JP5384862B2 - Display device - Google Patents

Description

  The present invention relates to a liquid crystal display device including a backlight.

  The basic configuration of the liquid crystal display device is a combination of a liquid crystal panel for controlling the transmittance in pixel units and a backlight. The liquid crystal panel can be formed with a screen by controlling the amount of backlight in units of pixels by applying a drive signal based on the video signal.

In general, the backlight is configured to emit light with a uniform amount of light over the entire display screen using a fluorescent lamp. In recent years, light emitting diodes (LEDs), organic LEDs (OLEDs), and the like have been adopted as light emitting means instead of fluorescent lamps. In the present specification, for the sake of simplicity, these light emitting means are collectively referred to as LEDs.
The LED is a semiconductor light-emitting element, and is advantageous in that it is small, light, and has a long life. Furthermore, since an LED can have a relatively narrow spectral distribution having RGB (red, green, blue) main wavelengths as optical characteristics, it can be used as a light source for three primary colors. White light emitting LEDs have been developed and can be used as well. Further, LEDs are known to have high-speed response as optical characteristics.

As a driving method of the LED, a pulse width modulation method (PWM) is known in which average luminance is controlled by controlling a pulse width for applying a current while maintaining a constant current by utilizing high-speed response. ing. Further, there is pulse frequency modulation (PFM) for variably controlling the light emission frequency. In addition, there is a drive system having both PWM and PFM characteristics.
In the following description, the driving method for controlling the pulse width, the pulse frequency, the pulse density, etc. will be collectively referred to as PWM.

  However, there are instability factors such as characteristic variations due to manufacturing lots and characteristic fluctuations due to temperature and the like. For this reason, in order to output stable luminance, a driving method that compensates for variations, fluctuations, and the like is required. In this specification, for the sake of simplicity, various instability factors are collectively referred to as characteristic fluctuations.

Conventionally, a method of accurately driving an LED (or OLED) by a pulse width modulation method has been disclosed (for example, see Patent Document 1). This Patent Document 1 shows a specific numerical example. When a 12-bit pulse width modulation driver having a smaller number of bits than 16-bit illumination data is used, pulse width modulation is performed at 16 times the display update rate. Is to be driven.
That is, Patent Document 1 performs pulse width modulation driving using data obtained by increasing the number of single bits equal to the value of the least significant 4 bits after using the most significant 12 bits as basic data. A method for driving 16 bits is disclosed.
JP 2006-106762 A

  However, in the case of Patent Document 1, the PWM drive frequency has to be increased by an amount corresponding to the resolution of the lower-order data divided into upper and lower parts at the bit position. In general, the higher the drive frequency, the more necessary it is to improve the operation response of the drive circuit, and the more noise tends to be generated.

  In the case of a liquid crystal display device using light emitting means such as an LED or OLED as a backlight light source, it is possible to improve the moving image quality of the display screen and to reduce power consumption by controlling the amount of light. For this reason, a new driving method for controlling the amount of light using the high-speed response of the light emitting means is required.

  In order to improve the moving image quality of the display screen, the backlight lighting period is set so as to be repeatedly turned on and off instead of always turning on the backlight. In addition, it is necessary to increase the accuracy of PWM driving for controlling the light amount in this short lighting period.

  The factor that determines the accuracy of the PWM drive required here is the resolution of the brightness of the backlight. Further, there is a setting range for compensating for the characteristic variation of the light emitting means with the drive signal. Since both are independent signals, the combined result of both is obtained in a multiplied form. For example, if the former is 8 bits and the latter is 8 bits, the combination of both is a 16-bit signal. In order to perform PWM driving with such high bit accuracy, it can be obtained by increasing the PWM cycle or increasing the PWM frequency. However, as described above, shortening the lighting period of the backlight is contrary to increasing the PWM period. Further, increasing the PWM frequency increases the operating speed of the drive circuit and increases the power consumption, which is contrary to the purpose of reducing the power consumption as described above.

  The present invention shortens the backlight lighting period in order to improve the moving image quality, performs PWM driving to control the amount of backlight light in order to reduce the power consumption in the lighting period, and further emits light with the PWM output signal. It is an object of the present invention to provide a display device using a driving method of light emitting means that can compensate for characteristic variations of the means.

The present invention comprises a means for setting a target value of the emission of each cycle, and means for accumulating the accumulated values of the light emission quantity of the origination Hikarimoto child on the basis of the drive signal is a pulse signal by PWM method, a light emitting a display device which is characterized in that the device to perform periodic lighting, and means for storing the light emission amount of the calling Hikarimoto element by the pulse signal (characteristic value or characteristic matrix), the pulse every PWM cycle a counter in synchronization with the signal output to accumulate a light emission amount of the calling Hikarimoto element, means for comparing the magnitude of the target value and the accumulated value of the light emission, the target value of the emission until the cumulative value exceeds Means for outputting a PWM pulse; means for calculating an error between the target value of light emission and the accumulated value; and propagating the error to the next PWM period, and correcting the PWM period so that the accumulated error becomes zero. Characterized by .

  According to the present invention, the backlight lighting period is shortened in order to improve the moving image quality, the PWM driving is performed to control the backlight light amount in order to reduce the power consumption in the lighting period, and the PWM output signal Thus, it is possible to provide a display device using a driving method of the light emitting means that can compensate for the characteristic variation of the light emitting means.

  Hereinafter, modes for carrying out the present invention will be described.

An embodiment of a liquid crystal display device according to the present invention will be described with reference to FIG.
FIG. 1 is a diagram for explaining the basic circuit operation of the PWM drive circuit of the display device according to the present invention.
FIG. 1A shows the configuration of the down counter 1 and the down counter 2, and FIG. 1B shows an example of the bit configuration of the counter. The bit configuration example illustrated in FIG. 1B is an integer-represented digital value in which the left side is MSB (most significant bit) and the right side is LSB (least significant bit).
The counters 1 and 2 each have a determination signal that is turned on when the counter value is greater than 0 and turned off when the counter value is 0.

(A) As an initial setting, initial setting values 1 and 2 are written in the down counter 1 and the down counter 2, respectively.
Here, the magnitude of each set value is set value 1 ≦ set value 2.
At this time, the determination signal 1 of the counter 1 is ON.
(B) The down counters 1 and 2 count down one by one in synchronization with the drive frequency.
(C) When the down counter 1 becomes 0, the determination signal 1 is turned OFF.
(D) When the down counter 2 reaches 0, the process ends and returns to (a).

A series of these operations is PWM (pulse width modulation), and the determination signal becomes a PWM output signal. The determination signal is an ONOFF signal based on a ratio of values set in the down counter 1 and the down counter 2.
Here, the terms are arranged, the drive frequency is the PWM frequency Fp, the cycle data set in the counter 2 is the PWM cycle Pp, the period data of the output signal set in the counter 1 is the duty Pd, and the determination signal 1 is the PWM output signal Pw. In other words. The setting of the PWM period Pp and the duty Pd that counts down at the PWM frequency Fp determines the ON and OFF periods of the PWM output signal Pw.
The circuit configuration is not unique, and a similar PWM output signal Pw can be obtained using an up counter instead of a down counter.

The PWM output signal Pw has a minimum period of 0, a maximum period of the PWM cycle Pp, and the count value of the PWM frequency Fp set as the PWM cycle Pp is the maximum resolution. In the above configuration, the PWM output signal Pw is a binary signal represented by ON / OFF. However, when the drive target includes an element having a low-frequency pass characteristic, the PWM output signal Pw is regarded as an analog signal averaged over time. Can do.
The signal amplitude is determined by the ratio Pp: Pd between the PWM period Pp and the duty Pd, and can be handled with a minimum value of 0 and a maximum value of 1. Alternatively, if the ratio between the two is the duty ratio Rd (Rd = Pd / Pp), the minimum value is 0% and the maximum value is 100%.

