TWI442377B - Liquid crystal backlight with dimming circuit - Google Patents

Description

Dimming circuit for liquid crystal backlight

The present invention relates to a liquid crystal backlight dimming circuit for controlling light emission of a backlight for a liquid crystal display device.

As an example of a conventional liquid crystal display device in which a backlight is built in, a display device disclosed in the following Patent Document 1 is known. The display device has a light detector that senses the brightness around the display device, a peripheral light sensing mechanism composed of an amplifier and a low-pass filter, and a control mechanism that adjusts the amount of light of the backlight according to the result perceived by the surrounding light sensing mechanism. The amount of backlight light corresponding to the surrounding environment is automatically obtained.

Patent Document 1: Japanese Patent Laid-Open No. 5-100027

In such a conventional display device, a fluorescent lamp such as a cold cathode tube is usually used as a backlight. On the other hand, research and development of display devices using light-emitting diodes (LEDs) as backlights have become increasingly active in terms of ease of integration. Thus, in the case where an LED is used as a backlight, errors in the luminous intensity of the LED, variations in temperature, and changes in time are problematic. Therefore, in order to stabilize the luminous intensity of the LED, similarly to the configuration of the conventional display device described above, it is necessary to perform a current conversion of the photodetector from the LED to monitor the luminous intensity of the LED. After being amplified into a voltage signal, the low-pass filter for averaging the voltage signal is input to the control unit via time, and the control unit feedback-controls the driving of the LED.

However, in the display device having the feedback control system described above, in order to obtain an average value of the luminous intensity, a low-pass filter having a large time constant is included in the feedback control system, so that the time constant of the feedback control system is subjected to a low-pass filter. Since the time constant is around, it is quite difficult to control the luminescence intensity with good responsiveness.

Accordingly, the present invention has been made in view of the above problems, and an object thereof is to provide a dimming circuit for a liquid crystal backlight which can improve the responsiveness of control of the light emission intensity of a backlight.

In order to solve the above problems, the liquid crystal backlight dimming circuit of the present invention controls a backlight illuminator for a liquid crystal display device, and includes: a photodetecting element that outputs a monitor current corresponding to the illuminance intensity from the backlight; a strength monitoring unit that inputs a monitor current and generates a light-emission intensity signal corresponding to a charge accumulation amount generated by the monitor current during a measurement period from the generation of the reset signal to the generation of the hold signal; and the control unit The luminous intensity signal generates a control pulse signal for controlling the illumination of the backlight by the pulse modulation control, and generates a reset signal such that the repetition period of the measurement period becomes N (N is an integer of 1 or more) of the period of the control pulse signal. And the signal is held and output to the luminous intensity monitoring unit.

According to the liquid crystal backlight dimming circuit, an illumination intensity signal corresponding to the integrated value of the monitoring current of the measurement time can be generated according to the monitoring current corresponding to the illumination intensity of the backlight, and the pulse intensity modulation control is performed by using the illumination intensity signal. And control the illumination of the backlight. At this time, the measurement time is repeated N times the period of the control pulse signal used for the pulse modulation control, so that the average value of the time of the illumination intensity of the backlight can be monitored in a short time and with high precision, and the luminous intensity is made. The control response is speeded up. In addition to the above-described dimming circuits for liquid crystal backlights, the following configurations may be further included.

First, it is preferable that the control unit generates the reset signal and the hold signal in such a manner that the measurement time includes a variation range of the pulse width of the control pulse signal. When such a configuration is provided, the monitoring accuracy of the backlight can be further improved, and the illumination control of the backlight with higher precision can be realized.

Further, the luminous intensity monitoring unit includes a charging amplifier that includes a voltage signal that accumulates electric charge in accordance with an input of the monitoring current to generate a voltage signal corresponding to the accumulated amount of electric charge, and discharges the electric charge according to the generation of the reset signal; The sample-and-hold circuit inputs a voltage signal and a hold signal, and outputs a voltage signal as a light-emission intensity signal in accordance with a point at which the signal is generated.

