Patent Publication Number: US-7916101-B2

Title: LED driving apparatus and method of controlling luminous power

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority from Japanese Patent Application Nos. JP 2004-296148 filed on Oct. 8, 2004 and JP 2005-227965 filed on Aug. 5, 2005, the disclosures of which are hereby incorporated by reference herein. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to an LED driving apparatus and a method of controlling luminous power. 
     A display, such as a liquid crystal display, that is not a self-emitting display is provided with a backlight. As such a backlight, one employing cold-cathode tubes as its light source has been known for example. In addition, in recent years, backlights employing light emitting diodes (LEDs), which consume less power, have also been known. 
     As a method of controlling the luminous power of such a backlight including LEDs, two kinds of methods have been known. One is a current value control method for controlling the level of a driving current supplied to the LEDs. Another is to implement pulse width modulation (PWM) control with keeping a driving current level constant. 
     In the current value control, the level itself of a driving current continuously supplied to the LEDs is changed to thereby achieve aimed luminous power. In the PWM control, the ON/OFF ratio of a driving current per unit time is changed to thereby achieve desired luminous power. 
     An example of documents disclosing the related art is Japanese Patent Laid-open No. 4-134486. 
     It has been known that the luminous efficiency of an LED varies depending on the value of a driving current therefor. 
     This respect will be described with reference to  FIGS. 7 to 9 . 
       FIG. 7  illustrates a graph of the relationship between forward voltages and forward currents of a certain LED. 
       FIG. 8  illustrates a graph of the relationship between the forward currents and luminous power of the LED. Specifically, 
       FIG. 7  shows the respective values of the forward voltages obtained when forward currents of certain values are applied to the LED.  FIG. 8  shows the respective values of the luminous power obtained when the forward currents of the certain values are applied to the LED. 
     Luminous efficiency is obtained by dividing luminous power by input power. Therefore, the luminous efficiency of the LED is obtained through the following procedure: a certain forward current value in the graph of  FIG. 7  is multiplied by the corresponding forward voltage value to obtain the input power; and the luminous power value in  FIG. 8  corresponding to the forward current value is divided by the obtained input power. 
       FIG. 9  illustrates a graph of the relationship between luminous efficiency that can be obtained through the above procedure and the forward current values. 
     As the graph shows, in the LED, the luminous efficiency increases as the forward current value increases from 50 mA to 100 mA, and the luminous efficiency decreases as the forward current value increases above 100 mA. The maximum luminous efficiency is obtained near a current value of 100 mA. 
     As is apparent from this graph, LEDs have characteristics in that the luminous efficiency varies depending on the value of the forward current (driving current). Specifically, the luminous efficiency of an LED is apt to increase as the driving current value increases until a certain current level, while above this level, the luminous efficiency is apt to decrease as the current value increases. 
     Since the luminous efficiency of an LED varies depending on the driving current value, the following problems arise in an LED driving apparatus that controls the luminous efficiency of an LED with any of the above-described methods: current value control and PWM control. 
     In the current value control, if the target value of the luminous power to be controlled is identical to the value corresponding to the current value providing the highest luminous efficiency like that shown in  FIG. 9 , the LED can be driven with the highest luminous efficiency. However, the target value does not necessarily correspond with the value, and therefore there is a possibility that the LED is driven with a low luminous efficiency. with a low luminous efficiency. 
     As for the PWM control, ON/OFF of a current is controlled with keeping the value of the current constant. Therefore, the constant current value must be the value corresponding to the maximum luminous power value in the allowable range thereof. 
     The current value for the maximum luminous power in the allowable range also does not necessarily correspond with the above current value for the highest luminous efficiency. Accordingly, there is a possibility that the LED is driven with a low luminous efficiency also in the PWM control. 
     Driving an LED with a low luminous efficiency requires unnecessary extra power higher than originally needed input power, which leads to the increase of power consumption. In addition, if an LED is driven with a low luminous efficiency, the heating value of the LED, a driving circuit thereof, and a power supply unit is apt to increase, which causes a problem that measures against the heating preclude the miniaturization of the device, for example. 
