JP2012174440A - Light control device and light control method for back light - Google Patents

Abstract

[Backlight dimming control device and dimming control method] Discharge of an output smoothing capacitor when dimming is OFF without causing an increase in cost and without causing discomfort to the screen appearance. I will provide a.
When a dimming OFF command is issued, the dimming PWM wave generated when the dimming is turned on is converted into a direct current whose amount of current per unit period is the same as the dimming PWM wave. By controlling the dimming of the LED matrix according to the converted direct current, the output smoothing capacitor C is discharged through the LED matrix. As a result, it is not necessary to provide a dedicated short circuit separately, thereby suppressing an increase in cost, enabling discharge to be performed in a short time through the low-resistance LED matrix, and even when discharging is performed through the LED matrix, The apparent luminance is not changed before and after the dimming OFF command is issued.
[Selection] Figure 3

Description

  The present invention relates to a dimming control device and a dimming control method for a backlight, and more particularly to a dimming control technique for an LCD (liquid crystal display device) using an LED (light emitting diode) as a backlight light source.

  Conventionally, cold cathode fluorescent lamps (CCFLs) have been used for LCD backlights, but recently, environmentally friendly backlights that do not use mercury are provided with LEDs as the light source. Has been. When an LED is used as the light source of the backlight, it is necessary to drive the LED with a constant current in order to minimize the change in chromaticity. As a method for driving an LED with a constant current, there are a step-up constant current drive method, a step-down constant current drive method, and a step-up / step-down constant current drive method.

  The number of LEDs used for the backlight light source varies depending on the display size of the LCD, the required luminance, and the like. For example, in the case of a vehicle-mounted LCD that is classified into a medium and small size, 5 to 36 LEDs are used as a backlight light source. The plurality of LEDs are arranged in a matrix, and the number of parallel lines is 1 to 4 and the number of series is 5 to 9 in the mainstream.

  FIG. 4 is a diagram illustrating a configuration example of a conventional LED driver circuit based on a step-up / step-down constant current driving method. In FIG. 4, 51 is a constant current drive IC, 52 is a step-down circuit, 53 is a step-up circuit, and 54 is an LED matrix. The LED matrix 54 is composed of a total of 24 LEDs with 3 in parallel and 8 in series. One row (eight) LEDs are configured by modules in which LEDs of the same standard are connected in series, and an LED matrix 54 is configured by connecting three modules (LED rows) 54-1 to 54-3 in parallel. Is done.

  The constant current drive IC 51 includes an internal power supply 61, a step-down control unit 62, a step-up control unit 63, an overvoltage detection unit 64, an overcurrent detection unit 65, an abnormality detection / notification unit 66, a current setting unit 67, and a PWM setting. Unit 68 and first to third constant current / dimming control units 69-1 to 69-3. The step-down circuit 52 includes a step-down driving transistor Tr1, a diode D1, and a coil L. Further, the booster circuit 53 includes a coil L that is also used as the step-down circuit 52, a booster drive transistor Tr2, and a diode D2. On the output side of the booster circuit 53, an output smoothing capacitor C for ripple smoothing and voltage dividing resistors R1 and R2 are provided.

  The constant current drive IC 51 operates by obtaining power from the internal power supply 61, and controls the step-down drive transistor Tr1 of the step-down circuit 52 to step down the input voltage VIN to a lower voltage. Further, by controlling the step-up driving transistor Tr2 of the step-up circuit 53, the voltage output from the step-down circuit 52 is stepped up to a higher voltage and applied to the LED matrix 54. At this time, the ripple of the applied voltage is suppressed by the output smoothing capacitor C provided at the output stage of the booster circuit 53.

  The constant current drive IC 51 has a built-in protection function for preventing serious damage caused by overvoltage or overcurrent. The overvoltage protection function is realized as follows. That is, the output voltage VOUT of the booster circuit 53 is fed back to the overvoltage detection unit 64 of the constant current drive IC 51 via the voltage dividing resistors R1 and R2. The overvoltage detection unit 64 monitors the feedback voltage and, when detecting that the feedback voltage is greater than a predetermined threshold (overvoltage), notifies the abnormality detection / notification unit 66 to that effect.

  The overcurrent is detected by detecting the current flowing through the step-down driving transistor Tr1. Specifically, using the resistor R0 connected to the source of the step-down driving transistor Tr1, the voltage value generated at the resistor R0 is monitored by the overcurrent detection unit 65 of the constant current driving IC 51. When the overcurrent detection unit 65 detects that the monitored voltage value is larger than a predetermined threshold (overcurrent), the overcurrent detection unit 65 notifies the abnormality detection / notification unit 66 to that effect. The abnormality detection / notification unit 66 notifies the outside of the constant current drive IC 51 that an abnormality has occurred when an overvoltage detection or overcurrent detection is notified from the overvoltage detection unit 64 or the overcurrent detection unit 65.

