JP2011029437A - Led driving system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an LED driving system that controls a power supply voltage for an LED at high speed. <P>SOLUTION: The LED driving system includes: an LED power supply in which, when a control signal to a control terminal is not more than a first value, boosting operation is performed, and when the control signal is not less than the first value, the boosting operation is stopped, and when the control signal is not less than the second value which is not less than the first value, the system is put in the state of system down; a plurality of LED light sources in which an output terminal of the LED power supply is connected to each anode side terminal; an LED driver in which each current source is connected to each of the LED light sources through each current terminal to pass current to each of the LED light sources; and a control circuit in which, when any of voltages of a plurality of current terminals is lower than the minimum output reference voltage, a first control signal lower than the first value is added to the control terminal, and when all the voltages of a plurality of current terminals are higher than the minimum output reference voltage, a second control signal higher than the first value and lower than the second value is added to the control terminal. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an LED drive system.

  In recent years, light-emitting diode (LED) light sources have been used in backlights on the back of liquid crystal displays such as liquid crystal televisions.

  The background to this is the global trend of halogen-free environmental conservation. Thereby, replacement with a cold cathode tube (CCFL: Cold Cathode Fluorescent Lamp) is progressing to an LED light source.

  White LED light sources for backlights are those that use red, blue, and green (RGB) three primary colors as light sources, those that use complementary colors obtained by blue LEDs and phosphors, and RGB that are all ultraviolet. There are LED light sources and phosphors that can be used as light sources.

  The plurality of LED light sources are turned on by supplying a LED power supply voltage to each anode side by an LED stable voltage power supply, and a constant current from each cathode side by an LED driver.

  Although the number of LED light sources varies depending on the size of the monitor, since LED light sources are more expensive than CCFLs, reduction of system cost has become a popular issue. Therefore, in order to reduce the number of LED drivers and reduce the cost of the system, a plurality of LEDs are connected in series within an allowable range within the rated voltage of the LED driver. In order to obtain sufficient luminance, a plurality of LEDs are connected in series.

  At this time, the fluctuation and variation of the total forward voltage (total Vf) of the LED light source in which m LEDs are connected in series with respect to the lighting current are also m times that of the single lamp. That is, the total Vf has characteristics that vary greatly depending on the magnitude of the current that flows when the LED light source is turned on, the location of the LED light source that is turned on, and the temperature characteristics.

  Therefore, when the LED light source to which a constant LED power supply voltage is supplied is turned on, if the total Vf increases, the voltage at the current terminal required for the LED driver to flow current decreases, and the current also decreases. There is a possibility that. In order to avoid this, the LED driving system is configured to increase or decrease the LED power supply voltage from the LED stable voltage power supply with respect to the Vf fluctuation.

  In other words, a minimum voltage detection circuit that detects the minimum voltage from the voltages of the plurality of current terminals is provided so that the voltage at the current terminal of the LED driver can secure the minimum voltage that can drive the lighting current, and the minimum voltage detected (Analog voltage) is fed back to the LED stable voltage power supply to supplement the LED power supply voltage and avoid lighting problems due to insufficient voltage.

  In general, in this system, when the voltage fed back to the LED stable voltage power supply falls below the threshold value, the LED power supply voltage rises. Conversely, when the voltage fed back to the threshold voltage rises, the LED power supply voltage drops. Operates with negative feedback.

  However, the conventional LED drive system cannot control the LED power supply voltage at high speed.

  As the LED driving system, for example, the one described in Patent Document 1 is known.

JP 2006-303093 A

  The objective of this invention is providing the LED drive system which can control the power supply voltage for LED at high speed.