     SUMMARY OF THE INVENTION 
     In consideration of the above problem, it is desirable to provide an LED driving apparatus that has the following configuration. 
     Specifically, the LED driving apparatus includes drive means for driving an LED to emit light, and control means that controls a driving current supplied from the drive means to the LED. The control means implements luminous power control by controlling an ON/OFF ratio of the driving current if a target value of luminous power of the LED is smaller than a predetermined value. The control means implements luminous power control by controlling a level of the driving current if the target value is equal to or larger than the predetermined value. 
     In addition, according to an embodiment of the present invention, a method of controlling luminous power has the following feature. 
     Specifically, as a method of controlling the luminous power of an LED, an ON/OFF ratio of a driving current is controlled if a target value of the luminous power of the LED is smaller than a predetermined value, and a level of the driving current is controlled if the target value is equal to or larger than the predetermined value. 
     According to an embodiment of the present invention, a light emitting diode (LED) driving apparatus includes a drive section for driving an LED to emit light, and a control section that controls a driving current supplied from the drive section to the LED. The control section implements luminous power control by controlling an ON/OFF ratio of the driving current if a target value of luminous power of the LED is smaller than a predetermined value. The control section implements luminous power control by controlling a level of the driving current if the target value is equal to or larger than the predetermined value. 
     According to an embodiment of the present invention, the method of controlling the driving of an LED is switched between the above-described current value control and the PWM control according to a target value of luminous power. Specifically, according to the above configurations, the control method can be switched to the PWM control if the target value is smaller than the luminous power value providing the highest luminous efficiency of an LED. If the target value is equal to or larger than the predetermined value, the control method can be switched to the driving current control. 
     If such luminous power control is possible, the PWM control can be implemented in which a driving current level is kept constant at the level providing the highest luminous efficiency of the LED when the target value is smaller than the predetermined value. Thus, the LED can be driven with the highest luminous efficiency. 
     In addition, if the target value is equal to or larger than the predetermined value, switching to the current value control can obtain desired luminous power with as little power as possible. If the PWM control is implemented even when the target value is equal to or larger than the predetermined value, the driving current level inevitably needs to be larger compared with the case of implementing the current value control. The larger current level results in the driving of an LED with a lower luminous efficiency as is apparent from the characteristic diagram of  FIG. 9 . Therefore, switching to the current value control allows an LED to be driven most efficiently. 
     As described above, according to an embodiment of the present invention, luminous power control is switched between the PWM control and the current value control depending on a target value of luminous power. Thus, an LED can be driven with the highest luminous efficiency possible. 
     In addition, this control method switching can minimize the power consumed to drive an LED, and further can minimize the heating value of an LED, a driving circuit thereof, and a power supply unit. Thus, the device can be miniaturized. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other objects of the invention will be seen by reference to the description, taken in connection with the accompanying drawings, in which: 
         FIG. 1  is a block diagram illustrating an example of the configuration of a liquid crystal display provided with an LED driving apparatus according to an embodiment of the invention; 
         FIG. 2  is a block diagram illustrating the configuration of an LED driving apparatus according to a first embodiment of the invention; 
         FIG. 3  is a circuit diagram illustrating the internal configuration of the LED driving circuit; 
         FIG. 4  is a flow chart illustrating processing operation for achieving the operation of the first embodiment; 
         FIG. 5  is a block diagram illustrating the configuration of an LED driving apparatus according to a second embodiment of the invention; 
         FIG. 6  is a flow chart illustrating processing operation for achieving the operation of the second embodiment; 
         FIG. 7  is a diagram illustrating a graph of the relationship between forward voltages and forward currents of a certain LED; 
         FIG. 8  is a diagram illustrating a graph of the relationship between the forward currents and luminous power of the certain LED; and 
         FIG. 9  is a diagram illustrating a graph of the relationship between the forward currents and luminous efficiency of the certain LED. 
     
    
    
     DETAILED DESCRIPTION 
     Best modes (embodiments hereinafter) for carrying out the invention will be described below. 
