JP2011100331A - Apparatus and method for driving multiple loads, led driving device using the same, and video display device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a constant current supply device that has multiple channels and can change successive current peak value during driving, thereby preventing variations between drive currents. <P>SOLUTION: The constant current supply device stores a reference current that is adjusted in a variable reference current source in current storage parts that are provided in each channel, and converts the reference current into the drive currents in a constant current drive part. The multiple current storage parts for each channel enable each channel to store individual current information. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to LED backlight driving of a liquid crystal display.

  Currently, liquid crystal displays have been used in various electronic devices such as small terminals such as mobile phones and large televisions, and are now one of the important display devices. This liquid crystal display requires a backlight as a light source separately from the liquid crystal panel because the liquid crystal itself does not emit light. Until now, cold cathode fluorescent lamps have been the mainstream of backlights. However, due to the recent increase in interest in environmental problems, the environmental problems of cold cathode fluorescent lamps using mercury, which is a harmful substance, have become issues. Therefore, LEDs are currently attracting attention as light sources that can replace cold cathode fluorescent lamps. Using an LED as a backlight can solve the problem of using a cold cathode tube, in addition to the advantage that it does not contain harmful substances. An issue when using a cold cathode tube is the contrast of the screen. Previous LCDs used a cold-cathode tube to project the entire surface, so even if the LCD shutter was completely closed, the backlight light on the back side of the LCD panel leaked to the front, creating a complete black. I couldn't. Therefore, it has a problem that the contrast is low with respect to a self-luminous plasma display or the like. On the other hand, since the LED is a point light source with respect to the cold cathode tube, the brightness can be adjusted for each LED, and dark video information can be made more prominent by reducing the brightness of the LED or turning it off. In general, the brightness is not adjusted for each LED, but the screen is divided into multiple areas, one LED row is arranged for each area, and the brightness of the screen is changed for each divided area. This method is called local dimming. When this local dimming is used, by adjusting the backlight to the brightness that matches the video information set for each area, it is possible to express black more and to add more contrast to the video. Become.

  However, there are also problems when using LEDs as a backlight. This is a variation in brightness in the display surface when the entire surface is projected, which has been a problem even when a cold cathode tube is used. Even when an LED is used as a backlight, naturally, it is necessary to eliminate the luminance variation in the display surface when the entire surface is projected, and for this purpose, each divided area needs to have a uniform luminance. As an LED drive device, since a drive device is generally assigned to each area, it is necessary to eliminate luminance variations in each channel of the drive device. Here, the brightness of the LED is determined by the amount of flowing current. That is, in order to reduce the luminance variation, it is necessary to increase the relative accuracy of the current flowing through each driving device. There are two main reasons for the luminance variation. One is a variation in circuit characteristics in each driving device, and the other is a variation in LED characteristics. As the former variation in circuit characteristics, variation in resistance elements provided in the circuit, variation in amplifiers, and the like can be considered. Moreover, as the latter variation in LED characteristics, variation in light emission intensity with respect to current and variation due to deterioration with time can be considered.

  FIG. 11 shows a conventional load driving device in FIG. As a backlight of the display panel 110, a plurality of LED arrays 120a, 120b,. The LED row 120a is driven by the LED driving device 100a, the LED row 120b is driven by the LED driving device 100b, and the LED row 120n is driven by the LED driving device 100n. The LED string is also called a channel.

The LED driving devices 100a, 100b,... 100n are originally designed to generate the same amount of driving current, but due to variations in resistance values and variations in amplification, between LED driving devices, There may be variations in drive current. In this case, even if the LED incorporated in the plurality of LED rows 120a, 120b,... 120n is a high-quality LED having no variation in emission intensity, the channel due to the variation in driving current from the LED driving device Each backlight has a different brightness, and a display panel with uniform brightness cannot be provided.
Conversely, if there is a variation in the light emission characteristics of the LEDs incorporated in the plurality of LED rows 120a, 120b,... 120n, the drive current from the LED driving devices 100a, 100b,. Even if it is not uniform, the brightness of the backlight differs for each channel due to variations in LED characteristics, and a display panel with uniform brightness cannot be provided.
Furthermore, if there is a variation in drive current and a variation in the light emission characteristics of the LED, it is needless to say that a display panel with uniform brightness cannot be provided.

JP2009-70878 JP2004-219955 JP2006-6056

  An object of the present invention is to provide a display panel having uniform brightness by equalizing the brightness of the backlight for each channel.

