JP5899002B2 - Light emitting diode drive circuit control method and light emitting device - Google Patents

Description

  The present invention relates to a light emitting diode drive circuit control method and a light emitting device.

  A light emitting device such as a backlight is used for a display device such as a liquid crystal display. The backlight includes, for example, a plurality of light emitting diodes (hereinafter referred to as “LEDs”). The LED is a diode that emits light by applying a forward voltage Vf having a predetermined magnitude. In general, due to manufacturing variations, the minimum Vf required for light emission differs for each LED.

  Patent Document 1 describes an LED driver (driving circuit) that emits light by applying a voltage to an LED. This LED driver emits light by applying a reference voltage to each LED. Since the reference voltage is common to all the LEDs, the reference voltage is set to be equal to or greater than the largest value among the minimum necessary Vf. Therefore, when the LED driver described in Patent Document 1 is used, an unnecessarily large voltage is applied to an LED having a relatively small minimum Vf, and the current generated in each LED varies. As a result, the light emission luminance of each LED varies.

  An LED driver that solves such a problem is described in Patent Document 2. The LED driver described in Patent Document 2 includes a bipolar transistor. While applying a reference voltage to each LED and absorbing a minimum necessary variation in Vf by the bipolar transistor, a current having the same value is supplied to each LED. In order to suppress variations in light emission luminance.

JP 2008-77892 A JP 2007-96287 A

  The LED driver described in Patent Document 2 unifies the current value generated in each LED by absorbing a variation in Vf necessary for light emission by a bipolar transistor. Therefore, in an LED having a relatively small minimum required Vf, a reference voltage larger than the Vf is applied, so that power consumption by the bipolar transistor is increased and extra power is consumed.

  In order to solve such problems, in recent years, more advanced LED drivers (high-function LED drivers) have been proposed. For example, upon receiving a current and voltage adjustment command, the high function LED driver first applies a reference voltage value to the LED while performing constant current control. Then, the cathode side voltage of the LED is detected. When the detected voltage is low, the applied voltage is increased by one step, and the cathode side voltage of the LED is detected again. When the detected voltage is high, the applied voltage is lowered by one step, and the cathode side voltage of the LED is detected again. The high-performance LED driver repeats this and applies a voltage having a value that is a minimum required Vf or a little larger than that to each LED. As a result, variations in light emission luminance are suppressed, and an increase in power consumption is suppressed.

  The adjustment of current and voltage by the high function LED driver as described above is performed for each LED connected to the high function LED driver. For example, in the case of an LED backlight for local dimming that adjusts the brightness for each small area of the screen, a plurality of such high-function LED drivers are provided, so one adjustment of current and voltage by the high-function LED driver is performed. If it is completed sequentially, it takes a lot of time to activate the backlight. In the future, as the area to be subjected to local dimming becomes smaller and the number of areas increases accordingly, this problem becomes more prominent.

  The present invention is for solving such a problem, and for an LED light-emitting device including a plurality of LED drivers for driving LEDs, the time until the adjustment of all currents and voltages of the LED drivers is completed is shortened. It is an object of the present invention to provide a method for controlling a driving circuit of a light emitting diode and a light emitting device.

The present invention is caused by variation in applied voltage that is minimum required for each light emitting diode to emit light by adjusting a plurality of light emitting diodes, a voltage applied to each light emitting diode, and a current generated in each light emitting diode. A method of controlling each drive circuit in a light emitting device comprising a plurality of drive circuits capable of performing an adjustment operation for suppressing variations in light emission luminance and a control circuit for controlling the plurality of drive circuits,
After the control circuit outputs an adjustment command for causing at least two or more of the drive circuits to perform the adjustment operation in parallel and causing one drive circuit to perform the adjustment operation, the adjustment operation by the drive circuit is performed. Before the completion, the control circuit outputs the adjustment command to another drive circuit to which the adjustment command is not output .

In the present invention, each drive circuit is connected as a slave having the control circuit as a master by a branched serial bus.
The control circuit outputs the adjustment command to all the drive circuits functioning as slaves in accordance with information indicating each drive circuit assigned to each drive circuit.