  According to one aspect of the present invention, it is connectable to an input-side power source as an input, and includes a control terminal and an output terminal that outputs an output voltage, and a control signal to the control terminal is predetermined. When the voltage is equal to or lower than the first value, the voltage boosting operation is performed. When the voltage is equal to or higher than the first value, the voltage boosting operation is stopped, and the control signal to the control terminal is equal to or higher than the first value. When the value is equal to or greater than the second value, the LED power source is in a system down state, the LED power source is connected to each anode side terminal, a plurality of LED light sources, a plurality of current sources, a plurality of current sources, A current terminal, wherein each current source is connected to each LED light source via each current terminal, and a current is supplied to each LED light source, and the voltages of the plurality of current terminals are input. The voltage of the plurality of current terminals In a first case where this is lower than a predetermined minimum output reference voltage, a first control signal lower than the first value is applied to the control terminal of the LED power supply to boost the LED power supply. In a second case where all of the voltages of the plurality of current terminals are higher than the lowest output reference voltage, a second higher than the first value and lower than the second value. An LED driving system is provided, comprising: a control circuit that adds a control signal to stop the LED power supply from performing a boosting operation.

  According to the present invention, the LED power supply voltage can be controlled at high speed.

It is a block diagram of the LED drive system which concerns on the 1st Embodiment of this invention. It is a wave form diagram showing the digital feedback signal of the LED drive system concerning a 1st embodiment of the present invention. It is a block diagram of the LED drive system which concerns on the 2nd Embodiment of this invention. It is a wave form diagram showing the digital feedback signal of the LED drive system concerning a 2nd embodiment of the present invention. It is a block diagram of the LED drive system of a comparative example.

  Prior to the description of the embodiments of the present invention, the background to which the inventors have made the present invention will be described.

  When the inventors control the LED power supply voltage based on the lowest voltage (analog voltage) detected by the lowest voltage detection circuit as in the LED drive system described in the background section, the LED power supply voltage is increased at high speed. I realized I couldn't control it.

  FIG. 5 is a block diagram of a comparative LED driving system. This system includes a LED stable voltage power source 1 (LED power source 1) such as a DC / DC converter, an LED driver 2 having a plurality of constant current drive lines (current sources 2a-1 to 2a-n), and a minimum voltage detection. The circuit 10, the controller 4, and a plurality of LED light sources 5-1 to 5-n each including a plurality of LEDs connected in series are provided.

  The LED power source 1 is configured in substantially the same manner as a known step-up DC / DC converter, for example, and functions as follows. That is, the LED power source 1 can be connected to an input side power source (not shown) as an input. The LED power source 1 further includes a feedback control terminal 6 as a control terminal and an output terminal 6A that outputs an output voltage. As a result, the LED power supply 1 enters a boosting operation (step-up operation) state or a boosting operation stop state according to the magnitude of the control signal to the feedback control terminal 6. That is, when the control signal to the feedback control terminal 6 is less than or equal to a threshold value (predetermined first value), the boost operation is performed, and when it is greater than or equal to the threshold value, the boost operation is stopped. When the control signal is less than or equal to the threshold value, the smaller the magnitude of the control signal, the higher the load driving capability and the higher the voltage boosting operation. That is, when the GND voltage is input, the boosting operation is performed at the highest speed.

  In addition, the LED power source 1 is a system power source that operates the system of the LED power source 1 when the control signal to the feedback control terminal 6 is equal to or higher than an overvoltage detection level (predetermined second value) that is equal to or higher than a threshold value. Is disconnected, and the system is in a system down state (shutdown state) in which the operation as a power source is stopped. It takes some time to go from the system down state to the system operating state again. In the step-up operation stop state, for example, the switching transistor is only in a state where the switching operation is stopped, and it is natural that the system does not go down.

  Each of the LED light sources 5-1 to 5-n is connected to the output terminal 6A of the LED power source 1 at the anode side terminal, and is supplied with the LED power source voltage Vd.

  In the LED driver 2, each current source 2a-1 to 2a-n is connected to a cathode side terminal of each LED light source 5-1 to 5-n via each current terminal, and each LED light source 5-1 to 5- A constant current is passed through n.

  The controller 4 is supplied with the operating power supply 8 to control the lighting line of the LED driver 2 and to control the luminance by PWM or the like.

