JP2008186668A - Led drive circuit and video image display device using it - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an LED drive circuit enabling compact and inexpensive configuration by reducing the number of components while maintaining high drive efficiency. <P>SOLUTION: A switching regulator circuit 5a is constituted of an input capacitor 51, an inductor 52, a controller IC 7, a switching FET 53, a diode 54, an output capacitor 55, cutting FETs 56a and 56b, and a sensing resistor 57. A current flowing through an LED string 26a or 26b is converted into a voltage value by the voltage drop of the sensing resistor 57, and fed back to the controller IC 7. A gate drive circuit 72 of the controller IC 7 is controlled based on feeding back results so that a constant current flows through the LED string by stably actuating an inner circuit. Moreover, by alternatively turning on and off the cutting FETs 56a, 56b in place of the controller IC 7, two LED strings are driven in a time-sharing manner by a driving signal forming circuit 6. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a driving circuit for causing a light emitting diode (hereinafter also simply referred to as an LED (Light Emitting Diode)) to emit light, and an image display device using light from the LED as backlight light of a liquid crystal display element. is there.

  In recent years, video display devices using transmissive liquid crystal display elements have become widespread in applications such as PC (Personal Computer) monitors and TV video display. The liquid crystal display element does not emit light itself, but a surface light source device called a backlight device is installed on the back side of the image display surface, and the image is displayed by transmitting light emitted from the backlight device.

  As light sources used in the backlight device, CCFL (Cold Cathode Fluorescent Lamp) (Cold Cathode Fluorescent Lamp), LED (Light Emitting Diode), organic EL (electroluminescence) element, and the like are known. LEDs have been mainly used in small backlight devices so far, but their response speed is faster than CCFL, they are Hg-free without using Hg (mercury), and the color reproduction range is wide. Therefore, as the luminous efficiency is improved, it has been used for a backlight device for a large-sized video display device.

  In a backlight device using LEDs, it is necessary to drive a large number of LEDs with current, and a constant current sink method and a switching regulator method are known as driving methods.

  Among them, a switching regulator system is known in which an LED is driven using a step-up switching regulator circuit (DC / DC converter) as disclosed in Patent Document 1.

FIG. 7 is a diagram for explaining the basic operation principle of a conventional LED drive circuit.
Referring to FIG. 7A, a conventional LED drive circuit 8a includes an input capacitor 81, a controller IC 82a, an inductor 83, a switching FET 84, a diode 85, an output capacitor 86, and a sense resistor 87 having a resistance value R _SEN . A step-up switching regulator circuit 8a is formed.

The operation will be described. The switching FET 84 is switched at a high speed, and the energy stored in the inductor 83 during that time is released to the output side to boost and output the voltage _VIN input from the outside, and a plurality of LEDs connected in series as a load ( In the following, an electric current is supplied to the LED string.

The ratio of the output voltage V _OUT to the input voltage V _IN , that is, the step-up ratio is determined by the ratio (duty) of the ON time and the OFF time of the switching FET 84. The current flowing through the LED string is converted into a voltage value by the voltage drop of the sense resistor 87 and fed back to the controller IC 82a.

In the controller IC 82a, the comparator 821 compares the sense voltage with the reference voltage V _REF and changes the setting of the gate drive circuit 822 based on the comparison result. The gate drive circuit 822 changes the output voltage V _OUT by changing the on time and the off time of the switching signal that drives the gate of the switching FET 84 based on the setting. By performing such feedback, it is controlled so that a constant current always flows through the LED string 88.

Since there is inter-individual variation and temperature change in V _F (forward voltage) of each LED, the voltage of the LED string at the time of constant current driving is not constant. However, in the switching regulator type LED driving circuit, in order to be able to maintain the output voltage V _OUT by performing feedback to the current detection and the output voltage V _OUT of the sense resistor always optimum value, higher drive than other driving methods such as a constant current sink system as It has the advantage that efficiency can be realized.

  On the other hand, in addition to the control operation of a normal switching regulator, there is also a request for inputting a PWM signal from the outside and PWM driving the load accordingly.

  However, in a step-up switching regulator circuit using a normal controller IC, the input side and the output side are not insulated as shown in FIG. There is no way to turn it off. Therefore, PWM drive cannot be executed.

  On the other hand, a technique is known that enables PWM drive in a step-up switching regulator circuit by improving the controller IC and adding components (Patent Documents 2 and 3).

FIG. 7B is a diagram for explaining a circuit example obtained by modifying a conventional LED drive circuit.
Referring to FIG. 7B, in the conventional LED drive circuit 8b, compared to the LED drive circuit described in FIG. 7A, an FET switch called a disconnect FET 89 is connected in series with the load, and the controller The difference is that the PWM control means 823 is provided in the IC 82b.

  When a PWM drive signal is input from the outside to the controller IC 82b, the PWM control unit 823 drives the gate of the cutting FET 89 in accordance with the waveform of the PWM drive signal.

  When the gate of the cutting FET 89 is turned off, the current flowing through the LED string 88 as a load is forcibly cut off.

  At this time, although the voltage value detected by the sense resistor 87 becomes zero, the PWM control means 823 inside the controller IC 82b temporarily sets the voltage level of the comparator 821 to prevent the feedback system from becoming unstable temporarily. Hold it.

As a result, the feedback system is stabilized, and a stable switching operation can be realized even if PWM driving is performed on the load.
JP 2002-244103 A Japanese Patent No. 3747036 Japanese Patent No. 3747037 JP 2004-253804 A

  On the other hand, a switching regulator type LED drive circuit requires components such as a controller IC, a coil, a switching FET, a diode, and an output capacitor for each circuit in principle.