Thus, since the PWM output signal Pw can be explained by the characteristics of both the binary signal and the analog signal, the meanings of both may be used in the following. Since the PWM output signal is determined by the ratio Pp: Pd between the PWM period and the duty, the magnitude of the PWM output signal is proportional to the duty while being inversely proportional to the PWM period.
Based on this relationship, a PWM cycle and a duty are set in order to obtain a target PWM output signal.

  In general, a television video signal is generated at 30 frames / second (60 fields / second) so as not to feel flicker. In a display that combines a liquid crystal panel and a backlight, even if the backlight blinks repeatedly in synchronization with the frame period, the blinking light does not feel in the visual system. The PWM output signal of the present invention can be used to drive the backlight of such a display.

The duty ratio Rd includes, as a counter value, a digital value (= Fp · Pp) that counts the PWM drive frequency Fp within the Pp period and a digital value (= Fp · Pp) that counts the PWM drive frequency Fp within the Pd period. Pd). In order to increase the resolution of the output signal, Fp may be increased or Pp may be increased.
However, if the drive frequency Fp is increased, there are problems in the circuit configuration such as increasing the response speed of the drive circuit, and noise is likely to occur. If the PWM period Pp is lengthened, the period of the output waveform becomes longer and the low frequency component increases, resulting in a lack of signal smoothness. Thus, there are limits to the set values of Fp and Pd.

Therefore, the present invention treats the PWM period Pp and the duty Pd as decimal data in order to increase the resolution of the PWM output signal. The PWM output signal is in units of pulses generated by counting the PWM frequency as an integer value. In this embodiment, it means that a value smaller than the minimum value of the output signal is used as a decimal for internal calculation. As a binary representation, a decimal point is set at the bit position of the minimum value of the output signal, and an operation including a fractional bit having a smaller weight is performed.
In the example of the down counter described above, the integer value 1 is counted in synchronization with the PWM frequency, but a decimal number can be obtained in synchronization with the PWM frequency.

FIG. 2 shows a configuration using the up counter 203 and the comparator 202. The PWM cycle Pp or the duty Pd can be periodically counted with the same configuration, and is stored in the register 204. The up-counter 203 adds a PWM period Pp or a duty Pd, which is a decimal value, in synchronization with the PWM drive frequency.
Then, the set value 201 indicating the upper limit is compared with the comparator 202, and if it is the same value, a determination signal is output. The comparison target is an integer value that is greater than or equal to the decimal point. When a determination signal having the same value is output, the integer part for which the determination has been completed is cleared to zero.

For example, a decimal is not necessary to express Pp: Pd = 1000: 99 as a ratio.
However, in order to represent the same ratio when Pp has an upper limit of 100, Pp: Pd = 100: 9.9, and Pd needs a decimal representation of 9.9.
Such an upper limit of Pp is caused by, for example, a limitation of a backlight lighting period to be described later.

Incidentally, since the output signal is a pulse signal synchronized with the drive frequency, it corresponds to a digital value represented by an integer.
Therefore, if an integer representation is output at the final stage while handling a digital value represented by a decimal point in an internal operation, a decimal error may occur.
For example, if decimal number 9.9 is replaced with integer 10 or 9, decimal errors of 0.1 and 0.9 occur, respectively. In this embodiment, high accuracy can be realized by preparing means for canceling an error in a combination of a plurality of output signals.

First, a numerical example in the case of expressing only Pd with a decimal point is shown.
Assume that Pp = 100 and Pd = 9.9. If the device configuration is such that the PWM period Pp can be set to 1000, if Pp = 1000 and Pd = 99, the same ratio can be realized while calculating both with integer values.
However, this does not hold when the PWM cycle Pp has an upper limit. Therefore, an up counter with an initial value of 0 is prepared and the following procedure is executed.

(A) First cycle The count is incremented by 1 from the initial state, and compared with Pd (= 9.9). When the integer part matches, the PWM output signal is inverted. Further, it is compared with Pp (= 100) while counting up one by one, and when the coincidence is reached, the PWM cycle is ended. In spite of the duty set value of 9.9, it is determined that there is a match at 9 in the integer part, and the output signal is generated, so that a minus error of 0.9 decimal is generated. At this stage, the ratio of Pd and Pp is 100: 9.

(B) Second period In order to compensate for the error generated in the preceding stage, the set value of the duty of the second period is corrected to 10.8 (= 9.9 + 0.9), and the count-up is performed again. The count is incremented by one and compared with Pd (= 10.8), and when the integer part matches, the PWM output signal is inverted. Further, it is compared with Pp (= 100) while counting up one by one, and when the coincidence is reached, the PWM cycle is ended. Since the set value of the duty is 10.8, it is determined that there is a match at 10 in the integer part, and the output signal is generated, so that a minus error of decimal fraction 0.8 occurs. At this stage, the ratio of Pd and Pp is 100: 10.

(C) Third period In order to compensate for the error generated in the preceding stage, the set value of the duty of the third period is corrected to 10.7 (= 9.9 + 0.8), and the count-up is performed again. The count is incremented by one and compared with Pd (= 10.7), and when the integer part matches, the PWM output signal is inverted. Further, it is compared with Pp (= 100) while counting up one by one, and when the coincidence is reached, the PWM cycle is ended. Since the set value of the duty is 10.7, it is determined that there is a match at 10 in the integer part, and an output signal is generated. Therefore, a decimal error of 0.7 occurs. At this stage, the ratio of Pd and Pp is 100: 10.

(D) Fourth and subsequent cycles Although details of the subsequent steps are omitted, the set value of the corrected duty is 10.0 in the tenth cycle, and the decimal error is 0. During the 10 cycles, an output signal is produced with a combination of a ratio of 100: 9 once and 100: 10 9 times, which averages 100: 9.9. At this stage, the error is 0, which is the same as the procedure (a) returning to the operation procedure of the first cycle and repeating it, so that the average output as a whole also becomes a ratio of 100: 9.9.

Next, an operation example when the PWM cycle Pp is expressed with a decimal point will be described. Assume that Pp = 99.9 and Pd = 10. If the device configuration is such that the PWM period Pp can be set to 999, if Pp = 999 and Pd = 100, the same ratio can be realized while calculating with both integer values.
However, this does not hold when the PWM cycle Pp has an upper limit. Therefore, an up counter with an initial value of 0 is prepared and the following procedure is executed.

(A) First cycle The count is incremented by 1 from the initial state and compared with Pd (= 99.9). When the integer part matches, the end of the PWM cycle is determined. Even though the setting value of the PWM cycle is 99.9, it is determined that the values coincide with each other in the integer part 99, so that a minus error of 0.9 decimal is generated. Since the count of the duty Pd = 10 is completed within this period, the ratio of Pd to Pp is 99:10 at this stage.
(B) Second period In order to compensate for the error generated in the preceding stage, the set value of the second period Pp is corrected to 100.8 (= 99.9 + 0.9), and the count-up is performed again. The count is incremented by one and compared with Pp (= 100.8), and when the integer part matches, the end of the PWM cycle is determined. Since the set value of the PWM period Pp is 100.8 and it is determined that the values coincide with each other in the integer part 100, a minus error of decimal fraction 0.8 has occurred. At this stage, the ratio of Pd and Pp is 100: 10.
(C) Third period In order to compensate for the error generated in the preceding stage, the set value of the third period Pp is corrected to 100.7 (= 99.9 + 0.8), and the count-up is performed again. The count is incremented by one and compared with Pp (= 100.7), and when the integer part matches, the end of the PWM cycle is determined. Since the set value of the PWM cycle Pp is 100.7 and it is determined that the values match at 100 in the integer part, a minus error of 0.7 is generated as a decimal. At this stage, the ratio of Pd and Pp is 100: 10.
(D) Fourth and subsequent cycles Although details of the subsequent steps are omitted, the set value of the corrected duty is 100.0 in the tenth cycle, and the decimal error is zero. During the 10 cycles, an output waveform is created with a combination of a ratio of 99:10 once and 100: 10 9 times, which averages 99.9: 10. At this stage, the error is 0, which is the same as returning to the procedure (a) and returning to the operation procedure of the first cycle, so that the average output as a whole becomes a ratio of 99.9: 10.