In this case, since the voltage signal corresponding to the integral value of the monitor current after the reset signal is generated can be output from the charging amplifier, and the level of the voltage signal at the time of generating the hold signal is output by the sample hold circuit, the responsiveness can be improved. A time average of the luminous intensity of the backlight is obtained.

Further, preferably, the luminous intensity monitoring unit includes a charging amplifier that stores a charge in accordance with an input of the monitoring current to generate a voltage signal corresponding to the accumulated amount of the electric charge as a luminous intensity signal, and causes the electric charge according to the generation of the reset signal. The first switch of the discharge; and the second switch are connected between the photodetection element and the charge amplifier, and input a monitor current to the charge amplifier in synchronization with the generation of the hold signal.

In this way, the voltage signal set to the integral value of the monitor current from the generation of the reset signal to the generation of the hold signal can be outputted by the charge amplifier, so that the time average value of the light-emission intensity of the backlight can be obtained with good responsiveness.

Furthermore, it is preferable that the luminous intensity monitoring unit includes a current frequency conversion circuit that converts into a pulse signal having a repetition frequency corresponding to the monitoring current in accordance with an input of the monitoring current; and a counter that inputs the pulse signal and the weight a signal is set, after the reset signal is generated, the pulse signal is counted and output as a light intensity signal; and a switch is connected between the light detecting element and the counter, and the pulse signal is input to the counter in synchronization with the generation of the hold signal. .

In this case, the photodetection element can output a pulse signal having a repetition frequency corresponding to the received light intensity via the current frequency conversion circuit, and the pulse signal is input from the switch between the photodetection element and the counter, and is input in synchronization with the generation of the hold signal. The counter, and the counter counts the pulse signal after the reset signal is generated. With such a configuration, the time average of the luminous intensity of the backlight can also be obtained with good responsiveness.

Effect of invention

According to the dimming circuit for liquid crystal backlight of the present invention, the responsiveness of controlling the luminous intensity of the backlight can be improved.

Hereinafter, a suitable embodiment of the dimming circuit for liquid crystal backlight of the present invention will be described in detail based on the drawings. In the description of the drawings, the same or corresponding portions will be denoted by the same reference numerals, and the description will be omitted.

[First embodiment]

1 is a functional block diagram of a liquid crystal display device 1 according to a first embodiment of the present invention, and FIG. 2 is a circuit diagram showing a main part of the liquid crystal display device 1 of FIG. As shown in FIG. 1, the liquid crystal display device 1 includes a liquid crystal panel 2, a backlight 3 disposed opposite the liquid crystal panel 2, an LED drive circuit 4, and a liquid crystal backlight dimming circuit 5. The backlight 3 is provided with a plurality of light-emitting diodes (LEDs) that illuminate the liquid crystal panel 2 with white light. The backlight 3 is connected to the LED driving circuit 4 for supplying driving current to the LED provided in the backlight 3. The LED driving circuit 4 is configured to be supplied to the backlight 3 by pulse modulation control (hereinafter referred to as PWM control). The current is turned on/off to adjust the intensity of the light irradiated by the backlight 3. More specifically, the LED drive circuit 4 turns on/off the current supplied to the LED of the backlight 3 in synchronization with the on/off of the control pulse signal for PWM control input from the liquid crystal backlight dimming circuit 5.

The liquid crystal backlight dimming circuit 5 connected to the LED drive circuit 4 is for controlling the light emission intensity of the backlight 3 to a desired intensity, and is provided by the photodetection integrated circuit 6, the controller (control unit) 7, and the memory 8. Composition. As shown in FIG. 2, the photodetecting integrated circuit 6 includes an emission intensity monitoring unit 13 including a photodiode (photodetecting element) 9, a charging amplifier 10, a sample and hold circuit 11, and an A/D converter 12. Hereinafter, each constituent element of the liquid crystal backlight dimming circuit 5 will be described in detail.

