JP2010015728A - Backlight dimming control device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an inexpensive backlight dimming control device capable of setting and adjusting an luminosity optionally and miniaturizing the size without using a constant voltage circuit and making the luminosity of a LED dependent on a supplied voltage. <P>SOLUTION: The backlight dimming control device includes a light-emitting circuit 20 composed of one or more LEDs connected in series, which emit light by electric power from a power source +B, a current control circuit 21 serially connected to the light-emitting circuit 20 and making an emitter current of a transistor TR variable by controlling a base voltage of the transistor TR in an emitter follower connection, a current output circuit 22 serially connected to the current control circuit 21, determining its operating current, and determining an operating current of the transistor TR0 of the light-emitting circuit 20, and a modulation circuit 23 carrying out PWM of the current of the light-emitting circuit 20. For example, a lighting period is increased for a current reduced by a voltage drop of the power source +B by detecting a voltage supplied to the light-emitting circuit 20 from the power source +B by a voltage detection circuit 10, and by adjusting a pulse width of PWM based on the detected voltage by the voltage detection circuit 10. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a backlight dimming control device for a backlight that illuminates the back of a liquid crystal screen used in a car audio device, a navigation device, various displays, a mobile phone, a mobile terminal, and the like of an automobile.

  As a conventional backlight dimming control device that is widely used, a backlight circuit that arranges LED series circuits composed of transistors connected in series with light emitting diodes (hereinafter simply referred to as “LEDs”) in a plurality of rows and in a planar shape. The equipment used is known. In this known backlight dimming control device, a transistor connected in series with an LED is ON / OFF controlled by pulse width modulation (hereinafter simply referred to as “PWM signal”). The brightness of the LED, that is, the brightness of the backlight is adjusted by the ratio (duty ratio) between the ON time and the OFF time of the PWM signal.

FIG. 1 is an electric circuit diagram of a well-known vehicle backlight circuit, in which (a) is a circuit not having a constant voltage circuit, and (b) is a circuit having a constant voltage circuit. is there. FIG. 2 shows a current waveform (a) and a voltage waveform (b) of a backlight circuit for a vehicle.
As shown in FIG. 2A, when the output (voltage) of the vehicle battery 1 is directly used, the backlight circuit 2 of the liquid crystal display in which LEDs are connected in series, as shown in FIG. When the PWM signal is turned on, a predetermined current I flows, and when the PWM signal is turned off, the predetermined current I that has been energized until then is cut off.

At this time, even if the internal resistance of the battery 1 is ignored, the power supply voltage E of the battery 1 decreases by the voltage drop R · I due to the wiring resistance R, and the backlight circuit of the liquid crystal display The power supply voltage + B applied to 2 is + B = E−R · I (V). When this is illustrated, the current I is as shown in FIG. 2A, and the power supply voltage + B is as shown in FIG. 2B.
Furthermore, in the LED illumination of the backlight circuit 2 of the in-vehicle liquid crystal display, when the power supply voltage + B is the vehicle battery 1, there is a voltage fluctuation due to the change in the amount of power generation due to the engine speed, etc. FIG. When the power supply voltage + B is directly used from the vehicle battery 1 or the generator, the luminance of the backlight circuit 2 fluctuates due to the voltage fluctuation. Therefore, normally, as shown in FIG. 1B, a constant voltage circuit 3 is interposed on the load side of the battery 1 of the vehicle.

  However, as shown in FIG. 1B, the backlight dimming control in which power is supplied to the backlight circuit 2 through the constant voltage circuit 3 needs to increase the output capacity of the constant voltage circuit 3. There is a problem that increasing the capacity increases the cost. Also, depending on the output capacity and internal resistance of the constant voltage circuit 3, if the load side instantaneously causes a larger current change, this results in a voltage fluctuation as shown in FIG. There is a possibility that fluctuations in luminance occur in the light circuit 2. At the same time, in the case where another device is connected to the output of the constant voltage circuit 3, if an instantaneous voltage fluctuation as shown in FIG. In particular, there is a possibility of affecting the audio circuit and the image circuit. As a countermeasure against this, it is necessary to connect a large-capacitance capacitor or the like to the power supply of the backlight circuit 2 in order to prevent voltage fluctuations that affect the audio circuit and the image circuit. There was a problem.