  In the present invention, the control circuit repeatedly performs polling for confirming completion of the adjustment operation in the order in which the adjustment instructions are output to the drive circuits, and the adjustment operation by all the drive circuits is performed. It is characterized by confirming completion.

The present invention also includes a plurality of light emitting diodes,
By adjusting the voltage applied to each light-emitting diode and the current generated in each light-emitting diode, an adjustment operation is performed to suppress variations in light emission luminance caused by variations in the minimum applied voltage required for each light-emitting diode to emit light. A plurality of drive circuits capable of,
A control circuit for controlling the plurality of drive circuits, wherein at least two or more drive circuits perform the adjustment operation in parallel, and output an adjustment command for causing one drive circuit to perform the adjustment operation; The light emitting device includes: a control circuit that outputs the adjustment command to another drive circuit to which the adjustment command is not output before the adjustment operation by the drive circuit is completed. It is.

  According to the present invention, since at least two or more drive circuits perform the adjustment operation in parallel by the control circuit, it is possible to shorten the time until all the adjustment operations of the drive circuits are completed.

Further, the control circuit, after outputting the adjustment command to a single drive circuit, before the control operation by the driving circuit is completed, it outputs an adjustment command to the other drive circuit adjustment command is not output Therefore, the circuit scale can be reduced as compared with the case where at least two or more drive circuits perform the adjustment operation in parallel , thereby performing the adjustment operation by each drive circuit simultaneously.

  Further, according to the present invention, each drive circuit is connected as a slave having a control circuit as a master by a branched serial bus, so that only one control circuit is required, and the configuration of the light emitting device is simplified. can do.

  Further, according to the present invention, the control circuit repeatedly performs polling in the order in which the adjustment commands are output, so the number of repeated polling is reduced to confirm that the adjustment operation by all the drive circuits is completed. Can do.

Further, according to the present invention, it is possible to shorten the time until all the adjustment operations of the respective drive circuits are completed, and it is possible to shorten the startup time.
In addition, after the control circuit outputs the adjustment command to one drive circuit, before the adjustment operation by the drive circuit is completed, the adjustment command is output to another drive circuit to which the adjustment command is not output. Therefore, the circuit scale can be reduced as compared with a configuration in which the adjustment operation is performed in parallel in at least two drive circuits, and the adjustment operation by each drive circuit is performed simultaneously.

It is a figure which shows the LED light-emitting device 1 which concerns on this invention. 4 is a wiring system diagram showing a connection state between a driver control IC chip 5 and a plurality of drivers 4. FIG. 3 is a circuit diagram showing a part of the configuration of a driver 4. FIG. It is a flowchart which shows the control method which concerns on this invention. It is a figure which shows a mode that adjustment operation by each driver 4 is performed in parallel.

  FIG. 1 shows an LED light emitting device 1 according to the present invention. The LED light emitting device 1 includes a plurality of light emitting units 3 including LEDs 2, twelve high-function LED drivers (hereinafter referred to as “drivers”) 4, a driver control IC (Integrated Circuit) chip 5, a RAM ( A random access memory (Random Access Memory) 6, a ROM (Read Only Memory) 7, and a power supply unit 8 are provided. For example, in a liquid crystal display, the LED light emitting device 1 is installed in a backlight that irradiates light from the back to a liquid crystal panel.

  The light emitting unit 3 is a row of six LEDs 2 connected in series (hereinafter referred to as “ch”). A total of 192 light emitting units 3 are provided, and every 16 light emitting units 3 are connected to different drivers 4. Each light emitting unit 3 is connected to a power supply unit 8. In the present embodiment, the light emitting unit 3 exists from 1ch to 192ch, and in this way, in the liquid crystal display, for example, the luminance can be adjusted for each of 192 small areas divided into 16 × 12.