  The abnormality detection / notification unit 66 also performs abnormality determination of the LED matrix 54 through the first to third constant current / dimming control units 69-1 to 69-3 in addition to the above-described abnormality determination of overvoltage and overcurrent. . That is, the first to third constant current / dimming control units 69-1 to 69-3 are applied to the voltages VLED1 to VLED1 applied in the predetermined abnormality detection period from the start of lighting (dimming ON) of the LED matrix 54, respectively. The VLED 3 is detected, and the detected voltage is notified to the abnormality detection / notification unit 66. The abnormality detection / notification unit 66 determines that the voltages VLED1 to VLED3 are the threshold voltages of the constant current drive IC 51 (for example, 0.4 [V]) based on the notifications from the constant current / dimming control units 69-1 to 69-3. ), It is determined that the cathode side of the LED arrays 54-1 to 54-3 is in contact with the ground, and the abnormality is notified to the outside of the constant current drive IC 51.

  Dimming the LED matrix 54 is performed by the current setting unit 67, the PWM setting unit 68, and the first to third constant current / dimming control units 69-1 to 69-3. The PWM setting unit 68 generates a pulse wave having a constant current set by the current setting unit 67 and having a duty ratio set at a time ratio corresponding to the dimming rate. The constant current / dimming control units 69-1 to 69-3 are respectively supplied.

  The first constant current / dimming control unit 69-1 pulses the first LED row 54-1 constituting the LED matrix 54 based on the constant current pulse wave generated by the PWM setting unit 68. The dimming of the first LED array 54-1 is controlled by intermittently supplying a constant current according to the wave duty ratio.

  The second constant current / dimming control unit 69-2 pulses the second LED row 54-2 constituting the LED matrix 54 based on the constant current pulse wave generated by the PWM setting unit 68. The dimming of the second LED array 54-2 is controlled by intermittently supplying a constant current according to the wave duty ratio.

  The third constant current / dimming control unit 69-3 pulses the third LED row 54-3 constituting the LED matrix 54 based on the constant current pulse wave generated by the PWM setting unit 68. The dimming of the third LED array 54-3 is controlled by intermittently supplying a constant current according to the wave duty ratio.

  When the LED matrix 54 is lit by turning on the dimming of the first to third constant current / dimming control units 69-1 to 69-3, the boosting control unit 63 drives the boosting drive transistor Tr2. Then, the booster circuit 53 is operated. At this time, the boost control unit 63 performs PWM control of ON / OFF of the boost drive transistor Tr2 using a pulse wave with a fixed period generated from an oscillator (not shown), thereby changing the input voltage VIN to a predetermined output voltage VOUT. Boost the pressure.

  For example, the input voltage VIN is a power supply voltage supplied from a head unit (not shown) mounted on the vehicle, and is assumed to be, for example, 8.0 [V]. If the forward voltage VF is 3.0 [V] when a constant current of 80 [mA] is passed through the LEDs used in the first LED array 54-1, the first LED array 54-1. As a whole, the forward voltage is 3.0 × 8 = 24.0 [V].

  Temporarily, the forward voltage VF of the LED used in each of the first to third LED rows 54-1 to 54-3 varies and is used for the second LED row 54-2. Assuming that the forward voltage VF when a constant current of 80 [mA] flows through the LEDs is 3.3 [V], the entire second LED array 54-2 is 3.3 × 8 = 26.4 [V]. The forward voltage is Similarly, assuming that the forward voltage VF is 3.6 [V] when a constant current of 80 [mA] is applied to the LEDs used in the third LED array 54-3, the third LED array 54- The total of 3 is 3.6 × 8 = 28.8 [V].

  When the feedback voltage at which the current becomes 80 [mA] is 1.0 [V], the output voltage VOUT of the booster circuit 53 is 28.8 + 1.0 = 29.8 [V]. In this case, the voltage VLED1 applied to the first constant current / dimming control unit 69-1 is 29.8-24.0 = 5.8 [V]. The voltage VLED2 applied to the second constant current / dimming control unit 69-2 is 29.8-26.4 = 3.4 [V]. The voltage VLED3 applied to the third constant current / dimming control unit 69-3 is 29.8-28.8 = 1.0 [V].