In Patent Document 1, the current value of the LED is accurately grasped so as not to be affected by the repetitive pulse frequency of the PWM signal, the output of the constant voltage circuit is stabilized by feedback control, and the LED is adjusted to a desired brightness. Trying to stabilize.
That is, according to a constant voltage circuit that generates a drive signal for driving an LED, and a drive current value, a drive voltage value, or a luminance of the LED when the LED is driven by the drive signal. A measurement circuit that measures the measured signal, a sampling circuit that samples the measurement signal measured by the measurement circuit, and a control circuit that controls at least the sampling timing according to the waveform of the measurement signal, and the LED A feedback signal is output to the constant voltage circuit as a guide for how to change the current or voltage applied to. In other words, the driving voltage value applied to the LED is monitored by the measurement circuit, and the constant voltage circuit is controlled so that it is constant.

As described above, in Patent Document 1, the drive voltage value applied to the LED is monitored by the measurement circuit so that the luminance of the LED can be maintained at a predetermined value, and control is performed so that this always becomes a desired value. To do. Therefore, by controlling the sampling timing according to the waveform of the measurement signal, and selecting and sampling the waveform of the desired measurement signal, the measurement signal can be sampled with high accuracy and the luminance of the LED can be maintained at a predetermined value. Is.
JP 2007-287964 A

  In general, the LED adjusts the brightness of the LED by the PWM signal, but it is necessary to sample the measured values such as current and voltage with high accuracy in a short time regardless of whether the repetition pulse frequency of the PWM signal is high or low. There is. Therefore, in Patent Document 1, for example, when the detected waveform has waveform distortion such as overshoot, undershoot, and ringing, the problem that the measured value cannot be sampled accurately will be solved by feedback control. It is what.

However, although the technique of Patent Document 1 performs feedback control using the actual LED current output from the constant voltage circuit, waveform distortions such as overshoot, undershoot, and ringing are originally internal to the constant voltage circuit. There is a high possibility that it is caused by the resistance and the control system, and there is a problem that if the constant voltage circuit has a desired output capacity, the entire device becomes expensive. The constant voltage circuit becomes a load when viewed from the power source side of the battery or the generator. In particular, when the load of the circuit after the constant voltage circuit becomes large, the constant voltage circuit is also added from the power source side and the load becomes larger. Therefore, the control to compensate for this is limited to measures such as connecting a large capacity capacitor. However, there is a problem that the use of a capacitor that satisfies the requirement increases the size.
Therefore, generally, when the lowest voltage of the fluctuating power supply voltage is set as the operating voltage and the difference from the highest power supply voltage is large, the series resistor R in FIG. As a result, the desired minimum voltage is obtained. In particular, when the difference between the highest and lowest voltage is larger than the forward voltage (forward voltage drop) Vf of the LED, energy loss of one LED or more is generated in the steady state in the design according to the lowest operating voltage. become.

  Therefore, the present invention was made to solve such a problem, and the brightness of the LED can be set and adjusted to an arbitrary brightness without depending on the supplied voltage without using a constant voltage circuit, Another object of the present invention is to provide an inexpensive backlight dimming control device that can be miniaturized and has low energy loss.

The backlight dimming control device according to claim 1 includes one or more series-connected LED series circuits that emit light by power supplied from a power source, and light emission formed by connecting one or more LED series circuits in parallel. A circuit and a current control circuit connected in series to the light emitting circuit and controlling the base voltage of the emitter-follower-connected transistor, the emitter current of the transistor being settable, and the current control circuit connected in series; A current output circuit for determining the energization current, a current output circuit for determining the energization current of the transistor of the light emitting circuit, and a modulation circuit for PWMing the current of the light emitting circuit, and a voltage supplied from the power source to the light emitting circuit. Detected by the voltage detection circuit, and adjusts the PWM pulse width according to the voltage value detected by the voltage detection circuit Than is.
Here, a current adjusting resistor may be directly connected to one or more series-connected LED series circuits constituting the light emitting circuit, or may be omitted. That is, it is sufficient if the current value of the LED is limited to a predetermined value corresponding to the power supply voltage, and a diode can be used. Further, it can be omitted when the circuit impedance is large. The power source for supplying the power can be an in-vehicle battery or a generator output.
Further, the current control circuit for controlling energization of the LED current is variable by controlling the base voltage of the emitter-follower connected transistor, and may be any circuit as long as the input resistance becomes very large.
The current output circuit that variably controls the energization current of the transistor of the LED series circuit may be any circuit that is connected in series to the current control circuit and determines the energization current that is energized to the current control circuit.
Further, the voltage detection circuit detects a voltage supplied from the power source to the light emitting circuit, and may have any high impedance.
Furthermore, since the modulation circuit blinks the current of the light emitting circuit with a PWM signal, it may be connected to the + B side of the series circuit, the ground side, or between them. In any case, the modulation circuit may increase the voltage broadly to compensate for the decrease in the current of the LED series circuit when the voltage drops, for example, depending on the voltage value detected by the voltage detection circuit. On the contrary, the modulation is made narrowly so that the brightness of the LED of the light emitting circuit becomes constant.

