Patent Grant
9788376
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.
The technology herein relates to an LED backlight driving circuit, a liquid crystal display device, and a method of driving a driving circuit.
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.
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.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of disclosure.
The above and further objects and features will more fully be apparent from the following detailed description with reference to the accompanying drawings.
<Configuration of First Embodiment>
In the liquid crystal display device 1, the LED control circuit 4 and the LED circuit 7 configure the LED backlight driving circuit. The LED backlight 6 includes a backlight unit 13 (which is not illustrated in detail) including a backlight chassis in which a light guide plate converting light emitted from the LED 70 into 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 LED 70 in addition to the LED circuit 7.
<Description of Operation of First Embodiment>
If the dimming determination circuit 10 determines the dimming ratio X to be equal to or less than Y [%] (here, 20%) (S2: YES), the dimming ratio of the current control signal 10a to 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 circuit 11. At the same time, the ON/OFF control signal 10b is set to ON (S6) and transferred to the sequence control circuit 12. If the ON/OFF control signal 10b is 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 circuits 11a, 11b, and 11c are the same circuits, and
In the operation of the constant current circuit 11a, when the current control signal 10a is input, the same voltage level is generated in the current setting resistor Ra. Thus, a ch1 current 7b-1 serving 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 current 7b-1 can arbitrarily be varied based on the value of the current control signal 10a. Further, ON/OFF of the electric current is controlled by switching the connection of the switch 15a at the previous stage of the constant current circuit 11a by the switching signal 12a from the sequence control circuit 12 in the constant current circuit 11a.
As the current control signal 10a is input to the constant current circuits 11a, 11b, and 11c via the switch 15, the current values of the currents 7b-1, 7b-2, and 7b-3 are driven to be in proportion to the parallel number. As the switching signals 12a, 12b, and 12c are input to the switches 15a, 15b, and 15c, 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 signal 9 input from the outside is obtained. The ON period and the OFF period are constant.
Further, if the dimming signal 9 is 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 switch 15 has been described, but ON/OFF may be switched by setting the current value to 0.
In
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 currents 7b-1, 7b-2, and 7b-3 is 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 currents 7b-1, 7b-2, and 7b-3 are sequentially supplied to the respective LED columns. At this time, the values of the currents 7b-1, 7b-2, and 7b-3 are set to be values obtained by multiplying the setting Max current by the dimming ratio and the number of channels (three times in
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 in
<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 in
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.
<Configuration of Second Embodiment>
<Description of Operation of Second Embodiment>
Similarly to the first embodiment,
<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 in
Further, in the second embodiment, the sequence driving may be performed by switching of the switch 16a or the switch 16c instead of the switch 16b. Further, the sequence driving may be performed by switching of two or all of the switches 16a, 16b, and 16c. Furthermore, switching in the sequence driving may be controlled using the sequence control circuit 12. 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 circuits 11a, 11b, and 11c to the LED circuit 7.
<Configuration of Third Embodiment>
An LED backlight driving circuit of a third embodiment has a similar configuration to that of the second embodiment (
<Description of Operation of Third Embodiment>
At the time of the constant current driving, the switch 17a is connected with the ch1 current 7b-1, and the switch 17c is connected with the ch3 current 7b-3, and at the time of the sequence driving, an operation is performed such that the switches 17a and 17c are connected with the ch2 current 7b-2.
Thus, at the time of the sequence driving, the outputs of the constant current circuits 11a, 11b, and 11c overlap, the current values are added, and driving is performed with an LED current of three times.
The dimming signal 9 is input to the dimming determination circuit 10 (S10). In other words, the dimming determination circuit 10 acquires the dimming signal 9. The dimming determination circuit 10 calculates the dimming ratio X % based on the acquired dimming signal 9 (S11), and determines whether or not the dimming ratio X is equal to or less than Y [%] (here, 20%) (S12). If the dimming determination circuit 10 determines the dimming ratio X to be equal to or less than Y (S12: YES), the ON/OFF control signal 10b is set to ON (S13) and transferred to the sequence control circuit 12. As a result, the sequence driving is performed. If the dimming determination circuit 10 determines the dimming ratio X to be neither equal to nor less than Y (S12: NO), the ON/OFF control signal 10b is 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>
The switch 17a may function as a switch that performs switching between the ch1 current 7b-1 and the ch3 current 7b-3, and the switch 17b that switches the connection between the constant current circuits 11b and the ch2 current 7b-2, the ch3 current 7b-3 may be installed instead of the switch 17c. At this time, the sequence driving can be performed by performing an operation such that both the switch 17a and the switch 17b are connected to the ch3 current 7b-3. Thus, the values of the constant current circuits 11a, 11b, and 11c are added, and driving can be performed with the LED current of three times.
The switch 17c may function as a switch that performs switching between the ch1 current 7b-1 and the ch3 current 7b-3, and the switch 17b that switches the connection between the constant current circuits 11b and the ch1 current 7b-1, the ch2 current 7b-2 may be installed instead of the switch 17a. At this time, the sequence driving can be performed by performing an operation such that both the switch 17b and the switch 17c are connected to the ch1 current 7b-1. Thus, the values of the constant current circuits 11a, 11b, and 11c are added, and driving can be performed with the LED current of three times.
In addition, the switch 16 may have any configuration as long as the driving current in which the outputs of all the constant current circuits 11a, 11b, and 11c overlap is sequentially supplied to the LED circuit 7.
In the first to third embodiments, the LED control circuit 4 includes the three constant current circuits 11a, 11b, and 11c, 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 groups 7a-1, 7a-2, . . . , and 7a-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.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2015-116803 | Jun 2015 | JP | national |
| Number | Name | Date | Kind |
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| 20100219764 | Yamasaki et al. | Sep 2010 | A1 |
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| 20130250215 | Sasaki | Sep 2013 | A1 |
| 20150216009 | Lee | Jul 2015 | A1 |
| Number | Date | Country |
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| 2008-047494 | Feb 2008 | JP |
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| 5030623 | Sep 2012 | JP |
| Number | Date | Country | |
|---|---|---|---|
| 20160366740 A1 | Dec 2016 | US |