  On the other hand, when the dimming is turned off and the LED matrix 54 is turned off, the operations of the first to third constant current / dimming control units 69-1 to 69-3 are stopped and the boost driving transistor Tr2 is turned off. As a result, the operation of the booster circuit 53 is stopped. Here, when the forward voltage VF of the diode D2 of the booster circuit 53 is 0.5 [V], the terminal voltage of the output smoothing capacitor C when the boosting operation is stopped is 8.0−0.5 = 7.5 [7.5]. V]. Therefore, the accumulated charge in the output smoothing capacitor C is discharged through the voltage dividing resistors R1 and R2 and the overvoltage detection unit 64 from 29.8 [V] when dimming is ON to 7.5 [V] when dimming is OFF. .

  The time required for this discharge can be shortened as the values of the voltage dividing resistors R1 and R2 are reduced. However, when the resistance value is reduced, there arises a problem that the circuit current (power) increases as a load. In addition, DC-DC switching noise is likely to occur, and there is a problem that erroneous detection and malfunction in the overvoltage detection unit 64 are likely to occur. Therefore, in practice, the values of the voltage dividing resistors R1 and R2 cannot be reduced.

  As a result, the conventional LED driver circuit has a problem that the time required for the discharge becomes long to some extent. For example, if the values of the voltage dividing resistors R1 and R2 are R1 = 36 [kΩ] and R2 = 2 [kΩ], respectively, and the capacitance value of the output smoothing capacitor C is 40 [μF], the output smoothing capacitor C The time required for the discharge until the terminal voltage is changed from 29.8 [V] to 7.5 [V] is 2.1 seconds.

  Thus, when the discharge time at the time of dimming OFF becomes long, when the LED matrix 54 is turned on at a low dimming rate after the dimming is turned off, there is a problem that an unlit LED row may be generated. This problem will be described below with reference to the drawings. FIG. 5 shows that when the value of the voltage dividing resistors R1 and R2 is reduced, the discharge time can be shortened, and after the dimming OFF command is issued until the next dimming ON command is issued. The operation example when the discharge necessary for the terminal voltage of the output smoothing capacitor C to be 7.5 [V] is completed is shown.

  The pulse wave of the step-up PWM shown in FIG. 5A is a PWM wave supplied from the step-up control unit 63 to the step-up drive transistor Tr2. That is, when a dimming ON command is issued from the head unit to the constant current drive IC 51, the boost control unit 63 supplies the boost PWM wave of FIG. 5A to the boost drive transistor Tr2. In response to this, the output voltage VOUT of the booster circuit 53 (= terminal voltage of the output smoothing capacitor C) gradually rises as shown in FIG. 5C, and stabilizes when it reaches 29.8 [V]. . As the output voltage VOUT rises gradually, the voltages VLED1 to VLED3 of the first to third constant current / dimming control units 69-1 to 69-3 are gradually increased as shown in FIGS. To rise.

  On the other hand, when the dimming OFF command is issued from the head unit to the constant current drive IC 51, the boost control unit 63 stops the supply of the boost PWM wave in FIG. In response to this, the output voltage VOUT of the booster circuit 53 (= terminal voltage of the output smoothing capacitor C) gradually decreases due to the discharge as shown in FIG. 5C, and reaches 7.5 [V]. Stabilize. As described above, FIG. 5 shows an example in which the discharge time is short. Therefore, the terminal of the output smoothing capacitor C is in a period from when the dimming OFF command is issued until the next dimming ON command is issued. The discharge necessary for the voltage to drop from 29.8 [V] to 7.5 [V] is completed.

  When the output voltage VOUT gradually decreases, the voltages VLED1 to VLED3 of the first to third constant current / dimming control units 69-1 to 69-3 also gradually decrease as shown in FIGS. I will do it. When the output voltage VOUT reaches 7.5 [V], the voltages VLED1 to VLED3 of the first to third constant current / dimming control units 69-1 to 69-3 are all 0.4 [V]. It has fallen to the following values.

  The pulse wave of the dimming PWM shown in FIG. 5B is a PWM wave output from the PWM setting unit 68. The duty ratio of the dimming PWM wave is determined according to the dimming rate supplied from the head unit to the constant current drive IC 51. FIG. 5B shows a dimming PWM wave when the dimming rate is 0.5%. The first to third constant current / dimming control units 69-1 to 69-3 cause a constant current of 80 [mA] to flow during a period in which the dimming PWM wave is turned on, thereby causing the operations shown in FIGS. The voltages VLED1 to VLED3 of the first to third constant current / dimming control units 69-1 to 69-3 shown in (f) are respectively 5.8 [V], 3.4 [V], and 1.0 [V]. V].