The above procedure can be extended. Both the duty Pd and the PWM cycle Pp can be handled in decimal point representation, and the count-up value can be handled in decimal point representation. Not only an up counter but also a down counter can be used for the device configuration. Even if a numerical value expressed by a decimal point is treated as an integer value obtained by multiplying an appropriate coefficient, the same operation is performed.
In any case, it is a common feature that the error generated in a single PWM cycle is propagated to the subsequent PWM cycle, and the operation parameter is corrected by the propagated error. As described above, the present invention can realize high accuracy by providing the error propagation procedure and the correction procedure so that the accumulated error is 0 while allowing the error within a single PWM period.

It is assumed that the driving target by the PWM output signal is a light emitting element such as an LED, and the light emission luminance Y of the light emitting element is in the relationship of the product of the coefficient K and the drive signal D.
Y = K ・ D
Here, if the light emission luminance of the LED fluctuates at a ratio R (initial value 1.0), if R is larger than 1.0, the luminance is increased, and if it is smaller than 1.0, the luminance is decreased.
Y = K ・ R ・ D
It becomes. In order to realize the target value Yt of the light emission luminance, the drive signal may be multiplied by 1 / R.
Yt = K ・ R ・ (D / R)
If the drive signal D is a PWM output signal, the setting value of the PWM duty Pd is multiplied by (1 / R) or the PWM cycle Pp is multiplied by R. Since the present invention can set the PWM duty Pd and the PWM period Pp independently of the frame period Pf and the backlight lighting period Pe, as described above, either compensation calculation can be used.

For example, when Pp = 100, Pd = 9.9, and the light emission luminance of the LED changes by 1% to R = 1.01, the former method:
Pd = 9.9 / 1.01 = 9.80119 ...
In the latter method,
Pp = 100 × 1.01 = 101
It becomes.

For example, when Pp = 99.9, Pd = 10, and the light emission luminance of the LED changes by 1% to R = 1.01, the former method:
Pd = 10 / 1.01 = 9.90099
In the latter method,
Pp = 99.9 × 1.01 = 100.899
It becomes.

In the division, the number of digits of the calculation result may not be fixed, and the configuration of the calculation circuit is more difficult than the multiplication. In the present invention, the compensation operation is performed by multiplication by setting the PWM period Pp as a compensation target. As a result, the number of digits of the operation result is fixed, and the mounting becomes easy.
As a result, as shown in FIG. 3, it is easy to compensate the compensation calculation inside the backlight 301 including the backlight component, and as a result, it is not necessary to disclose the characteristic variation to the outside. The characteristic variation includes means 304 for calculating a correction coefficient by inputting sensor signals such as temperature, luminance, and elapsed time.

A means 305 for generating a signal of the PWM period Pp to be compensated using the result is provided. By providing a command input unit 302 from the outside and a PWM duty generation unit 303 related to the light emission amount, a PWM output signal is generated at 306 and stable light emission using the light emission unit 307 is performed.
Since the light emission amount control and the characteristic variation compensation can be handled separately as separate values of the PWM duty Pd and the PWM period Pw, the configuration is simplified. The light emission amount control is received as a command related to the PWM duty Pd from a control device arranged outside, and the characteristic variation caused by the light emitting element inside the backlight can be compensated by setting the PWM cycle Pw inside the backlight. Thus, it can be seen from the outside of the backlight as a backlight having stable characteristics.

Next, the backlight lighting period Pe will be described with reference to FIG.
Since a conventional screen display by a CRT (cathode ray tube) is formed by scanning an electron beam, each pixel emits light in an impulse manner. On the other hand, a display such as a liquid crystal panel is called a hold-type display device because it holds pixel signals at a frame period.

  The difference in the display principle between the impulse type and the hold type is the difference in the frequency components of the display screen, and the impulse type can display higher frequency components. As a result, it is said that the impulse-type display is superior to the hold-type in the image quality of the moving image display. For example, if the backlight is continuously lit using a fluorescent lamp, the characteristics of the hold-type liquid crystal panel appear on the display screen.

  In the present embodiment, the backlight lighting period set for each frame period Pf is shorter than the frame period, and is turned on and off alternately. Realize features. In the impulse backlight lighting period Pe, a PWM output signal with high resolution is generated by the above-described PWM method that performs error propagation, and the backlight emission amount can be adjusted with high accuracy. Yes.

The amount of light emitted from the backlight can be variably set according to the characteristics of the video signal, the brightness of the environment where the display device is placed, the preference of the viewer, and the like.
For example, to display a dark screen, there is an effect of reducing power consumption by suppressing the amount of light emitted from the backlight. When the response speed of the light source is sufficient as in the case of an LED, it is possible to control the light emission amount with high accuracy while setting the backlight light emission amount independently for each frame and turning it on for a short time. Then, by forming a display screen with a combination of the backlight emission amount and the transmittance setting of the liquid crystal panel, the image quality of the moving image is improved and the power consumption is reduced.

The frame period Pf is a numerical value defined by the video signal.
The backlight lighting period Pe is synchronized with the frame period Pf, but the time position is arbitrary and can be determined in consideration of the response time of the liquid crystal element of the liquid crystal panel. Although the relationship with the screen structure is not shown, the screen may be divided into a plurality of regions, and the time position of the backlight lighting period Pe may be changed for each region.
If Pe = Pf, the lighting is continuous.
By satisfying Pe <Pf and sufficiently shortening Pe, impulse-type display characteristics can be obtained. The backlight lighting period Pe and the PWM cycle Pp may be the same or not. The maximum number of output pulses in the Pe period is determined by the product of the PWM period Pp and the PWM frequency Fp.

The PWM period Pp can be set asynchronously without depending on the frame period Pf and the backlight lighting period Pe.
In the present embodiment, an output error is allowed in a single PWM cycle Pp, and the error is propagated to the next PWM cycle Pp to cancel the accumulated error. As described above, by providing the error propagation means and using a plurality of PWM periods, an output having no error from the set backlight emission amount is realized.

  To cancel the error, a plurality of frame periods are required. Therefore, when it is determined that the original video signal has a large change and the error cannot be visually recognized like a scene change, the signal response can be speeded up by turning off error propagation. Such switching of response time can be realized by providing switch means for turning on and off error propagation based on the characteristics of the video signal.

  The numerical value range that the backlight lighting period Pe can take is 0 or more and the frame period Pf or less, but is practically determined based on the desired luminance of the display screen and the moving image display characteristics. The higher the backlight emission luminance, the shorter the backlight lighting period Pe.

In FIG. 5, the relationship between the backlight lighting period Pe and the PWM cycle Pp is classified and illustrated.
(A) Pe = Pp
(B) Pe = Pp · k (k is an appropriate integer value)
(C) Pe ≦ Pp
In (a), Pe and Pp have the same value, so the circuit configuration is simplified. Pe (= Pp) may be fixedly set, but a different value can be set for each frame period. Since the output waveform is generated a plurality of times within the backlight lighting period Pe, the above (b) is effective in averaging signals. In (c), the PWM cycle Pp is arbitrarily set under the condition that it is shorter than the backlight lighting period Pe. As described above, the PMW cycle Pp can be set for fluctuation compensation. In any configuration, the error is canceled by propagating the error component generated in the PWM cycle to the next cycle.

