the transformer  520  to optimize the load efficiency of a low intensity, a medium intensity, or a high intensity state. 
   In the foregoing specification the invention has been described with reference to specific exemplary embodiments thereof. It will, however, be evident that various modification and changes may be made thereto without departure from the broader spirit and scope of the invention as set forth in the appended claims. The specification and drawings are, accordingly, to be regarded in an illustrative rather than restrictive sense.