  The lowest voltage detection circuit 10 monitors the voltage of each current terminal of the LED driver 2 when the LED is on, and detects and outputs the lowest voltage among them. This minimum voltage is divided by the voltage dividing resistors 7 a and 7 b through the buffer and fed back to the feedback control terminal 6.

  At each current terminal of the LED driver 2, a difference voltage between the LED power supply voltage Vd and the total Vf of the LED light sources 5-1 to 5-n is generated. As described above, the lowest voltage detection circuit 10 detects and outputs the lowest voltage among the differential voltages, and transmits a direct current (DC) voltage value to the feedback control terminal 6, thereby providing an insufficient LED power supply. The voltage Vd increases.

  In this LED drive system, the LED light sources 5-1 to 5-n differ in LED power supply voltage, the number of LEDs connected in series, and the drive current value depending on the request of the target set. Further, since the LED has excellent responsiveness, for example, in applications such as a backlight of a liquid crystal television (LCD-TV), improving the responsiveness of the backlight is effective for improving the image quality. For example, when a bright image is displayed on a black background, it is effective to improve the image quality to turn on only the backlight on the back of the bright image. Therefore, it is required to control at a high speed the lighting, extinguishing, luminance change, etc. of part or all of the backlight according to the displayed image.

  At this time, in the system of this comparative example, depending on the lighting state of the LED light source, an overvoltage may be applied to the feedback control terminal 6 and the luminance of each line or all LEDs may not be sufficiently obtained, or all the LEDs may be turned off. The inventor knew independently that there was.

  The reason is that generally, the feedback control terminal 6 of the LED power supply 1 often has a threshold value of less than 1V, and as described above, a voltage that greatly exceeds this threshold value and exceeds the overvoltage detection level. This is because the LED power supply 1 enters a system down state in which the operation as a power supply is stopped, assuming that an overvoltage is detected.

This will be described in detail below.
For example, in the general standard of white LED used as a light source, the forward voltage Vf indicates 2.6 to 3.8 V with respect to the reference current (10 to 20 mA). Therefore, when eight LEDs are connected in series, the total Vf is 20.8 to 30.4V. Further, the voltage of each current terminal of the LED driver 2 is 1.0 to 10.6 V when the power supply voltage for LED is controlled assuming that the minimum voltage is 1 V. At this time, the timing at which each line turns on an arbitrary LED light source and extinguishes by PWM control is different for each line.

  Here, for example, the LED light sources of a plurality of lines including the minimum Vf line (Vf = 20.8 V) and the maximum Vf line (Vf = 30.4 V) are turned on, and the LED power supply voltage Vd is based on the maximum Vf line. Consider the moment when the maximum Vf line and the like are extinguished and the control is switched to the minimum Vf line from the state where the voltage is controlled to 31.4V. At this time, since the voltage of the current terminal in the minimum Vf line is 10.6 V (= 31.4 V−20.8 V), the voltage obtained by dividing 10.6 V is applied to the feedback control terminal 6. Actually, the forward voltage Vf of the LED is selected by an arbitrary rank number, and the method of reducing 10.6V is performed. However, depending on the influence of overvoltage detection and the difference between the LED power supply 1 and the like, The inventor independently learned that prompt response is hindered.

  Also, consider the moment of switching from a state where only one line is driven to a state where ten lines are driven. When there is almost no difference in the maximum Vf in each of these states, even if the load changes by 10 times, the minimum voltage detected by the minimum voltage detection circuit 3 at that moment does not change and is the same in the system of this comparative example. . Therefore, the speed at which the LED power source 1 boosts the LED power source voltage does not change, and the transient response of the LED power source 1 is delayed. Therefore, the inventor has also independently known that the brightness of each line or all LEDs cannot be obtained, or that each line or all LEDs may flicker.

  The inventors have made the present invention based on the above-mentioned unique knowledge.

  Embodiments of the present invention will be described below with reference to the drawings. These embodiments do not limit the present invention.

(First embodiment)
A first embodiment of the present invention will be described with reference to FIGS. In this embodiment, the power supply voltage for LED is controlled using two control signals of high level and low level.