  A certain period of time after the dimming ON command is issued is set to an abnormality detection period in which the cathode side of the first to third LED rows 54-1 to 54-3 is not in contact with the ground. This abnormality detection period is determined using a timer count or a time constant. For example, in the case of the timer count type, when the period of the step-up PWM wave is 500 [kHz] and the count value is 32770, the abnormality detection period is 65.54 [msec].

  In the example of FIG. 5G, even when the LED matrix 54 is turned on at a low dimming rate (0.5%) after dimming OFF, within 65.54 [msec] after the dimming ON command is issued. The voltage VLED1 to VLED3 of the first to third constant current / dimming control units 69-1 to 69-3 exceeds 0.4 [V] during the ON period of the dimming PWM wave emitted from . Therefore, the abnormality detection / notification unit 66 determines that there is no abnormality. As a result, the first to third LED rows 54-1 to 54-3 are all lit normally.

  In contrast, in FIG. 6, by increasing the values of the voltage dividing resistors R1 and R2, the discharge time of the output smoothing capacitor C becomes longer, and after the dimming OFF command is issued, the dimming ON is next performed. An example of operation in the case where the discharge necessary for the terminal voltage of the output smoothing capacitor C to be 7.5 [V] is not completed before the command is issued is shown.

  That is, as shown in FIG. 6C, the output voltage VOUT of the booster circuit 53 (= terminal voltage of the output smoothing capacitor C) is the next command to turn on the dimming after the dimming OFF command is issued. Until it is discharged, the voltage does not drop to 7.5 [V], and the discharge is not completed. Therefore, as shown in FIGS. 6D to 6F, the voltages VLED1 to VLED3 of the first to third constant current / dimming control units 69-1 to 69-3 are set to 0.4 [V ] Something that has n’t fallen below

  When there is one of the voltages VLED1 to VLED3 that has not decreased to a value of 0.4 [V] or less, and when a dimming ON command is issued next, the same boosting as in FIG. When the boosting operation is performed by the PWM wave, the output voltage VOUT may rise too rapidly and a large overshoot may occur. Therefore, in order to avoid a rapid voltage rise and prevent the occurrence of overshoot, the boost control unit 63 synchronizes the boost PWM wave with the ON period of the dimming PWM wave, as shown in FIG. 6A. It occurs intermittently.

  Then, as shown in FIG. 6C, the output voltage VOUT gradually increases in a manner synchronized with the ON period of the dimming PWM wave. Accordingly, the first to third constant current / dimming control units 69 in the ON period of the dimming PWM wave generated within the abnormality detection period of 65.54 [msec] after the dimming ON command is issued. Some of the voltages VLED1 to VLED3 of −1 to 69-3 do not exceed 0.4 [V]. Here, the voltage VLED3 of the third constant current / dimming control unit 69-3 shown in FIG. 6 (f) does not exceed 0.4 [V].

  If the third voltage VLED3 does not exceed 0.4 [V], the abnormality detection / notification unit 66 determines that an abnormality has occurred in the third LED array 54-3, and the third LED array 54- 3 is controlled not to light up. As a result, there arises a problem that only the first and second LED rows 54-1 and 54-2 are lit, and the third LED row 54-3 is not lit.

  A technique has been proposed in which the discharge time of the output smoothing capacitor C can be shortened without reducing the values of the voltage dividing resistors R1 and R2 (see, for example, Patent Document 1). In the technique disclosed in Patent Document 1, an LED array is separately provided as a short circuit for discharging the charge accumulated in the output smoothing capacitor C, so that the accumulated charge in the output smoothing capacitor C is rapidly discharged through the path of the short circuit. It is possible to let you.

JP 2008-205036 A

  However, the technique described in Patent Document 1 has a problem that the cost increases because a dedicated short circuit must be provided separately. Further, in the technique described in Patent Document 1, since a current flows rapidly through the LED array as a short circuit in response to a dimming OFF command, the liquid crystal display is turned off after being instantly brightened. For this reason, there has been a problem that the brightness of the screen flickers and the user feels uncomfortable.

  The present invention has been made to solve such a problem, and discharges accumulated charges of the output smoothing capacitor when a dimming OFF (extinguishment) command is issued without incurring an increase in cost. An object of the present invention is to be able to perform in a short time without giving a sense of incongruity to the appearance on the screen.

  In order to solve the above-described problem, in the present invention, when a turn-off command for setting the dimming rate to zero is issued while turning on a backlight in which a plurality of light emitting diodes are arranged in a matrix, the backlight A constant-current pulse wave generated according to the dimming rate when the lamp is turned on is converted into a direct current whose amount of current per unit period is the same as that of the pulse wave. By controlling the light, the output smoothing capacitor is discharged through the plurality of light emitting diodes.