  In this respect, in a backlight device used for a large-sized video display device, it is necessary to drive hundreds to thousands of LEDs in order to ensure necessary luminance.

  As an example, when 1,000 LEDs are driven, if the output voltage is set to about 70 V and the number of LEDs connected in series is 20, 50 LED strings are driven.

  When all of the drive circuits are constituted by independent switching regulator circuits, the number of parts becomes enormous, and there is a problem that the backlight device becomes large and expensive.

FIG. 8 is a diagram illustrating a conventional LED drive circuit that reduces the number of drive circuits.
Referring to FIG. 8A, in order to reduce the number of drive circuits, the boost voltage ratio of the boost switching regulator circuit is increased to increase the output voltage, and the series of LEDs that can be connected to one LED drive circuit. It is possible to increase the number as much as possible.

  In the previous example, when the output voltage is doubled to 140 V, the number of connectable LEDs is doubled to about 40, so the number of drive circuits can be halved to 25.

  However, when increasing the output voltage, it is necessary to use an expensive member having a high withstand voltage for a switching FET, a diode, an output capacitor, and a connector for connecting the LED driving circuit and the LED.

  Further, when the output voltage increases, it is necessary to add a protection circuit component for safety measures. For this reason, when the output voltage exceeds a certain level, there is a possibility that the material price may be higher than when the output voltage is low.

  Further, as another method, it is conceivable to use a controller IC incorporating a plurality of circuits as shown in FIG.

  In FIG. 8B, since a controller IC including two circuits is used, it is possible to configure two switching regulator type LED drive circuits with one controller IC. Can be halved.

  However, in this circuit configuration, a plurality of switching FETs, diodes, coils, output capacitors, and the like are still necessary, so that the cost reduction effect is limited.

  Furthermore, as another method, a configuration as shown in Patent Document 4 is also conceivable. Specifically, a plurality of LED strings are connected to one step-up switching regulator type LED drive circuit, and a cut FET provided in one row serving as a reference among them, and a cut FET of another LED string, Configure a current mirror circuit.

  By switching the cutting FET of the LED string serving as a reference at a high speed, all the LED strings connected in the current mirror can be driven with substantially the same average current value.

However, in this driving method, the output voltage of the switching regulator circuit is common to all LED strings, and therefore cannot correspond to V _F (forward voltage) of each LED. That is, V _F (forward voltage) variation for each LED string is not absorbed.

In this case, since the output voltage is determined by the LED string having the largest sum of V _F (forward voltage) of the LED, this corresponds to the difference between the sum of V _F of the LED string and the sum of V _F of other LED strings. Any excess voltage drop will result in heat loss.

Therefore, if V _F (forward voltage) variations in the LED of each LED string is large, in order to drive efficiency is degraded heat generation is increased, utilizing the height of efficiency, which is the original feature of the switching regulator system There is a drawback that it cannot be done.

  The present invention has been made in order to solve the above-described problems, and reduces the number of parts while maintaining high driving efficiency, and uses an LED driving circuit that enables a compact and inexpensive configuration. An object of the present invention is to provide a video display device.

  An LED driving circuit according to the present invention is an LED driving circuit for driving a plurality of LED strings each including at least one light emitting diode and connected in parallel to each other, and each of the plurality of LED strings. A switching type power conversion circuit for supplying a predetermined current to a plurality of LED strings, a plurality of disconnecting switches provided corresponding to the plurality of LED strings, respectively, for cutting off a passing current flowing through the corresponding LED strings, and a passage An adjustment circuit that adjusts the power conversion circuit so as to stabilize the passing current based on a feedback signal according to the current, and a plurality of disconnecting switches that are sequentially opened and closed based on a signal input from the outside. And a control circuit for driving each of the LED strings in a time-sharing manner.

  Preferably, the power conversion circuit and the adjustment circuit correspond to at least one step-up switching regulator circuit.

  In particular, the switching regulator circuit outputs a voltage in the range of 50V to 100V.

  Preferably, the plurality of LED strings correspond to two or three LED strings. The control circuit drives two or three LED strings in a time-sharing manner.

  Preferably, the control circuit outputs a disconnection switch drive signal for sequentially opening / closing each of the plurality of disconnection switches based on a pulse width modulation signal input from the outside.

  In particular, when the number of time divisions is N, the control circuit includes an N frequency dividing circuit that receives a pulse width modulation signal.

In particular, the frequency of the pulse width modulation signal is set in the range of 150 Hz to 500 Hz.
In particular, the control circuit forcibly turns off the plurality of disconnect switches regardless of the state of the pulse width modulation signal in the case of a predetermined condition.

  Preferably, the control circuit is configured to determine whether an abnormality has occurred in any of the plurality of LED strings or the power conversion circuit based on an input signal from the outside, and based on the determination result of the determination unit, When it is determined that an abnormality has occurred, a notification unit that notifies the outside of the abnormality is provided.

  An image display device according to the present invention includes a transmissive liquid crystal panel including liquid crystal display elements, and a backlight device for outputting backlight light of the liquid crystal panel. The backlight device includes the LED drive circuit described above.

  According to the LED drive circuit according to the present invention, by driving a plurality of LED strings in a time-sharing manner in the switching type LED drive circuit, it is possible to connect a large number of LEDs without increasing the number of components and to reduce the member price. Make it possible.