The photodetecting integrated circuit 6 is fixed so that the photo sensing region of the photodiode 9 is disposed in the vicinity of the liquid crystal panel 2, and the photodiode 9 generates a monitoring current corresponding to the intensity of light incident on the photo sensing region. The intensity of light irradiated to the liquid crystal panel 2 by the backlight 3 is detected.

A charge amplifier 10 in which the amplifier A and the capacitor C are connected in parallel is connected to the cathode electrode of the photodiode body 9. The monitor current generated by the photodiode 9 is input to the charging amplifier 10, and the charge is accumulated in the capacitor C of the charging amplifier 10 by the monitoring current, and a voltage signal corresponding to the accumulated amount of charge is generated as approximately proportional to the amount of the charge. The voltage signal is monitored and output. Further, a switch 14 is connected between the input and the output of the charging amplifier 10, and the switch 14 discharges the electric charge accumulated in the charging amplifier 10 in synchronization with the generation of the pulse of the reset signal output from the controller 7.

Further, the sample-and-hold circuit (S/H) 11 connected to the output of the charging amplifier 10 receives the sample-and-hold signal (hold signal) from the controller 7, and cooperates with the generation of the pulse of the sample-and-hold signal to maintain the monitoring of the input by the charging amplifier 10. The voltage of the voltage signal is output as a luminous intensity signal. The sample-and-hold circuit 11 is connected to the controller 7 via an A/D converter (A/D) 12, and the illumination intensity signal output from the sample-and-hold circuit 11 is converted into a digital signal and output after the sample-and-hold signal is generated. To controller 7.

The controller 7 generates a control pulse signal for PWM control based on the luminous intensity signal output from the A/D converter 12, so that the luminous intensity of the backlight 3 becomes a desired luminous intensity. At this time, the controller 7 can read out the parameter for displaying the brightness of the screen of the liquid crystal panel 2 from the memory 8 at any time, and feedback control the pulse width of the control pulse signal to present the luminous intensity corresponding to the parameter. That is, the controller 7 sets the pulse width of the control pulse signal to be longer when the luminous intensity indicated by the luminous intensity signal is smaller than the luminous intensity indicated by the parameter, and the luminous intensity indicated by the luminous intensity signal is greater than that indicated by the parameter. In the case of the luminous intensity, the pulse width of the control pulse signal is set to be short. The control pulse signal is generated by a clock generated by a clock source built in the controller 7, or by a clock input from the external controller to the controller 7, in the same manner as the time interval between the pulses, and on the other hand, according to the set The pulse width is changed by the duty ratio. Here, the parameters read by the controller 8 from the memory 8 may be parameters input from the outside or parameters loaded into the program stored in the controller 7.

Further, the controller 7 generates a reset signal for specifying a time for resetting the charge accumulated in the charge amplifier 10 and a sample hold signal for specifying the time for holding the monitor voltage signal output from the charge amplifier 10 by using the clock. . The controller 7 sends the reset signal and the sample and hold signal to the switch 14 of the charging amplifier 10 and the sample and hold circuit 11, respectively.

Next, various signals which are input and output to the liquid crystal backlight dimming circuit 5 will be described with reference to FIG.

First, the control pulse signal for controlling the illumination of the backlight is generated by the controller 7 in a pulse wave pattern which is repeated at a constant period T1, and is sent to the LED drive circuit 4 (Fig. 3(a)). Thereby, the light system irradiated by the backlight 3 is turned on/off in synchronization with the on/off of the pulse waves of the control pulse signal (FIG. 3(b)). At this time, although the pulse width of each pulse wave of the control pulse signal fluctuates due to the PWM control, it is controlled to a pulse width variation range having a time width of time T2 (<T1).

