JP4969867B2 - Image display device and control method thereof - Google Patents

Description

  The present invention relates to an image display device using a booster circuit and a control method therefor, and more particularly to an image display device that operates by efficiently switching the operation method of the booster circuit and a control method therefor.

  In recent years, organic EL displays have attracted attention in mobile phone devices. The organic EL display is a self-luminous display and has features such as high brightness, wide viewing angle, ultra-thinness, and power saving. Some organic EL elements emit light in a single color such as green, blue, and red, and a multi-color display can be created by combining them. In addition, since it is a self-luminous type, a backlight is not required, so that power consumption can be suppressed. However, the organic EL display consumes power as the light emission luminance of the pixel to be displayed increases, and the power consumption increases as the ratio of pixels having a high luminance value among all the pixels increases. Further, when power is supplied to an organic EL display such as a cellular phone device, it cannot be driven with a 1-cell battery voltage, so a boost power source that boosts the voltage of the battery is essential.

  Conventionally, a booster power supply uses a DC-DC converter (boost circuit). Based on the output current and efficiency of the DC-DC converter, the current flowing in the coil constituting the booster circuit is turned on / off by a switching element or the like to operate to obtain a desired booster. Typical examples of this operation method include a pulse width modulation method for adjusting a pulse width, a burst method for performing an on operation for a time required for boosting, and a frequency modulation method for adjusting a frequency. . In addition, a power supply device capable of selecting these methods and a power supply device capable of automatically switching the modulation method according to an output current have been proposed.

  In an organic EL display, which is a typical application of a self-luminous element, the current consumption varies depending on the brightness of the display image as described above. The organic EL display has a lighting period and a light-off period, and the current consumption during the lighting period depends on the brightness of the screen to be displayed. Theoretically no current flows. Therefore, the current consumption of the organic EL display includes a no-load state due to the off period and a current variable state due to the on period. Since the current consumption changes during the on-period, the pulse width modulation type DC-DC converter has poor efficiency at low load. Also, depending on the operating frequency and current consumption, a discontinuous state in which the boosting is stopped and started is repeated, and noise components increase. On the other hand, in the burst method, the efficiency at the time of high load is poor, and the ripple component of the output voltage becomes large in order to detect the drop in the output voltage and control the DC-DC converter. As for the frequency modulation method, since the frequency fluctuates, flicker or noise occurs in the organic EL display depending on the operating frequency.

  There is also a technique for improving the efficiency by switching the operation of the DC-DC converter at the time of low load and high load in contrast to the one that operates by one operation method as described above. This is because the DC-DC converter switches the operation method depending on the current consumption, operates in the burst method or frequency modulation method at low load, and operates in the pulse width modulation method at high load. However, in the above switching method, when the operating current of the organic EL display changes at any time, there is a problem of noise and flickering in the burst method and frequency modulation method at low load, and the pulse width modulation method also operates discontinuously. There is a similar problem in the situation.

  Aiming to solve these problems, we have prepared multiple power sources for organic EL displays (ie, using multiple DC-DC converters), and always have the most efficient power supply circuit for the current consumption of organic EL displays. Prior art (refer to patent document 1) which improves efficiency and reduces power consumption of a power supply circuit by selecting is also proposed.

JP 2003-280584 A

However, even the conventional technology using the above-described plurality of power supplies has the following problems.
(1) In order to switch and operate DC-DC converters with different efficiencies, multiple DC-DC converters do not always operate, so at least one of the multiple DC-DC converters is the DC that is the issue -Since it is necessary to design the efficiency peak of the DC converter according to the maximum drive current, the power supply circuit has a large current capacity.
(2) Since a plurality of circuits are required, a circuit is added to a power supply circuit having a large current capacity, so that the size is not reduced as a result.
(3) In addition, since the circuit operation is based on current detection feedback, the switching timing is shifted on the screen where the previous frame is black and the next frame is white. The DC-DC converter will operate. In the off period, current is consumed by the self-loss of the boost power supply. When turning off the step-up power supply during the off period, if the power is turned on at the same time as the organic EL display is turned on, there is a problem that it takes a long time to rise up to the required voltage. However, it is necessary to maintain the output voltage, and the pulse width modulation method has a problem that this period is most inefficient.