  In addition, since a voltage higher than the input voltage can be generated by using a step-up type switching regulator circuit, a large number of LEDs can be connected with a smaller number of drive circuits than in the case where no step-up is performed.

  In addition, since the output voltage is between 50V and 100V, circuit components with a withstand voltage of 100V or less can be used. That is, it is not necessary to use a circuit component having a high breakdown voltage of 100 V or higher, and the LED driving circuit can be configured with a relatively small cost and a small area.

  Furthermore, the number of time-division driving of the LED strings is set to 2 or 3, and the PWM duty for driving each LED string is not excessively reduced, thereby preventing a reduction in driving efficiency due to an increase in the peak value of the driving current. be able to.

  In addition, since a disconnection switch drive signal for performing time-division drive is generated based on an external pulse width modulation signal, there is no need to change a signal input to the backlight device from the video display device side. In addition, even when a plurality of switching regulator circuits are provided in the LED driving circuit, only one control circuit is required, so that time-division driving can be realized while minimizing the addition of components on the backlight device side.

  In addition, it is possible to easily generate a disconnect switch driving signal for driving a plurality of LED strings from a pulse width modulation signal by using a small number of components such as flip-flops constituting a frequency dividing circuit, so that the LED driving circuit is configured at low cost. can do.

  In addition, by setting the frequency of the pulse width modulation signal between 150 Hz and 500 Hz, it is possible to suppress flickering of the backlight device and realize accurate light control.

  In addition, when an abnormality such as overheating or overcurrent occurs in either the LED string or the power conversion circuit, the disconnect switch can be forcibly turned off, thus realizing an LED drive circuit that is safe against failures. can do.

  In addition, the drive signal generation circuit identifies any abnormality in the LED string or the power conversion circuit and notifies the outside of the abnormality, thereby preventing the damage when the abnormality occurs and increasing the safety of the LED drive circuit. Can do.

  In addition, since the video display device according to the present invention uses the above LED drive circuit as a backlight device, it is possible to realize a video display device that is low in price and low in power consumption.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. The present invention is not limited to the following embodiment.

FIG. 1 is a diagram illustrating an external configuration of video display device 1 according to the embodiment of the present invention.
Referring to FIG. 1, a video display device 1 according to an embodiment of the present invention includes a video unit 11, a transmissive liquid crystal panel 12, an optical sheet 13, a power supply circuit unit 14, a backlight device 2, and the like.

  The liquid crystal panel 12 incorporates a liquid crystal driver circuit and has a pixel structure with a number of TFT (thin film transistor) elements arranged in a matrix.

  The backlight device 2 is disposed on the back side as viewed from the viewer of the liquid crystal panel 12. The optical sheet 13 is disposed between the backlight device 2 and the liquid crystal panel 12 and has functions such as reducing luminance unevenness of the backlight device 2 that is a light source and improving luminance in the front direction.

  The power supply circuit unit 14 is a switching power supply that obtains a plurality of DC voltages from a voltage of AC 100 V supplied from a household outlet. The obtained DC voltage is used as a power source for the video unit 11 and the backlight device 2.

FIG. 2 is a schematic block diagram of video display device 1 according to the embodiment of the present invention.
With reference to FIG. 2, the video display principle of video display device 1 according to the embodiment of the present invention will be described.

  The video circuit unit 111 included in the video unit 11 decodes a video signal input from an external antenna, video equipment, or the like, converts the video signal into a signal format and timing suitable for display on the liquid crystal panel 12, and the liquid crystal panel 12 It transmits to the liquid crystal driver circuit (vertical driver circuit 121 and horizontal driver circuit 122) mounted therein.

  The liquid crystal driver circuits (vertical driver circuit 121 and horizontal driver circuit 122) switch a TFT element to select a specific pixel and change the transmittance of the liquid crystal element corresponding to the pixel.

  Then, with the backlight device 2 arranged on the back side of the liquid crystal panel 12 and emitting light, all the pixels from the upper left to the lower right of the liquid crystal panel 12 are fixed according to the signal input from the video circuit unit 111. By scanning in order and changing the transmittance of the liquid crystal of each pixel, an image can be displayed.

  Furthermore, this image display is performed for each fixed field period shorter than the response time of the human eye, and the moving image is displayed by sequentially updating the images.

  The video unit 11 and the power supply circuit unit 14 are circuit boards on which various circuit components are mounted on a printed circuit board, and are arranged on the back side of the liquid crystal panel 12.

Next, the configuration of the backlight device 2 will be described.
Backlight device 2 according to the embodiment of the present invention includes a plurality of LED boards 21, a driver board 22, LED boards, a harness 23 that connects the LED boards and the driver board, and a temperature sensor 24.

  The LED substrate 21 is disposed on the back side of the liquid crystal panel 12 with the optical sheet 13 interposed therebetween, and the driver substrate 22 is further disposed at the same position on the back surface as the video unit 11 and the power supply circuit unit 14.

The temperature sensor 24 is disposed in the vicinity of the LED substrate 21.
A plurality of LEDs 25 are mounted on the LED substrate 21, and the LEDs are connected in series by a pattern on the substrate. The LED 25 is a type of white LED that obtains white light by combining a blue LED chip and a yellow phosphor. The LED substrate 21 is a so-called aluminum base substrate in which an aluminum plate is used as a base material and a pattern made of copper foil is formed on the surface separated from the insulating layer, and heat generated by the LED 25 is transmitted through the copper foil and the aluminum plate. Efficiently escapes to the outside.