     Initially, one example of the configuration of a liquid crystal display equipped with a light emitting diode (LED) driving apparatus of an embodiment will be described referring to  FIG. 1 . 
     Referring to  FIG. 1 , AC power supply is input from an AC power supply input terminal tAC to the liquid crystal display. 
     A power supply circuit  6  is fed with the AC power supply from the AC power supply input terminal tAC to produce a DC voltage. The power supply circuit  6  then supplies the produced DC voltage as the illustrated DC power supply to a signal processing circuit  7 , a panel driving circuit  8 , a controller  2 , and an LED driving circuit  3 . 
     Furthermore, video signals are input from a video input terminal tv to the liquid crystal display. The video signals are supplied to the signal processing circuit  7 . The signal processing circuit  7  implements required signal processing for the supplied video signal to thereby obtain a signal necessary for controlling the driving of a liquid crystal panel  5 . 
     The signal processing circuit  7  supplies to the panel driving circuit  8  a signal necessary for controlling the driving of the panel. The panel driving circuit  8  drives the liquid crystal panel  5  based on the signal. 
     In addition, the signal processing circuit  7  extracts a luminance signal from the input video signal and supplies the luminance signal to the controller  2 . 
     The controller  2  is, for example, a micro computer including a central processing unit (CPU), a read only memory (ROM), a random access memory (RAM) etc., and implements overall control of the liquid crystal display. 
     The controller  2  adjusts the luminous power of LEDs  4   a  provided in a backlight  4  according to information of an average picture level (APL) (average luminance) calculated based on the luminance signal supplied from the signal processing circuit  7 . According to the luminous power control based on the information of an APL, when the APL is lower than a certain level for example, the luminous power of the backlight is decreased to a predetermined value (one-tenth, for example) to thereby achieve high contrast. 
     In addition to the adjustment based on an APL, the controller  2  adjusts the luminous power of the LEDs  4   a  in the backlight  4  also in response to the operation by a user via a user interface (I/F)  9 . 
     The user I/F  9  comprehensively includes a command receiver that receives a command signal from an operating element provided on the outer surface of the case of the liquid crystal display, or a remote controller. For example, a user operates a knob operating element for brightness adjustment provided on the outer surface of the case, and thereby can input to the liquid crystal display an instruction as to the luminous power of the backlight  4 . Alternatively, a user selects an item of brightness adjustment from a configuration menu displayed on the screen of the liquid crystal panel  5  to thereby carry out operational input according to an instruction via the screen. Thus, an instruction as to the luminous power of the backlight  4  can be input. 
     If an instruction is thus input by a user via the user I/F  9 , the controller  2  controls the luminous power of the backlight  4  in response to the instruction input information. 
       FIG. 2  is a block diagram that picks up and illustrates, of the configuration shown in  FIG. 1 , part relating to control of the luminous power of the LED  4   a  in the backlight  4 . 
     Referring to  FIG. 2 , the LED driving circuit  3  is fed with DC power supply from the power supply circuit  6  shown in  FIG. 1 , and supplies a driving current to the LED  4   a  in the backlight  4  based on the DC power supply. 
     The LED driving circuit  3  is provided with a current value control terminal t 1  and an ON/OFF control terminal t 2 . The respective terminals receive a control signal from the controller  2  as shown in the drawing. With a configuration to be described later, the LED driving circuit  3  changes the level of a driving current supplied to the LEDs  4   a  in response to the control signal supplied to the current value control terminal t 1 . In addition, in response to the control signal supplied to the ON/OFF control terminal t 2 , the LED driving circuit  3  changes the ON/OFF timing of a driving current with keeping the level of the driving current constant. 
     Note that the controller  2  and the LED driving circuit  3  constitute the LED driving apparatus  1  as the first embodiment as shown also in  FIG. 2 . 
     The controller  2  sets the target value of the luminous power of the LED  4   a  according to APL information calculated based on a luminance signal from the signal processing circuit  7  shown also in  FIG. 1 , and according to an instruction input value if an instruction as to the brightness adjustment is input from the user I/F  9 . The controller  2  then supplies a control signal to the current value control terminal t 1  or the ON/OFF control terminal t 2  of the LED driving circuit  3  so that the luminous power of the target value is obtained. 
