Abstract:
An apparatus for and a method of controlling a brightness and energy consumption of a back light in a portable display. A determination is made whether an input voltage of the portable display is provided by a DC power source or batteries and a control voltage is adjusted to control a duty cycle of a pulse width modulator which supplies power to the back light based on the determination. An initial value of the control voltage may be initially set according to power provided by one of the DC voltage source and the batteries so that the control voltage is automatically changed where the input voltage is provided from the other of the DC voltage source and the batteries.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This application claims the priority of Korean Patent Application No. 2002-35159, filed Jun. 22, 2002 in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference.  
           [0002]    1. Field of the Invention  
           [0003]    The present invention relates to an apparatus for and a method of operating a portable display device, and more particularly, to an apparatus for and a method of reducing energy consumption of a battery operated portable display device by adjusting a brightness of a back light.  
           [0004]    2. Description of the Related Art  
           [0005]    As portable display devices are widely used, reducing an energy consumption, which is directly related to a playing time of the portable display devices, has become a concern. In particular, since most portable display devices having screens use liquid crystal displays (LCDs), which consume a large amount of energy, the portable display devices using LCDs contradict a need for reducing energy consumption. Thus, an inverter control technique of adjusting the brightness of the back light, which consumes a great portion of the energy consumed by the LCD, is necessary. When operating the portable display devices using a direct current (DC) power source derived from a commercial energy supply, the energy consumption is not a significant problem; however, when operating the portable display devices using batteries, the energy consumption should be reduced.  
           [0006]    [0006]FIG. 1 is a block diagram illustrating a structure of a conventional back light driving device which is formed of a brightness adjusting unit  100 , an inverter  101 , a transformer  102 , and an LCD  103  including a back light  103 - 1 . The inverter  101  generates a pulse width modulation (PWM) waveform and a pulse frequency modulation (PFM) waveform in correspondence to a brightness adjusting signal and supplies the waveforms to the transformer  102 . The back light  103 - 1  is located behind the LCD  103  and provides illumination in order for a user to clearly recognize images on the LCD  103 . The transformer  102  supplies power to the back light  103 - 1  using the PWM waveform, which is output from the inverter  101 . The brightness adjusting unit  100  adjusts an input voltage Vcc by using a variable resistor VR 1  and outputs a brightness adjusting voltage signal to the inverter  101 .  
           [0007]    When the input voltage Vcc is lowered, a PWM frequency output from the inverter  101  is reduced and a duty ratio is increased, so the amount of current flowing to the back light  103 - 1  is increased. Here, a battery outputs a voltage level equal to or less than a DC voltage level of power being supplied from the DC power source. In addition, when using batteries, the voltage level output from the battery is gradually lowered as energy stored in the battery is depleted. Accordingly, when batteries are used to operate the portable display device, the amount of current flowing to the back light  103 - 1  is increased compared to the case where the DC power source is used, so battery life is reduced.  
         SUMMARY OF THE INVENTION  
         [0008]    The present invention provides an apparatus for and a method of reducing energy consumption by controlling a brightness of a back light in a battery operated portable display device.  
           [0009]    According to an aspect of the present invention, an apparatus for reducing energy consumption in a portable display device having a back light which provides illumination for a display of the portable display device comprises: a determination unit, which determines whether a voltage of a power source, which drives the portable display device, is lowered by checking the power source for a predetermined period; and a back light driving control unit, which controls a driving current of the back light to maintain a brightness of the back light to be constant where the voltage of the power source is lowered.  
           [0010]    According to an aspect of the present invention, the back light driving control unit may comprise a brightness adjusting unit, which outputs a brightness adjusting signal to maintain the brightness of the back light to be constant where the voltage of the power source is lowered; and a back light driving unit, which controls and outputs the driving current of the back light to maintain the brightness of the back light to be constant, in response to the brightness adjusting signal.  
           [0011]    According to an aspect of the present invention, the constant brightness and current maintained in the back light driving control unit may be a reference brightness and current, respectively, as supplied from a constant voltage power source.  