In this case, the pulse wave of the reset signal reset by the charging amplifier 10 is between the pulse width variation range of the control pulse signal and the next pulse width variation range, so that the pulse wave generation period becomes T1 by the controller 7 Produced (Fig. 3(c)). Thereby, the charge accumulated in the charging amplifier 10 is discharged before the input of the control pulse in synchronization with the generation of the pulse of the reset signal output from the controller 7, so that each pulse corresponding to the corresponding control pulse signal can be accumulated in the charging amplifier 10. The charge of the luminous intensity of the wave is output as a luminous intensity signal (Fig. 3(d)).

Further, the pulse wave of the sample-and-hold signal is generated by the controller 7 in a pulse wave generation period of T1 before the generation of the reset signal between the pulse width variation range of the control pulse signal and the next pulse width fluctuation range ( Figure 3 (e)). As a result, in the measurement period P1 from the generation of the reset signal to the generation of the sample-and-hold signal, the monitor voltage signal determined by the amount of charge accumulated in the charge amplifier 10 is output as the light-emission intensity signal. In this case, the repetition period of the measurement period P1 coincides with the period T1 of the control pulse signal, and each measurement period P1 is set such that the pulse width variation range of the control pulse signal is included.

Further, the controller 7 may generate a reset signal and a sample hold signal so that the repetition period of the measurement time defined by the reset signal and the sample hold signal becomes N times (N is an integer of 1 or more) of the period T1 of the control pulse signal. 4 is a diagram showing various signals outputted to the liquid crystal backlight dimming circuit 5 in a case where the repetition period of the measurement time is three times the period T1 of the control pulse signal.

The repetition frequency of the measurement time is twice the frequency shown in Fig. 3 (Fig. 4(a)), and the illumination output is synchronized with the control pulse signal, and is repeated twice as fast as the frequency shown in Fig. 3 (Fig. 4(b)). In the case of Fig. 4, the reset signal has a pulse wave having a period of 3 × T1 across three pulse width variation ranges (T2) (Fig. 4(c)). Thereby, the electric charge corresponding to the luminous intensity of the three consecutive pulse waves corresponding to the control pulse signal is accumulated in the charging amplifier 10 in stages, and is output as a monitor voltage signal (FIG. 4(d)).

Further, before the generation of the reset signal of the time zone in which the pulse width variation range of the control pulse signal is generated, the pulse wave of the control sample-and-hold signal is generated by the controller 7 in a pulse wave generation period of 3 × T1 (Fig. 4(e)). As a result, the measurement period P2 is set so as to include the pulse width variation range of the control pulse signal for three consecutive times.

Further, it is preferable that the upper limit of the repetition period of the measurement period set by the controller 7 is set within a range that does not impair the response time of the response time of the backlight 3. Further, it is preferable to set the range in which saturation of the analog amplifier 10 and saturation of the A/D converter 12 are not caused. For example, in general, in a television set, dozens of still images can be reflected in one second, and in the case of the NTSC (National Television Standards Committee), there are 30 (the field of view is 60). ). Therefore, for the integration time of 1 time, when it is desired to control the backlight in each field of view, it is required to perform lighting of 1/60 second or less. Measurement. Assuming that the field of view of M times is required before the control is stabilized, 1/M times 1/(60.M) is required further. Assuming M=5, 1/300 second is required. Generally, in the case of a large-screen monitor, the backlight is composed of a plurality of blocks (the number of block divisions is B). When one detector is responsible for the control of all blocks, the integration time needs to be further required to reach 1/B. In the above example, in the case of B = 9, the integration time is 1/2700 seconds = 370 μs. Assume that if the PWM repetition frequency is 20 kHz, the PWM pulse period is 50 μs, so N=370 μs/50 μs≒7, and the repetition period during the measurement period is 7 times of the PWM pulse period. Thus, it is preferable to set the value of N according to the specifications and requirements of the backlight system (for example, the required response time, the number of block divisions, and the pulse period of the PWM).