The current control circuit of the backlight dimming control device according to claim 2 applies an output obtained by D / A (digital / analog) conversion to a base of a transistor having an emitter-follower connection, and generates an emitter current of the transistor. Energization control is performed.
Here, the D / A converted output applied to the base is for digitally adjusting the brightness, but is applied as an analog output to the base of a transistor having the digitally processed output as an emitter-follower connection. Yes, even the signal processing and input / output do not specify analog or digital.

The current output circuit of the backlight dimming control device according to claim 3 constitutes a current mirror circuit.
Here, as the current mirror circuit, a wider type current mirror using a bipolar transistor, a base current compensation type current mirror, a MOS-FET wider current mirror, a Wilson current mirror, a high precision Wilson current mirror, a super linear circuit, etc. It can be implemented as a circuit.

  The current control circuit of the backlight dimming control device according to claim 4 performs pulse width modulation by intermittently connecting a base voltage of a transistor of the current control circuit. At this time, by using the output of the current control circuit as a PWM signal, it can also serve as a modulation circuit, and the circuit configuration can be simplified.

According to the backlight dimming control device of claim 1, the LED series circuit formed by connecting one or more LEDs that emit light by the power supplied from the power supply in series is one circuit or two or more circuits connected in parallel. A current control circuit in which the emitter current of the transistor of the LED series circuit is variable by controlling a base voltage of the transistor connected in series to the light emitting circuit and connected to the emitter-follower, and in series with the current control circuit A current output circuit for determining an energization current connected to the current control circuit and determining an energization current of the transistor of the light emitting circuit; and a voltage detection for detecting a voltage supplied from the power source to the light emitting circuit. And a modulation circuit for PWMing the current of the light emitting circuit according to the voltage value detected by the voltage detection circuit. It is intended to.
Therefore, in the LED series circuit formed by connecting one or more light emitting circuits in series, the emitter current of the transistor can be varied by controlling the base voltage of the transistor of the LED series circuit in the emitter-follower connection of the current control circuit. Thus, the brightness can be adjusted. In addition, a power supply voltage that supplies power to the light emitting circuit is detected by a voltage detection circuit, and a PWM duty ratio (ratio of pulse width of on pulse to off pulse) of the modulation circuit that supplies current to the light emitting circuit is adjusted based on the power supply voltage. Therefore, the brightness of the light emitting circuit can be controlled to a predetermined value by widening the pulse width by the amount by which the applied voltage of the light emitting circuit is reduced.
Moreover, it is possible to design by setting the operating voltage of one or more LED series circuits connected in series to the highest power supply voltage, eliminating the need to increase the value of the series resistance, and the power supply voltage is the forward direction of the LED. If there is a variation greater than the voltage drop, one or more LEDs can be connected instead of connecting a series resistor, minimizing energy loss for voltage regulation.
Therefore, the brightness of the LED can be set and adjusted to an arbitrary brightness without depending on the supplied voltage without using a constant voltage circuit. In addition, since a constant voltage circuit and a large-capacitance capacitor are not required, it is possible to reduce the size, reduce the component cost, reduce energy loss, and reduce the cost.

  The current control circuit of the backlight dimming control device according to claim 2 can apply the D / A converted output to the base of a transistor having an emitter-follower connection to control energization as the emitter current of the transistor. Thus, in addition to the effect of the first aspect, the signal processing of the voltage detection circuit for detecting the power supply voltage and the modulation circuit for pulse width modulating the current of the light emitting circuit can be performed by a single microcomputer.

  Since the current output circuit of the backlight dimming control device according to claim 3 is a current mirror circuit, in addition to the effect according to claim 1 or 2, the current output circuit is limited to the number of LEDs arranged. Therefore, a wide surface can be made uniform brightness and uniform brightness.