  In order to achieve the above object, the load driving device according to the present invention supplies a plurality of loads and a specific driving current adjusted to each of the plurality of loads, and therefore has a unique adjustment value for each of the plurality of loads. A load current control unit having a table storing therein, a variable reference current source that generates a unique reference current for each load based on an adjustment value read from the table, and a reference from the variable reference current source A current holding unit that holds current information and generates a copied reference current based on the reference current information within a predetermined time frame, and generates an adjusted specific drive current based on the copied reference current A constant current drive unit is provided, only one variable reference current source is provided, a plurality of current holding units are provided, and a plurality of constant current drive units are provided.

  Furthermore, the load driving method according to the present invention includes a plurality of loads, only one variable reference current source, a plurality of current holding units, and a plurality of constant current driving units, and is adjusted to each of the plurality of loads. A method for supplying a specific drive current, the step of storing a specific adjustment value for each of a plurality of loads in a table provided in the load current control unit, and based on the adjustment value read from the table Generating a unique reference current for each load from the variable reference current source, holding information on the reference current from the variable reference current source in the current holding unit, and information on the reference current within a predetermined time frame. Generating a reference current copied based on the reference current, and generating a regulated unique drive current based on the reference current copied in the constant current drive unit. That.

  According to the present invention, since the load current control unit having the table storing the unique adjustment values for each of the plurality of loads is provided, the characteristics of the LEDs in each channel are learned in advance, and according to the learning results. The light emission luminance can be adjusted for each channel using a unique adjustment value. Therefore, it is possible to provide a display panel with uniform brightness by equalizing the brightness of the backlight for each channel.

  Further, according to the present invention, it is possible to copy current while maintaining high relative accuracy for each channel by adopting a current copier method as a method of copying current to each channel. In addition, the current information storage function of the current copier system can be used to store different adjusted reference currents in each channel. As a result, the drive current of each channel can be set individually and sequentially.

From the above, it is possible to always maintain high current relative accuracy between channels without increasing the circuit scale.
In addition, the drive current can be changed even when the LED is always lit without PWM control.

1 is a block diagram of a constant current supply circuit according to a first embodiment of the present invention. 1 is a circuit diagram of a constant current supply circuit in Embodiment 1 of the present invention. The circuit diagram which shows another structure of the variable reference current source in Example 1 of this invention Block diagram of constant current supply circuit in Embodiment 2 of the present invention Block diagram of constant current supply circuit in Embodiment 3 of the present invention The chart figure which shows the drive timing of each load in Example 3 of this invention The chart figure which shows the drive timing of each load in Example 3 of this invention The chart figure which shows the drive timing of each load in Example 3 of this invention The chart figure which shows the drive timing of each load in Example 3 of this invention The chart figure which shows the drive timing of each load in Example 2 of this invention Block diagram showing a conventional load driving device

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

Example 1
Hereinafter, an apparatus and a method for driving a plurality of loads in an embodiment of the present invention, an LED driving apparatus and an image display apparatus using the apparatus and method will be described with reference to the drawings.
The liquid crystal display panel using the constant current supply circuit according to the present invention is divided into a plurality of segments, and each segment is arranged with one or a plurality of light emitting elements such as light emitting diodes (LEDs). Has been. These light emitting element groups provide a backlight of a liquid crystal display. By using the constant current supply circuit according to the present invention, each light emitting element group constitutes one channel and can be independently controlled and driven.
When high-quality light-emitting elements are used and there is no variation in the light-emitting characteristics of the light-emitting elements, the constant current supply circuit according to the present invention adjusts the drive current supplied to each channel, and the same for all channels. A constant drive current can be supplied.
Further, even if the light emission characteristics of the light emitting elements vary, the constant current supply circuit according to the present invention adjusts the drive current supplied to each channel to emit light from any light emitting element group at a desired light emission level. I can do it. Therefore, uniform light emission luminance can be obtained over the entire panel of the liquid crystal display. Note that the present invention can be used for a heat generating element instead of a light emitting element.

FIG. 1 shows a configuration of a constant current supply circuit according to an embodiment of the present invention. The constant current supply circuit shown in FIG. 1 includes a variable reference current source 4, n (n is an arbitrary integer) current holding unit 9a, ... (only 9a and 9b are shown in the figure), n constant current driving units 10a,... (Only 10a and 10b are shown in the figure), n loads 13a,... (Only 13a and 13b are shown in the figure) and the load current control unit 2. A power source 20 is connected to the current holding units 9a. The load 13a is a light emitting element group, for example, an LED load.
Here, the subscript a and the subscript b correspond to channel 1 and channel 2, respectively, and one light emitting element group can be controlled for each channel. In addition, in the case where the current holding unit 9 is used to express only numbers without adding the subscripts a and b, the channel is not limited and generally indicates itself.