FIG. 6 shows an example of the video signal and the operation timing of the display device.
FIG. 6 shows a time axis connecting the backlight lighting periods Pe of consecutive frames in order to make the generation of the PWM signal easier to understand when the backlight lighting period Pe is shorter than the frame period Pf. The PWM cycle Pp is shown using the time axis.
FIG. 6 shows a frame period Pf (= 1 / frame frequency), a backlight lighting period Pe (Pe ≦ frame period Pf), and further, a PWM frequency Fp as a backlight operation timing related to backlight driving. , PWM cycle Pp, PWM output signal Pw (binary signal), PWM output period Pd (Pd ≦ Pp), and duty ratio Rd (Rd = Pd / Pp).
As shown in the figure, the frame period Pf, the backlight lighting period Pe, and the PWM period Pp can be set asynchronously.

Generally, a display device is made at 30 frames / second or an integral multiple thereof. In the operation of the liquid crystal display, the screen is rewritten in a frame cycle, and the screen is displayed during the frame cycle.
Since a conventional CRT (cathode ray tube) screen display is formed by scanning an electron beam, each pixel emits light in an impulse manner. On the other hand, a display such as a liquid crystal panel is called a hold-type display device because it holds pixel signals at a frame period.
The difference in the display principle between the impulse type and the hold type is the difference in the frequency components of the display screen, and the impulse type can display higher frequency components. As a result, it is said that the impulse-type display is superior to the hold-type in the image quality of the moving image display.

  Here, Pe = Pf when the backlight is configured by light emitting means that is always continuously lit, such as a fluorescent lamp. This is the same property as the 0th-order hold type output known in the technical field of DA converter (digital to analog signal conversion). If this property is explained in the frequency domain, the low frequency component passes but the high frequency component is suppressed. If this property is described in terms of display image quality, blurring will be observed when there is movement in the display screen.

In the present embodiment, the backlight lighting period Pe is made shorter than the frame period Pf to produce an impulse type output. As a result, high-frequency components are allowed to pass, and there is an effect of eliminating motion blur. Specific numerical examples of the backlight lighting period Pe will be shown. If the frame frequency of the display device is 120 Hz, the frame period is 8.33 ms. The horizontal size (number of pixels) of the display screen is set to 1920 pixels defined by the high-definition video signal.
Here, it is assumed that the object displayed on the display screen moves in the horizontal direction by a half of the screen, that is, 960 pixels (= 1920 pixels / 2) per second. Accordingly, the time required for the object to move a distance of 1 pixel is 1.04 ms (= 1 second / 960 pixels). Here, if the backlight is always lit, the object is displayed in the same position during the period of 8.33 ms which is the frame period.

In the next frame period, the image is displayed at a position separated by about 8 pixels (= 8.33 ms / 1.04 ms). The operation to display at such a jumping position is the same operation as the 0th-order hold of the DA converter described above, and the high frequency component is suppressed and blur is observed.
Therefore, the backlight lighting period Pe is set to 1 ms or less in order to display only the period in which the moving object is located in one pixel every frame period. Visually, it is possible to display a moving image without blur, which is equivalent to watching a process in which an object continuously moves while the strobe is lit. In this embodiment, the backlight lighting period Pe is set to 1 ms or less at the time position at the end of the frame period in which the response of the liquid crystal element is stable.

  The operation of the present embodiment will be generalized. If the number of pixels in the horizontal direction of the screen is H and the object moves 1 / S of the screen in the horizontal direction for 1 second, the period in which the object is located in 1 pixel is S / H (seconds), so one frame period The backlight lighting period Pe is set to S / H (seconds) or less. Thereby, there is an effect of eliminating the blur of the moving image display.

  Note that there are cases where the number of frames per second of the display device is set to 25 frames / second or 30 frames / second, or an integer multiple thereof, based on television broadcasting standards (PAL, NTSC, etc.). Further, the frame rate of the display device may be set at an integer multiple of 24 frames / second by simulating 24 frames / second of movie screening. Here, when the low frame rate is set, if the backlight is turned on once for each frame, visual flicker may be felt. Therefore, in the case of a low frame rate, flickering due to backlight lighting is performed by lighting a plurality of times per frame.

  The following description describes a specific configuration in the case of one impulse-type lighting per frame, but any of them can be applied to a case where any plurality of impulse-type lighting is performed per frame.

Further, the backlight light amount control by the PWM drive method is performed in the backlight lighting period Pe.
The PWM drive outputs a binary PWM output signal Pw in synchronization with the PWM frequency Fp during the period of the PWM cycle Pp. Here, the PWM cycle Pp may be the same value as the backlight lighting period Pe or a different value. Thus, the PWM periods Pp may be continuous in time or dispersed. The PWM output signal Pw is a combination of pulses synchronized with the PWM frequency Fp, and may be continuous or distributed. The maximum number of pulses of the PWM output Pw within the period Pp is determined by a value obtained by multiplying the period of the PWM cycle Pp by the PWM frequency Fp. In the PWM cycle Pp, the total of the periods in which the PWM output signal Pw = 1 is represented by the PWM output period Pd.

The minimum of the PWM output period Pd is 0, and the maximum is the PWM cycle Pp. A ratio Rd = Pd / Pp between the PWM period Pp and the PWM output period Pd is called a duty ratio. The duty ratio Rd is unitless, the minimum is 0, and the maximum is 1.
Such brightness control is applied to the backlight of a liquid crystal display. By calculating the minimum brightness required for display from the video signal and controlling it as the brightness of the backlight, it is effective in reducing power consumption. In the present embodiment, impulse type light emission is performed as described above, and brightness control is performed by the PWM method during the impulse light emission period.

Thereby, in the liquid crystal display, motion blur is eliminated and backlight light amount control is performed. The present embodiment is a new method for generating the PWM output signal.
A procedure for controlling the brightness by driving the light emitting means using the PWM output signal will be described. The brightness to be set is set to the duty ratio Rd by converting the minimum brightness value to 0 and the maximum brightness value to 1. The PWM output period Pd within the PWM cycle Pp is determined by multiplying the duty ratio Rd set above by the PWM cycle Pp.

A value obtained by multiplying the PWM output period Pd by the PWM frequency Fp is the number of pulses of the PWM output signal Pw. The number of pulses is appropriately arranged (continuous or distributed) within the PWM cycle Pp, and the PWM output signal Pw is output. The PWM output signal Pw is used to turn on / off the light emitting means.
Since the PWM pulse which is the minimum unit of the PWM output signal Pw is synchronized with the PWM frequency Fp, if the PWM frequency Fp is a sufficiently high frequency, the blinking due to the on / off cannot be visually detected, and the average Feels bright. Thus, the brightness of the light emitting means can be set.

  In this embodiment, in order to finely set the average brightness, means for arbitrarily setting the PWM cycle Pp is used together with the duty ratio Rd described above. Since the multiplication of both determines the number of PWM pulses, the effect of being able to make a finer setting compared to a single set value (for example, only the duty ratio Rd) is clear. In order to realize this, the circuit configuration is characterized by including a rewritable register for storing the duty ratio Rd and the PWM cycle Pp.

By the way, there is a backlight lighting system called field sequential. This is because, in color display using a liquid crystal panel, the backlight is lit independently of RGB without providing a color filter on the liquid crystal panel, and the transmittance of the liquid crystal panel is controlled in synchronization with the RGB lighting. Color display is performed by combining RGB brightness.
The backlight lighting method called field sequential improves the transmittance by eliminating the need for a color filter in the liquid crystal panel, thereby realizing a bright screen display. Here, in order to perform color image display while lighting the backlight sequentially in RGB, as shown in FIG. 7, the lighting time of each of the plurality of types of lighting 1, 2, 3, 4 is shortened and repeated. Lights up. It goes without saying that the present invention can be used for such an impulse-type illumination lighting independent of each color.

