LED backlight driving circuit, liquid crystal display device, and method of driving a driving circuit

An example LED backlight driving circuit includes: an LED circuit including a plurality of LED columns that are connected in parallel, each of LED column including one or more LEDs that are connected in series; an LED control circuit connected to constant current circuits corresponding to a parallel number of the LED columns, the LED control circuit including a circuit that controls ON/OFF of a driving current of the LED and a dimming determination circuit that outputs a control signal capable of arbitrarily setting the driving current according to a dimming signal. The LED control circuit performs control based on first driving in which dimming is performed by varying a current value of the driving current of the LED and second driving in which the ON/OFF of the driving current is controlled in addition to the varying of the current value.

CROSS-REFERENCE TO RELATED APPLICATIONS

This non-provisional application claims priority under 35 U.S.C. §119(a) on Patent Application No. 2015-116803 filed in Japan on Jun. 9, 2015, the entire contents of which are hereby incorporated by reference.

FIELD

The technology herein relates to an LED backlight driving circuit, a liquid crystal display device, and a method of driving a driving circuit.

BACKGROUND

In a liquid crystal display device using a LED backlight, methods of controlling brightness of an LED are roughly divided into two types of methods, that is, a pulse current dimming scheme and a constant current dimming scheme. In the pulse current dimming scheme (hereinafter, “PWM dimming”), visual brightness is controlled by changing a percentage of an ON period and an OFF period of an electric current, that is, a duty ratio while maintaining a current value of an electric current flowing through an LED to be constant. In the constant current scheme (hereinafter, “constant current dimming”), visual brightness is controlled by changing a current value of an electric current flowing an LED.

In the PWM dimming, switching control of the ON period is consequential, and thus an accurate adjustment is possible, and an LED current is constant during the ON period. Further, in the PWM dimming, since there is no change in characteristics of the LED at the time of lighting, it is easy to control chromaticity or the like, and the PWM dimming is widely used as a current dimming scheme. However, in the PWM dimming, a dimming ratio is restricted according to a rising/falling time of a driving current, and thus the dimming ratio may not be sufficiently obtained. As a solution to this problem, there is a technique of increasing a dimming range by simultaneously controlling a pulse and an electric current such that a driving current value is decreased while decreasing the duty ratio of the PWM.

In addition, the PWM dimming has a problem in that flickering is seen by some people. Further, in the PWM dimming, as the current value of the electric current flowing through the LED increases (luminance increases), a current change at the time of ON/OFF increases, and thus ripples are likely to overlap at a power source circuit side. Thus, in the PWM dimming, there is a problem in that a ringing sound is likely to be generated in a circuit member such as a capacitor or a coil. For this reason, there are recently cases where, in order to prevent flickering of the LED or an ON/OFF change of the LED current, a constant current dimming scheme of increasing only the driving current without performing pulse width modulation of the driving current and controlling luminance of the LED is used. In the constant current dimming scheme, the voltage/current ripples, the ringing sound, and visibility are improved, but since an electric current is controlled in an analogue manner, an electric current error has directly influence on luminance characteristics of the LED. Thus, it is harder to perform control at a low luminance side (a low current value) than in the PWM dimming, and there is a problem in that luminance is likely to be uneven.

In the case of dimming an LED backlight with a plurality of parallel LED circuits, it is necessary to simultaneously perform dimming of a plurality of current sources, that is, a plurality of constant current circuits. If the dimming is performed according to the constant current dimming scheme, due to individual differences of the constant current circuits, there is a difference in the driving current value, and thus the respective LED columns differ in luminance. Thus, the constant current dimming scheme has a problem in that the in-plane luminance of the entire LED backlight becomes uneven.