FIG. 2 is obtained by replacing each part of FIG. 1 with a specific circuit.
The load 13a is composed of k light emitting elements (k is an arbitrary integer) connected in series, for example, LED loads 22a,..., And a drive current 12a,.
The load current control unit 2 includes a channel specifying unit 23, a table 24, a switching signal generator 25, and a PWM control signal generator 26.
In the table 24, for each of the plurality of channels, the address information of the channel and the information of the current value obtained by learning about the channel are recorded. The current value information obtained by learning is output as the reference current control signal 3 for each channel. Learning will be described in detail later. The reference current control signal 3 represents current value information which is an m-bit digital signal (m is an integer and matches the number of input bits of the DAC).

The switching signal generator 25 generates a signal for controlling on / off switching of the switches SW1, SW2, and SW3 included in the current holding unit 9. The switching control signal is output as a current holding control signal 8 to each of the current holding units 9a,. In FIG. 2, the current holding control signal 8 is shown as a bundle of signal lines, but a bus structure may be used.
The PWM control signal generator 26 generates duty information for each channel based on the luminance information included in the video signal, and generates the PWM control signal 11 based on the duty information. The PWM control signal 11 is output to each of the constant current drive units 10a,... At a predetermined timing. In FIG. 2, the PWM control signal 11 is shown as a bundle of signal lines, but a bus structure may be used.

The channel designating unit 23 designates a channel so that the table 24, the switching signal generator 25, and the PWM control signal generator 26 operate in cooperation with each other for the designated channel.
The variable reference current source 4 includes a digital analog converter (hereinafter referred to as DAC) 21, an amplifier AMP1, a resistor R1, and an NMOS transistor Tr1. The adjusted reference current 7 is generated by converting the output of the DAC 21 into a voltage / current. Here, the output of the DAC 21 is determined by m-bit digital data which is the reference current control signal 3 from the load current control unit 2. The m-bit digital data is a learning value adjusted in advance so as to eliminate variations between channels. The reference voltage generated by the DAC 21 is voltage-current converted by the amplifier AMP1, the NMOS transistor Tr1, and the resistor R1, and supplied from the drain as the adjusted reference current 7. In FIG. 2, the drain output draws the adjusted reference current 7.

The current holding unit 9a includes three switches SW1a, SW2a, SW3a, a capacitor C1a, and a PMOS transistor Tr2a. This is a current copier circuit.
The current holding unit 9a performs three operations, which are a current input operation, a current holding operation, and a current output operation, respectively. Each operation is performed by switching the switches SW1a, SW2a, and SW3a by the current holding control signal 8 within the time frame assigned to the channel 1. First, the reference current 7 is passed in a state in which the switch SW1a and the switch SW2a are closed and the switch SW3a is opened. Then, the gate voltage necessary for the reference current 7 to flow the reference current 7 between the source and drain of the PMOS transistor Tr2a is gradually accumulated in the capacitance C1a between the gate and source of the PMOS transistor Tr2a. This is the current input operation. Subsequently, when the gate-source voltage has settled and the switches SW1a and SW2a are opened, the voltage of the gate of the PMOS transistor Tr2a is held by the capacitor C1a. This is the current holding operation. Subsequently, when the switch SW3a is closed, the current determined by the voltage held at the gate, that is, the reference current 7 input earlier flows as it is as the output current 70a. This is the current output operation. In this way, the current copier circuit is a circuit that holds the information of the reference current 7 in the capacitor C1a, and outputs the reference current 70a that is reproduced by accurately reproducing the current from the information.

In addition to the above operation of the current holding unit 9a in the time frame assigned to the channel 1, the same operation as described above is performed in the current holding unit 9b in the time frame assigned to the channel 2 as well. . However, each current holding unit does not simultaneously perform a current input operation.
Here, it is known that the current holding units 9a,... Can read the adjusted reference current 7 with higher accuracy if the PMOS transistors Tr2a,.

  The constant current driving unit 10a includes resistors R2a and R3a, switches SW4a and SW5a, an amplifier AMP2a, and an NMOS transistor Tr3a. The reference voltage is obtained by flowing the adjusted reference current 70a supplied from the current holding unit 9a through the resistor R2a. The operation principle of the voltage-current conversion of the reference voltage is the same as that of the variable reference current source 4. In addition, the switch SW4 and the switch SW5 are provided for PWM control of the drive current 12a. When turning on the drive current 12a, SW4a is closed and SW5a is opened as in the state shown in FIG. On the other hand, in order to turn off the drive current 12a, although not shown, the switch SW4a is opened and the switch SW5a is closed. Although not shown as another PWM control method, there is also a method of dimming by PWM control by inserting a switch on the drive current 12a wiring and turning the switch on / off.