According to the liquid crystal backlight dimming circuit 5, the charging amplifier 10 can output a monitor voltage signal corresponding to the integrated value of the monitor current from the photodiode 9 after the reset signal is generated, and the sample hold circuit 11 holds the sample hold signal. At the time of generation, the level of the voltage signal is monitored and output as a luminous intensity signal. In the controller 7, the illumination intensity of the backlight 3 is controlled by PWM control using the illumination intensity signal. At this time, the measurement time from the generation of the reset signal to the generation of the sample-and-hold signal is repeated N times the period of the control pulse signal used for the PWM control, so that the average value of the time of the illumination intensity of the backlight can be short-time and It is monitored with high precision, and the response of the control of the luminous intensity is increased. This is because the feedback control system does not include components with a large time constant such as a low-pass filter. In particular, even in the case where the liquid crystal backlight dimming circuit 5 is required to be set in the light and dark of each image frame displayed on the liquid crystal panel 2, the high-speed light emission control of the backlight that meets the requirements can be performed.

Further, since the measurement time is set so as to include the fluctuation range of the pulse width of the control pulse signal, the monitoring accuracy of the illumination intensity of the backlight 3 can be further improved, and the illumination control of the backlight 3 with higher precision can be realized.

[Second embodiment]

Next, the second embodiment will be described focusing on the difference from the first embodiment.

Fig. 5 is a circuit diagram showing a liquid crystal display device 21 including a liquid crystal backlight dimming circuit 25 of the second embodiment, and Fig. 6 is a circuit diagram of the current/frequency conversion circuit 30 of Fig. 5. As shown in FIG. 5, in the photodetecting integrated circuit 26 of the liquid crystal backlight dimming circuit 25, the cathode current of the photodiode 9 is connected to convert the monitor current from the photodiode 9 to have a corresponding The current/frequency conversion circuit 30 for monitoring the frequency of the current value, the output of the current/frequency conversion circuit 30 is connected to the controller 27 via the switch 32 and the counter 31. The current/frequency conversion circuit 30, the switch 32, and the counter 31 constitute the luminous intensity monitoring unit 23.

The circuit configuration of the current/frequency conversion circuit 30 includes, for example, an integration circuit 33 and a comparator 34 as shown in FIG. The integrating circuit 33 includes a capacitor 36 interposed between the output terminal and the inverting input terminal of the operational amplifier 35, and a switch 37 that can short-circuit the both ends of the capacitor 36. This switch 37 resets the charge accumulated in the integrating circuit 33 in accordance with the output of the comparator 34. Further, the output terminal of the operational amplifier 35 is connected to one of the terminals of the comparator 34, and the reference potential Vref is applied to the other terminal. The inverting input terminal of the operational amplifier is input to the monitor current of the photodiode 9.

In such a current/frequency conversion circuit 30, in accordance with the amount of charge accumulated in the integrating circuit 33, the comparator 34 causes the voltage generated at the output of the integrating circuit 33 to exceed the threshold value Vref of the comparator 34, and the comparator 34 is used. The output is switched from a high level to a low level, and the switch 37 is turned on to short-circuit both ends of the capacitor 36. Thus, the voltage generated by the output of the integrating circuit 33 becomes lower than the threshold value Vref of the comparator 34, and the output of the comparator 34 is switched from the low level to the high level, the switch 37 becomes the power-off, and the integrating circuit 33 starts to accumulate the charge. . At this time, the larger the monitoring current is, the shorter the output of the comparator can be switched, and the monitoring pulse signal written by the current/frequency conversion circuit 30 becomes a pulse waveform having a repetition frequency proportional to the value of the monitored current branch current. .