  According to the present invention configured as described above, since the output smoothing capacitor is discharged through a plurality of light emitting diodes originally provided as a backlight light source, there is no need to separately provide a dedicated short circuit, which increases the cost. Can be suppressed. Further, since the discharge is not performed through the voltage dividing resistor as in the conventional case, it is possible to avoid the disadvantage that the discharge speed becomes slow due to the value of the voltage dividing resistor which cannot be reduced, which is smaller than the voltage dividing resistor. Discharge can be performed in a short time through the light emitting diode of resistance value.

  Furthermore, according to the present invention, the discharge is not performed in such a manner that a large current flows through the plurality of light emitting diodes at once, but the unit period compared to the pulse wave generated according to the dimming rate when the backlight is turned on. In accordance with the direct current converted in such a manner that the amount of current per unit does not change, discharging is performed while turning on the plurality of light emitting diodes. Therefore, even when discharge is performed through a plurality of light emitting diodes, the apparent luminance does not change before and after the turn-off command is issued, and it is possible to avoid giving a sense of incongruity to the appearance.

  As described above, according to the present invention, the discharge of the accumulated charge in the output smoothing capacitor when the dimming OFF (extinguishment) command is issued without causing an increase in cost also gives a sense of incongruity on the screen. And can be done in a short time. Thereby, when a plurality of light emitting diodes are turned on at a low dimming rate after the dimming is turned off, it is possible to prevent a row of light emitting diodes that are not lit.

It is a figure which shows the structural example of the LED driver circuit provided with the light control apparatus of the backlight by this invention. It is a figure which illustrates the light control PWM wave generated by the PWM setting part of this embodiment, and the direct current which converted this. It is a timing chart which shows the operation example of the dimming control apparatus of the backlight by this embodiment. It is a figure which shows the structural example of the conventional LED driver circuit by a buck-boost constant current drive system. It is a timing chart which shows the example of operation | movement of the conventional light control apparatus in the case where voltage dividing resistance is made small and discharge time is shortened. It is a timing chart which shows the operation example of the conventional light control apparatus when the voltage dividing resistance is enlarged and the discharge time becomes long.

  Hereinafter, an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a diagram illustrating a configuration example of an LED driver circuit according to the present embodiment including a backlight dimming control device according to the present invention.

  In FIG. 1, 1 is a constant current drive IC, 52 is a step-down circuit, 53 is a step-up circuit (corresponding to the drive circuit of the claims), 54 is an LED matrix (a plurality of light emitting diodes and backlights in the claims) Equivalent). The LED matrix 54 is composed of a total of 24 LEDs with 3 in parallel and 8 in series. A resistor R0 is connected to the input side of the step-down circuit 52. On the output side of the booster circuit 53, an output smoothing capacitor C for ripple smoothing and voltage dividing resistors R1 and R2 are provided. The step-down circuit 52, the step-up circuit 53, the LED matrix 54, the resistor R0, the output smoothing capacitor C, and the voltage dividing resistors R1 and R2 other than the constant current driving IC 1 are the same as those shown in FIG.

  The constant current drive IC 1 includes, as its functional configuration, an internal power supply 61, a step-down control unit 62, a step-up control unit 63, an overvoltage detection unit 64, an overcurrent detection unit 65, an abnormality detection / notification unit 66, a current setting unit 67, and a PWM setting. Unit 68 and first to third constant current / dimming control units 19-1 to 19-3. In the functional configuration of the constant current drive IC 1, the first to third constant current / dimming control units 19-1 to 19-3 correspond to the dimming control unit in the claims, and other than this The configuration is the same as that shown in FIG.

  Dimming the LED matrix 54 is performed by the current setting unit 67, the PWM setting unit 68, and the first to third constant current / dimming control units 19-1 to 19-3. The PWM setting unit 68 corresponds to the pulse generation unit of the claims, and is a constant current pulse wave set by the current setting unit 67, and has a duty ratio at a time ratio corresponding to the dimming rate. Are generated and supplied to the first to third constant current / dimming control units 19-1 to 19-3, respectively.

  The first constant current / dimming control unit 19-1 configures the LED matrix 54 based on the constant current pulse wave generated by the PWM setting unit 68 when the dimming backlight is turned on. The light control of the first LED array 54-1 is controlled by intermittently supplying a constant current to the LED array 54-1 according to the duty ratio of the pulse wave.

  The second constant current / dimming control unit 19-2 configures the LED matrix 54 based on the constant current pulse wave generated by the PWM setting unit 68 when the dimming backlight is turned on. The dimming of the second LED array 54-2 is controlled by intermittently supplying a constant current to the LED array 54-2 according to the duty ratio of the pulse wave.