  The power supply unit 8 generates a power supply voltage for causing each light emitting unit 3 to emit light. The voltage generated by the power supply unit 8 is a voltage based on 37.5 V. As will be described later, the voltage applied to each light emitting unit 3 is variable for each light emitting unit 3 by the driver 4. .

  The driver control IC chip 5 and the plurality of drivers 4 are connected via an SPI (Serial Peripheral Interface) bus 9. FIG. 2 shows a connection state between the driver control IC chip 5 and the plurality of drivers 4. The driver control IC chip 5 functions as a master, and the SPI bus 9 branches and is connected to each driver 4 individually, whereby each driver 4 functions as a slave. The SPI bus 9 has a chip selection line CS_N (N = 1 to 12), a clock line CLK, a slave input / master output line SIMO, and a slave output / master input line SOMI. As shown in FIG. 2, except for the chip selection line CS_N, it is common to each slave.

  The driver control IC chip 5 is also connected to the RAM 6, the ROM 7, and the power supply unit 8. The driver control IC chip 5 reads the driver control program data from the ROM 7, writes it in the RAM 6, develops it, and operates the power supply unit 8 when the backlight is activated. In addition, the driver control IC chip 5 outputs a target current value signal indicating a target current value of a current generated in each light emitting unit 3 to each driver 4. Thereafter, the driver control IC chip 5 performs an adjustment operation for each light emitting unit 3 to adjust the applied voltage in the vicinity of Vf that is the minimum necessary for light emission and to generate a current of the target current value. 4 outputs a signal (adjustment command) to be executed. After the adjustment is completed, the driver control IC chip 5 adjusts the luminance of each light emitting unit 3 with respect to each driver 4 based on image information input from a CPU (Central Processing Unit) of the liquid crystal display. The brightness adjustment signal is output.

  The RAM 6 is composed of a rewritable semiconductor memory such as a static random access memory (SRAM). The ROM 7 is, for example, a non-volatile and rewritable flash ROM, and stores program data for driver control. The ROM 7 stores an address corresponding to a terminal to which each chip selection line CS_N is connected, that is, information indicating each driver 4 assigned to each driver 4.

  Each driver 4 is an IC chip that controls the light emitting units 3 to emit light. The driver 4 includes a current register, in which each target current value indicated by each target current value signal input from the driver control IC chip 5 is set. When an adjustment command is input from the driver control IC chip 5, each driver 4 sets the current so that the forward current generated in each light emitting unit 3 becomes each target current value set in the current register. Control. At this time, the driver 4 performs adjustment so that the forward voltage applied to each light emitting unit 3 is in the vicinity of Vf that is the minimum necessary for light emission. Such adjustment of current and voltage is performed without using the driver control IC chip 5.

  Hereinafter, the adjustment operation of the current and voltage of the light emitting unit 3 by the driver 4 will be described in more detail. For example, when an adjustment command is input, the driver 4 applies a voltage with a reference voltage value to one of the 16 light emitting units 3 connected to the driver 4. Thereafter, the cathode side voltage of the light emitting unit 3 is detected. If the detected voltage is low, the applied voltage is increased by one step and the cathode side voltage of the light emitting unit 3 is measured again. When the detected voltage is high, the applied voltage is lowered by one step, and the cathode side voltage of the light emitting unit 3 is detected. The driver 4 repeats such an operation and adjusts the voltage applied to the light emitting unit 3 to be in the vicinity of the minimum necessary Vf, thereby absorbing the Vf variation of each LED 2. Each driver 4 performs such an adjustment operation on the 16 light emitting units 3 connected thereto. In the present embodiment, the cathode side voltage of the light emitting unit 3 is adjusted to 0.5V. When the adjustment operation is completed for all the light emitting units 3, each driver 4 sets a value indicating that the adjustment operation is completed in an adjustment completion register included in the driver 4.

  Note that the driver 4 according to the present invention may be a more advanced LED driver. A higher-function LED driver performs control so that the current value is deliberately higher than the target current value so that the applied voltage can be finely adjusted to absorb the minimum necessary variation in Vf. At this time, the LED driver reduces the duty ratio described later in accordance with the increased current in order to reduce the emission luminance that has increased with the increase in the current value.