In order to solve the above-described problems, an image display device according to a first invention
A booster circuit for boosting a DC power supply voltage by a plurality of operation methods;
A display unit that is driven by an output voltage from the booster circuit and that includes a plurality of pixels composed of self-luminous elements;
The operation mode of the boost circuit, the on-period of the self-luminous element, switching to the pulse width modulation method, the off interval, and a control unit that operates by switching to a burst mode or a frequency modulation,
It is characterized by having.

The image display apparatus according to the second invention,
Before SL control unit, said in conjunction with the off (dark) of the self-luminous element, from the pulse width modulation method, switching to the burst-mode or said frequency modulation scheme,
It is characterized by that.

The image display device according to the third invention,
Before SL controller, immediately before the self-luminous element is turned on (lit), from the burst mode or the frequency modulation scheme is switched to the pulse width modulation method,
It is characterized by that.
This is conceptually linked to the on-operation, that is, the switch immediately before turning on (lights up), but in reality, in the off section, preparations for turning on in consideration of the start-up time are made. The switching is performed before the ON operation only during a period in which the booster circuit can be sufficiently executed. It is preferable to use a vertical synchronizing signal as a synchronizing signal that satisfies this.

As described above, the solution of the present invention has been described as an apparatus, but the present invention can be realized as a method substantially corresponding to these, and the scope of the present invention also includes these. I want you to understand.
For example, the fifth invention that realizes the present invention as a method is:
A control method for an image display device having a display unit in which a plurality of pixels composed of self-luminous elements driven by an output voltage from a booster circuit that boosts a DC power supply voltage is provided,
The operation mode of the boost circuit, the on-period of the self-luminous element, switching to the pulse width modulation method, the off interval is intended to include the step of operating by switching to the burst mode or frequency modulation scheme.

  According to the present invention, the self-luminous element (organic EL display) constituting the display device can be operated by switching the operation system of the booster circuit to the operation system optimum for the off section in the off section. . For example, by switching from the pulse width modulation method to a burst method or frequency modulation method that is efficient at a low current such as an off interval, it becomes possible to improve efficiency and reduce power consumption. Further, by switching the operation method in the off section of the self-luminous element, efficiency improvement and power consumption improvement can be achieved without affecting the display screen.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. A typical example of an image display device using a self-luminous element will be described by applying the present invention to a portable electronic device (portable terminal device). FIG. 1 is a block diagram showing an image display function block of a portable electronic device to which the present invention is applied. As shown in the figure, an image display functional block (device) includes a battery 11 such as a lithium ion battery, a booster circuit 12 having a DC-DC converter that boosts the power supply voltage of the battery 11, and an organic EL element based on the output voltage. An organic EL display 13 that displays a picture by driving a self-luminous display that is arranged in a matrix, a CPU 14 that receives input image data and decodes it in frame units, receives the decoded image data, and receives a required signal The signal processing unit 15 performs processing and supplies the signal to the organic EL display 13. Although not shown, the organic EL display 13 has a drive circuit.

  The signal processing unit 15 applies the output voltage of the booster circuit 12 on and off in synchronization with the display timing of image data of sequential frames to the organic EL display 13 and supplies a vertical synchronization signal to the CPU 14. .

  Further, the signal processing unit 15 has a frame buffer 19 for storing the display image of the next screen. The CPU 14 not only passes the image data to the signal processing unit, but also generates a frequency setting signal to the booster circuit 12 and a mode setting signal for setting the operation method based on the image data. That is, the signal processing unit 15 outputs a frequency setting signal and a mode setting signal synchronized with the vertical synchronizing signal from the signal processing unit 15 to the boosting circuit 12 to control the setting of the boosting circuit supplied to the display driving circuit. As a result, the operating state is optimized and power consumption at the battery end is reduced. As described above, portable electronic devices that are required to be able to be used for a long time without being charged as much as possible are optimal for the application of the present invention.