  In addition to the white LED described above, the LED used in the backlight device is, for example, a combination of an ultraviolet LED chip and a phosphor, or each of R (red), G (green), and B (blue). It is also possible to combine primary color LEDs.

  In addition to the aluminum base substrate described above, a general-purpose glass epoxy substrate or the like may be used as the LED substrate as long as there is no problem with the heat dissipation of the LED.

  It is difficult to form the LED substrate 21 having a length exceeding 350 mm due to the restriction of the work size at the time of manufacture. In a large video display device, since the horizontal dimension of the display screen is 1000 mm or more, one LED substrate cannot cover the end of the display screen, and the substrate needs to be divided.

  In this example, as an example, two LED boards are connected as a set by a harness, and both ends thereof are connected to the driver board 22 by another harness. That is, one LED string is formed by two LED substrates. In this example, LED strings 26a and 26b provided corresponding to one switching regulator 5a are representatively shown. Since the other LED strings have the same configuration, detailed description thereof will not be repeated.

  FIG. 3 is a schematic block diagram illustrating a configuration of the LED drive circuit formed on the driver substrate according to the embodiment of the present invention.

  Referring to FIG. 3, LED drive circuit 3 mounted on driver board 22 according to the embodiment of the present invention includes drive signal generation circuit 6 and a plurality of boost type switching regulator circuits (hereinafter simply referred to as switching regulator circuit). Also called). Here, switching regulator circuits 5a to 5c driven by the drive signal generation circuit 6 are shown, and the configuration of the switching regulator circuit 5a will be described representatively.

  The switching regulator circuit 5a according to the embodiment of the present invention includes an input capacitor 51, an inductor 52, a controller IC 7, a switching FET 53, a diode 54, an output capacitor 55, disconnecting FETs 56a and 56b, and a sense resistor 57.

  In the switching regulator circuit 5a according to the present embodiment, two LED strings 26a and 26b are connected as loads, and the two LED strings are turned on and off alternately by turning on and off FET switches 56a and 56b called disconnection FETs. Divided drive.

  In the following, the operation in a state where only one LED string is driven will be described. Details of the time-division drive operation will be described later.

The switching regulator circuit 5a according to the embodiment of the present invention steps up the switching FET 53 at high speed and releases the energy stored in the inductor 52 during that time to the output side, thereby boosting the voltage V _IN input from the outside. And supply current to the LED string 26a or 26b as a load.

The ratio of the output voltage V _OUT to the input voltage V _IN , that is, the step-up ratio is determined by the ratio of the ON time and the OFF time of the switching FET 53. The current flowing through the LED string 26a or 26b is converted into a voltage value by the voltage drop of the sense resistor 57 and fed back to the controller IC7.

Inside the controller IC 7, the comparator 71 compares the sense voltage based on the sense resistance value R _SEN with the reference voltage V _REF, and changes the setting of the gate drive circuit 72 based on the comparison result.

  The gate drive circuit 72 changes the output voltage by changing the on time and the off time of the switching signal for driving the gate of the switching FET 53 based on the setting.

  By performing such feedback, control is performed so that a constant current always flows through the LED string 26a or 26b.

  The switching frequency fSW of the switching FET 53 is normally set to a value of about several hundred kHz. When the switching frequency fSW is increased, the inductance value of the inductor 52 can be reduced to reduce the circuit size, but the switching loss in the switching FET 53 and the diode 54 increases. In the present embodiment, in consideration of this relationship and balance with other circuit constants, the switching frequency fSW = 400 (kHz) is set as an example.

  In this example, since the operation of the step-up switching regulator circuit 5a is used, it is possible to drive a number of LEDs 25 connected in series with the same number of circuits as compared with the case where no step-up is performed. . That is, there is an advantage that the price of the switching regulator circuit 5a can be reduced and the video display device 1 can be made inexpensive.

In order to obtain an output voltage V _OUT higher than the input voltage V _IN , in addition to the boosting type (boost type) switching regulator circuit, a buck-boost type (buck boost type) and a SEPIC (Single-Ended Primary Inductance Converter) ) Type switching regulator circuit is also possible. In this example, a boosting type switching regulator circuit having higher driving efficiency than that of the buck-boost type and advantageous in terms of the number of parts will be described as an example.

  Further, the switching regulator circuit 5a according to the present embodiment does not incorporate the switching FET 53 in the controller IC 7, but has an external configuration. Therefore, the drive current for driving the switching FET 53 does not flow inside the controller IC 7.

  On the other hand, when a switching element is incorporated in an IC in a switching regulator circuit or when a current sink type LED driving IC is used, a driving current flows inside the IC. This may increase the IC chip temperature. For this reason, there is a restriction on the output power per drive circuit from the viewpoint of the thermal rating of the IC chip.

  In the present embodiment, the switching element is not provided inside the controller IC 7, but is provided externally, so that the output power can be increased in the case of time-division driving, which is a more advantageous configuration. It is possible.

Here, the output voltage V _OUT of the switching regulator circuit will be described.
As the set value of the output voltage V _OUT , if the voltage is too high, the components used for the switching regulator circuit 5 need to have a high withstand voltage as described above, and the price of the circuit member may be expensive. In particular, when the voltage exceeds 100 V, it is necessary to consider the withstand voltage of the connector and the substrate, safety measures, and the like, so that it is difficult to reduce the cost when viewed from the entire backlight device 2. Therefore, the output voltage V _OUT is set to 100 V or less.