FIG. 8 shows a configuration example of a drive circuit in the case where an LED is used as the light emitting means.
The driving circuit shown in FIG. 8 operates to stably drive the LED by maintaining the current value constant.
The current I flowing through the LED 2004 has a relationship of I = V / R using the voltage V generated at both ends of the resistor R2005 connected in series.
The constant current circuit 2001 controls the current I2006 so as to have a preset current value based on the feedback voltage V2007.
In order to control the light emission amount of the LED 2004 under the condition of the current I set in this way, the current is turned on / off based on a PWM (pulse width modulation) signal 2008 using the switch 2002. In this embodiment, a target value 1001 and a characteristic value 1002 are set in the PWM signal calculation circuit 2000 to create the PWM signal 1003. A specific configuration will be described next.

In this embodiment, a target value 1001 and a characteristic value 1002 are set in the PWM signal calculation circuit 2000 to create the PWM signal 1003.
FIG. 9 schematically shows a PWM signal calculation procedure.
In FIG. 9, the horizontal axis is a time axis indicating the PWM cycle. The vertical axis represents the LED light emission amount, and shows the target value and cumulative value of light emission for each period.
The PWM signal Pw is a binary signal that changes in synchronization with the PWM frequency Pf, and its shortest is referred to as a PWM pulse.

The minimum unit of the LED light emission amount is the light emission amount of one pulse period of the PWM signal, and this is the LED characteristic value W. In the drawing, a plurality of PWM pulses generated within the PWM period are numbered as (1), (2), (3),... And a characteristic value W is stacked in the vertical axis direction together with the pulse number. Here, if the target value of the light emission amount is Wt, the PWM pulse number Pw for realizing this is an integer value obtained by dividing the target value T by the characteristic value W.
Pw = INT (Wt / W)
Here, INT is an integer operation and is rounded up.

The actual light emission amount is a value obtained by multiplying Pw and W, and an error occurs in the target value Wt.
EW = (Pw · W−Wt)
become.
In the present embodiment, the error EW generated in a certain PWM cycle is propagated to the next PWM cycle. In FIG. 9, the error propagation procedure is indicated by arrows.
Since the error does not actually emit light at the propagation destination, it is shown in white in FIG. In the PWM cycle of the propagation destination, the light emission amount by the actual PWM pulse is accumulated with the propagated error value as an initial value.
If the accumulated value exceeds the target value, the error propagation procedure is performed again. By repeating this procedure, the target value can be achieved on average. In the above, the target value Wt has been described as a constant value, but it goes without saying that it can be changed for each period.

FIG. 10 shows a configuration example of the PWM signal calculation circuit 2000.
In FIG. 10, a target value register 1015 is a register for setting a target value of the light emission amount of the light emitting element, and has an input terminal 1001 that can be rewritten from the outside, and a terminal 1008 that outputs the register contents to the outside.
The characteristic value register 1010 is a register for setting the characteristic value of the light emission amount of the light emitting element, and has an input terminal 1002 that can be rewritten from the outside and a terminal 1004 that outputs the contents of the register to the outside.

The comparator 1014 is a means for comparing the magnitude relationship between the target value register 1015 and the cumulative value register 1013. The comparator 1014 inputs the output signal 1008 and the output signal 1007, and sets the magnitude determination result as a binary signal of the PWM signal 1003.
If you rewrite
IF (signal 1008 ≧ signal 1007) THEN PWM = 1,
ELSE PWM = 0,
In the above formula, the inequality sign ≧ may be replaced by>.

  The adder 1011 adds the contents of the characteristic value register 1010 and the cumulative value register 1013 under the condition of the PWM signal 1003 = 1 in synchronization with the PWM frequency in the PWM cycle, and the operation result is again stored in the cumulative value register 1013. Write operation. If the PWM signal 1003 = 0, the above addition operation is stopped or the signal 1004 is switched to 0.

The subtractor 1012 subtracts the value of the target value register 1015 from the value of the accumulated value register 1013 at the end of the PWM cycle, and stores the accumulated value in the accumulated value register 1013 so that the calculation result becomes the initial value of the next PWM cycle. Write operation. Note that the subtraction result of the subtractor 1012 is always 0 or more from the above condition.
The PMW signal 1003 is initialized to 0 before the start of the PWM cycle, and is set to 0 or 1 based on the above determination within the PWM cycle. The ratio of the period between 0 and 1 indicates the result of pulse width modulation.

In the above description, the characteristic value is added to the cumulative value register 1013. However, the characteristic value may be subtracted from the cumulative value register 1013. In this case, at the beginning of the PWM period, an addition value of the error propagated from the target value and the previous PWM period is set in the accumulated value register 1013, and the characteristic value is subtracted in synchronization with the pulse of the PWM signal.
Then, instead of comparing the target value register 1015 and the cumulative value register 1013 using the comparator 1014, it may be determined whether or not the cumulative value register 1013 is 0 or less. This configuration has an effect that the comparator 1014 is unnecessary.

Next, the procedure for driving the red, green, and blue three-color light emitting means by the PWM method will be described with reference to FIG.
It is known that the emission wavelength distribution can be replaced with tristimulus values X, Y, and Z in consideration of human visual sensitivity. If the numerical value obtained by converting the emission wavelength distribution of the red, green and blue light emitting means into XYZ is written as the characteristic value of the light emitting means, the following is obtained.
Here, the light emitting means is an LED.
Red LED = (Xr, Yr, Zr)
Green LED = (Xg, Yg, Zg)
Blue LED = (Xb, Yb, Zb)
By combining these, the target values (Xt, Yt, Zt) and the drive signals (Pr, Pg, Pb) can be related as a characteristic matrix.

(Formula 1)

  The output of the red, green, and blue LEDs can be expressed by the product of the drive signal of the red, green, and blue light emitting means and the XYZ characteristic matrix. In general, the drive signal of the light emitting means for realizing the output target value can be calculated by multiplying the target value by the inverse of the characteristic matrix.

(Formula 2)

However, since the calculation of the matrix inverse is complicated, in this embodiment, the drive signal is obtained by directly using the components of the characteristic matrix. Here, the constituent elements of the XYZ characteristic matrix are such that red LED has X as the main component, green LED has Y as the main component, blue LED has Z as the main component, and the remaining components have small values. For example, it can be regarded as a diagonal matrix.
Therefore, for simplification of explanation, it is assumed that the XYZ characteristic matrix has only diagonal components (Xr, Yg, Zb), and the red, green, and blue LEDs have one pulse of the PWM signal, and the Xr, Yg, and Zb components respectively. Shall be output. Here, for simplicity, the target value is set to a constant white value and XW = YW = ZW. However, it can be arbitrarily set for each period.

(Formula 3)

In FIG. 11, the horizontal axis represents the PWM cycle. The vertical axis shows the operation of the cumulative value register within the PWM cycle.
In the first period, the component is added to the accumulated value register of XYZ in synchronization with the pulse output of each of the red, green, and blue LEDs. For each of the XYZ accumulated value registers, the accumulated value is compared with the target value.
For each of XYZ, the corresponding red, green and blue LED pulses are output until the accumulated value> the target value. A value obtained by subtracting the target value from the accumulated value at the end of the first cycle is set as an error value.

  Then, the first cycle error value is propagated to be regarded as light emission of the second cycle, and the error value of the first cycle is set as the initial value of the accumulated value register of the second cycle. The error value set as the initial value is not a value that actually emits light in the second period, and is shown in white in the drawing.