  After the above adjustment operation, the driver 4 performs luminance control by a pulse width modulation (PWM) method for each light emitting unit 3 in accordance with the luminance adjustment signal input from the driver control IC chip 5. The frequency of PWM is 1560 Hz, for example.

  FIG. 3 is a circuit diagram showing a part of the configuration of the driver 4. FIG. 3 shows a portion of the driver 4 corresponding to one light emitting unit 3. The driver 4 includes a transistor Tr1, a transistor Tr2, a resistor Rs, a differential amplifier Amp, and a voltage adjustment circuit Va for each channel. In the present embodiment, the transistors Tr1 and Tr2 are P-type field effect transistors, but may be constituted by N-type field effect transistors.

  The six LEDs 2, the transistors Tr1 and Tr2, and the resistor Rs are connected in series in this order between the power supply unit 8 and the ground. Specifically, the anode of the first LED 2 is connected to the power supply unit 8, and the cathode of the first LED 2 is connected to the anode of the second LED 2. The second to sixth LEDs 2 are connected to the cathode of the LED 2 at each stage and the anode of the LED 2 at the next stage of each stage. The cathode of the sixth LED 2 is connected to the drain of the transistor Tr1, the source of the transistor Tr1 is connected to the drain of the transistor Tr2, the source of the transistor Tr2 is connected to one end of the resistor Rs, and the other end of the resistor Rs. Is connected to ground. The resistor Rs is a resistor element.

  A PWM signal that indicates the pulse width of the duty ratio that is instructed from the driver control IC chip 5 is input to the gate of the transistor Tr1. The duty ratio is a ratio of the pulse width that is turned on in one period of the PWM signal, and is expressed as a percentage. When the PWM signal is on, that is, at a high level, the transistor Tr1 becomes conductive, and when the PWM signal is off, that is, at a low level, the transistor Tr1 is disconnected. Therefore, the driver control IC chip 5 can change the time during which the light emitting unit 3 is lit in one cycle by changing the pulse width.

  The output terminal of the differential amplifier Amp is connected to the gate of the transistor Tr2. The target current value signal is input from the driver control IC chip 5 to the non-inverting input terminal of the differential amplifier Amp, and one end of the resistor Rs is input to the inverting input terminal of the differential amplifier Amp. The transistor Tr2 and the differential amplifier Amp are controlled so that the current value of the current generated in the light emitting unit 3 matches the target current value.

  The voltage adjustment circuit Va is connected to the power supply unit 8 and detects the voltage between both terminals of the resistor Rs. When the detected voltage is low, the applied voltage is adjusted higher by one step. If it is high, the applied voltage is adjusted one step lower.

  As the driver 4 configured in this manner, for example, iW7032 or iW7040 manufactured by iWatt can be used.

  The LED driver control method according to the present invention is a control method of the LED light emitting device 1 configured as described above, and is performed by the driver control IC chip 5. FIG. 4 is a flowchart showing this control method.

  In this control method, first, the driver control IC chip 5 sets a numerical value “1” to a variable i that is a soft counter (step S1). Next, the driver control IC chip 5 outputs an adjustment command to the i-th driver 4 via the SPI bus 9 (step S2). Next, the driver control IC chip 5 determines whether or not the value of the variable i is less than 12 (step S3). If it is less than 12, the process proceeds to step S4. If not less than 12, the process proceeds to step S5. . In step S4, the value of variable i is incremented by 1, and the process returns to step S2. As a result, the adjustment command is output to the other driver 4 that has not output the adjustment command.

  The adjustment instruction output by the driver control IC chip 5 is performed without much time. This is shown in FIG. As shown in FIG. 5, the adjustment command is output so that the adjustment operation by each driver 4 partially overlaps in time. That is, the adjustment operation by each driver 4 is performed in parallel. If the adjustment operation by each driver 4 is not performed partly in parallel, but all are performed simultaneously, it is necessary to prepare the driver control IC chips 5 and the SPI buses 9 as many as the number of drivers 4, which is realistic. Absent.