  Here, the current information stored by the current copier circuit cannot be held permanently. This is because the charge held in the capacitor C1a between the gate and the source of the PMOS transistor Tr2a changes with time due to leakage current existing in the capacitor C1a, the switch SW1a, and the like including the gate electrode of the PMOS transistor. When the voltage between the gate and source of the PMOS transistor Tr2a changes, the output current of the current holding unit changes, so that the drive current that should have been adjusted to eliminate the variation between the channels eventually changes. Therefore, it is necessary to periodically refresh the gate-source voltage of the PMOS transistor Tr2a, that is, the stored current information. The refresh timing is a period in which the target current holding unit 9a is not in the current output operation and the other current holding units 9b˜ are not in the current input operation. In other words, the timing of the current input operation such as the refresh of the current information and the change of the current peak value by analog dimming is preferably set to the off period of PWM control in which the drive current is turned off.

Next, information on the current value obtained by learning for each channel will be described.
First, a case where a high-quality light-emitting element having no variation in light emission characteristics is used will be described.
A test signal is input as the video signal 1. As the test signal, for example, a luminance signal having a predetermined level is input on the entire screen. In the table 24, a temporary reference current control signal 3 as an initial value before adjustment is set for all channels. The temporary reference current control signal 3 is used for each channel, and it is checked by using an ammeter or the like whether or not the drive current 12 obtained based on the temporary reference current control signal 3 has a predetermined level of current value. If a predetermined level of current is not obtained, the provisional reference current control signal 3 is increased or decreased until a predetermined level of current is obtained. Then, the reference current control signal 3 at the time when a predetermined level of current is obtained is written in the table 24 as the adjusted reference current control signal 3. In this way, the adjusted reference current control signal 3 obtained by learning is written in the table 24 for all channels. In this case, the drive current supplied to all channels is the same constant drive current. Since high-quality light-emitting elements that do not vary in light-emitting characteristics are used, a display panel with uniform brightness can be provided.
Next, a case where a low-quality light emitting element having a variation in light emission characteristics is used will be described.
A test signal is input as the video signal 1. As the test signal, for example, a luminance signal having a predetermined level is input on the entire screen. In the table 24, a temporary reference current control signal 3 as an initial value before adjustment is set for all channels. Using the temporary reference current control signal 3 for each channel, display with the drive current 12 obtained based on the temporary reference current control signal 3, and check whether or not a predetermined level of brightness has been obtained, using a luminance meter or the like . If the predetermined level of luminance is not obtained, the provisional reference current control signal 3 is increased or decreased until the predetermined level of luminance is obtained. Then, the reference current control signal 3 at the time when a predetermined level of luminance is obtained is written in the table 24 as the adjusted reference current control signal 3. In this way, the adjusted reference current control signal 3 obtained by learning is written in the table 24 for all channels. In this case, the drive current supplied to all channels is a constant drive current that is individually adjusted according to variations in the light emission characteristics. Thereby, a display panel with uniform brightness can be provided.

Next, the operation of the entire drive device when driving the load LED will be described.
First, the load current control unit 2 receives the video signal 1. Channel information is generated based on the video signal 1. The channel information includes address information of the current holding unit 9 in which current is to be stored, duty information for PWM control of each channel, and the like. Information on the current value to be stored is read from the table 24.
The load current control unit 2 performs the following three functions based on the above three information (address information, duty information, and current value information).

  The first function is to input the reference current control signal 3 obtained by learning of the channel addressed from the table 24 to the variable reference current source 4 and store it in the channel from the variable reference current source 4. The current value is input to the current holding unit 9 in the next stage. The current value to be stored at this time is a current value for adjusting the drive current 12 of each channel so that the light emission luminance of the light emitting element does not vary between channels.

  The second function is to input the current holding control signal 8 from the switching signal generator 25 to the current holding unit 9 of the addressed channel based on the address information, and the current input and current of the current holding unit 9 of that channel. Preparing and controlling the holding and current output operations.

The third function is that the PWM control signal generator 26 inputs the PWM control signal 11 to each constant current drive unit 10 based on the duty information of the addressed channel, and the drive current 12 is PWM-controlled and output. That is.
Based on the adjusted reference current control signal 3, the variable reference current source 4 outputs an adjusted reference current 7 that has been subjected to current value correction so as to eliminate variations in backlight luminance between channels. Inside the variable reference current source 4, the peak value of the output current of the variable reference current source is finely adjusted by the reference current control signal 3 input from the load current control unit 2.