           [0012]    According an aspect of the present invention, a method of reducing energy consumption in a portable display device comprises determining whether a voltage of a power source is lowered by checking the power source, which operates the portable display device, for a predetermined period; and controlling a driving current of a back light to be constant to maintain a constant brightness of the back light, which provides illumination for a display of the portable display device, if it is determined that the voltage of the power source is lowered.  
           [0013]    According to an aspect of the present invention, the constant brightness and the constant driving current of the back light may be a reference brightness and a reference current as provided by a power source which provides a constant voltage.  
           [0014]    According to an aspect of the present invention, where the voltage of the power source is lowered, the method may further comprise outputting a back light brightness adjusting signal to maintain the brightness of the back light to be constant and controlling the driving current to maintain the brightness of the back light to be constant in response to the back light brightness adjusting signal.  
           [0015]    According to an aspect of the present invention, where the voltage of the power source is constant, the method may further comprise (b- 1   b ) outputting a back light brightness adjusting signal to maintain a present brightness and maintaining the driving current of the back light in response to the back light brightness adjusting signal.  
           [0016]    Additional aspects and/or advantages of the invention will be set forth in part in the description which follows, and, in part, will be obvious from the description, or may be learned by practice of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    The above and/or other aspects and advantages of the present invention will become more apparent by describing in detail an embodiment thereof with reference to the attached drawings in which:  
         [0018]    [0018]FIG. 1 is a block diagram illustrating a structure of a conventional back light driving device;  
         [0019]    [0019]FIG. 2 is a block diagram illustrating a structure of an apparatus for reducing energy consumption according to an embodiment of the present invention;  
         [0020]    [0020]FIG. 3 is a detailed diagram illustrating a brightness adjusting unit and a battery detection unit of FIG. 2;  
         [0021]    [0021]FIGS. 4A through 4C illustrate waveforms for explaining the apparatus for reducing the energy consumption of FIG. 2; and  
         [0022]    [0022]FIG. 5 is a flowchart illustrating a method for reducing energy consumption according to the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0023]    Reference will now be made in detail to the present embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout.  
         [0024]    [0024]FIG. 2 is a block diagram illustrating a structure of a display device incorporating an apparatus for reducing energy consumption according to the present invention. The display device comprises a video source device  200 , a scaler  201 , a timing control unit  202 , a liquid crystal display (LCD)  203  having a back light  203 - 1 , a user interface control unit  204 , a battery detection unit  205 , a brightness adjusting unit  206 , an inverter  207 , and a transformer  208 .  
         [0025]    The video source device  200  generates video sources and processes the video sources into video signals to be displayed. Examples of the video source device  200  include video processors such as, for example, a DVD player, a camcorder, and a laptop computer. The scaler  201  adjusts the video signals, which are output from the video source device  200 , according to a scale of the LCD  203 . The timing control unit  202  holds the video signals output from the scaler  201  and outputs the video signals to the LCD  203 .  
         [0026]    The user interface control unit  204  displays reproduction information such as, for example, inputs of front keys and remote controller keys, which are arranged for an interface between a user and the portable display device, or a reproduction time on a fluorescence light tube (FLT) (not shown). In addition, the user interface control unit  204  determines whether the portable display device is operated using a DC power source or batteries. When the portable display device is operated using the DC power source, the level of a power supply voltage is constant; however, when the portable display device is operated using batteries, the level of the power supply voltage gradually decreases. Thus, the user interface control unit  204  may determine whether the portable display is operated using the DC power source or batteries by checking for the level of the power supply voltage for a predetermined period.  
         [0027]    The battery detection unit  205  detects a power source determination signal, i.e., a signal indicating whether the portable display device is operated using the DC power source or batteries, output from the user interface control unit  204  to control the output of the brightness adjusting unit  206 .  