  In step S <b> 5, the driver control IC chip 5 sets a numerical value “1” to the variable i. Next, the driver control IC chip 5 checks the adjustment completion register of the i-th driver 4 via the SPI bus 9, and if the adjustment is completed, sets an adjustment completion flag in the RAM 6 (step S1). S6). Next, the driver control IC chip 5 determines whether or not the value of the variable i is less than 12 (step S7). If it is less than 12, the process proceeds to step S8. If not less than 12, the process proceeds to step S9. . In step S8, the value of variable i is incremented by 1, and the process returns to step S6. As a result, all drivers 4 are polled for completion of adjustment.

  In step S9, the driver control IC chip 5 checks the RAM 6 to check whether or not the adjustment completion flag is set for all the drivers 4, and if the flag is set, ends the control for adjustment. If not, the process returns to step S5.

  Assuming that the driver control IC chip 5 requires 1 ms for each driver 4 to output an adjustment command and 200 ms for each driver 4 to perform the adjustment operation, the control as described above is performed. When the method is executed, the time taken to complete the adjustment operation of all the drivers 4 is 200 ms + 1 ms × 12 = 212 ms. On the other hand, when the adjustment operation by each driver 4 is completed one by one, the total time is (200 ms + 1 ms) × 12 = 2412 ms. Therefore, according to the control method of the present invention, the backlight adjustment sequence can be completed quickly, and the start-up time when the backlight is activated can be greatly reduced. Here, the above-mentioned 1 ms or 200 ms is only an example, and it may be shorter than 1 ms or 200 ms, but even in such a case, the startup time at the time of starting the backlight is greatly reduced. be able to.

  The adjustment operation by each driver 4 is shortened if at least two of the adjustment operations overlap in time. As shown in FIG. 5, if the adjustments by all the drivers 4 are overlapped in time, the time is greatly reduced.

1 LED light emitting device 2 LED
3 Light Emitting Unit 4 Driver 5 IC Chip for Driver Control 6 RAM
7 ROM
8 Power supply unit 9 SPI bus

Claims (4)

By adjusting the voltage applied to each light-emitting diode and the current generated in each light-emitting diode, variations in light emission luminance caused by variations in the minimum applied voltage required for each light-emitting diode to emit light A method of controlling each drive circuit in a light-emitting device comprising a plurality of drive circuits capable of performing an adjustment operation to suppress the control and a control circuit for controlling the plurality of drive circuits,
After the control circuit outputs an adjustment command for causing at least two or more of the drive circuits to perform the adjustment operation in parallel and causing one drive circuit to perform the adjustment operation, the adjustment operation by the drive circuit is performed. Before the completion, the control circuit outputs the adjustment command to another drive circuit to which the adjustment command is not output . Each drive circuit is connected as a slave having the control circuit as a master by a branched serial bus,
3. The light emitting diode according to claim 2, wherein the control circuit outputs the adjustment command to all the drive circuits functioning as slaves in accordance with information indicating each drive circuit assigned to each drive circuit. A method for controlling the drive circuit.   The control circuit repeatedly performs polling for confirming completion of the adjustment operation in the order in which the adjustment instructions are output to each drive circuit, and confirms that the adjustment operation by all the drive circuits is completed. 4. The method of controlling a drive circuit for a light emitting diode according to claim 2, wherein: A plurality of light emitting diodes;
By adjusting the voltage applied to each light-emitting diode and the current generated in each light-emitting diode, an adjustment operation is performed to suppress variations in light emission luminance caused by variations in the minimum applied voltage required for each light-emitting diode to emit light. A plurality of drive circuits capable of,
A control circuit for controlling the plurality of drive circuits, wherein at least two or more drive circuits perform the adjustment operation in parallel, and output an adjustment command for causing one drive circuit to perform the adjustment operation; The light emitting device includes: a control circuit that outputs the adjustment command to another drive circuit to which the adjustment command is not output before the adjustment operation by the drive circuit is completed. .

Images