Although the details of each operation of the current holding unit 9 have been described above, the current input operation, the current output operation, and the current holding operation are performed, and which operation is performed is determined by the content of the received current holding control signal 8. The current input operation is an operation in which the current holding unit 9 and the variable reference current source 4 are electrically connected so that the current holding unit 9 can read the information of the adjusted reference current 7. In the current output operation, the current holding unit 9 and the constant current driving unit 10 are electrically connected, and the output current of the current holding unit 9, that is, the adjusted reference current 70 read and stored is supplied to the constant current driving unit 10. It is an operation to output. The current holding operation is an operation in which the current holding unit 9 is not electrically connected anywhere and only holds current information.
When a current holding control signal 8 is input to the current holding unit 9 in a certain channel, the current holding unit 9 first moves to a current input operation, and the adjusted reference current 7 output from the variable reference current source 4 is input. . At this time, the current holding unit 9 of the other channel is determined by the driving state of the load, the current holding unit 9 of the channel driving the load performs a current output operation, and the current holding unit 9 of the channel not driving the load Holding operation is in progress.

In a certain channel, after the current holding unit 9 reads and stores the current information, the current holding control signal 8 is input from the load current control unit 2, and the current holding unit 9 shifts from the current input operation to the current holding operation. Further, the current holding control signal 8 is inputted, and the current output operation is started, and a current 70 equal to the adjusted reference current 7 is supplied to the constant current driving unit 10.
The constant current drive unit 10 generates a drive current 12 from a current 70 equal to the adjusted reference current 7 input from the current holding unit 9. Further, the drive current 12 subjected to PWM control is supplied to the load 13 based on the duty information input from the load current control unit 2. According to an example, the reference current 7 and the current 70 are about 100 μA, and the drive current 12 is about 50 mA.

  In addition, as described above, in a channel that does not supply the drive current 12 to the load for a certain period, the current holding unit 9 performs a current holding operation. Therefore, if no other current holding unit performs a current input operation, the current holding unit 9 It is possible to read the peak value of the current when the operation is shifted to and the load is driven next.

  As described above, the LED driving device is configured as shown in FIG. In an LED driving device, particularly an LED driving device for a backlight, luminance variations on the screen can be eliminated when the luminance setting of all areas is the same. In other words, it is possible to control the LED brightness to be equal among all the channels that are driven by the backlight. As described above, since the luminance of the LED is basically determined by the current flowing through the LED, the relative accuracy of the LED driving current among all the channels that are driven by the backlight can be increased. In this embodiment, the variation between the drive currents of the respective channels is examined in advance, and the current information that has been subjected to the variation correction based on the information from the table 24 provided in the load current control unit 2 is used as the reference current control signal 3. It is possible to increase the current relative accuracy between all channels with one DAC.

In addition, since the peak value of the drive current can be changed during driving, it is possible to sequentially correct the deterioration of the value of the drive current 12 as described above, and PWM dimming and analog dimming as brightness adjustment methods. Both are possible, and the degree of freedom in luminance setting can be increased compared to the case of only PWM dimming.
Even if the same driving current is applied, the luminance of the LED is lowered due to deterioration over time, and the luminance at the beginning of driving cannot be maintained. Furthermore, the degree of deterioration over time differs for each LED element. Therefore, the current brightness information of the LED is captured by a brightness sensor or the like, and the reference current correction signal is sequentially determined based on the brightness information so that the brightness of each channel is uniformly required. By mounting, it is possible to cancel the influence of aging of the LED.

  The variable reference current source 4 may have the configuration shown in FIG. In this configuration, the adjusted reference current 7 is generated by combining the correction current 31 determined by the reference voltage generated by the DAC 30 and the predetermined reference current 33 generated by the DAC 32. In the configuration shown in FIG. 2, when one DAC 21 corrects the adjusted reference current 7 with high accuracy, the number of DACs may be one, but a DAC with a large number of bits with high accuracy and high resolution is required. The circuit scale becomes large. On the other hand, in the configuration shown in FIG. 3, the function of the DAC 21 shown in FIG. 2 is shared by two DACs. The DAC 30 in FIG. 3 has fine voltage steps and finely adjusts the adjusted reference current 7 as the final output. Playing the role of current value correction, the DAC 32 has a large voltage step, and plays the role of analog dimming of the LED load brightness that greatly changes the adjusted reference current 7. By dividing the DAC into two functions as described above, accuracy and resolution are not required for each DAC as in the configuration of FIG. 2, and therefore the number of bits of each DAC can be reduced, so that the size can be reduced. it can. Further, when analog dimming is not performed during driving, the reference current 33 may be constant, so the DAC 32 can be changed to a voltage source that supplies a desired reference voltage (not shown).