In the second period, on the basis of the initial value, corresponding red, green, and blue LED pulses are output until the cumulative value of the light emission amounts of the red, green, and blue LEDs exceeds the target value. Similarly, the error value of the second cycle is regarded as light emission of the third cycle, and the error value of the second cycle is set as the initial value of the cumulative value register of the third cycle.
In the above description, the XYZ characteristic matrix is described as having only diagonal components. However, since the spectrum distribution of an actual LED has a width, it has components other than diagonal components. Even if there are components other than diagonal, the present invention operates in the same way as long as they are small values. With one pulse of red LED, Xr, Yr, Zr components are added to the accumulated value registers of XYZ, and with one pulse of green LED, Xg, Yg, Zg components are added to the accumulated value registers of XYZ, One pulse of the blue LED adds Xb, Yb, and Zb components to the accumulated value registers of XYZ. By adding Yr, Zr, Xg, Zg, Xb, and Yb, which are components other than these diagonals, the color reproduction characteristics of the LED can be calculated correctly.

In this embodiment, there is a procedure for propagating an error value generated in a certain cycle to the next cycle. And there exists an effect which implement | achieves the precision of the output which cannot be implement | achieved in a single period as an average output. Further, since the drive signal can be generated by directly using the components of the characteristic matrix of the light emitting means for each color, there is an effect of simplifying the calculation procedure. Further, the fact that the calculation is simple provides an effect of simplifying the circuit configuration for executing the calculation.
As a variation of the above, an apparatus configuration in which each XYZ component is subtracted in synchronization with a pulse instead of addition may be used. At the beginning of the PWM period, the target value and the added value of the error propagated from the previous PWM period are set in the accumulated value register, and the characteristic value is subtracted in synchronization with the pulse of the PWM signal. In this subtraction procedure, the comparison between the accumulated value and the target value may be performed by determining whether the accumulated value register is 0 or less.

Further, as another variation, it can be applied to light emission of four colors in which white (W) is added to three colors of red, green, and blue (RGB). The characteristic value of the white LED is as follows.
White LED = (XW, YW, ZW)
With one pulse of the white LED, XW, YW, and ZW components are added to the accumulated value registers of XYZ.
Similar to the above procedure, the cumulative value and the target value are compared for each of the XYZ cumulative value registers. If cumulative value> target value for each of XYZ, the corresponding RGB and W pulse output is stopped. Thus, the calculation of the PWM pulse for realizing the target value can be easily realized using the RGBW4 colors.

As described above, in this embodiment, by providing the error propagation procedure, the target value can be achieved with high accuracy and the PWM frequency can be suppressed to a low level.
Even if the PWM period is shortened and the number of PWM pulses in the PWM period is set to be small, the accuracy is high.

FIG. 12 is a configuration example of the present embodiment in the case where light is emitted by combining the three primary colors of red, green, and blue.
For each of XYZ, the cumulative value and the target value are compared, and the operation procedure for outputting the drive signal (PWM signal) based on the comparison result is the same as the configuration of the white LED described above. The difference is that the XYZ characteristic values are exchanged between the three colors of red, green, and blue.
Here, the target values are Xt, Yt, and Zt.
The characteristic value of the red LED is Xr, Yr, Zr, the characteristic value of the green LED is Xg, Yg, Zg, and the characteristic value of the blue LED is Xb, Yb, Zb. The PWM output signal of the red LED is Pr, the PWM output signal of the green LED is Pg, and the PWM output signal of the blue LED is Pb. The cumulative value of X is Xa, the cumulative value of Y is Ya, and the cumulative value of Y is Ya.

The PWM signals Pr, Pg, and Pb are compared with the target values Xt, Yt, and Zt and the magnitudes of the cumulative values Xa, Ya, and Za, and are determined to output a pulse if the cumulative value does not reach the target value.
If you rewrite
IF (Xt> Xa) Pr = 1
ELSE Pr = 0
IF (Yt> Ya) Pg = 1
ELSE Pg = 0
IF (Yt> Ya) Pb = 1
ELSE Pb = 0

If the PWM signal Pr = 1, Xr is added to the accumulated value Xa, Yr is added to the accumulated value Ya, and Zr is added to the accumulated value Za. Zg is added to the accumulated value Za, and if the PWM signal Pb = 1, Xb is added to the accumulated value Xa, Yb is added to the accumulated value Ya, and Zb is added to the accumulated value Za.
If you rewrite
IF (Pr == 1) {Xa + = Xr, Ya + = Yr, Za + = Zr}
IF (Pg == 1) {Xa + = Xg, Ya + = Yg, Za + = Zg}
IF (Pb == 1) {Xa + = Xb, Ya + = Yb, Za + = Zb}

The variation can be applied to light emission of four colors in which white (W) is added to three colors of red, green, and blue (RGB). W has characteristic values XW, YW, and ZW of XYZ. The target value of XYZ is compared with the cumulative value. If the cumulative value does not reach the target value for all of XYZ, W is output as a pulse.
If you rewrite
IF ((Xt> Xa) &(Yt> Ya) &(Zt> Za)) PW = 1
ELSE PW = 0

Then, XW, YW, and ZW are added to the accumulated values Xa, Ya, and Za of XYZ in one pulse of the white LED.
If you rewrite
IF (PW == 1) {Xa + = XW, Ya + = YW, Za + = ZW}

  In this way, using the RGBW4 colors, a PWM signal for driving the light emitting means to calculate the target value is calculated. The present invention has an effect of providing a simple device configuration without using the characteristic matrix inverse calculation means.

FIG. 13 shows a configuration example of red in the PWM signal calculation circuit for controlling red, green, and blue LED emission.
Based on the red, green, and blue LED characteristics, the X emission amount is Xr, Xg, and Xb, and the red, green, and blue LED properties are Y, Yr, Yg, and Yb, and the red, green, and blue three color LEDs Based on the characteristics, the light emission amount of Z is Zr, Zg, Zb. The red PWM signal PR is set to 1 when the X target value and the cumulative value are not reached, and the green PWM signal Pg is set to 1 when the Y target value and the cumulative value are not reached. The blue PWM signal Pb is set to 1 when the target value of Z and the accumulated value are not reached by comparison.

Then, a switcher 1020 is prepared, and if Pr = 1, the characteristic value 1010Xr is passed to the adder 1011. If Pg = 1, the characteristic value 1010Xg is passed to the adder 1011. If Pb = 1, the characteristic value 1010Xb. Is passed to the adder 1011. The adder 1011 adds the signal and the value of the cumulative value register 1013 and writes the result to the cumulative value register 1013 again.
In this way, a procedure for repeating the addition of the characteristic values of each color, which is a component of the characteristic matrix, in synchronization with the output of the PWM pulse is realized.

FIG. 14 shows a configuration example of the drive circuit 2020 when an LED is used as the light emitting means in this embodiment.
In FIG. 14, the LED drive circuit 2020 includes a communication unit 2011, and performs signal exchange with an external management unit via a communication path. The content of the signal includes a characteristic value and a target value of the driving light emitting means.
These signals are stored in the storage means 2012. The PWM signal calculation circuit 200 reads the characteristic value and target value of the light emitting means from the storage means 2012, and generates and outputs a PWM signal based on the procedure described above.

A constant current circuit 2001 generates a current for constant current driving of the light emitting means. Then, the switch 2002 is used to turn on and off the current based on the PWM signal and output it. The output current I2006 flows through the LED 2004 and the resistor R2005 as light emitting means, and outputs light.
Here, the voltage across the resistor R2005 is fed back to the constant current circuit 2001. The oscillation circuit 2013 generates a timing signal necessary for circuit operation including the PWM drive frequency. Although not shown, the LEDs can be three types of red, green and blue, or four types of red, green, blue and white. Or the light emission means based on another principle may be sufficient.