  FIG. 1 is a block diagram of an LED light source driving system according to the first embodiment of the present invention. This system includes a detection result digital output circuit 3 (control circuit) instead of the minimum voltage detection circuit 10 in the comparative example of FIG. Since the other circuit configuration is the same as that of the comparative example of FIG. 5, the same reference numerals are given to the same elements and the description thereof is omitted. This system is used, for example, for a backlight of a liquid crystal television or a PC monitor.

  The detection result digital output circuit 3 includes a plurality of comparators 3a-1 to 3a-n and comparators 3a-1 to 3a-n that respectively compare the voltage of each current terminal of the LED driver 2 with the lowest output reference voltage 3c. A multi-input AND gate circuit 3b that performs an AND operation on each output.

  The cathode side terminal of the LED light source 5-1 is connected to the current terminal of the LED driver 2 and the non-inverting input terminal of the comparator 3a-1. Similarly, the cathode side terminals of the LED light sources 5-2 to 5-n are connected to the current terminals of the LED driver 2 and the non-inverting input terminals of the comparators 3a-2 to 3a-n.

  The voltage dividing resistors 7a and 7b are connected in series, one end is connected to the output of the AND gate circuit 3b, and the other end is grounded. A connection point between the voltage dividing resistor 7 a and the voltage dividing resistor 7 b is connected to the feedback control terminal 6 of the LED power source 1.

  Next, the operation of the LED light source driving system of FIG. 1 will be described. The description of the same operation as that of the comparative example of FIG. 5 is omitted.

  Each of the comparators 3a-1 to 3a-n outputs a low level (GND) when the voltage of the current terminal of the connected LED driver 2 is the minimum output reference voltage 3c or less, and when the voltage is the minimum output reference voltage 3c or more. Outputs a high level.

  FIG. 2 is a waveform diagram showing the digital feedback signal Vfb of this LED drive system. In the figure, the horizontal axis represents time, and the vertical axis represents voltage.

  Until the time t1, all the LED light sources 5-1 to 5-n are turned off. The digital feedback signal Vfb is at a high level.

  At time t <b> 1, based on the control of the controller 4, a constant current is supplied to light a predetermined one of the LED light sources 5-1 to 5-n. Here, when there is even one line in which the voltage at the current terminal is less than the minimum output reference voltage 3c, the output of the comparator connected to that line becomes low level. This low level signal is input to the AND gate circuit 3b, the output of the AND gate circuit 3b becomes low level, and the digital feedback signal Vfb also becomes low level. Then, the LED power source 1 performs the boosting operation of the LED power source voltage Vd with the maximum capacity based on the digital feedback signal Vfb. This is because, as described above, the LED power source 1 performs the boosting operation with the maximum capacity when the GND voltage is input to the control terminal.

  Next, at time t2, when the LED power supply voltage Vd becomes high and the voltages of all the current terminals become higher than the minimum output reference voltage 3c, the outputs of all the comparators 3a-1 to 3a-n become high level, The digital feedback signal Vfb returns to the high level.

  As described above, according to the present embodiment, the signal to the feedback control terminal 6 is replaced with the logic signal generated by the detection result digital output circuit 3 from the voltage of the current terminal of each line. The voltage of the control terminal 6 is limited to two types obtained by dividing the logic operation voltage (VIH = VDD and VIL = GND) which is the main power source of the AND gate circuit 3b. Therefore, if the voltage VH is set to a value that does not cause overvoltage, application of overvoltage to the feedback control terminal 6 can be prevented. Thereby, even when the maximum Vf line is extinguished and the control is switched to the minimum Vf line from the state in which the LED power supply voltage is controlled with the maximum Vf line as a reference, no overvoltage is applied to the feedback control terminal 6. Therefore, at this moment, the LED power source 1 does not stop the operation as a power source, and can respond faster than the system of the comparative example of FIG.