For example, when the LED backlight is driven using two constant current circuits A and B driving 100 mA at dimming of 100%, due to an error between the circuits, if the driving current of 101 mA and the driving current of 99 mA flow through the circuits A and B, respectively, at dimming of 100%, an error between the circuits is 2 mA, and a luminance difference with respect to the driving current is about 2%. However, when 11 mA and 9 mA flows through the circuits A and B, respectively, at dimming of 10%, the luminance difference with respect to the driving current is close to 20% even there is the same error, that is, 2 mA. In other words, when the difference (error) in the current between the constant current circuits is almost constant regardless of the dimming ratio, as the driving current value is decreased at the time of low dimming (at the time of a low current), the ratio of the current difference of the driving current value between the constant current circuits with respect to the driving current value is increased, and the current difference is likely to be particularly remarkably seen as a luminance difference, leading to luminance unevenness. Since an error of a current value of a neighboring LED is often viewed as unevenness even at 10%, depending on an in-plane luminance design of the entire backlight, it is difficult to use low dimming of less than 20% in the constant current circuit having the error of 2 mA.

As the solutions to this problem, there are a technique of averaging a difference in luminance by alternately switching a current source and an LED through a switch and performing driving in a time division manner and a technique of removing a difference between current sources by driving a plurality of LEDs in a time division manner with respect to one current source.

In the former, it is solved by alternately switching a current source and an LED when a parallel number is 2, but as the parallel number increases, control and a combination of alternate driving become more complicated, and thus a circuit size is likely to increase significantly. In the latter, the ON period of the electric current for each of the LEDs that are connected in parallel is necessarily the reciprocal of the parallel number, and a maximum luminance of the backlight is commonly equal to or less than half of the luminance when the LED is constantly turned on, and thus it is difficult to use luminous efficiency sufficiently. Further, the known example is under the assumption of a method of performing dimming according to a time interval or a pulse width and thus deals with neither a problem nor a solution at the time of low dimming in a dimming scheme based on an increase in an electric current.

As a solution to a variation in brightness at the time of low luminance dimming in the constant current dimming, there is a technique of causing an electric current of a pulse form to flow through an LED at the time of low luminance and performing dimming by changing an average value (that is, a duty ratio or a frequency) of pulse waveforms. This known example is an effective method in securing linearity and reproducibility of dimming-luminance characteristics of an LED. However, the method of performing dimming based on the duty ratio or the frequency is the same scheme as the conventional PWM dimming, and still has the problem such as a noise, a ringing sound, flicking, or ripples. Further, there is a problem in that since the LED current value (peak value) is constant when dimming is performed based on a pulse average value, the differences in the electric current at the time of low luminance is not improved when a plurality of parallel LEDs are driven by a plurality of constant current circuits.

SUMMARY

According to one aspect of an embodiment of the present disclosure, an electric current of an LED is controlled such that at the time of low luminance (small current) driving influencing a difference in in-plane luminance, ON/OFF of a driving current of an LED is controlled by performing switching control of each constant current circuit, and switching to a method of performing sequence driving of an LED is performed.

An current ON period (a pulse width) of systems (parallel) at the time of sequence driving is set to the reciprocal of the number of systems (the parallel number), and the current value is set to a current value proportional to the number of systems (the parallel number) necessary for desired luminance. Further, control is performed such that an OFF period (an extinction period) is not provided in terms of the entire LED circuit.

The above and further objects and features will more fully be apparent from the following detailed description with reference to the accompanying drawings.

DETAILED DESCRIPTION OF NON-LIMITING EXAMPLE EMBODIMENTS

First Embodiment

<Configuration of First Embodiment>

FIG. 1is a diagram illustrating a configuration of an example of LED backlight driving circuit according to a first embodiment. LEDs70of an LED circuit7are dimmed based on a voltage and an electric current generated by an LED control circuit4. The LED control circuit4includes a constant current circuit11, a dimming determination circuit10, a sequence control circuit12, and an anode voltage generating circuit14. As the anode voltage generating circuit14applies a voltage to an anode side of the LEDs, and the constant current circuit11causes an electric current to flow from a cathode side of each LED column, the LEDs70are turned on. Further, luminance of each LED column is controlled by varying the current value. The dimming determination circuit10decides whether or not sequence control is performed, and decides an LED driving current value. When the sequence control is performed, dimming of the entire LED circuit7is performed such that the sequence control circuit12performs ON/OFF control of each constant current circuit, and performs ON/OFF control of an LED driving current.