     The controller  2  of the embodiment stores in advance information of the forward current value (driving current level) providing the highest luminous efficiency and the luminous power value (predetermined value) obtained from the forward current value as to the LED  4   a.    
     The controller  2  switches between luminous power control attributed to supply of a control signal to the current value control terminal t 1  (current value control) and luminous power control attributed to supply of a control signal to the ON/OFF control terminal t 2  (PWM control) based on the result of comparison between the stored luminous power value as the predetermined value and the set target value. This switching will be described later in detail. 
     In addition, in the first embodiment, the ROM of the controller  2  stores, in correspondence with target values of luminous power, information of driving current values and the ON/OFF ratios of a driving current for obtaining the corresponding luminous power. 
     Specifically, if the controller  2  implements the current value control, the controller  2  obtains information of a driving current value based on the set target value and the stored correspondence information, and supplies the current value information to the current value control terminal t 1  of the LED driving circuit  3  to thereby control luminous power. When the current value control is implemented, as a control signal applied to the ON/OFF control terminal t 2 , an ON control signal that turns on supply of a driving current is applied. 
     Furthermore, if the PWM control is implemented, the controller  2  obtains information of an ON/OFF ratio based on the set target value and the stored correspondence information, and supplies an ON/OFF control signal to the ON/OFF control terminal t 2  based on the information to thereby control luminous power. When implementing the PWM control, such current value information is supplied to the current value control terminal t 1  that the current value is kept constant at the value of the forward current providing the highest luminous efficiency of the LED  4   a.    
     In this case, such ON/OFF ratio information for the PWM control is supplied that the ratio of length of the ON period to the total length of the ON and OFF periods equals the value obtained by dividing the target value of luminous power by the luminous power value as the predetermined value. 
       FIG. 3  illustrates the internal configuration of the LED driving circuit  3  shown in  FIG. 2 . 
     Since the DC power supply shown in  FIG. 2  is supplied across a capacitor Ci, a DC voltage is stored across the capacitor Ci. A switching element Q 1 , a diode D 1 , and a choke coil L 1  constitute a down converter employing the voltage across the capacitor Ci as operation power supply. The operation of the down converter produces an LED driving voltage of a DC voltage across a smoothing capacitor Co. Thus, a DC driving current is supplied to the LED  4   a  shown in  FIG. 2 . 
     As the switching element Q 1 , an MOS-FET is adopted. 
     The switching element Q 1  is driven and controlled by a control circuit  12 . 
     The control circuit  12  is supplied with an ON/OFF control signal from the ON/OFF control terminal t 2  as shown in the drawing, and turns on and off of the switching element Q 1  based on the ON/OFF control signal. 
     In addition, input to the control circuit  12  is a feedback signal from an error amplifier (E/A)  11 . 
     A current detection resistor R 1  incorporated in the output line of an LED driving voltage detects the level of a driving current supplied to the LED  4   a  and inputs the level to the error amplifier  11 . In addition to the detected current level, input to the error amplifier  11  is a current value control signal that has been input via the current value control terminal t 1  illustrated also in  FIG. 2  and has been converted into an analog signal by a D/A converter  10 . The error amplifier  11  outputs a signal according to the difference between the input driving current level and level of the current value control signal. 
     The control circuit  12  controls the operation of the switching element Q 1  according to the output signal from the error amplifier  11 , to thereby control so that the level of a driving current supplied to the LED  4   a  is kept constant at the value corresponding to the current value control signal supplied to the current value control terminal t 1 . 
     Here, as described above, the luminous efficiency of the LED changes depending on the level of a supplied driving current (forward current). Specifically, as shown in  FIG. 9 , the luminous efficiency is apt to increase as a forward current value increases until a certain forward current value, and above the value, the luminous efficiency is apt to decrease as a forward current value increases. 
     Since the luminous efficiency changes depending on the forward current value, the above-described current value control and PWM control involve a possibility that the LED  4   a  is driven with a low luminous efficiency. 