  Further, the CPU 14 sequentially calculates the luminance information of the input image data in units of frames, and in synchronization with the display timing of the frame, the CPU 14 sends a voltage control signal (not shown) to the booster circuit 12 based on the calculation result of the luminance information in the frame. To control the output voltage. The output voltage control of the booster circuit 12 by the voltage control signal from the CPU 14 is executed in synchronization with the off period of voltage application to the organic EL display 13 by the voltage application timing signal from the signal processing unit 15 or the vertical synchronization signal. To do.

  Further, the CPU 14 includes a luminance information calculation unit 16, a consumption current calculation unit 17, and a RAM 18. The signal processing unit 15 is provided with a frame buffer 19 for one frame, the luminance information calculation means 16 sequentially calculates the luminance information of the input image data in units of frames, and the image data of the frame for which the luminance information is calculated Once stored in the buffer 19, it is displayed on the organic EL display 13.

  The RAM 18 stores a set voltage table (not shown) indicating the correspondence between the brightness information and the set voltage, and reads the set voltage corresponding to the calculation result of the brightness information from the RAM 18 to display the frame. In synchronization with the timing, the output voltage of the booster circuit 12 is controlled by a voltage control signal (not shown) from the CPU 14 so as to become the set voltage read from the RAM 18. Therefore, in this embodiment, the display control means is configured including the CPU 14 and the signal processing unit 15.

  In the present embodiment, the luminance information calculation means 16 of the CPU 14 calculates the average luminance value of the image data for each frame as the luminance information. Further, the RAM 18 stores a current value-frequency setting table indicating the optimum switching frequency for the consumption current value calculated from the luminance average value as shown in FIG. 5, for example. With reference to this current value-frequency setting table, an optimum frequency corresponding to the average brightness value calculated by the brightness information calculating means 16 is obtained, and the voltage is boosted so as to operate in a pulse width modulation system using the obtained frequency. The circuit 12 is controlled.

  When the screen to be synchronized and the voltage control signal are controlled by the CPU, it is not necessary to take the vertical synchronization signal, and the CPU may also be used for the operation of the signal processing unit, or in the organic EL panel. There may be a signal processing unit and a frame buffer. Furthermore, there may be a plurality of frame buffers instead of one.

  With such a configuration, when the organic EL display is in the on period (during lighting), the organic EL display is driven by the pulse width modulation method, the current consumption of the organic EL display is calculated from the average luminance of the screen data, and the booster circuit is calculated from the calculated current. When the operating frequency is changed and the current consumption is small relative to the displayed image, noise and flicker are improved by setting the frequency so that the booster circuit is in a continuous state. Maintain efficiency and improve efficiency by lowering frequency.

FIG. 2 is a graph showing the relationship between current consumption and efficiency of the organic EL display used in this example. As shown in the figure, the frequency modulation method is efficient even at a low load. On the other hand, in the pulse width modulation method, the efficiency is low at a low load, and the efficiency is improved at a high load. The present invention utilizes such characteristics of the EL device, in conjunction with the EL element to be low load OFF period (OFF period), the operation mode of the boost circuit, from "pulse width modulation", " switching to the burst mode and frequency modulation scheme ", in which further switch to immediately before the on-period from the" bursting method and frequency modulation scheme "to" pulse width modulation ", promote efficient operation. Further, when the load is the same, the efficiency is better when the operating frequency is lower. However, the current threshold values in the continuous state and the discontinuous state maintain the continuous state even at a low load when the operating frequency is high.

  FIG. 3 shows a diagram of the DC-DC converter in a continuous state (a) and a discontinuous state (b). In the discontinuous state, the frequency component increases, thereby generating noise and ripple due to voltage fluctuation in the output voltage. In the present invention, the occurrence of a discontinuous state is prevented by switching the operation method and setting the optimum frequency.

  FIG. 4 is a flowchart showing an example of processing in the image display apparatus of the present invention. As shown in the figure, image data is acquired in step S20, and then the process proceeds to step S22 to calculate a current value consumed by the display from one frame of image data. Based on the current value obtained in step S22, the corresponding operating frequency is determined with reference to the setting table of FIG. The image data is transferred to the frame buffer (step S26), the booster circuit is set (step S28), and the image is displayed (step S30).