FIG. 2is a diagram illustrating an overall configuration of a liquid crystal display device according to the first embodiment. A liquid crystal display device1includes an LCD panel5, an LED backlight6, and a control circuit2. The LED circuit7is mounted in the LED backlight6. The control circuit2includes an LCD control circuit3and the LED control circuit4. Based on a display signal8, the LCD control circuit3transfers a signal, a voltage, or the like to the LCD panel5and control a display of the LCD. Based on a dimming signal9, the LED control circuit4applies, for example, a driving signal and a voltage for dimming the LED backlight6to the LED circuit7. LCD control circuit3, LED control circuit, and LED circuit7perform operations based on the program stored in a recording medium30such as a CD (Compact Disc)-ROM, a DVD (Digital Versatile Disc)-ROM, a BD (Blu-ray® Disc), a hard disc drive, or a solid state drive which corresponds to a portable medium as a computer readable medium. For example, the LED control circuit4performs an operation which will be described later based on a program stored in a storage medium30and executing the program through a CPU.

In the liquid crystal display device1, the LED control circuit4and the LED circuit7configure the LED backlight driving circuit. The LED backlight6includes a backlight unit13(which is not illustrated in detail) including a backlight chassis in which a light guide plate converting light emitted from the LED70into a surface light source is accommodated and a reflecting sheet, a prism sheet, and the like that are arranged on the back surface and the front surface of the light guide plate and used for effectively using the light emitted from the LED70in addition to the LED circuit7.

FIG. 3is a diagram illustrating a connection relation between the internal components of the LED control circuit4and the LED circuit7. The LED control circuit4includes the dimming determination circuit10, the constant current circuit11, the sequence control circuit12, and the anode voltage generating circuit14. The LED circuit7is configured such that two or more columns of LED groups7a-1to7a-nin which one or more LEDs70are connected in series are connected in parallel. As the anode voltage generating circuit14applies the voltage to the anode side of the LED group of the LED circuit7, the cathode sides of the LEDs70connected in parallel are connected to the constant current circuit11, and the constant current circuit11causes the electric current to flow, the LED70is turned on. The dimming determination circuit10and the sequence control circuit12generate a control signal for driving the constant current circuit11based on a dimming ratio input from the dimming signal9. Here, the dimming ratio indicates the duty ratio of the dimming signal9.

FIG. 4is a diagram illustrating an arrangement of the LEDs70in the LED backlight6. The LED circuit7is arranged in a line on one end or both ends of the LED backlight6, and the LED groups7a-1to7a-nare arranged together in units of blocks in order from the end. As the LED groups are individually driven, the entire plane of the LED backlight6emits light.

<Description of Operation of First Embodiment>

FIG. 1illustrates a configuration when the LED circuit7has a configuration in which a parallel number is 3. The constant current circuit11includes constant current circuits11a,11b, and11cof three channels according to the parallel number. The dimming determination circuit10decides whether or not the sequence control is performed based on the dimming signal9given from the outside. The dimming determination circuit10generates a current control signal10a, and the current control signal10ais input to the respective constant current circuit11. The dimming determination circuit10further generates an ON/OFF control signal10band a synchronous signal10c, and inputs the ON/OFF control signal10band the synchronous signal10cto the sequence control circuit12. The sequence control circuit12generates switching signals12a,12b, and12cfor the respective LED columns based on the ON/OFF control signal10band the synchronous signal10c. The switching signals12a,12b, and12care input to the constant current circuits11a,11b, and11cof the respective LED columns of the constant current circuit11.

FIG. 5illustrates an operation flow of the dimming determination circuit10, andFIG. 6illustrates the details of the constant current circuit11a. The dimming determination circuit10determines whether or not information Y of a dimming ratio X of the dimming signal9applied from the outside is a certain constant dimming ratio. A dimming ratio serving as a determination threshold value is a constant value held in the dimming determination circuit10. Here, the threshold value is assumed to be 20% as an example. As illustrated inFIG. 5, the dimming signal9is input to the dimming determination circuit10(S1). In other words, the dimming determination circuit10acquires the dimming signal9. The dimming determination circuit10calculates the dimming ratio X % based on the acquired dimming signal9. Thereafter, the dimming determination circuit10determines whether or not the dimming ratio X is equal to or less than Y [%] (here, 20%) (S2). If the dimming determination circuit10determines the dimming ratio X to be neither equal to nor less than Y (S2: NO), the current value of the same percentage as the input dimming ratio X is set as the dimming ratio of the current control signal10a(S3). Similarly, the ON/OFF control signal10bis set to OFF (S4), the sequence driving is not performed, and the LED driving current is constantly set to ON. In other words, the driving current of the current value set based on the dimming ratio is supplied to each LED column. It is referred to a “first driving scheme.”