Although not shown, the DAC 30 is a current source whose output changes in a large step, while the DAC 32 is a current source whose output changes in a step smaller than the large step width. The output current sum of the two current sources can also be used as the output.
Further, the DAC 30 is driven by a temporary reference current control signal 3 (set to a low value so that a positive adjustment value is always added) as an initial value before adjustment, and the DAC 32 is driven by a positive adjustment value. You may make it do. In this case, only positive adjustment values may be recorded in the table 24.
In the constant current supply circuit of this embodiment, the load may be a heating element used in a print head of a thermal printer.

(Example 2)
FIG. 4 shows a configuration of a constant current supply circuit according to the embodiment of the present invention.
The constant current supply method in the present embodiment is the same as that in the first embodiment. The difference from the first embodiment is that the number of current holding units is larger than the number of constant current driving units, and a plurality of current holding units are connected to one constant current driving unit.

  The constant current supply circuit shown in FIG. 4 has a variable reference current source 4 and p (p is an arbitrary integer) current holding units 91a and 92a, n (n is an arbitrary integer and smaller than p) constant current drive. The unit 10a, n loads 13a, and the load current control unit 2 are configured. Here, the load is, for example, an LED load. The relationship between p and n is, for example, p = 2n or 2p = 3n.

  In FIG. 2 shown in the first embodiment, it has been described that the next current information is input to the current holding unit 9a while the drive current 12a is off. However, if the off-period of the drive current 12a is shorter than the time during which the voltage of the capacitor C1a and the gate of the PMOS transistor Tr2a is a desired value, the correct information cannot be read and the incorrect drive current 12a is output. Also, in the constant lighting mode in which the drive current 12a is not turned off, once the current information is read, the current information stored during the drive cannot be refreshed, so the drive current 12a changes due to deterioration of the stored information in the current holding unit 9a. Resulting in. If the current information is changed halfway, it is necessary to electrically disconnect the constant current drive unit 10a connected to the current holding unit 9a to which the current information is to be changed and stop the drive current in order to read the current information again. Come out. In the case of an LED load, when the drive current 12a is turned off, the instantaneous luminance decreases, which is inappropriate as a backlight. According to the present embodiment, the current information can be accurately read even in a state where the constant lighting mode and the drive current off period are short and sufficient time cannot be taken to update the current information.

  In the present embodiment shown in FIG. 4, there are two current holding portions 91a and 92a for one constant current driving portion 10a. Now, it is assumed that the current holding unit 91a performs a current output operation, supplies the adjusted reference current 71a stored in the constant current driving unit 10a, and generates the driving current 12a. Meanwhile, since the current holding unit 92a is not performing a current output operation, the current holding unit 92a can read current information of the next drive current 12a and new current information that has not deteriorated with time. When it is desired to refresh the current information or when the current peak value of the drive current 12a is switched, the current holding unit 91a is changed from the current output operation to the current holding operation or the current input operation by the current holding control signal 8, and the current holding unit 92a Is set to a current output operation, the newly stored adjusted reference current 72a is supplied to the constant current drive unit 10a, and a new drive current 12a is generated. FIG. 10 shows the timing of current holding control in the constant lighting mode when p = 2n. As shown in FIG. 10, the current holding unit 91a and the current holding unit 91b alternately and continuously supply the reference currents 71a and 72a, and always supply a constant drive current to the load 13a. It is possible to reliably refresh to a desired current value even in a driving state in which an off period sufficient to read current information cannot be taken while driving a load by the above method. Also in the always-on mode, it is possible to change the current peak value of the drive current 12a by correcting the deterioration of the current holding portion with time and the LED luminance change with time immediately after the load is turned on in the same way by the same method.

(Example 3)
FIG. 5 shows a configuration of a constant current supply circuit that suppresses current variation according to the embodiment of the present invention.
The constant current supply method in the present embodiment is the same as that in the first embodiment. The difference from the first embodiment is that a matrix switch unit 60 is provided. The matrix switch unit 60 enables analog dimming and fine adjustment of the current peak value even when the full load is always in the lighting mode as in the second embodiment.

  The constant current supply circuit shown in FIG. 5 includes a variable reference current source 4, x (x is an arbitrary integer) current holding units 9a,... 9e, and y (y is an arbitrary integer) constant current driving unit 10a. ,... 10d and z loads (z is an arbitrary integer) 13a,... 13d, load current control section 2, and matrix switch section 60. For example, the load is an LED load. The relationship between x, y, and z may be z <y or z <x. As will be described later, z <y or z <x may be satisfied. Furthermore, z = y or z = x may be sufficient.

  In the present embodiment, the matrix switch unit 60 includes a first matrix switch unit 61 that switches a connection relationship between the current holding unit 9 and the constant current drive unit 10, and a connection between the constant current drive unit 10 and the load 13. A second matrix switch unit 62 for switching the relationship is included. The first matrix switch unit 61 is a switch that allows any one of the reference currents 70a,... 70d copied from the current holding units 9a,... 9e to flow to any one of the constant current drive units 10a,. The second matrix switch unit 62 is a switch that allows any one of the drive currents 12a to 12d from the constant current drive units 10a to 10d to flow to any one of the loads 13a to 13d. The load current control unit 2 outputs a path control signal 63 to change the connection destination of the matrix switch unit 60.