  Since the current control circuit of the backlight dimming control device according to claim 4 performs pulse width modulation by intermittently modulating the base voltage of the transistor of the current control circuit. In addition to the effect described in claim 3, the current control circuit can also be used as the modulation circuit, and the circuit configuration can be simplified.

[Embodiment 1]
Hereinafter, embodiments of the present invention will be described with reference to the drawings. Note that, in the embodiments, the same symbols and the same reference numerals in the drawings are the same or corresponding functional parts, and therefore, redundant description is omitted here.

  FIG. 3 is an overall configuration diagram of the backlight dimming control device according to the first embodiment of the present invention, and FIG. 4 is a main circuit diagram of the backlight dimming control device according to the first embodiment of the present invention. FIG. 5A is a characteristic diagram showing the relationship between the voltage and pulse width of the backlight dimming control device according to Embodiment 1 of the present invention, and FIG. 5B is a characteristic showing the relationship between the power supply voltage and the operable voltage range. FIG. 6 is an explanatory diagram of a PWM signal waveform of the backlight dimming control device according to the first embodiment of the present invention, and FIG. 7 is a program executed by the main control circuit of the backlight dimming control device according to the first embodiment of the present invention. It is a flowchart of.

In the figure, the power supply voltage + B is a voltage of an energy source supplied from a battery and / or a generator. The voltage of the power supply voltage + B is detected by the voltage detection circuit 10 including an A / D conversion circuit. The output of the voltage detection circuit 10 is input to a main control circuit 11 composed of a microcomputer (CPU). The main control circuit 11 comprising a microcomputer is connected to a light / dark controller 12 having a switch for up / down a built-in counter. The output of the counter built in the main control circuit 11 is connected to the digital input of the D / A conversion circuit 13.
In the present embodiment, when a microcomputer with terminals for analog input and analog output is used as the main control circuit 11, the voltage detection circuit 10 and the D / A conversion circuit 13 composed of an A / D conversion circuit are used. It can be omitted and used with a microcomputer with analog input / output terminals.

  The resistor R0 to which the power supply voltage + B is applied and the LED01, LED02,..., LED0m of a circuit in which a plurality of LEDs are connected in series constitute an LED series circuit S0. The LED series circuit S0 includes an LED series circuit S1 composed of a required number of resistors R1, LED11, LED12,..., LED1m, an LED series circuit S2 composed of resistors R2, LED21, LED22,. ..., a resistor Rn and an LED series circuit Sn composed of LEDn1, LEDn2, ..., LEDnm are connected in parallel. Here, the resistors R0, R1, R2,... Are arranged as necessary and are not necessarily required. The LED series circuit S0 or the LED series circuits S1, S2,. The n is an integer and is the number of LED series circuits. The m is an integer and is the number of LEDs connected in series.

  Here, a plurality of LED01, LED02,..., LED0m of the LED series circuit S0 connected in series on the collector side of the transistor TR are connected in series with a plurality of LED series circuits S1, S2,. LED11, LED12, ..., LED1m, LED21, LED22, ..., LED2m, ..., LEDn1, LEDn2, ..., the number of LEDs connected in series is generally reduced by at least one. is there. That is, the number is determined by the voltage drop between the collector and emitter of the transistor TR and the degree of amplification correction by the resistor r0. The base of the transistor TR0 and the ground are connected via a base-emitter resistance of the transistor TR0 itself, and the resistors r0 and r1 correct them.

  The cathode side of LED01 of the LED series circuit S0 of the light emitting circuit 20 is connected in series to the collector side of the transistor TR, the emitter side of the transistor TR is grounded, and is connected to the collector side of the transistor TR0 via a resistor r0. Yes. The transistor TR has an emitter-follower connection, and the current i0 between the collector and the emitter of the transistor TR is determined by the voltage applied via the base resistor r1. This emitter-follower connected transistor TR constitutes a current control circuit 21. In other words, the current control circuit 21 is configured to set the emitter current of the transistor TR by controlling the base voltage of the transistor TR connected in series to a specific LED series circuit S0 of the light emitting circuit 20 and having an emitter-follower connection.