  Further, as described above, the voltage of the feedback control terminal 6 includes the voltage VIH (when the LED power supply voltage Vd is sufficient for all lines) and the voltage VIL (at least one of the lines is insufficient for the LED power supply voltage Vd). 2), and is not affected by the Vf difference of each line. That is, since this voltage is not an analog output such as a buffer as in the comparative example of FIG. 5 but a logic output, the LED power supply 1 can always be performed with the maximum capacity when performing the boosting operation. Therefore, the response to the load fluctuation can be made faster than the system of the comparative example of FIG.

  That is, regardless of the LED lighting state, the LED power source 1 can be controlled without applying an overvoltage to the feedback control terminal 6 and a stable power source voltage is supplied to the load LED light source. Can be realized.

  When the voltage VIH (that is, the power supply voltage VDD of the AND gate circuit 3b) is equal to or lower than the overvoltage detection level, the voltage dividing resistors 7a and 7b are not used and the output terminal of the AND gate circuit 3b is used as the feedback control terminal 6. You may connect.

(Second Embodiment)
Next, a second embodiment of the present invention will be described with reference to FIGS. This embodiment is different from the first embodiment in the configuration of the detection result digital output circuit.

  FIG. 3 is a block diagram of an LED light source driving system according to the second embodiment of the present invention.

  The detection result digital output circuit 30 includes comparators 3a-1 to 3a-n, an n-input 1-output multiplexer 3d, an n-ary counter 3e, and an oscillator 3f. The comparators 3a-1 to 3a-n compare the voltage of each current terminal of the LED driver 2 with the lowest output reference voltage 3c. The multiplexer 3d selects and outputs one of the comparison results of the comparators 3a-1 to 3a-n. The n-ary counter 3e counts the reference clock signal and outputs an n-ary count value, thereby sequentially specifying the comparison results selected by the multiplexer 3d. The oscillator 3f outputs a reference clock signal to the n-ary counter 3e. Since the other circuit configuration is the same as that of the first embodiment of FIG. 1, the same reference numerals are given to the same elements and the description thereof is omitted.

  The detection result digital output circuit 30 interlocks the n-ary counter value based on the oscillation signal of the oscillator 3f and the LED output number, and repeatedly outputs the detection results of the comparators 3a-1 to 3a-n in order. That is, this system operates so that the output width of the low level gradually increases as the number of lines with insufficient voltage at the current terminal of the LED driver 2 increases. Therefore, the system of FIG. 1 can have a simple soft start function of the LED power source 1.

  For example, when the voltage of the current terminal of only one line is insufficient, the period during which the low level of the digital feedback signal Vfb is output is shorter than that of the system of FIG. 1, so the LED power supply voltage Vd is higher than that of the system of FIG. The pressure is increased at a slow speed.

  FIG. 4 is a waveform diagram showing the digital feedback signal Vfb of this LED drive system. In the figure, the horizontal axis represents time, and the vertical axis represents voltage.

  Until the time t1, all the LED light sources 5-1 to 5-n are turned off. The digital feedback signal Vfb is at a high level.

  At time t <b> 1, based on the control of the controller 4, a constant current is supplied to light a predetermined one of the LED light sources 5-1 to 5-n. Here, when there is even one line in which the voltage at the current terminal is less than the minimum output reference voltage 3c, the output of the comparator connected to that line becomes low level. Thereby, the multiplexer 3d outputs the voltage VIL only for a predetermined period. Therefore, the digital feedback signal Vfb is at a low level only for a predetermined period. Then, the LED power supply 1 boosts the LED power supply voltage Vd with the maximum capability only during the period when the digital feedback signal Vfb is at the low level.

  Thereafter, until time t2, the number of lines with insufficient voltage at the current terminal changes, and a low level is output accordingly.

  Next, at time t2, when the LED power supply voltage Vd becomes high and the voltages of all the current terminals become higher than the minimum output reference voltage 3c, the outputs of all the comparators 3a-1 to 3a-n become high level. Then, the multiplexer 3d continuously outputs the voltage VIH, and the digital feedback signal Vfb returns to the high level.