     Therefore, in the present embodiment, the forward current value providing the highest luminous efficiency (the best current value) and the luminous power (predetermined value) obtained from the forward current value are obtained in advance referring to the result of an experiment, like that shown in the characteristic diagram of  FIG. 9 , as to the relationship between forward current values and luminous efficiency of the LED  4   a . When the LED  4   a  is driven to emit light with luminous power smaller than the luminous power of the predetermined value, luminous power is controlled by the PWM control with keeping the driving current value at the best current value. When the LED  4   a  is driven to emit light with a luminous power value equal to or larger than the predetermined value, the current value control is implemented. 
     Thus, when the LED  4   a  is driven to emit light with luminous power smaller than the predetermined value, the LED  4   a  can be driven with the constant best current value invariably. Therefore, the LED  4   a  can be driven with the highest luminous efficiency. 
     In addition, also in the case of driving the LED  4   a  to emit light with at least the luminous power of the predetermined value, the LED  4   a  can be driven with as high luminous efficiency as possible. Specifically, if the PWM control is maintained even for a luminous power value equal to or larger than the predetermined value, the driving current level for the PWM control, which is constant, must inevitably be set higher than the level of a driving current employed when implementing the current value control for the luminous power value. The increase of the driving current level decreases the luminous power as is apparent from the characteristic of  FIG. 9 . Therefore, when driving the LED  4   a  to emit light with at least the luminous power of the predetermined value, the current value control allows the driving of the LED  4   a  with a higher luminous efficiency. 
     Thus, the luminous power control of the embodiment can drive the LED  4   a  with a condition providing as high luminous efficiency as possible invariably. Accordingly, power consumption due to the driving of the LED  4   a  can be minimized. In addition, if the LED  4   a  can be driven to emit light with a condition providing as high luminous efficiency as possible invariably, the heating value of the LED  4   a  itself, the LED driving circuit  3 , and the power supply circuit  6  (refer to  FIG. 1 ) that supplies DC power supply to the LED driving circuit  3  can be minimized. It therefore is prevented that the size of the device is increased to address the heating. 
       FIG. 4  is a flow chart illustrating processing operation for achieving the operation of the first embodiment. 
     The processing operation shown in  FIG. 4  is executed based on a program stored in an ROM or the like incorporated in the controller  2 , for example. 
     Referring to  FIG. 4 , in a step S 101 , processing of comparing a set target value and the above-described predetermined value is executed. 
     Subsequently, in a step S 102 , processing of determining whether or not the target value is smaller than the predetermined value is executed based on the result of the comparison processing of the step S 101 . 
     If the positive determination that the target value is smaller than the predetermined value is obtained in the step S 102 , the processing sequence moves to a step S 103 , in which processing of setting the PWM control is executed. Specifically, in order to implement, as the PWM control, the ON/OFF control of a driving current with keeping the driving current value constant at the above-described best current value, initially a current value control signal for indicating the best current value is supplied to the current value control terminal t 1  of the LED driving circuit  3 . 
     Sequentially, in a step S 104 , the ON/OFF ratio according to the luminous power as the target value is set. Specifically, information of the ON/OFF ratio associated with the input target value is retrieved from correspondence information stored in an ROM or the like. An ON/OFF control signal based on the ratio information is then supplied to the ON/OFF control terminal t 2  to thereby control luminous power by the PWM control. 
     Alternatively, if the negative determination that the target value is not smaller than the predetermined value (the target value is equal to or larger than the predetermined value) is obtained in the step S 102 , processing of setting the current value control is executed in a step S 105 . Specifically, in order to implement the current value control and therefore continuously supply a driving current, an ON control signal is initially supplied to the ON/OFF control terminal t 2 . 
     Sequentially, in a step S 106 , the current value according to the luminous power as the target value is set. Specifically, information of the current value associated with the input target value is retrieved from the correspondence information. A current value control signal based on the current value information is then supplied to the current value control terminal t 1  to thereby control luminous power by the current value control. 