  FIG. 6 is a timing chart showing the driving state of the current holding unit 9 when the loads 13a, 13b, 13c, and 13d are in the always-on mode. The constant current driver 10a is connected to the load 13a, the constant current driver 10b is connected to the load 13b, the constant current driver 10c is connected to the load 13c, and the constant current driver 10d is connected to the load 13d. In the constant current supply circuit shown in FIG. 5, the number of current holding units 9a,... 9e is one more than the number of constant current driving units 10a,... 10d and loads 13a,. ing. Therefore, by inputting the current peak value to be changed using the surplus current holding unit and switching the current holding unit by driving the current holding unit storing the current information with the matrix switch unit 60 and the channel to be changed, The current peak value can be changed while the lamp is lit. For example, in the load 13a in the section a shown in FIG. 6, the current holding unit 9a supplies current to the constant current driving unit 10a to drive the load 13a. In this section a, the current holding unit 9e remains, so that new current information for driving the load 13a is input to the current holding unit 9e during the section a, and the constant current driving unit is switched by the matrix switch unit 60 when the section b is switched. By switching the connection between 10a and the current holding unit 9a to the current holding unit 9e, the current peak value can be changed while the load 13a is driven. As described above, in the circuit shown in this embodiment, the current information can be refreshed and the current peak value can be changed during driving even in the always-on mode. In the second embodiment, a plurality of current holding units are necessary for the constant current driving unit of each channel. However, in this embodiment, since only the number of channels + 1 current holding units is required, the number of current holding units is reduced. It is possible to reduce. In the timing chart of FIG. 6, the constant current supply circuit of FIG. 5 does not need to change the connection between the constant current driving unit 10 and the load 13. The scale can be reduced.

Further, by using the matrix switch unit 60, it is possible to reduce the number of current holding units and constant current driving units with respect to the number of loads under a specific driving condition. An example of the driving condition is shown in the timing chart of FIG. According to the timing chart shown in FIG. 7, each load is driven with 50% duty, the rise of each channel is delayed by 1/4 period, two loads are always driven, and the remaining two loads are driven. There will be no. FIG. 7 also shows information on the current holding unit and the constant current driving unit that drive each load. Here, each current holding unit 9 and the constant current driving unit 10 are connected by a matrix switch unit 60. The current holding unit 9a is connected to the constant current driving unit 10a, the current holding unit 9b is connected to the constant current driving unit 10b, and It is assumed that the current holding unit 9c is connected to the constant current driving unit 10c.
In the timing chart of FIG. 7, the current holding unit 9a and the constant current driving unit 10a drive the load 13a in the interval c, and the current holding unit 9a and the constant current driving unit 10a do not drive any load immediately after the interval c. . On the other hand, the current holding unit 9a and the constant current driving unit 10a drive the load 13d in the section d, and further drive the load 13c in the section e.

  By introducing the matrix switch unit 60 in this way, a current holding unit that is not used for a given period can be driven by being allocated to a channel that requires the next set current, thereby efficiently. Each part can be used. As a result, for example, in the timing chart shown in FIG. 7 in which one or more loads are always turned off at any time, the number of constant current driving units or the number of current holding units x with respect to the number z of loads. The number can be reduced. That is, z> y or z> x may be satisfied.

  However, as described in the second embodiment, a predetermined time is required to accurately read current information in the current holding unit. Therefore, for example, consider a timing chart as shown in FIG. In the timing chart shown in FIG. 8, the current holding unit and the constant current driving unit for driving each load are switched each time as in the timing chart shown in FIG. Here, at timing 80, the current holding unit 9c needs to switch from the driving condition of the load 13d to the driving condition of the load 13c. However, as described above, the driving current is turned off for a predetermined time in order to accurately read the current information. There is a need. This method can be used if the reading time required to turn off the drive current is sufficiently short with respect to the cycle of the drive current and there is no problem such as LED flickering on the load side. On the other hand, if the reading time is not negligible with respect to the drive current cycle and there is a problem such as LED flickering, the problem is to increase only one current holding part and start according to the timing chart shown in FIG. Can be solved. In the timing chart shown in FIG. 9, the current holding unit that drives each load is fixed, and only the constant current driving unit is switched by the matrix switch unit each time. Although this method depends on the amount of current driving the load and the adjusted reference current, the circuit scale of the constant current driving unit 10 is generally much larger than that of the current holding unit 9. Therefore, the same number of current holding units 9 as the loads 13 are required, but it can be said that it is advantageous because the number of constant current driving units 10 having a larger circuit scale can be reduced.