  As described above, according to this embodiment, the low-level output width of the digital feedback signal Vfb is gradually increased as the number of lines with insufficient voltage at the current terminal increases. The power supply voltage Vd can be boosted gradually. Further, as in the first embodiment, even when the LED power supply voltage Vd is controlled with the maximum Vf line as a reference, the maximum Vf line is extinguished and the control is switched to the minimum Vf line. Since no overvoltage is applied to the control terminal 6, the LED light sources 5-1 to 5-n can be turned on / off and the brightness can be changed faster than in the comparative example.

  If the voltage VIH (that is, the power supply voltage VDD of the multiplexer 3d) is equal to or lower than the overvoltage detection level, the voltage dividing resistors 7a and 7b are not used and the output terminal of the multiplexer 3d is connected to the feedback control terminal 6. good.

  The embodiment of the present invention has been described in detail above, but the specific configuration is not limited to the above embodiment, and various modifications can be made without departing from the scope of the present invention.

1 Stable voltage power supply for LED (LED power supply)
2 LED drivers 2a-1 to 2a-n Current sources 3, 30 Detection result digital output circuit (control circuit)
3a-1 to 3a-n comparator 3b AND gate circuit 3c minimum output reference voltage 3d multiplexer 3e n-ary counter 3f oscillator 4 controller 5-1 to 5-n LED light source 6 feedback control terminals 7a and 7b voltage dividing resistors

Claims (7)

When it is connectable to an input-side power source as an input, and includes a control terminal and an output terminal that outputs an output voltage, and the control signal to the control terminal is equal to or less than a predetermined first value When the boosting operation is performed and the value is equal to or higher than the first value, the boosting operation is stopped, and when the control signal to the control terminal is equal to or higher than the second value equal to or higher than the first value, the system is stopped. LED power supply that goes down,
A plurality of LED light sources, each of which is connected to the output terminal of the LED power source;
An LED driver, comprising: a plurality of current sources; and a plurality of current terminals, wherein each current source is connected to each LED light source via each current terminal, and a current flows to each LED light source;
In the first case where the voltages of the plurality of current terminals are input and one of the voltages of the plurality of current terminals is lower than a predetermined minimum output reference voltage, the first lower than the first value In the second case, a control signal is added to the control terminal of the LED power supply to cause the LED power supply to perform a boosting operation, and in the second case, all of the voltages of the plurality of current terminals are higher than the minimum output reference voltage. A control circuit that applies a second control signal that is higher than a first value and lower than the second value to stop the LED power supply from stepping up;
An LED driving system comprising:   The LED driving system according to claim 1, wherein the plurality of LED light sources are provided on a back surface of a liquid crystal display and used as a backlight.   By controlling the LED driver and passing a current to a predetermined one of the LED light sources, the lighting, extinguishing, and luminance change of the LED light sources are controlled based on an image displayed on the liquid crystal display. The LED driving system according to claim 2, further comprising a controller. The control circuit includes:
A plurality of comparators, wherein the voltage of each current terminal is input to each non-inverting input terminal, and the lowest output reference voltage is input to each inverting input terminal;
An AND circuit that receives each output signal of each comparator and outputs a logical product of them to the control terminal of the LED power supply;
The LED drive system according to any one of claims 1 to 3, further comprising: The control circuit includes:
An oscillation circuit that outputs a clock signal;
A counter that counts the clock signal and outputs an n-ary count value;
A plurality of comparators, wherein the voltage of each current terminal is input to each non-inverting input terminal, and the lowest output reference voltage is input to each inverting input terminal;
A multiplexer that sequentially outputs any of the output signals of the comparators to the control terminal of the LED power supply based on the n-ary count value;
The LED drive system according to any one of claims 1 to 3, further comprising:   The LED driving system according to claim 4 or 5, further comprising a voltage dividing resistor that divides an output signal of the AND circuit or the multiplexer and outputs the divided signal to the control terminal of the LED power source.   The said LED power supply is comprised as what has the capability to perform a pressure | voltage rise operation, when the said control signal to the said control terminal is a said 1st control signal, From Claim 1 characterized by the above-mentioned. Item 7. The LED drive system according to any one of Items 6 to 7.

Images