         [0028]    The brightness adjusting unit  206  outputs a back light brightness adjusting signal having a level which responds to the determination signal from the battery detection unit  205 . Referring to FIG. 3, the brightness adjusting unit  206  attenuates an input voltage Vcc and outputs a back light brightness adjusting signal VBLA. When the battery detection unit  205  outputs a determination signal indicating that a battery is present, the driving current of the back light  203 - 1  increases, so the brightness adjusting unit  206  outputs the back light brightness adjusting signal for lowering the driving current of the back light  203 - 1  to the driving current of the back light  203 - 1  of the DC power source level. When the battery detection unit  205  outputs the determination signal indicating that DC power is present, the brightness adjusting unit  206  maintains the brightness and the driving current of the back light  203 - 1  at a predetermined level.  
         [0029]    The inverter  207  generates the PWM waveform for adjusting the brightness of the back light  203 - 1  corresponding to the brightness adjusting signal from the brightness adjusting unit  206  and outputs the PWM waveform to the transformer  208 . When the input voltage Vcc is lowered, the PWM frequency output from the inverter  207  is reduced (i.e., a period T is increased) and a duty ratio (ΔT/T) is increased by an amount which is proportional to the reduction in the input voltage Vcc, so an amount of current used is increased as shown in FIG. 4B. In addition, the inverter  207  checks for the state of the back light  203 - 1  to perform a shutdown function for preventing the portable display device from being damaged by over current.  
         [0030]    The transformer  208  modulates the PWM waveform output from the inverter  207  to operate the back light  203 - 1 .  
         [0031]    Referring to FIG. 3, when the portable display device is operated using the DC power source, the battery detection unit  205  outputs a low level battery enable signal to the brightness adjusting unit  206 . The low level battery control signal causes the transistor TR 1  to turn On, placing a resistor R 1  in parallel with a series combination of a variable resistor VR 1  and a resistor R 3 . The input voltage Vcc is thus attenuated based on the resistor R 2  and a parallel combination of R 1  and (VR 1 +R 3 ). The variable resistor VR 1  may then be adjusted to establish a reference level for the back light brightness adjusting signal V BLA  which is provided to the inverter  207 . When the portable display device is operated using batteries, the user interface control unit  204  outputs the power source determination signal to the battery detection unit  205 . Accordingly, the battery detection unit  205  outputs the high level battery enable signal. When the battery enable signal is at the high level, the transistor TR 1  is turned Off and the input voltage Vcc, is attenuated based on R 2  and (VR 1 +R 3 ) to provide the back light brightness adjusting signal VBLA. With the transistor TR 1  turned Off, the attenuation is reduced and the back light brightness adjusting signal V BLA  increases. As a result, the brightness of the back light  203 - 1  where batteries are used is equal to the brightness of the back light  203 - 1  where the DC power source is used. In other words, the brightness of the back light  203 - 1  is kept uniform, regardless of whether batteries are used or the DC power source is used by the back light brightness adjusting signal V BLA  from the brightness adjusting unit  206 . The back light brightness adjusting signal V BLA , which is initially adjusted using the DC power source and the brightness adjusting unit  206 , is adjusted by the brightness adjusting unit  206  in response to the battery enable signal, output to the inverter  207 , and the inverter  207  lowers the duty ratio of the PWM modulation i.e., the back light brightness signal, corresponding to the adjusted back light brightness adjusting signal V BLA . When the duty ratio of the PWM modulation is lowered stably, the driving current of the back light  203 - 1  is reduced, which lowers the energy consumption, and the brightness of the back light  203 - 1  where batteries are used becomes the same as the brightness of the back light  203 - 1  where the DC power source is used. Alternatively, the variable resistor VR 1  may be adjusted while the battery enable signal is at the low level.  
         [0032]    When a signal is input from the user interface control unit  204  indicates a DC power source is again being used, the battery detection unit  206  outputs the low level battery enable signal. When the battery enable signal is at the low level, the transistor TR 1  is again turned On and the input voltage Vcc is attenuated based on the resistor R 2  and a parallel combination (VR 1 +R 3 ) and R 1  to output the voltage V BLA , to control the brightness of the back light  203 - 1 . As a result, the back light brightness adjusting signal output to the inverter  207  is maintained at a predetermined level. The adjusted driving current of the back light  203 - 1  is output to the inverter  207 , and the inverter  207  outputs a PWM waveform having a stable duty ratio within a design specification standard in response to the brightness adjusting signal V BLA .  