Returning to Fig. 5, the switch 32 connected between the current/frequency conversion circuit 30 and the counter 31 is connected to the output of the current/frequency conversion circuit 30 and the input of the counter 31 when the hold signal is input from the controller 27 to keep the signal on. The monitoring pulse signal is input to the counter 31. Further, the switch 32 may be inserted between the photodiode 9 and the current/frequency conversion circuit 30 in a case where the monitoring pulse signal generated by the current/frequency conversion circuit 30 is proportional to the monitor current.

The counter 31 receives the pulse signal for monitoring, counts the number of pulses of the monitoring pulse signal, and outputs the result of the counting to the controller 27. Further, in the counter 31, a reset signal is input from the controller 27, and the number of counts of the counter 31 is reset in accordance with the generation of the number of pulses of the reset signal.

Next, various signals that are input and output in the liquid crystal backlight dimming circuit 25 will be described in detail with reference to FIG.

The pulse wave of the reset signal of the reset counter 31 is between the pulse width variation range of the control pulse signal having the time width T2 and the next pulse width variation range, so that the pulse wave generation period becomes the T1 mode by the controller 27. Produced (Fig. 7(c)). Thereby, the number of counts of the counter 31 is reset before the input of the control pulse in synchronization with the generation of the pulse of the reset signal output from the controller 27, and thus corresponds to the charge accumulation amount of each pulse wave corresponding to the control pulse signal. The count number is counted by the counter 31 and output (Fig. 7(d)).

Further, the hold signal is in a cut-off state before the generation of the reset signal between the pulse width variation range of the control pulse signal and the next pulse width variation range, and is turned on during the period including the other pulse width variation range. The generation period of the pulse wave is T1 and is generated by the controller 27 (Fig. 7(e)). As a result, in the measurement period P3 after the generation of the reset signal to the level at which the hold signal is changed from the on-change to the cut-off level, the light-emission intensity signal corresponding to the count number of the charge amount accumulated in accordance with the monitor current is output. In this case, the repetition period of the measurement period P3 is set to coincide with the period T1 of the control pulse signal, and each measurement period P3 is set to include the pulse width variation range of the control pulse signal.

Here, similarly to the first embodiment, the controller 27 may have a repetition period of the measurement time P3 defined by the reset signal and the hold signal N times the period T1 of the control pulse signal (N is an integer of 1 or more) The method generates a reset signal and a hold signal.

In the liquid crystal backlight dimming circuit 25, the photodiode 9 can output a monitoring pulse signal having a repetition frequency corresponding to the received light intensity via the current frequency conversion circuit 30, and the monitoring pulse signal is applied to the photoelectric The switch 32 between the diode 9 and the counter 31 is input to the counter 31 in synchronization with the generation of the hold signal, and the counter pulse 31 counts the monitor pulse signal after the reset signal is generated. With such a configuration, the time average value of the luminous intensity of the backlight 3 can also be obtained with good responsiveness.

Further, the present invention is not limited to the above-described embodiments. Fig. 8 is a circuit diagram showing a liquid crystal display device 41 according to a modification of the present invention. The liquid crystal backlight dimming circuit 45 shown in the same figure is different from the first embodiment in the configuration of the luminous intensity monitoring unit 53 included in the photodetecting integrated circuit 46. That is, the switch 54 is connected between the cathode electrode of the photodiode 9 and the input of the charging amplifier 10. The switch 54 is input with a hold signal by the controller 47, and is connected to the photodiode 9 in synchronization with the conduction state of the hold signal. Between the cathode electrode and the input of the charging amplifier 10, the monitor current of the photodiode 9 is input to the charging amplifier. In the liquid crystal backlight dimming circuit 45, the waveforms of the various signals that are input and output are the same as those of the second embodiment (see FIG. 7).