On the other hand, if the output voltage V _OUT is too low, the number of connectable LEDs 25 is reduced, so that the price merit is still poor compared to the case where the voltage is not boosted. In general, since a DC voltage of up to about 48V can be generated from a commercial AC voltage of AC100V by a switching power supply, if it is lower than 50V, it is better to use the output voltage of the switching power supply of the power supply circuit unit 14 without performing boosting It will be cheap.

Therefore, in the present embodiment, it is considered that a voltage of 50V to 100V is suitable as the set value of the output voltage V _OUT of the switching regulator circuit.

In the embodiment of the present invention, as an example, the output voltage V _OUT is set to about 85 V, and 24 white LEDs 25 (12 per LED board × 2) are connected in series to form one LED string 26. Configure.

  In addition, the LED drive circuit 3 according to the embodiment of the present invention has a configuration capable of PWM driving of the LED string by an external PWM signal input as described in the configuration of FIG. 7B. Specifically, the controller IC 7 includes a PWM control unit 73, which will be described later, but the PWM control unit 73 causes the feedback system to become temporarily unstable when both the gates of the disconnecting FETs 56a and 56b are off. In order to prevent this, it is assumed that the voltage level of the comparator 71 is temporarily held. As a result, the feedback system is stabilized, and a stable switching operation can be realized even if PWM driving is performed on the load.

  In the LED drive circuit 3 according to the embodiment of the present invention, two sets of LED strings and cutting FETs are connected to one controller IC as compared with the configuration of the LED drive circuit of FIG. The difference is that the gate of the cutting FET is not connected to the controller IC.

The operation of time division driving according to the embodiment of the present invention will be described below.
FIG. 4 is a diagram illustrating drive signal generation circuit 6 according to the embodiment of the present invention.

  Referring to FIG. 4, drive signal generation circuit 6 according to the embodiment of the present invention includes a PWM signal generator 41, a D flip-flop 42, logic gates AD0 to AD3, and a determination unit 43. Such a circuit can be realized by using an individual logic IC or a device such as a CPLD (Complex Programmable Logic Device).

  The dimming signal 60 is input from the external video circuit unit 11 to the drive signal generation circuit 6 according to the embodiment of the present invention.

  The dimming signal 60 is a digital signal having a value from 0 to 100 and indicating the ratio [%] of the luminance level of the backlight light to the maximum luminance level.

  The PWM signal generator 41 generates a PWM signal 61 having a period of tPWM, that is, a frequency fPWM = 1 / tPWM and a duty (= tON / tPWM) equal to the value of the dimming signal.

  The PWM signal 61 generated from the PWM signal generator 41 is input to the PWM controller 73 of the switching regulator circuit. The PWM signal 61 is input to the CLK terminal of the D flip-flop 42. Further, the output terminal / Q of the D flip-flop 42 and the input terminal D are connected, and the D flip-flop 42 operates as a divide-by-2 circuit.

  The logic gate AD0 receives the PWM signal 61 and the input of the output signal from the output terminal Q of the D flip-flop 42, and outputs an AND logic operation result to the logic gate AD2. The logic gate AD1 receives the PWM signal 61 and the input of the output signal from the output terminal Q of the D flip-flop 42, and outputs an AND logic operation result to the logic gate AD3. The logic gate AD2 receives the output signal of the logic gate AD0 and the inverted signal of the output signal of the determination unit 43, and outputs the AND logic operation result as a cut FET drive signal. The logic gate AD3 receives the output signal of the logic gate AD1 and the inverted signal of the output signal of the determination unit 43, and outputs the AND logic operation result as a cut FET drive signal. In this example, as an example, a cut FET drive signal that is an output signal of the logic gate AD2 is input corresponding to the gate of the cut FET 56a. Further, it is assumed that a cut FET drive signal that is an output signal of the logic gate AD3 is input corresponding to the gate of the cut FET 56b.

  As will be described later, the determination unit 43 sets the abnormality signal to the “H” level when an abnormality is detected. On the other hand, the determination unit 43 sets the abnormal signal to the “L” level during normal times. Therefore, in the normal time, the “H” level, which is the inverted signal, is input to the input nodes of logic gates AD2 and AD3. In other words, in normal times, the logic gates AD2 and AD3 function as buffer circuits for the output signals of the logic gates AD0 and AD1, and the cutting FET drive signal is output with the same waveform as the output signals of the logic gates AD0 and AD1. It will be.

FIG. 5 is a timing chart for explaining the operation of time-division driving.
In FIG. 5A, the signal waveform of the PWM signal 61 is shown. When this PWM signal 61 is input to the CLK terminal of the D flip-flop 42, a signal waveform (divided by 2) 62 having a period twice as shown in FIG. 5B is output from the Q terminal. At this time, the D flip-flop 42 operates as a divide-by-2 circuit that receives a PWM signal.

  By inputting this divide-by-two signal 62 and the original PWM signal 61 to the logic gates AD0 and AD1 in the subsequent stage, every other PWM pulse as shown in (c1) and (c2) of FIG. Two signal waveforms 63a and 63b as shown in FIG. As described above, this signal waveform is input to the gates of the disconnect switches 56a and 56b as a disconnect FET drive signal.

  Thereby, cutting FET can be turned on / off alternately. That is, the two LED strings 26a and 26b can be driven in a time division manner.

  As described above, the original PWM signal 61 is input to the controller IC 7 but is not connected to the gates of the cut FETs 56a and 56b.