  The third constant current / dimming control unit 19-3 configures the LED matrix 54 based on the constant current pulse wave generated by the PWM setting unit 68 when the dimming backlight is turned on. The light control of the third LED array 54-3 is controlled by intermittently supplying a constant current to the LED array 54-3 according to the duty ratio of the pulse wave.

  When the LED matrix 54 is lit by turning on the dimming of the first to third constant current / dimming control units 19-1 to 19-3 as described above, the boosting control unit 63 causes the boosting driving transistor to be turned on. The booster circuit 53 is operated by driving Tr2. At this time, the boost control unit 63 performs PWM control of ON / OFF of the boost drive transistor Tr2 using a pulse wave with a fixed period generated from an oscillator (not shown), thereby changing the input voltage VIN to a predetermined output voltage VOUT. Boost the pressure.

  Here, it is assumed that the input voltage VIN is 8.0 [V] and the constant current set by the current setting unit 67 is 80 [mA]. The forward voltage VF is 3.0 [V] when a constant current of 80 [mA] is supplied to the LEDs used in the first LED array 54-1, and the second LED array 54-2 is applied. The forward voltage VF is 3.3 [V] when a constant current of 80 [mA] is passed through the LED used, and the LED used for the third LED array 54-3 is 80 [mA]. When the forward voltage VF when a constant current is supplied is 3.6 [V] and the feedback voltage at which the current becomes 80 [mA] is 1.0 [V], the output voltage VOUT (output) of the booster circuit 53 The terminal voltage of the smoothing capacitor C) is 29.8 [V].

  On the other hand, when the dimming is turned off and the LED matrix 54 is turned off, the operation of the booster circuit 53 is stopped by turning off the booster drive transistor Tr2. However, in the present embodiment, the operations of the first to third constant current / dimming control units 19-1 to 19-3 are not stopped. Here, when the forward voltage VF of the diode D2 of the booster circuit 53 is 0.5 [V], the terminal voltage of the output smoothing capacitor C when the boosting operation is stopped is 8.0−0.5 = 7.5 [7.5]. V]. Therefore, the accumulated charge in the output smoothing capacitor C is discharged from 29.8 [V] when dimming is ON to 7.5 [V] when dimming is OFF. As will be described below, this discharge is performed through the LED matrix 54 in which dimming is controlled by the first to third constant current / dimming controllers 19-1 to 19-3.

  That is, the first constant current / dimming control unit 19-1 is configured to output a constant current pulse generated by the PWM setting unit 68 when a backlight extinction command is issued with a dimming rate of zero (dimming OFF). The wave (dimming PWM wave) is converted into a direct current whose amount of current per unit period is the same as that of the dimming PWM wave. Then, the first constant current / dimming control unit 19-1 controls the dimming of the first LED array 54-1 constituting the LED matrix 54 in accordance with the converted direct current, thereby the first constant current / dimming control unit 19-1. The accumulated charge in the output smoothing capacitor C is discharged through the LED array 54-1.

  FIG. 2 is a diagram illustrating a dimming PWM wave generated by the PWM setting unit 68 and a direct current obtained by converting the dimming PWM wave. FIG. 2A shows the dimming PWM wave generated by the PWM setting unit 68 when the dimming rate is 0.5%. The constant current flowing during the period when the dimming PWM wave is ON is 80 [mA]. Further, one pulse period T of the dimming PWM wave is set as the above-described unit period. In this case, the amount of current per unit period T of the dimming PWM wave generated by the PWM setting unit 68 is 80 × 0.5 / 100 × T = 0.4 T [mA].

  As described above, the first constant current / dimming control unit 19-1 applies the dimming PWM wave as shown in FIG. 2A to the current per unit period T as shown in FIG. The direct current is converted so that the amount becomes 0.4 T [mA] which is not different from the dimming PWM wave. Then, the first constant current / dimming control unit 19-1 controls the dimming of the first LED array 54-1 according to the DC current converted as shown in FIG. The accumulated charge in the output smoothing capacitor C is discharged through the LED array 54-1.

  Similarly, the second constant current / dimming control unit 19-2 uses a constant current dimming PWM wave generated by the PWM setting unit 68 as a unit when a dimming OFF backlight turn-off command is issued. The amount of current per period is converted into a direct current that is the same as that of the dimming PWM wave. Then, the second constant current / dimming control unit 19-2 controls the dimming of the second LED row 54-2 constituting the LED matrix 54 in accordance with the converted direct current, thereby the second constant current / dimming control unit 19-2. The accumulated charge in the output smoothing capacitor C is discharged through the LED array 54-2.