  Since the collector and base of the transistor TR0 connected in series to the current control circuit 21 are short-circuited and the transistor TR0 uses the characteristics of the diode between the emitters, the forward voltage (forward voltage) of the transistor TR0 is used. (Drop) Vf is determined by the forward current If and is as shown in the characteristic diagram of FIG. Since the forward voltage (forward voltage drop) Vf of the transistor TR0 is the base voltage of the other transistors TR1, TR2,..., TRn, the transistors TR1, TR2,. The currents i1, i2,..., In S1, S2,..., Sn are logically equal to the current i0 specified by the LED series circuit S0 (exactly “approximately equal”). This transistor TR0 is connected in series to the current control circuit 21, determines the energization current i0 to be passed through the current control circuit 21, and is connected in series to the LED series circuits S1, S2,. A current output circuit 22 for determining the currents i1, i2,..., In of the transistors TR1, TR2,. The current output circuit 22 of the present embodiment includes LED series circuits S 0, S 1, S 2,... That specify energized currents i 0, i 1, i 2,. A current mirror circuit that equalizes the energization currents of the Sn transistors TR0, TR1, TR2,.

  That is, the current i0 between the collector and the emitter of the transistor TR is determined by the power supply voltage + B, which becomes the base-emitter voltage of the transistor TR0. At the same time, it becomes the base voltage of the transistors TR1, TR2,..., TRn, and since the base-emitter voltage of the transistor TR0 is applied, the transistors TR1, TR2,. .., In, and the brightness of the LEDs of the LED series circuits S1, S2,..., Sn connected in series is also proportional to the current i0. In FIG. 3, the currents of the LED series circuits S1, S2,..., Sn are individually shown as currents i1, i2,..., In, but the base-emitter voltage of the transistor TR0 is the transistor TR1. , TR2,..., TRn, the currents i1, i2,..., In are all substantially equal to the current i0.

  Further, the transistor TR0 and the emitters of the transistors TR1, TR2,..., TRn connected in series to the LED series circuits S0, S1, S2,. Is connected to ground through The switching element FET is a normally-off type P-type MOSFET having a source terminal connected to the power supply side and a drain terminal connected to the ground. In the switching element FET, a PWM signal which is an output of the main control circuit 11 is led to a gate terminal via a gate resistance rf. This switching element FET constitutes a modulation circuit 23 that, together with the main control circuit 11, performs PWM (pulse width modulation) on the current of the light emitting circuit 20 according to the value of the voltage detected by the voltage detection circuit 10. Note that the relationship between the detected voltage value and the pulse width of the PWM on-time of the light emitting circuit 20 may use a map or may be calculated.

In this embodiment, the input of the transistor TR via the base resistor r1 of the current control circuit 21 is the analog output of the D / A conversion circuit 13 that inputs the output of the counter built in the main control circuit 11, and the LED Since the brightness is determined, when implementing the present invention, a base voltage may be applied using a unique circuit. For example, the output of a predetermined circuit (not shown) may be artificially varied with a volume or the like to obtain a predetermined voltage. In any case, depending on the magnitude of the voltage applied to the base of the transistor TR, the LED current passes through the currents i0, i1, i2, ..., in the LED series circuits S0, S1, S2, ..., Sn. It only has to be able to adjust the brightness.
However, as in the present embodiment, the main control circuit 11 is set when there is data of the LED series circuits S0, S1, S2,..., Sn currents i0, i1, i2,. The magnitude of the voltage applied to the base of the transistor TR can be changed with reference to the brightness, and the on / off pulse width can be controlled by the PWM signal.

  In the backlight dimming control device of this embodiment, the current between the collector and the emitter of the transistor TR is determined as the base voltage by the dimming of the transistor TR, that is, the base voltage for controlling the brightness, and this is the transistor TR0. At the same time, the voltage drop between the base and emitter of the transistor TR0 is caused by a plurality of LED series circuits S1, S2,..., Sn transistors TR1, TR2,. .., TRn, the base-emitter voltage of the transistors TR1, TR2,..., TRn is the same as the base-emitter voltage drop of the transistor TR0, and the LED series circuits S1, S2,. The brightness of the LEDs is also the currents i1, i2,..., Sn of the LED series circuits S1, S2,. By making n equal to the current i0, a plurality of LED series circuits S1, S2,..., Sn LED11, LED12,. .., LED1m, LED21, LED22,..., LED2m,..., LEDn1, LEDn2,.