  FIG. 5 is a current value-frequency setting table showing the optimum switching frequency for the consumption current value calculated from the luminance average value, which is used in the processing of the flowchart of FIG. The best condition for achieving both display noise and low power consumption is that the switching frequency of the pulse width modulation method is the minimum switching frequency that can maintain a continuous state with respect to the current consumption of the display from the efficiency graph of FIG. Is to work. In the setting table of FIG. 5, since the minimum switching frequency that can maintain a continuous state is set for the current consumption value, the switching frequency is high when the current consumption is low, and the switching frequency is low when the current consumption is high. As a result, low power consumption is achieved by reducing noise and improving efficiency. This value is set by the user depending on the circuit configuration and circuit design. Although not shown in the table, the temperature condition may be added to FIG. Or you may use the calculation formula correct | amended by the apparatus temperature etc. which measured the data of FIG.

  FIG. 6 is a timing chart showing the setting timing in the present invention. As shown in the figure, there is a delay of two frames from the image processed by the CPU to the image displayed on the display. It is necessary to set the frequency and the operation mode in a period until the display emits light. Therefore, taking the image 1 as an example, the frequency is set at the same time as the light emission stop of the image 0 (that is, in conjunction with the off state), the mode is set to the frequency modulation method, and the frequency is set to the image 1 setting. Since the circuit cannot be followed by changing the mode after the display is turned on (lit), the mode is set to the pulse width modulation method in synchronization with the vertical sync signal. The pulse width modulation method can be operated with a frequency setting corresponding to the current value.

  Here, when the light emission is stopped, the mode is set to the frequency modulation method, and the frequency setting is changed because the display is in the off period, so even if there is a voltage fluctuation, the display on the screen is not affected. . The frequency setting is effective in the pulse width modulation method, but is not affected even if the operating frequency is changed in the frequency modulation method in which the frequency varies with the current consumption.

  Although the present invention has been described based on the drawings and examples, it should be noted that those skilled in the art can easily make various modifications and corrections based on the present disclosure. Therefore, it should be noted that these variations and modifications are included in the scope of the present invention. For example, functions included in each member, each means, each step, etc. can be rearranged so as not to be logically contradictory, and a plurality of means, steps, etc. can be combined or divided into one. Is possible.

It is a block diagram which shows the image display functional block of the portable electronic device to which this invention is applied. It is a graph which shows the relationship between the consumption current and efficiency of the organic EL display used in the present Example. It is a figure of a DC-DC converter in a continuous state (a) and a discontinuous state (b). It is a flowchart which shows an example of the process in the image display apparatus of this invention. It is a current value-frequency setting table showing the optimum switching frequency for the consumption current value calculated from the luminance average value. It is a timing chart which shows the setting timing in this invention.

Explanation of symbols

11 Battery 12 Booster Circuit 13 Organic EL Display 14 CPU
15 Signal Processing Unit 16 Luminance Information Calculation Unit 17 Current Consumption Calculation Unit 18 RAM
19 Frame buffer

Claims (4)

A booster circuit for boosting a DC power supply voltage by a plurality of operation methods;
A display unit that is driven by an output voltage from the booster circuit and that includes a plurality of pixels composed of self-luminous elements;
A control unit that switches the operation method of the booster circuit to the pulse width modulation method in the ON section of the self-light-emitting element and switches to the burst method or the frequency modulation method in the OFF section;
An image display device comprising: The image display device according to claim 1,
The control unit is switched from the pulse width modulation method to the burst method or the frequency modulation method in conjunction with the self-light emitting element being turned off.
An image display device characterized by that. The image display device according to claim 1 or 2,
The control unit switches from the burst method or the frequency modulation method to the pulse width modulation method immediately before the light emitting element is turned on.
An image display device characterized by that. A control method for an image display device having a display unit in which a plurality of pixels composed of self-luminous elements driven by an output voltage from a booster circuit that boosts a DC power supply voltage is provided,
The operation method of the booster circuit is switched to the pulse width modulation method in the on section of the self-luminous element, and is switched to the burst method or the frequency modulation method in the off section.
A method for controlling an image display device including:

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