If the dimming determination circuit10determines the dimming ratio X to be equal to or less than Y [%] (here, 20%) (S2: YES), the dimming ratio of the current control signal10ato be transferred is set to be in proportion to the number of channels (here, three times since the number of channels is 3) (S5) and transferred to the constant current circuit11. At the same time, the ON/OFF control signal10bis set to ON (S6) and transferred to the sequence control circuit12. If the ON/OFF control signal10bis ON, the current value set based on the dimming ratio X is controlled to be the current value based on the parallel number of the LED columns and sequentially supplied to the respective LED columns. It is referred to as a “second driving scheme.” In other words, switching between the first driving scheme and the second driving scheme is controlled according to the dimming ratio.

The constant current circuits11a,11b, and11care the same circuits, andFIG. 6representatively illustrates a relation between the constant current circuit11aand a switch15a. The constant current circuit11aincludes an FET110, an operational amplifier111, and a current setting resistor Ra. A drain of the FET110is connected to the cathode side to the LED70, and a source thereof is connected to the current setting resistor Ra. An output of the operational amplifier111is connected to a gate of the FET110. A non-inverting input terminal of the operational amplifier111is connected to the switch15a, and an inverting input terminal is connected to a connection point of the source of the FET110and the current setting resistor Ra. The switch15ahas an ON/OFF function of the current control signal10aon the non-inverting input terminal of the operational amplifier111of the constant current circuit11a. When the current control signal10ais ON, the dimming determination circuit10and the non-inverting input terminal are connected, and when the current control signal10ais OFF, the non-inverting input terminal is grounded. The constant current circuit11band a switch15b, as well as the constant current circuit11cand a switch15chave a similar relation.

In the operation of the constant current circuit11a, when the current control signal10ais input, the same voltage level is generated in the current setting resistor Ra. Thus, a ch1 current7b-1serving as the LED driving current is expressed by the following equation:
7b−1[A]=10a[V]/Ra[Ω]

Here, since a resistance value of Ra is constant, the current7b-1can arbitrarily be varied based on the value of the current control signal10a. Further, ON/OFF of the electric current is controlled by switching the connection of the switch15aat the previous stage of the constant current circuit11aby the switching signal12afrom the sequence control circuit12in the constant current circuit11a.

FIGS. 7 and 8illustrate a timing chart of an overall operation of the LED control circuit4. First, when dimming is performed in a state where the dimming signal9is less than 100%, the synchronous signal10chaving the ON period of the reciprocal of the parallel number is generated based on the cycle of the dimming signal9. Further, the ON/OFF control signal10bis generated based on the determination result according to the dimming ratio. When the dimming signal9becomes the determination threshold value (20%) of the determination circuit10, the ON/OFF control signal10bis set to ON. The sequence control circuit12generates the switching signals12a,12b, and12cfor performing the sequence driving by the constant current circuit11based on the two signals. The dimming determination circuit10generates the current control signal10afor driving with the current value proportional to the parallel number, and inputs the current control signal10ato the constant current circuit11.

As the current control signal10ais input to the constant current circuits11a,11b, and11cvia the switch15, the current values of the currents7b-1,7b-2, and7b-3are driven to be in proportion to the parallel number. As the switching signals12a,12b, and12care input to the switches15a,15b, and15c, the ON period is adjusted and driven to be the reciprocal of the parallel number. The in-plane luminance is controlled such that luminance corresponding to the same dimming ratio as the dimming signal9input from the outside is obtained. The ON period and the OFF period are constant.

Further, if the dimming signal9is an analog voltage or the like rather than the pulse signal, a circuit that generates a reference signal according to a pulse based on an input signal may be set at a previous stage. Here, the example in which ON/OFF of the electric current is switched using the switch15has been described, but ON/OFF may be switched by setting the current value to 0.