In the present embodiment, the operation of the circuit configuration is managed by setting numerical values of XYZ.
In general, color characteristics such as LED emission characteristics, white chromaticity, and three primary colors of a display device are often expressed by three numerical values (X, Y, Z). (X, Y, Z) is a standard expression method determined on the basis of human vision, and can be used as a grounded absolute value. Managing the operation of the light emitting device with (X, Y, Z) means that the light emitting characteristics can be managed as a grounded absolute value.
External management means connected via the communication path manages the set values of (X, Y, Z). At this time, a plurality of driving circuits 2020 may be connected. Next, the designation method of (X, Y, Z) is shown.

FIG. 15 shows coordinate points on the xy chromaticity diagram. A procedure for setting a target value using the coordinate points shown in FIG. 15 will be described.
In FIG. 15, the horizontal axis x and the vertical axis y are x = X / (X + Y + Z) and y = Y / (X + Y + Z), respectively, and the Y axis is perpendicular to the paper surface. There is. If (x, y, Y) is given in the chromaticity diagram, (X, Y, Z) is obtained from X = x · (Y / y) and Z = (1−xy) · (Y / y). It is done.

In general, the light emission characteristics of the light emitting means, the three primary colors of the display device, or the white chromaticity (white point) are given as xy chromaticity points. In this embodiment, it is possible to specify the color emitted by the light emitting means by using such a point (x, y) on the chromaticity diagram.
By converting these numerical values to (X, Y, Z) and setting them as target values, the above-mentioned PWM signal is output and the light emitting means is driven to output the target color. In this way, there is an effect that it is possible to set a target value of a color to be emitted using chromaticity coordinates which is a technical standard method for color designation and is widely used.

In the present embodiment, the characteristic variation of the light emitting means is corrected by being incorporated in the above-described PWM signal generation procedure.
The operation principle will be described with reference to FIG. Here, it is assumed that the LED characteristic values at temperatures T0 and T1 (T0 <T1) are C0 and C1, and both are known. Specific numerical values of C0 and C1 are set by the characteristic values related to X, Y, and Z described above. The LED characteristic Ck at an arbitrary temperature Tk between the temperatures T0 and T1 is assumed to be on a straight line connecting the two points. Assuming a proportional relationship, the following holds.
(Tk−T0): (T1−Tk) = (Ck−C0): (C1−Ck)
Solving this, Ck = (C1. (Tk-T0) + C0. (T1-Tk)) / (T1-T0)
It becomes.

In this embodiment, in the PWM signal calculation procedure described above, the characteristic values C1 and C0 are switched based on the temperature ratio (Tk−T0) :( T1−Tk), C1 is the number of times (Tk−T0), and C0 is set. If the PWM signal is output repeatedly using the number of times (T1−Tk), the accumulated value becomes (C1 · (Tk−T0) + C0 · (T1−Tk)). If Tk = T0, only C0 is used, and if Tk = T1, only C1 is used.
When the cumulative value is divided by (T1-T0) to obtain the average value, it becomes the same value as Ck. The present invention is characterized in that in the PWM signal calculation procedure, the characteristic values C1 and C0 are switched based on the temperature ratio (Tk−T0) :( T1−Tk). Thus, the present invention has an effect that the characteristic variation can be easily corrected without obtaining the LED characteristic Ck of the arbitrary temperature Tk by calculation.

  When temperature characteristics are different among a plurality of types of LEDs, a characteristic value may be prepared for each LED type. Although the above example shows the characteristic variation due to the temperature, the present invention is not limited to the temperature, and can be used to correct the characteristic variation due to the variation factor based on the cumulative value of the driving time, the driving voltage, the driving current, and the like. In order to correct by using the above procedure when there are a plurality of types of variation factors, a characteristic value combining the variation factors may be provided in a tabular form in advance.

There is a backlight lighting method called field sequential. In color display using a liquid crystal panel, the backlight is lit independently for each primary color (for example, red, green, and blue) without providing a color filter in the liquid crystal panel, and the liquid crystal panel is synchronized with the lighting of the primary color. By controlling the transmittance of the light, the brightness of the primary colors is combined and color display is performed.
By eliminating the need for color filters in the liquid crystal panel, the transmittance is improved and a bright screen display is realized. Here, the backlight is turned on repeatedly in the primary colors. It goes without saying that the present invention can be used for such primary color independent impulse lighting.

A constant current driving method is known in order to stably emit an LED, which is a semiconductor element. The higher the current value, the brighter the image. However, the factor that determines the upper limit is temperature. The temperature includes the environmental temperature and the element itself generates heat. The temperature rise of the element itself depends on the power consumption of the element itself. The power consumption is a product of voltage and current in continuous driving, and is averaged over time in PWM driving.
Therefore, in this embodiment, the upper limit of the drive current for PWM drive is set higher than the drive current defined for continuous drive. The PWM drive current is set under the condition that the power consumption is the same as that of continuous drive, in other words, under the condition that the temperature rise of the element is the same as in the steady operation. Thus, in order to realize the target light emission luminance, the relationship between the backlight lighting period Pe, the PWM period Pp, and the drive current can be variably set.

By realizing short-time light emission such as flash synchronized with the frame period Pf, the frequency component included in the display screen is determined not by the frame period Pf but by the backlight lighting period Pe. Display including the same high frequency component can be performed.
In this embodiment, by controlling the brightness with high accuracy using a PWM drive circuit including an error propagation means while maintaining high brightness by variable control of the drive current while being such a short-time drive, Features as an impulse light emission type display device can be realized.

The backlight lighting period Pe can be set in accordance with the movement of the screen content by analyzing data of a plurality of frames that are temporally adjacent to the input video signal. For example, the high-frequency characteristics of the display screen can be improved by turning the backlight on an impulse type on a screen with a lot of movement, and the low-frequency characteristics of the display screen can be improved by turning the backlight on on a screen without movement. Thus, the characteristics of the impulse-type and hold-type display devices can be provided together.
Further, the display screen may be divided into a plurality of regions and the amount of backlight light may be controlled independently, and the display screen contrast can be improved and the power consumption can be reduced at the same time.

By the way, average light emission luminance becomes disadvantageous compared with continuous lighting by shortening the backlight lighting period Pe. The present invention increases the light emission luminance by setting the drive current higher than the rated value during continuous lighting while maintaining the temperature conditions of the light emitting elements during continuous lighting.
In addition, since the LED has a small size of the light emitting element itself, it is easy to improve the luminance by increasing the number of light emitting elements in terms of the device configuration. Further, the backlight lighting period may be variable based on the analysis result of the video signal. On a screen with little movement, the luminance can be improved by making the backlight lighting period longer than the above setting. At this time, the drive current can be kept low. In this manner, a high-accuracy PWM output signal can be generated with a simple device configuration, and a display device with high image quality can be realized.

FIG. 17 shows a circuit configuration example in which the screen 801 is divided into a plurality of areas, and independent PWM drive circuits 803 are arranged in the respective areas.
In FIG. 17, the screen 801 is divided into 3 × 4 in the vertical direction, and the backlight emission amount corresponding to each area is independently controlled. The instruction of the backlight emission amount is transmitted as a command string via the serial communication path 802, and a PWM drive circuit 803 that operates in response to the command and LEDs (not shown) are arranged in each region. ing.

Here, the synchronization signal indicating the frame period can also be incorporated in the command sequence of the serial communication path 802. Alternatively, although not shown, it is possible to prepare a signal line for a synchronization signal that is simultaneously transmitted to all the PWM drive circuits. Thereby, all the regions can operate while sharing the time axis.
A constant current source is prepared to supply current to the current switches in each region.
Then, a management circuit for instructing operation parameters is prepared for the PWM drive circuit in each region. The management circuit and the PWM drive circuit in each area are wired with a transmission path for transmitting the instruction signal.