  Further, the third constant current / dimming control unit 19-3 outputs a constant current dimming PWM wave generated by the PWM setting unit 68 in a unit period when a backlight extinction command for dimming OFF is issued. The amount of current per contact is converted into a direct current that is the same as that of the dimming PWM wave. Then, the third constant current / dimming control unit 19-3 controls the dimming of the second LED array 54-3 constituting the LED matrix 54 in accordance with the converted direct current, thereby the second constant current / dimming control unit 19-3. The accumulated charge in the output smoothing capacitor C is discharged through the LED array 54-3.

  FIG. 3 is a timing chart showing an operation example of the backlight dimming control device according to the present embodiment. The pulse wave of the boost PWM shown in FIG. 3A is a PWM wave supplied from the boost controller 63 to the boost drive transistor Tr2. That is, when a dimming ON command is issued from the head unit to the constant current drive IC 1, the boost control unit 63 supplies the boost PWM wave shown in FIG. 3A to the boost drive transistor Tr 2.

  In response to this, the output voltage VOUT of the booster circuit 53 (= terminal voltage of the output smoothing capacitor C) gradually increases as shown in FIG. 3C and becomes stable when it reaches 29.8 [V]. . As the output voltage VOUT rises gradually, the voltages VLED1 to VLED3 of the first to third constant current / dimming control units 19-1 to 19-3 are gradually increased as shown in FIGS. To rise.

  On the other hand, when the dimming OFF command is issued from the head unit to the constant current drive IC 1, the boost control unit 63 stops the supply of the boost PWM wave in FIG. Further, the first to third constant current / dimming control units 19-1 to 19-3 respectively convert the constant current dimming PWM wave generated by the PWM setting unit 68 to a DC current of 0.4 [mA]. And controlling the dimming of the first to third LED rows 54-1 to 54-3 according to the converted direct current, through the first to third LED rows 54-1 to 54-3. The accumulated charge in the output smoothing capacitor C is discharged.

  In response to this, the output voltage VOUT of the booster circuit 53 (= terminal voltage of the output smoothing capacitor C) gradually decreases due to the discharge as shown in FIG. 3C, and reaches 7.5 [V]. Stabilize. In the present embodiment, discharge is not performed through the voltage dividing resistors R1 and R2 having a large resistance value, but is performed through the LED matrix 54 having a small resistance value, so that the discharge can be performed in a short time.

  Specifically, in the present embodiment in which the discharge is performed through the LED matrix 54 while the discharge takes 2.1 seconds in the conventional technique in which the discharge is performed through the voltage dividing resistors R1 and R2, about 520 [msec]. The discharge is completed. As described above, since the discharge time is short, the terminal voltage of the output smoothing capacitor C is changed from 29.8 [V] after the dimming OFF command is issued until the next dimming ON command is issued. The discharge required to drop to 7.5 [V] has been completed.

  When the output voltage VOUT gradually decreases, the voltages VLED1 to VLED3 of the first to third constant current / dimming control units 19-1 to 19-3 also gradually decrease as shown in FIGS. I will do it. When the output voltage VOUT reaches 7.5 [V], the voltages VLED1 to VLED3 of the first to third constant current / dimming control units 19-1 to 19-3 are all 0.4 [V]. It has fallen to the following values.

  The pulse wave of the dimming PWM shown in FIG. 3B is a PWM wave output from the PWM setting unit 68. The duty ratio of the dimming PWM wave is determined according to the dimming rate supplied from the head unit to the constant current drive IC 1. FIG. 3B shows a dimming PWM wave when the dimming rate is 0.5%. The first to third constant current / dimming controllers 19-1 to 19-3 flow a constant current of 80 [mA] during a period in which the dimming PWM wave is turned on, and thereby, FIG. The voltages VLED1 to VLED3 of the first to third constant current / dimming control units 19-1 to 19-3 shown in (f) are respectively 5.8 [V], 3.4 [V], and 1.0 [V]. V].

  A certain period of time after the dimming ON command is issued is set to an abnormality detection period in which the cathode side of the first to third LED rows 54-1 to 54-3 is not in contact with the ground. For example, in the case of the timer count type, if the period of the step-up PWM wave is 500 [kHz] and the count value is 32770, the abnormality detection period is 65.54 [msec].

  In the example of FIG. 3G, even when the LED matrix 54 is turned on at a low dimming rate (0.5%) after dimming OFF, within 65.54 [msec] after the dimming ON command is issued. The voltage VLED1 to VLED3 of the first to third constant current / dimming control units 19-1 to 19-3 all exceed 0.4 [V] during the ON period of the dimming PWM wave emitted from . Therefore, the abnormality detection / notification unit 66 determines that there is no abnormality. As a result, the first to third LED rows 54-1 to 54-3 are all lit normally.