InFIG. 1, the switching signals are generated in the order of12a,12b, and12c, and thus driving is performed sequentially from the end of the display if the relation withFIG. 4is considered, but the order of ON/OFF is not restricted as long as control is performed such that the in-plane LEDs can uniformly be driven at a time average.

FIG. 9illustrates a relation of a driving scheme, a dimming LED current, and luminance viewed at a time axis. InFIG. 9, a horizontal axis indicates a time, and a vertical axis indicates an LED current value and a dimming ratio. InFIG. 9, with the passage of time, the dimming ratio changes three times, and the dimming ratio decreases each time the dimming ratio changes. First, the dimming ratio is driven to be 100%, and the dimming ratio is changed to Y % by the two changes. Thereafter, the dimming ratio is changed to be smaller than Y % by the third change. Further, first, driving is performed at a constant current, and after the dimming ratio is changed twice, that is, when the dimming ratio is Y %, the sequence driving starts. At this time, as illustrated inFIG. 9, there is an error between channels in the values of the currents7b-1,7b-2, and7b-3.

The LED backlight driving circuit changes the current value according to the change in the dimming ratio. When the constant current driving circuit is used, if the dimming ratio is 100%, each of the currents7b-1,7b-2, and7b-3is a setting Max current value. Thereafter, as the dimming ratio decreases, the current value decreases.

If the dimming ratio is changed to be Y % or less by the two changes, and the sequence driving starts, the currents7b-1,7b-2, and7b-3are sequentially supplied to the respective LED columns. At this time, the values of the currents7b-1,7b-2, and7b-3are set to be values obtained by multiplying the setting Max current by the dimming ratio and the number of channels (three times inFIG. 9).

As described above, in the region where the dimming ratio is Y % or less, dimming is performed with the current value proportional to the number of channels (here, three times) as illustrated inFIG. 9, and the ON period is sequentially driven to be the reciprocal of the number of channels (here, 1/3 cycle), and thus desired dimming can be performed.

<Description of Effects of First Embodiment>

The relation between the dimming ratio and the LED driving current according to the configuration of the first embodiment is illustrated inFIG. 10, and the relation between the dimming ratio and the error of the LED driving current is illustrated inFIG. 11. A solid line indicates the case where the sequence driving of the first embodiment is performed when the dimming ratio is Y % or less, and a dotted line indicates the case where driving is performed based on only the constant current dimming without performing the sequence driving. If the sequence driving of the first embodiment is performed at the time of low dimming of Y % or less, a current error with respect to an ideal value is relatively smaller than if not performed. In other words, the error between the constant current circuits decreases, and thus the in-plane luminance difference can be suppressed.

Further, in the first embodiment, even when the parallel number of the LED circuits is large, it is enough by only sequential driving, and thus control is simple. Further, since sequential driving is performed at only the low dimming side, even if the parallel number of the LED circuits is increased, the luminance does not decrease, and it is possible to suppress the in-plane luminance difference caused by the current difference while maintaining the existing luminance design.

Further, the power change of the entire circuit viewed at the time axis is not repetition of ON/OFF of electric power as in the PWM but consistently constant, and thus a noise or a ringing sound is unlikely to occur. Further, the backlight does not repeat lighting and extinction as in the PWM, and some LEDs are constantly in the lighting state, and thus flickering or ripples are unlikely to occur.

Second Embodiment

<Configuration of Second Embodiment>

FIG. 12is a diagram illustrating a configuration of an LED backlight driving circuit according to a second embodiment. A different point withFIG. 1lies in that the sequence control circuit is not installed, and a switch16is installed at a stage behind the constant current circuit11instead of the switch15at the stage ahead of the constant current circuit11. Further, the ON/OFF control signal10band the synchronous signal10cfrom the dimming determination circuit10are input to the switch16. The switch16installed behind the constant current circuit11operates as a switch of switching a connection between the constant current circuits11a-11cand the LED circuit7.