The backlight emission amount is calculated by setting the number of display frames to 30 frames / second, and an instruction is transmitted prior to displaying each frame.
If the operation parameters of the PWM drive circuit are 16 bits × 3 types, the total is 48 bits, and for 6 PWM drive circuits, 48 bits × 6 = 288 bits, and if this is transmitted within 33 ms. Since it is good, a transmission rate of about 10 kbit / second is sufficient. Even if the frame frequency is increased 10 times and the number of area divisions is increased 10 times to reach about 10 Mbit / second, a relatively simple serial transmission method can be used.

  Therefore, the instruction data is transmitted in common to all PWM drive circuits as an address + data structure in which an address signal for distinguishing six PWM drive circuits is added. The receiving side PWM drive circuit can determine the instruction addressed to itself by the added address signal. Thereby, the transmission path between the management circuit and the PWM drive circuit can be easily wired.

FIG. 18 shows an internal configuration example of the PWM drive circuit 803 for each region arranged in each region.
The PWM drive circuit 803 illustrated in FIG. 18 includes a communication unit 804 that inputs a command and a synchronization signal using a serial communication path 802, a management unit 805 that analyzes the input command and issues an instruction to the operation of the internal circuit. It comprises a memory 806 for storing commands and operating states, a PWM signal output circuit 807 for generating a PWM output waveform based on the commands, and a current switch circuit 808 for driving the light emitting means.

The light emitting means shown in FIG. 18 can use an LED or an OLED. In addition, the current switch circuit 808 illustrated in FIG. 18 can include means for receiving a current supply from an external power supply, although not illustrated. Further, the current switch circuit 808 shown in FIG. 18 includes a clock circuit for causing the entire circuit to operate synchronously.
Although not shown, the current switch circuit 808 can be connected to a sensor for measuring temperature, luminance, elapsed time, and the like, and can be provided with a measurement signal input means. Then, a characteristic variation is detected based on the sensor signal, and a PWM output signal is generated so as to compensate for the variation. Thereby, a stable characteristic is realized by a combination of the driving circuit and the light emitting means. There is no need to notify the external device of the characteristic variation.

As shown in FIG. 19, the drive current can be varied along with the backlight lighting period Pe. For example, if the ambient illuminance is high, the backlight lighting period Pe is set to be long in order to prioritize the brightness of the display screen, while if the environmental illuminance is low, priority is given to the moving image characteristics of the display screen to shorten the backlight lighting period Pe. Control to set.
Also, a means for analyzing the content of the video signal can be prepared, and the backlight lighting period Pe can be variably set based on a measured value such as the presence or absence of a motion component or an area. For example, if the video signal is determined to be a still image, the frame period and the backlight lighting period Pe are made to coincide with each other to make a hold-type display, and the luminance is maintained by suppressing the drive current.

If it is determined that the video signal is a moving image, the backlight lighting period Pe is shortened to make an impulse-type display, and the drive current is increased to maintain the luminance. Further, the backlight brightness is modulated by calculating the necessary backlight emission amount from the measurement result of the video signal and setting the PWM duty ratio in each lighting period Pe.
Assuming that the power consumption per hour of the light-emitting element and the temperature rise of the element are in a substantially proportional relationship, when performing short-time lighting, a drive current higher than the rated value defined during continuous lighting is set.
In this embodiment, the driving current at the time of impulse type display is set higher than the rated current at the time of continuous lighting, and the driving of the light emitting means is performed together with the backlight lighting period Pe based on the measurement result of the environmental illuminance and the video signal. The current is variably set.

It is a figure for demonstrating operation | movement of the basic circuit of the PWM drive circuit of the display apparatus which concerns on this invention. It is a figure which shows the structure using an up counter and a comparator. It is a block diagram of a backlight. It is a figure for demonstrating a backlight lighting period. It is a figure which classify | categorizes and shows the relationship between a backlight lighting period and a PWM period. It is a figure which shows a video signal and the operation timing of a display apparatus. It is a figure which shows the state which shortens each lighting time of multiple types of illumination, and lights repeatedly. It is a figure which shows the structural example of the drive circuit in case LED is used as a light emission means. It is a figure which shows the calculation procedure of a PWM signal typically. It is a figure which shows the structural example of a PWM signal calculation circuit. It is a figure for demonstrating the procedure which drives the light emission means of 3 colors of red, green, and blue by a PWM system. It is a figure which shows the structural example in the case of making it light-emit combining red green blue primary colors. It is a figure which shows the structural example of red among the PWM signal calculation circuits which control LED light emission of red, green, and blue. It is a figure which shows the structural example of the drive circuit at the time of using LED as a light emission means. It is a figure which shows the coordinate point on xy chromaticity diagram. It is a figure for demonstrating the operation principle which incorporates and correct | amends the characteristic fluctuation | variation of a light emission means in the production | generation procedure of a PWM signal. It is a figure which shows the example of a circuit structure which divides | segments a screen into several area | region and arrange | positions an independent PWM drive circuit in each area | region. It is a figure which shows the internal structural example of the PWM drive circuit for every area | region arrange | positioned in each area | region. It is a figure for demonstrating that a drive current can be made variable with a backlight lighting period.

Explanation of symbols

101 …………………… Down Counter 1
102 …………………… Down Counter 2
103 …………………… Set value 1
102 …………………… Set value 2
201 …………………… Set value 202 …………………… Comparator 203 …………………… Up counter 204 …………………… Register 301 ……………… …… Backlight 302 …………………… Command input means 303 …………………… PWM duty setting means 304 …………………… Sensor input means 305 …………………… PWM cycle setting means 306 ........................... PWM output signal generation means 307 ........................... Light emission means 803 .......................................... PWM drive circuit 804 ........................... Communication means 805 ……………… Management means 806 ............ Memory 807 ........................... PWM signal output circuit 807 ........................... Current switch circuit 1001. ……………… Target value 1002 ………………… Characteristic value 1003 …………… ... PWM signal 1004 ... Output 1005 of characteristic value register 1010 ... ... Output 1006 of adder 1011 ... ... Output 1007 of subtractor 1012 ... ... Output 1008 of cumulative value register 1013 ......... Output 1010 of target value register 1015 ........... Characteristic value register 1011 ........... Adder 1012 .... Subtractor 1013 ........... Cumulative value register 1014 ........... Comparator 1015 ........... Target value register 2000 ................ PWM signal calculation circuit 2001 ........ ………… Constant current circuit 2002 …………………… Switch 2004 ………………… LED
2005 ………………… Resistance R
2006 …………………… Current I
2007 …………………… Feedback voltage V
2008 …………………… PWM signal

Claims (2)

Means for setting a target value of light emission for each cycle;
Means for accumulating a cumulative value of the light emission amount of the light emitting element based on a drive signal which is a pulse signal by a PWM method;
A display device characterized in that the light emitting element is periodically lit.
Means for storing a light emission amount (characteristic value or characteristic matrix) of the light emitting element by the pulse signal;
A counter for accumulating the light emission amount of the light emitting element in synchronization with the pulse signal output every PWM cycle;
Means for comparing the target value of the light emission and the magnitude of the cumulative value;
Means for outputting a PWM pulse until the accumulated value exceeds the target value of light emission;
Means for calculating an error between the target value of the light emission and the cumulative value;
A display device, wherein the error is propagated to the next PWM cycle, and the PWM cycle is corrected so that the accumulated error becomes zero. The display device
A display device that combines a liquid crystal display panel and a backlight that is a light emitting element that is disposed on the back surface of the liquid crystal display panel and emits light with a uniform amount of light over the entire display screen from the back surface of the liquid crystal panel ,
Periodic lighting of the backlight , which is the light emitting element,
The display device according to claim 1, wherein the display device is synchronized with a frame period of the display device.

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