Further, Fig. 9 is a circuit diagram showing a liquid crystal display device 61 according to another modification of the present invention. The liquid crystal backlight dimming circuit 65 shown in the figure is incorporated in the light-emission intensity monitoring unit 73 included in the photodetecting integrated circuit 66, and the differential amplifier 67 is inserted between the sample-and-hold circuit 11 and the A/D converter 12. The difference is different from the first embodiment (see FIG. 2), and the other configurations are the same. In other words, when one of the differential amplifiers 67 is input, the light-emission intensity signal is input from the sample-and-hold circuit 11, and is input to the other side, and the reference potential Vref is input. In this manner, the differential amplifier 67 converts the difference signal between the luminous intensity signal and the reference potential Vref to the controller 7 by the A/D converter 12, and the controller 7 performs feedback control by reducing the difference signal to generate PWM control. Control pulse signal. Thus, in the controller 7, it is no longer necessary to read the parameters from the memory 8. Further, the differential amplifier 67 can also be inserted between the charging amplifier 10 and the sample and hold circuit 11.

Fig. 10 is a view showing various signals which are input and outputted in the liquid crystal backlight dimming circuit of Fig. 2, wherein Fig. 10(a) is a control pulse signal, and Fig. 10(b) is a light emission output from a photodiode, 10(c) is a reset signal, FIG. 10(d) is a monitor voltage signal of the charge amplifier, and FIG. 10(e) is a time chart of the sample hold signal. The time charts of Figs. 10(a) to 10(d) are the same as those of Fig. 3.

Here, the controller 7 of the liquid crystal backlight dimming circuit 5 can also generate two sample hold signals before and after the reset signal between the pulse width variation ranges of the control pulse signals for PWM control (Fig. 10 (e )). In this case, the sample hold circuit 11 holds the monitor voltage signal outputted from the charge amplifier after the pulse width variation range, so that the controller 7 can obtain the light emission intensity by the difference between the monitor voltage signals of both sides. As a result, the influence of residual errors, noise, and the like generated in the charging amplifier 10 and the like can be reduced, and the luminous intensity of the backlight 3 can be more accurately grasped.

Further, in the liquid crystal backlight dimming circuits 5, 45, and 65, the A/D converter may not be provided, and the controllers 7 and 47 may generate the control pulse signal using the luminous intensity signal of the analog signal.

Further, in the first embodiment, a plurality of light-emitting diodes (LEDs) that emit white light are used as the backlight, but a plurality of LEDs of three colors (R, G, B) or more may be used to form white. . In this case, it is preferable to set a dedicated PD (Photo Detector) (color PD) for each color and control them separately.

1, 21, 41, 61. . . Liquid crystal display device (liquid crystal display device)

3. . . Backlight

5, 25, 45, 65. . . Dimming circuit for liquid crystal backlight

7, 27, 47. . . Controller (control unit)

9. . . Photodiode (photodetection element)

10. . . Charging amplifier

11. . . Sample and hold circuit

13, 23, 53, 73. . . Luminous intensity monitoring unit

14, 32, 54. . . switch

30. . . Current/frequency conversion circuit

31. . . counter

Fig. 1 is a functional block diagram of a liquid crystal display device of a first embodiment of the present invention.

Fig. 2 is a circuit diagram showing the connection relationship of the dimming circuit for liquid crystal backlight of Fig. 1.

3 is a view showing various signals that are input and outputted in the liquid crystal backlight dimming circuit of FIG. 2, wherein (a) is a control pulse signal, (b) is a light-emitting output from a photodiode, and (c) is reset. The signal, (d) is the monitor voltage signal of the charge amplifier, and (e) is the time map of the sample hold signal.

4 is a view showing various signals that are input and outputted in the liquid crystal backlight dimming circuit of FIG. 2, wherein (a) is a control pulse signal, (b) is a light-emitting output from a photodiode, and (c) is reset. The signal, (d) is the monitor voltage signal of the charge amplifier, and (e) is the time map of the sample hold signal.

Fig. 5 is a circuit diagram showing a connection relationship of a dimming circuit for a liquid crystal backlight according to a second embodiment of the present invention.

Figure 6 is a circuit diagram of the current/frequency conversion circuit of Figure 5.