  Therefore, the controller IC 7 controls the internal circuit to operate stably and a constant current flows through the LED string in the same manner as when one LED string is driven. That is, the external drive signal generation circuit 6 turns on and off the disconnection FETs 56a and 56b alternately instead of the controller IC7.

  As a result, two LED strings 26a and 26b can be driven in a time division manner with respect to one controller IC7.

  Therefore, with the configuration of the LED drive circuit according to the present invention, a plurality of LED strings can be driven in a time-sharing manner in the switching regulator type LED drive circuit, and a large number of LEDs can be connected without increasing the number of components, It is possible to reduce the material price.

  Since the divide-by-2 circuit by the D flip-flop 42 shown in FIG. 4 is an asynchronous circuit, it is conceivable that a delay occurs in the output with respect to the input of the PWM signal.

  However, since the circuit scale according to the embodiment of the present invention is small, and the PWM frequency is relatively low as described later, the delay time hardly poses a problem. In this example, the asynchronous circuit is described, but it is naturally possible to design it as a synchronous circuit.

The switching regulator circuit 5 according to the embodiment of the present invention has a sense resistor 57 (resistance value R _SEN ) provided for detecting a passing current, and the sense resistor 57 includes two LED strings 26a and 26b. It is provided corresponding to. When the time-division driving is performed, the LED strings 26a and 26b are feedback-controlled by the controller IC 7 so that the passing currents having the same passing current value Io (= V _REF / R _SEN ) flow.

  Due to the variation between individual LEDs and temperature drift, the voltage corresponding to the passing current value Io is not equal in both LED strings 26a and 26b. However, when either LED string 26a or 26b is driven, switching is performed. The current can be driven while following the change of the output voltage by the feedback control function of the regulator circuit 5. The change of the output voltage in switching regulator circuit 5 according to the embodiment of the present invention is as shown by waveform 64 shown in FIG.

  In the no-load state in which neither of the two LED strings 26a, 26b is driven, the output voltage level is held by the PWM controller 73 as indicated by the hatched lines in FIG. In this example, as an example, it is assumed that the voltage level given to the comparator 73 is held when the PWM control unit 73 receiving the original PWM signal 61 receives the “L” level input.

When any one of the cut FETs 56a and 56b is turned on, the output voltage follows the load of the LED string, that is, according to the V _F (forward voltage) variation for each LED string. As a result, time-division driving is performed. The output voltage will then vary between the two values V _OUT 1 and V _OUT 2. That is, the voltage difference between the two LED strings can be absorbed and loss can be reduced.

Therefore, with the configuration of the LED drive circuit according to the present invention, it is possible to perform time-division driving corresponding to the V _F (forward voltage) variation of the LEDs of each LED string. That is, it is possible to reduce the number of parts while maintaining high driving efficiency, and to realize a small and inexpensive LED driving circuit.

  In addition, since the switching regulator circuit 5 is required to follow the voltage fluctuation described above for a time of Δt, the current flowing through each LED string is Δt after the cutting FET of each LED string is turned on. Signal waveforms such as 65a and 65b shown in (e1) and (e2) of FIG. 5 are not completely coincident with the passing current value Io, and are slightly larger or slightly smaller than the passing current value Io.

  For this reason, even if two LED strings are driven with the same PWM duty, the current time average values Iave1 and Iave2 are not exactly equal. However, as will be described later, if Δt is set to a sufficiently small value with respect to the PWM pulse period tPWM, the difference between the current values becomes almost negligible.

  Next, output current and output power of switching regulator circuit 5 according to the embodiment of the present invention will be described.

  In the embodiment of the present invention, time-division driving is performed with the number of time divisions N = 2 as an example, so that the duty of the drive current waveform of each LED string is a value up to 50%, and the time average value Iave of the current is The maximum value is 1/2 of the peak value Io of the output current. In order to provide the LED with the same average current Iave as in the case where time division driving is not performed (when N = 1) and to make the luminance of the backlight device equal, the passing current value Io to the LED string is twice that in the case where time division is not performed. It is necessary to.

  In the present embodiment, when the current time average value Iave is equal to the case where the time-division driving is not performed, the power input per LED string is also equal to the case where the time-division driving is not performed.

  However, when viewed from the drive circuit side, since two LED strings are connected to one circuit, the output power per drive circuit is twice that when not time-division driven.

  Next, consider the PWM signal and PWM frequency generated by the drive signal generation circuit 4.

  A dimming signal 60 for dimming the backlight device 2 is transmitted from the video circuit unit 111 as shown in FIG. In the video circuit unit 111, the luminance required for the backlight device 2 is determined based on the ambient brightness, the content of the video signal, and the like, and a corresponding dimming signal 60 is generated to generate a drive signal generation circuit in the backlight device 2. 4 to send.

  As described above, the drive signal generation circuit 4 generates the PWM signal 61 for driving the LED string based on the dimming signal. In addition to such a configuration, a CPU or the like may be provided in the backlight device 2 separately from the drive signal generation circuit, and a PWM signal may be generated therein, or the PWM signal may be directly generated inside the video circuit unit 111. May be generated and transmitted to the backlight device 2 side.

  In the present embodiment, the PWM frequency fPWM of the PWM signal 61 is set to a frequency sufficiently slower than the switching frequency fSW of the switching regulator circuit 5 itself. That is, since the width of the PWM pulse is sufficiently larger than the switching time, the drive pulse width can be adjusted without being affected by the timing of the switching operation.