     Through the above processing operation, luminous power control can be implemented by the PWM control when aimed luminous power is smaller than the predetermined value, and can be implemented by the current value control when aimed luminous power is equal to or larger than the predetermined value. 
       FIG. 5  illustrates the configuration of an LED driving apparatus  20  as a second embodiment of the invention. 
     The LED driving apparatus  20  of the second embodiment also implements switching of the luminous power control methods, implemented in the first embodiment. Furthermore, the LED driving apparatus  20  includes a luminous power sensor  21  in addition to the configuration of the LED driving apparatus  1  shown in  FIG. 2 . The luminous power sensor  21  is provided at a certain place in the backlight  4  so as to detect the luminous power of the LED  4   a  (the luminous power sensor  21  is represented with a dashed line in  FIG. 1 ). 
     Information of the luminous power detected by the luminous power sensor  21  is input to the controller  2 . 
     The controller  2  controls the luminous power of the LED  4   a  based on the luminous power value detected and input by the luminous power sensor  21  as well as based on the target value of luminous power set according to a brightness signal from the signal processing circuit  7  and an instruction input from the user I/F  9  as described above. 
     Specifically, the luminous power is controlled by changing a current value control signal supplied to the current value control terminal t 1  or an ON/OFF control signal supplied to the ON/OFF control terminal t 2  so that the value of the luminous power detected and input by the luminous power sensor  21  equals the target value. 
     Since luminous power is thus controlled based on the actually measured value of luminous power of the LED  4   a , the luminous power as the target value can be achieved more accurately even if there is variation in luminous power among the LEDs  4   a  for the same forward current value, for example. 
     This luminous power control can prevent variation in luminous power of the LEDs  4   a  among devices effectively. 
       FIG. 6  is a flow chart illustrating processing operation for achieving the operation of the LED driving apparatus  20  of the second embodiment. 
     The processing operation shown in  FIG. 6  is also executed based on a program stored in an ROM or the like incorporated in the controller  2 . 
     In steps S 201  and S 202 , as with the processing of the steps S 1 . 01  and S 102  shown in  FIG. 4 , processing of comparing a target value with the predetermined value and processing of determining from the compassion result whether or not the target value is smaller than the predetermined value, are implemented. 
     Subsequently, if the target value is smaller than the predetermined value and thus the positive determination is obtained, processing of setting the PWM control as processing of a step S 203  is executed as with the step S 103 . Alternatively, if the negative determination that the target value is not smaller than the predetermined value is obtained, processing of setting the current value control is executed in a step S 205  as with the step S 105 . 
     In this processing operation, in a step S 204  after the PWM control has been set, processing of controlling the ON/OFF ratio so that the sensed value equals the target value is executed. Specifically, the ratio of the ON/OFF control signal supplied to the ON/OFF control terminal t 2  of the LED driving circuit  3  is controlled so that the set target value equals the luminous power value from the luminous power sensor  21 . 
     Furthermore, in a step S 206  after the current value control has been set, processing of controlling the current value so that the sensed value equals the target value is executed. Specifically, the current value control signal supplied to the current value control terminal t 1  of the LED driving circuit  3  is controlled so that the set target value equals the luminous power value from the luminous power sensor  21 . 
     Through the above processing operation, the luminous power can be controlled more accurately based on the actually measured value while switching between the PWM control and current value control as the embodiment is implemented. 
     It should be noted that the invention is not limited to the above-described configurations of the embodiments. 
     For example, LEDs in a backlight of a liquid crystal display are driven to emit light in the embodiments. However, the invention can widely be applied to luminous power control of other LEDs. 
     In addition, the embodiments have the configurations for driving one LED to emit light for convenience of explanation. However, it is obvious that the similar luminous power control for plural LEDs can achieve the similar advantages. 
     Furthermore, the embodiments set a target value of luminous power according to APL information based on a brightness signal extracted from a video signal and operation by a user. However, factors for setting a target value of luminous power are not limited to these factors. The target value may be set based on other factors. 
     While preferred embodiments of the invention have been described using specific terms, such description is for illustrative purposes only, and it is to be understood that changes and variations may be made without departing from the spirit or scope of the following claims.