  The constant current supply device of the present invention has the effect that the current peak value can be adjusted successively so as to eliminate variations in the drive current of each channel even during driving, such as a backlight of a liquid crystal television, an organic EL, etc. This is useful for driving devices that require the relative accuracy of each channel.

1 channel information 2 load current control unit 3 reference current control signal 4 variable reference current source 7 adjusted reference current 8 current holding control signal 9 current holding unit 10 constant current driving unit 11 PWM control signal 12 driving current 13 load 21 DAC
22 LED load 60 Matrix switch

Claims (13)

A plurality of channels for supplying drive current to each of a plurality of loads;
A load current control unit including a table storing a specific adjustment value for each of the plurality of channels in order to supply the adjusted drive current to each of the plurality of channels;
A variable reference current source that generates a unique reference current for each channel based on the adjustment value read from the table;
A current holding unit for holding reference current information from the variable reference current source and generating a reference current copied based on the reference current information within a predetermined time frame;
Based on the copied reference current, a constant current drive unit that generates an adjusted drive current is provided,
Only one variable reference current source is provided,
A plurality of the current holding units are provided,
A plurality of the constant current driving units are provided,
A load driving device characterized by that.   Further, a first matrix switch unit is provided between the plurality of current holding units and the plurality of constant current driving units, and the reference current copied from any one of the current holding units can be supplied to any one of the constant current driving units. In addition, a second matrix switch unit is provided between the plurality of constant current driving units and the plurality of channels so that the driving current from any one of the constant current driving units can be supplied to any one channel. The load driving device according to claim 1, wherein   3. The load driving device according to claim 1 or 2, wherein the variable reference current source changes a peak value of an output current according to an input digital signal.   The variable reference current source includes a current source whose output changes in a large step and a current source whose output changes in a step smaller than the large step width, and outputs the sum of output currents of the two current sources. The load driving device according to claim 3, wherein The current holding unit is a current copier circuit,
3. The load driving device according to claim 1, wherein an output current of the variable reference current source is stored during a period in which the current copier circuit does not output a current. The constant current drive unit internally generates a current multiplied by a predetermined amount from the current input from the current holding unit,
Supplying the channel with the predetermined current as an output current of the constant current drive unit;
3. The load driving device according to claim 1, wherein PWM output is performed with the output current as a peak value. The number of the current holding unit, the constant current driving unit, and the channel is equal,
In the state where the drive current is PWM output in the constant current drive unit,
2. The load driving device according to claim 1, wherein the current holding unit stores and holds current information output from the constant current driving unit during an OFF period of PWM output. Having more current holding portions than the number of the channels;
Two or more current holding units are connected to one constant current driving unit,
In the state where the drive current is always supplied to the channel,
While one current holding unit stores current information, the other current holding unit supplies current to the constant current driving unit,
The load driving device according to claim 1, wherein the current holding unit that outputs a current to the constant current driving unit is switched. More than the number of the channels, and the current holding unit surplus with respect to two or more constant current drive units,
In a state where drive current is always supplied to all channels,
Current information that is to be updated is stored in the surplus current holding unit that does not output current at any arbitrary time, and is connected to the constant current driving unit of the column that is to be updated by the first matrix switch unit 3. The load driving device according to claim 2, wherein the connection between the current holding unit and the surplus current holding unit is switched. Having the constant current drive unit less than the number of the channels;
In the drive state where the drive current is PWM output in the constant current drive unit, and there is no timing at which all channels are simultaneously turned on at any time,
The current holding unit and the channel connected to the constant current driving unit whose driving current is turned off at a certain arbitrary time, and the current holding unit and the driving unit that hold current information to be turned on at the certain arbitrary time. 3. The load driving device according to claim 2, wherein the channel is switched. Equipped with multiple channels that supply drive current to each of multiple loads, only one variable reference current source, multiple current holding units, and multiple constant current drive units, adjust to each of multiple channels A method for supplying a driven current comprising:
Storing a unique adjustment value for each of a plurality of channels in a table provided in the load current control unit;
Generating a unique reference current for each channel from a variable reference current source based on the adjustment values read from the table;
Holding the reference current information from the variable reference current source in the current holding unit, and generating a copied reference current based on the reference current information within a predetermined time frame; and
And a step of generating an adjusted drive current based on the reference current copied in the constant current drive unit.   3. The load driving device according to claim 1, wherein the load is a light emitting element or a heat generating element.   The load driving method according to claim 11, wherein the load is a light emitting element or a heat generating element.

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