FIG. 13illustrates the details of the constant current circuit11. A constant current circuit11aincludes an FET110and an operational amplifier111, similarly to the first embodiment, and the constant current circuits11band11chave the same structure as the constant current circuit11a. The FET and the operational amplifier of each of the constant current circuits11band11chave the same configuration as the FET110and the operational amplifier111, and reference numerals thereof are omitted. The operations of the constant current circuits11a,11b, and11care similar to the first embodiment, and thus a description thereof is omitted. The subsequent stages of the constant current circuits11a,11b, and11care connected with7b-1,7b-2, and7b-3serving as ch1, ch2, and ch3 currents via the switch16, and outputs of the constant current circuits11a,11b, and11care connected with switches16a,16b, and16c, respectively. The ON/OFF control signal10band the synchronous signal10cfrom the dimming determination circuit10are input to the switch16. In the switch16, the switch16ahas an ON/OFF function of a connection between the constant current circuit11aand the ch1 current7b-1, and the switch16bhas a switching function of the constant current circuit11band the ch1, ch2, and ch3 currents7b-1,7b-2, and7b-3. The switch16chas an ON/OFF function of a connection between the constant current circuit11cand the ch3 current7b-3.

<Description of Operation of Second Embodiment>

FIGS. 14 and 15illustrate an overall operation timing of the LED circuit. A timing at which the sequence driving starts is similar to the first embodiment, but the paths of the switches16aand16cof the switch16are turned off (blocked) by the ON/OFF control signal10b. At the same time, sequential switching of the switch16bstarts. The constant current circuit11bis sequentially connected with the LED column of the respective channels of the LED circuit7, and thus the sequence driving of the electric current is performed using one constant current circuit. During the constant current operation, the switch16bdoes not perform the sequence driving and is in the state constantly connected with the ch2 current (7b-2).

Similarly to the first embodiment,FIG. 16illustrates a relation between the driving scheme and the dimming LED current using a horizontal axis as a time. Since in the region where the dimming ratio is Y % or less, dimming is performed with the current value proportional to the number of channels (here, three times) as illustrated inFIG. 16and only the constant current circuit11bis used, there is not error in the driving current value, and driving is performed with the same electric current sequentially in a cycle according to the reciprocal of the number of channels (here, 1/3 cycle).

<Description of Effects of Second Embodiment>

In the second embodiment, since driving is performed by one constant current circuit, an error at the time of small current (low dimming) is suppressed, and then it is possible to remove the difference in the electric current between the constant current circuits. The relation between the dimming ratio and the LED driving current according to the configuration of the second embodiment is illustrated inFIG. 17, and the relation between the dimming ratio and the error of the LED driving current is illustrated inFIG. 18. A solid line indicates the case where the sequence driving is performed if the dimming ratio is Y % or less, and a dotted line indicates the case where driving is performed based on only the constant current dimming without performing the sequence driving. As one constant current circuit is used at the time of low dimming of Y % or less, there is no error between the constant current circuits, and it is possible to prevent the occurrence of the in-plane luminance difference. Further, as the sequence driving is performed, similarly to the first embodiment, it is possible to suppress an increase in the error at the time of low dimming.

Further, in the second embodiment, the sequence driving may be performed by switching of the switch16aor the switch16cinstead of the switch16b. Further, the sequence driving may be performed by switching of two or all of the switches16a,16b, and16c. Furthermore, switching in the sequence driving may be controlled using the sequence control circuit12. In addition, the sequence driving may be performed by any configuration as long as the driving current proportional to the parallel number of the LED columns can be sequentially supplied from one of the constant current circuits11a,11b, and11cto the LED circuit7.

Third Embodiment

<Configuration of Third Embodiment>

An LED backlight driving circuit of a third embodiment has a similar configuration to that of the second embodiment (FIG. 12), but an internal configuration of the switch16of the LED control circuit4is different.FIG. 19is a diagram illustrating a relation between the internal configuration of the switch16and the constant current circuit11according to the third embodiment. Switches17aand17care installed instead of the switches16aand16cofFIG. 13. The switch17ais a switch that performing switching between the ch1 current7b-1and the ch2 current7b-2. The switch17cis a switch that performing switching between the ch2 current7b-2and the ch3 current7b-3. An FET and an operational amplifier of each of the constant current circuits11band11chave the same configuration as the FET110and the operational amplifier111, and reference numerals thereof are omitted.