  As described above in detail, in this embodiment, when a turn-off command for turning off the light control is issued while the backlight is turned on, the light is turned on according to the light control rate when the backlight is turned on. The constant current dimming PWM wave is converted into a direct current whose amount of current per unit period is the same as the dimming PWM wave. Then, the output smoothing capacitor C is discharged through the LED matrix 54 by controlling the dimming of the LED matrix 54 according to the converted direct current.

  According to this embodiment configured as described above, since the output smoothing capacitor C is discharged through the LED matrix 54 originally provided as a backlight light source, there is no need to separately provide a dedicated short circuit, which increases the cost. Can be suppressed. In addition, since the discharge is not performed through the voltage dividing resistors R1 and R2 as in the prior art, it is possible to avoid the disadvantage that the discharge speed becomes slow due to the value of the voltage dividing resistors R1 and R2 that cannot be reduced. Discharge can be performed in a short time through the LED matrix 54 having a smaller resistance value than the resistances R1, R2.

  Furthermore, according to the present embodiment, the discharge is not performed in such a manner that a large current flows through the LED matrix 54 at a stretch, but compared with the dimming PWM wave generated according to the dimming rate when the backlight is turned on. In accordance with the direct current converted in such a way that the amount of current per unit period does not change, the LED matrix 54 is discharged with the same brightness as when the dimming is ON. Therefore, even when a discharge is performed through the LED matrix 54, the apparent luminance does not change before and after the turn-off command is issued, and it is possible to avoid giving an uncomfortable appearance.

  As described above, according to the present embodiment, the discharge of the accumulated charge of the output smoothing capacitor C when the dimming OFF (extinguishment) command is issued without causing an increase in cost is also strange to the appearance on the liquid crystal screen. It can be performed in a short time without giving. Thereby, when the LED matrix 54 is turned on at a low dimming rate after the dimming is turned off, it is possible to prevent a row of unlit light emitting diodes from occurring.

  In the above embodiment, the step-up / down constant current drive type LED driver circuit has been described as an example, but the dimming control device of the present invention is also applied to the step-up constant current drive type and step-down constant current drive type LED driver circuits. It is possible to implement.

  In addition, each of the above-described embodiments is merely an example of implementation in carrying out the present invention, and the technical scope of the present invention should not be construed in a limited manner. That is, the present invention can be implemented in various forms without departing from the gist or the main features thereof.

1 Constant current drive IC
54 LED matrix 63 Boost control unit 67 Current setting unit 68 PWM setting unit 19-1 to 19-3 Constant current / dimming control unit C Output smoothing capacitor

Claims (3)

A dimming control device for controlling dimming of a backlight in which a plurality of light emitting diodes are arranged in a matrix,
A pulse generator that generates a constant-current pulse wave with a duty ratio set according to the dimming rate;
A dimming control unit that controls dimming of the plurality of light emitting diodes based on a pulse wave of a constant current generated by the pulse generation unit;
When a light extinction command with a dimming rate of zero is issued, the dimming control unit changes the constant current pulse wave generated by the pulse generation unit into a current amount per unit period that is the same as the pulse wave. Output smoothing provided on the output side of the drive circuit for driving the backlight through the plurality of light emitting diodes by controlling the dimming of the plurality of light emitting diodes according to the converted direct current A dimming control device for a backlight, wherein the capacitor is discharged. When the plurality of light emitting diodes are arranged in a matrix having a plurality of columns, the dimming control unit performs dimming according to the DC current having the same magnitude with respect to the light emitting diodes of the plurality of columns. 2. The backlight dimming control device according to claim 1, wherein the output smoothing capacitor is discharged through the plurality of light emitting diodes. A pulse generator that generates a constant-current pulse wave with a duty ratio set according to the dimming rate, and controls dimming of a plurality of light-emitting diodes based on the constant-current pulse wave generated by the pulse generator In a dimming control device comprising a dimming control unit, a dimming control method for controlling dimming of a backlight comprising the plurality of light emitting diodes arranged in a matrix,
When a light extinction command with a dimming rate of zero is issued, the dimming control unit changes the constant current pulse wave generated by the pulse generation unit into a current amount per unit period that is the same as the pulse wave. A first step of converting to no direct current;
An output smoothing capacitor provided on the output side of a drive circuit for driving the backlight through the plurality of light emitting diodes by controlling dimming of the plurality of light emitting diodes according to the direct current converted in the first step And a second step of performing a discharge of the backlight.

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