Fig. 7 is a view showing various signals which are input and outputted in the liquid crystal backlight dimming circuit of Fig. 5, wherein (a) is a control pulse signal, (b) is a light-emitting output from a photodiode, and (c) is reset. The signal, (d) is the output signal of the counter, and (e) is the time diagram of the hold signal.

Fig. 8 is a circuit diagram showing a connection relationship of a liquid crystal backlight dimming circuit according to a modification of the present invention.

Fig. 9 is a circuit diagram showing a connection relationship of a dimming circuit for a liquid crystal backlight according to another modification of the present invention.

Fig. 10 is a view showing a modification of various signals which are input and outputted in the liquid crystal backlight dimming circuit of Fig. 2, wherein (a) is a control pulse signal, (b) is a light-emitting output from a photodiode, (c) ) is a reset signal, (d) is the monitor voltage signal of the charge amplifier, and (e) is a time chart of the sample hold signal.

3. . . Backlight

4. . . LED drive circuit

5. . . Dimming circuit for liquid crystal backlight

6. . . Light detection integrated circuit

7. . . Controller (control unit)

8. . . Memory

9. . . Photodiode (photodetection element)

10. . . Charging amplifier

11. . . Sample and hold circuit

12. . . A/D converter

13. . . Luminous intensity monitoring unit

14. . . switch

Claims (3)

A liquid crystal backlight dimming circuit that sets N to an integer of 1 or more and controls a backlight illuminator for a liquid crystal display device, comprising: a photodetecting element that outputs a light corresponding to the backlight from the backlight The intensity monitoring unit receives the monitoring current and generates an emission intensity signal corresponding to the charge accumulation amount, which is generated during the measurement period from the generation of the reset signal to the generation of the hold signal. And the control unit generates a control pulse signal according to the illumination intensity signal, and generates the reset signal by repeating a period in which the repetition period of the measurement period becomes N times of a period of the control pulse signal And the holding signal is output to the light emission intensity monitoring unit, wherein the control pulse signal is used to control the light emission of the backlight by pulse width modulation control; and the light emission intensity monitoring unit includes: a charging amplifier, which is included in the Accumulating charges by inputting the aforementioned monitoring current to generate an accumulation amount corresponding to the aforementioned electric charge a voltage signal as a first light-emitting intensity signal, a first switch that discharges the charge in accordance with the generation of the reset signal; and a second switch connected between the light detecting element and the charging amplifier, and the holding signal Synchronizing and generating the aforementioned monitoring The stream is input to the aforementioned charging amplifier. A liquid crystal backlight dimming circuit that sets N to an integer of 1 or more and controls a backlight illuminator for a liquid crystal display device, comprising: a photodetecting element that outputs a light corresponding to the backlight from the backlight The intensity monitoring unit receives the monitoring current and generates an emission intensity signal corresponding to the charge accumulation amount, which is generated during the measurement period from the generation of the reset signal to the generation of the hold signal. And the control unit generates a control pulse signal according to the illumination intensity signal, and generates the reset signal by repeating a period in which the repetition period of the measurement period becomes N times of a period of the control pulse signal And the holding signal is output to the light emission intensity monitoring unit, wherein the control pulse signal is used to control light emission of the backlight by pulse width modulation control; and the light emission intensity monitoring unit includes: a current frequency conversion circuit. Converting to have a repetition frequency corresponding to the aforementioned monitoring current according to the input of the monitoring current a pulse signal; a counter that inputs the pulse signal and the reset signal, and after the reset signal is generated, counts the pulse signal and outputs the light intensity signal; and The switch is connected between the photodetecting element and the counter, and inputs the pulse signal to the counter in synchronization with the generation of the holding signal. The liquid crystal backlight dimming circuit according to claim 1 or 2, wherein the control unit generates the reset signal and the hold signal such that the measurement time includes a variation range of a pulse width of the control pulse signal.