<Description of Operation of Third Embodiment>

At the time of the constant current driving, the switch17ais connected with the ch1 current7b-1, and the switch17cis connected with the ch3 current7b-3, and at the time of the sequence driving, an operation is performed such that the switches17aand17care connected with the ch2 current7b-2.

Thus, at the time of the sequence driving, the outputs of the constant current circuits11a,11b, and11coverlap, the current values are added, and driving is performed with an LED current of three times.FIG. 20illustrates an operation of the dimming determination circuit10of the third embodiment. Unlike the flowchart ofFIG. 5of the first embodiment, the current control signal10adoes not vary according to the dimming ratio, and only determination of the operation of the ON/OFF control signal10bis performed.

The dimming signal9is input to the dimming determination circuit10(S10). In other words, the dimming determination circuit10acquires the dimming signal9. The dimming determination circuit10calculates the dimming ratio X % based on the acquired dimming signal9(S11), and determines whether or not the dimming ratio X is equal to or less than Y [%] (here, 20%) (S12). If the dimming determination circuit10determines the dimming ratio X to be equal to or less than Y (S12: YES), the ON/OFF control signal10bis set to ON (S13) and transferred to the sequence control circuit12. As a result, the sequence driving is performed. If the dimming determination circuit10determines the dimming ratio X to be neither equal to nor less than Y (S12: NO), the ON/OFF control signal10bis set to OFF (S14), the LED driving current is constantly set to ON, and the constant current driving is performed.

<Description of Effects of Third Embodiment>

FIGS. 21 and 22illustrate an overall operation timing of the LED circuit. The operation timing ofFIGS. 21 and 22differs from the timing chart ofFIGS. 14 and 15of the second embodiment in the operation of the current control signal10a, and it becomes a constant value regardless of the driving scheme, and thus similar effects to the second embodiment are obtained.

The switch17amay function as a switch that performs switching between the ch1 current7b-1and the ch3 current7b-3, and the switch17bthat switches the connection between the constant current circuits11band the ch2 current7b-2, the ch3 current7b-3may be installed instead of the switch17c. At this time, the sequence driving can be performed by performing an operation such that both the switch17aand the switch17bare connected to the ch3 current7b-3. Thus, the values of the constant current circuits11a,11b, and11care added, and driving can be performed with the LED current of three times.

The switch17cmay function as a switch that performs switching between the ch1 current7b-1and the ch3 current7b-3, and the switch17bthat switches the connection between the constant current circuits11band the ch1 current7b-1, the ch2 current7b-2may be installed instead of the switch17a. At this time, the sequence driving can be performed by performing an operation such that both the switch17band the switch17care connected to the ch1 current7b-1. Thus, the values of the constant current circuits11a,11b, and11care added, and driving can be performed with the LED current of three times.

In addition, the switch16may have any configuration as long as the driving current in which the outputs of all the constant current circuits11a,11b, and11coverlap is sequentially supplied to the LED circuit7.

In the first to third embodiments, the LED control circuit4includes the three constant current circuits11a,11b, and11c, but the number of constant current circuits is not limited thereto, and two or four or more constant current circuits may be arranged according to the number of the LED groups7a-1,7a-2, . . . , and7a-n.

As described above, according to the embodiment of the present disclosure, it is possible to control the in-plane luminance unevenness at the time of low dimming while utilizing the luminous efficiency of the existing backlight without increasing the number of LEDs or the maximum value of the driving current.

It should be understood that the embodiments disclosed herein are illustrative and non-restrictive in every respect. Since the scope of the present invention is defined by the appended claims rather than by the description preceding them, and all changes that fall within metes and bounds of the claims, or equivalence of such metes and bounds thereof are therefore intended to be embraced by the claims. That is, embodiments obtained by combining technique appropriately modified within the scope defined by the appended claims are also included in the technical scope of the present invention.

It is noted that, as used herein and in the appended claims, the singular forms “a”, “an”, “the” include plural referents unless the context clearly dictates otherwise.