  If the PWM frequency fPWM of the PWM signal 61 is too low, it may be recognized that the LED strings flicker alternately as flickering to the human eye. In general, the limit frequency at which blinking of light can be visually recognized as flickering is around 60 Hz. Therefore, the PWM frequency fPWM of the PWM signal 61 is 60 × N where N is the number of time divisions in time division driving. [Hz] or more is necessary.

  In this example, N = 2 as an example, so that it is 120 Hz or more. However, considering the margin, it is desirable to set it to 150 Hz or more.

  On the other hand, if the PWM frequency fPWM of the PWM signal 61 is too high, the switching regulator circuit cannot follow the voltage fluctuation between the two LED strings due to turning on / off of the cut FET. The time Δt required to follow is given by the following equation from the voltage difference ΔV between the two LED strings that are loads, the capacitance value Co of the output capacitor, and the magnitude Io of the charging current.

  In the present embodiment, if rough values of the voltage difference ΔV, the capacitance value Co, and the charging current magnitude Io are estimated to be 4 V, 5 μF, and 200 mA, respectively, Δt = 100 (μsec) is obtained.

  In order to dimm the backlight device with high accuracy, it is a requirement that the period tPWM of the PWM pulse for driving the LED string is sufficiently longer than the time Δt required for following. If the PWM pulse cycle tPWM is 20 times or more Δt, the PWM pulse cycle tPWM ≧ 2 (msec), and 500 Hz is obtained as the upper limit of the PWM frequency fPWM.

  From the above consideration, it is desirable that the PWM frequency is 150 Hz or more and 500 Hz or less. In the present embodiment, the PWM frequency is set to 300 Hz.

  The backlight device 2 according to the embodiment of the present invention has a function capable of forcibly stopping the driving of the LED string 26 when an abnormality occurs in order to improve safety.

  Specifically, a temperature sensor 24 is disposed in the vicinity of the LED string 26 of the backlight device 2. The temperature sensor 24 measures the temperature of the LED string 26 and outputs it to the drive signal generation circuit 4.

  The drive signal generation circuit 4 is provided with a determination unit 43 that determines whether or not the value input from the temperature sensor 24 is within the normal range.

  When it is determined that some trouble has occurred in the LED in the LED string 26 and the temperature is abnormally increased, the determination unit 43 outputs an abnormality signal 66 indicating an abnormality in the LED string.

  The abnormal signal 66 is input to a subsequent logic gate in the drive signal generation circuit 4, and operates to forcibly turn off the disconnecting FETs 56a and 56b. Specifically, if the determination unit 43 determines that there is an abnormality, it sets the abnormality signal 66 to the “H” level. In response to this, an "L" level signal is input to one of the input nodes of logic gates AD2 and AD3. Therefore, the cut FET drive signal that is the output signal of logic gates AD2 and AD3 is forcibly set to "L" level.

  Thereby, the drive of the LED string 26 is stopped, and the occurrence of an accident due to abnormal overheating of the LED string can be prevented.

  Here, as an example, the method of detecting an abnormality based on the detection of abnormal overheating by the temperature sensor 24 has been described, but it is also possible to detect an abnormality based on the detection of an overcurrent.

  For example, when a large current flows through the LED string due to an abnormality in the switching regulator circuit 3 or disconnection of the LED, a voltage drop in the sense resistor 57 increases. Therefore, the voltage value in the sense resistor 57 of the switching regulator circuit 3 is input to the determination unit 43 of the drive signal generation circuit 4 and the voltage value is compared with the voltage value serving as a threshold value. When it is determined that the voltage value is abnormally increased, the determination unit 43 outputs the abnormal signal 66 in the same manner as described above. It is also possible to turn off the cutting FETs 56a and 56b by this abnormal signal and forcibly stop the driving of the LED string 26.

The abnormal signal 66 is also transmitted from the drive signal generation circuit 4 to the video circuit unit 111.
In the video circuit unit 111, processing for notifying the user of the video display device 1 that an abnormality has occurred in the backlight device 2 is performed. For example, it is possible to notify the user that an abnormality has occurred by means such as generation of a beep sound or lighting of a pilot lamp, and request maintenance of the equipment. With such a configuration, the safety of the video display device can be further enhanced.

  In the above embodiment, the case where the number of time divisions in the time division driving of the LED string is two has been described. On the other hand, it is also possible to configure the LED drive circuit with a time division number of 3. In this case, it is necessary to change the drive signal generation circuit 6 shown in FIG.

FIG. 6 is a diagram for explaining the drive signal generation circuit 6 # when the number of time divisions is three.
Referring to FIG. 6, drive signal generation circuit 6 # includes D flip-flops 42a and 42b that replace D flip-flop 42 and logic gates that replace logic gates AD0 to AD3, as compared with drive signal generation circuit 6. The difference is that it includes AD4 to AD10.

  The drive signal generation circuit of FIG. 6 includes a divide-by-3 circuit composed of two D flip-flops and one logic gate for backlight dimming input from the video circuit unit 111. With the dimming signal as an input, it is possible to generate three cut FET drive signals that include one PWM pulse at a time.

  By inputting this signal to each of the three cut FETs, it is possible to drive three LED strings connected to one switching regulator circuit in a time division manner.

  In accordance with the same method, the number of time divisions can be increased to 4, 5, or more by changing the frequency divider 3 to the frequency divider 4 or the frequency divider 5 to drive a large number of LED strings in a time division manner. It is also possible to do.