         [0033]    [0033]FIG. 4A illustrates a PWM waveform generated in the inverter  207  when the portable display device is operated using the DC power source. Here, the PWM waveform of FIG. 4A has the stable duty ratio (ΔT/T) that is within the design specification standard.  
         [0034]    [0034]FIG. 4B illustrates a PWM waveform generated in the inverter  207  when the portable display device is operated using batteries. Here, the level of the input voltage is gradually lowered in the case where the portable display device is operated using batteries. When the level of the input voltage is lowered, the PWM frequency is reduced (T is increased), the duty ratio (ΔT/T) is increased, and the amount of current flowing to the back light  203 - 1  is increased. That is, the PWM frequency output from the inverter  207  is gradually reduced and the duty ratio(ΔT/T) is gradually increased so that the back light  203 - 1  requires an increasingly larger amount of energy.  
         [0035]    [0035]FIG. 4C illustrates a PWM waveform generated in the inverter  207  when the apparatus for reducing energy consumption is applied to the portable display device being operated using batteries. Referring again to FIG. 3, when the battery enable signal is at the low level, the transistor TR 1  is turned On and the brightness adjusting signal V BLA  obtained by dividing the input voltage Vcc is reduced. As a result, the brightness of the back light  203 - 1  to the inverter  207  is made equal to the brightness of the back light  203 - 1  in the case where the DC power source is used. In other words, the brightness of the back light  203 - 1  is the same, regardless of whether batteries are used or the DC power source is used, by controlling the brightness adjusting signal VBLA from the brightness adjusting unit  206 . The back light adjusting signal, which is adjusted to the signal in the case of using the DC power source by the brightness adjusting unit  206 , is output to the inverter  207 , and the inverter  207  lowers the duty ratio of the PWM as the back light brightness signal corresponding to the adjusted brightness adjusting signal. When the duty ratio of the PWM is lowered stably, the driving current of the back light  203 - 1  is reduced, which lowers the energy consumption and the brightness of the back light  203 - 1  becomes the same as the brightness of the back light  203 - 1  in the case where the DC power source is used.  
         [0036]    A method of reducing energy consumption will now be described with reference to FIG. 5. FIG. 5 is a flowchart illustrating a method for reducing energy consumption according to the present invention. Here, the method for reducing energy consumption comprises determining whether a battery is present in operation  500 . If a battery is determined to be present in operation  500 , the method further comprises, outputting a battery enable signal in operation  501 , controlling a driving current of a back light while maintaining the brightness of the back light in operation  502  in response to the battery enable signal and driving the back light in response to the controlled driving current in operation  505 . If a battery is determined not to be present in operation  500 , the method further comprises, outputting a high level battery enable signal in operation  503 , maintaining the driving current of the back light while maintaining the brightness of the back light in operation  504 , and driving the back light at the present level in step  505 .  
         [0037]    The determining of whether a battery is present in operation  500  comprises checking a level of the power supply voltage for a predetermined period.  
         [0038]    If it is determined that the portable display device is operated using batteries, the method further comprises outputting a battery enable signal having a first predetermined level and adjusting a brightness adjusting signal in response to the battery enable signal to maintain the brightness a display.  
         [0039]    If it is determined that the portable display device is being operated using the DC power source at operation  500 , the method further comprises outputting the battery enable signal having a second predetermined level and maintaining a brightness adjusting signal at a predetermined value in response to the battery enable signal to maintain the brightness a display.  
         [0040]    According to the present invention, when the portable display device is operated using batteries, the brightness of the back light, which consumes a large portion of the energy consumed by the LCD, is adjusted, so the energy consumption is reduced and the lifetime of the batteries is increased.  
         [0041]    While this invention has been particularly shown and described with reference to embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.