  When a large number of LED strings are driven by increasing the number of time divisions, the duty of the PWM pulse waveform that drives each LED string is reduced. In the time division number 4, the PWM duty is 25% or less, and in the time division number 5, the duty is 20% or less.

  In order to obtain the same light output, the current peak value needs to be increased as the duty decreases. In this respect, the current-light output characteristics of the LED are saturated when the output is extremely high, and therefore the same light output cannot be obtained even if the duty is 1 / N and the current peak value is N times. Therefore, if the number of time divisions is too high, the driving efficiency may be deteriorated. Further, the rated value of the forward current in the pulse driving of the normal LED is about 2 to 3 times the rated value in the case of DC driving.

  Therefore, in the present embodiment, it is considered desirable to set the time division number N to about 2 or 3 as an example in order to execute efficient time division driving.

  In the above embodiment, the configuration using the voltage mode control system in which the feedback control in the switching regulator circuit is performed based on the voltage across the sense resistor has been described. However, the current mode control system regulator circuit is used. Is of course possible.

  In the current mode control system, in addition to voltage feedback, a feedback path for limiting a current flowing in one switching cycle is further added, and a part of the controller IC and a part of peripheral circuits are changed. However, it is not necessary to change the basic configuration of the above-described embodiment such as the drive signal generation circuit. In the current mode control method, the circuit configuration is complicated compared to the voltage mode control method, but there is an advantage that the operation of the switching regulator circuit can be made more stable.

  In the above description of the embodiment, the parts other than the controller IC such as the drive signal generation circuit have been described as being composed of individual components, but it is naturally possible to integrate some or all of them as an integrated circuit as necessary. It is.

  The embodiments disclosed herein are illustrative and non-restrictive in every respect. The scope of the present invention is shown not by the above-described embodiment but by the scope of claims, and is intended to include all modifications within the meaning and scope equivalent to the scope of claims.

It is a figure explaining the external appearance structure of the video display apparatus 1 according to embodiment of this invention. 1 is a schematic block diagram of a video display device 1 according to an embodiment of the present invention. It is a schematic block diagram explaining the structure of the LED drive circuit formed in the driver board | substrate according to embodiment of this invention. It is a figure explaining the drive signal generation circuit 6 according to embodiment of this invention. It is a timing chart figure explaining operation of time division drive. It is a figure explaining drive signal generation circuit 6 # when the number of time divisions is three. It is a figure explaining the basic operation principle of the conventional LED drive circuit. It is a figure explaining the conventional LED drive circuit which reduces the number of drive circuits.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 Image display apparatus, 2 Backlight apparatus, 3 LED drive device, 5a-5c Switching regulator circuit, 6 Drive signal generation circuit, 7 Controller IC, 11 Image | video part, 12 Liquid crystal panel, 13 Optical sheet, 14 Power supply circuit part, 21 LED board, 22 driver board, 23 harness, 24 temperature sensor, 25 LED, 26a, 26b LED string, 51 input capacitor, 52 inductor, 53 switching FET, 54 diode, 55 output capacitor, 56a, 56b cutting FET, 57 sense Resistor, 71 Comparator, 72 Gate drive circuit, 73 PWM controller.

Claims (10)

LED driving circuits for driving a plurality of LED strings each including at least one light emitting diode and connected in parallel with each other,
A switching type power conversion circuit for supplying a predetermined current to each of the plurality of LED strings;
A plurality of disconnect switches provided corresponding to the plurality of LED strings, respectively, for cutting off a passing current flowing through the corresponding LED strings;
An adjustment circuit that adjusts the power conversion circuit to stabilize the passing current based on a feedback signal according to the passing current;
An LED driving circuit comprising: a control circuit that opens and closes each of the plurality of disconnecting switches in order based on a signal input from the outside, and drives each of the plurality of LED strings in a time-sharing manner.   The LED drive circuit according to claim 1, wherein the power conversion circuit and the adjustment circuit correspond to at least one step-up switching regulator circuit.   The LED driving circuit according to claim 2, wherein the switching regulator circuit outputs a voltage in a range of 50V to 100V. The plurality of LED strings correspond to two or three LED strings,
The LED driving circuit according to claim 1, wherein the control circuit drives two or three LED strings in a time-sharing manner.   2. The LED drive circuit according to claim 1, wherein the control circuit outputs a cut switch drive signal for sequentially opening and closing each of the plurality of cut switches based on a pulse width modulation signal input from the outside.   6. The LED drive circuit according to claim 5, wherein the control circuit includes an N frequency dividing circuit that receives the pulse width modulation signal when the number of time divisions is N. 7.   The LED drive circuit according to claim 5, wherein the frequency of the pulse width modulation signal is set in a range of 150 Hz to 500 Hz.   6. The LED drive circuit according to claim 5, wherein the control circuit forcibly turns off the plurality of disconnect switches regardless of a state of the pulse width modulation signal in a case of a predetermined condition. The control circuit includes:
A determination unit for determining whether an abnormality has occurred in any of the plurality of LED strings or the power conversion circuit based on an input signal from the outside;
The LED drive circuit according to claim 1, further comprising: a notification unit that notifies an abnormality to the outside when it is determined that an abnormality has occurred based on a determination result of the determination unit. A transmissive liquid crystal panel composed of liquid crystal display elements;
A backlight device for outputting backlight light of the liquid crystal panel,
The said backlight apparatus is a video display apparatus containing the LED drive circuit as described in any one of Claims 1-9.

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