The main control circuit 11 of the backlight dimming control device of the present embodiment is controlled as shown in FIG.
In FIG. 7, the arrival of the light / dark input signal from the light / darkness controller 12 is determined in step ST1, and when the light / dark input signal arrives in step ST1, the built-in counter is incremented or decremented in step ST2. Is changed, the analog value of the D / A conversion circuit 13 is changed, and the base voltage of the transistor TR is changed. As a result, the collector-emitter current i0 is determined and becomes the base-emitter voltage of the transistor TR0, so that the LED series circuits S1, S2,. The PWM signal can be blinked at a predetermined duty ratio. Therefore, the light emitting circuit 20 has a predetermined brightness.

In step ST1, the arrival of the light / dark input signal from the light / darkness controller 12 is determined. When the light / dark input signal does not arrive in step ST1, the current i0 between the collector and the emitter in which the light emitting circuit 20 is set to the predetermined brightness in step ST3. The digital value VB of the D / A conversion circuit 13 that outputs the base voltage of the transistor TR through which the current flows is stored in the base voltage memory MB as an index of brightness. Next, in step ST4, the power supply voltage + B detected by the voltage detection circuit 10 at this time is stored in the power supply voltage memory MV to confirm that the current brightness depends on the current power supply voltage + B. Store as VT.
At step ST5, the duty ratio for energizing the PWM signal output from the main control circuit 11 to the modulation circuit 23 is specified. This is stored as the on-pulse width PW in the on-pulse width memory MW of the PWM signal output from the main control circuit 11. The digital value VB stored in the base voltage memory MB, the voltage VT stored in the power supply voltage memory MV, and the on-pulse width PW stored in the on-pulse width memory MW are the reference for this routine. At this time, it becomes as shown in FIG. The threshold value of the current i0 in FIG. 6A is a value set in the routine of step ST1 and step ST2.

In step ST6, the voltage VT stored in the power supply voltage memory MV in step ST4 is subtracted from the current voltage Vx of the power supply voltage + B at that time, and it is determined whether the difference is within the allowable error ΔV. If it is within the range of the allowable error ΔV, the routine from step ST1 to step ST6 is repeatedly executed.
However, when the voltage VT stored in the power supply voltage memory MV in step ST4 is subtracted from the current voltage Vx of the power supply voltage + B at that time in step ST6 and it is determined that the difference is not within the allowable error ΔV, When the voltage VT stored in the power supply voltage memory MV is subtracted from the current voltage Vx of the power supply voltage + B in ST7, and the difference is a negative value, it means that the power supply voltage + B has dropped from the previous setting. Therefore, the voltage drop of the transistor TR0 of the current output circuit 22 is reduced. The voltage drop of the transistor TR0 of the current output circuit 22 is a decrease in the base voltage of the transistors TR1, TR2,..., TRn connected in series to the LED series circuits S1, S2,. The brightness of the light emitting circuit 20 decreases. Therefore, in order to compensate for this, the ON time of the ON pulse width PW of the PWM signal output from the main control circuit 11 is lengthened. At this time, the ON time of the ON pulse width PW is as shown in FIG. In FIG. 6B, the product W1 of the reduced current and the reference on-pulse width is substantially equal to the product W2 of the reduced current and the increased on-pulse width.

  Conversely, when the voltage VT stored in the power supply voltage memory MV is subtracted from the current voltage Vx of the power supply voltage + B in step ST7, and the difference is a positive value, the power supply voltage + B is higher than the previous setting. This means that the voltage drop of the transistor TR0 of the current output circuit 22 is increased. The voltage drop of the transistor TR0 of the current output circuit 22 is an increase in the base voltage of the transistors TR1, TR2,..., TRn connected in series to the LED series circuits S1, S2,. The brightness of the light emitting circuit 20 becomes brighter. Therefore, in order to compensate for this, the ON time of the ON pulse width PW of the PWM signal output from the main control circuit 11 is shortened. At this time, the ON time of the ON pulse width PW is as shown in FIG. In FIG. 6C, the product W3 of the increased current and the controlled on-pulse width is substantially equal to the product W4 of the reference current and the decreased on-pulse width.

  As shown in FIG. 5B, when the voltage VT stored in the power supply voltage memory MV is subtracted from the current voltage Vx of the power supply voltage + B and the difference is positive or negative, the power supply voltage + This means that B has risen or fallen, so that the voltage drop of the transistor TR0 in the current output circuit 22 becomes higher or lower. This increases or decreases the base voltage of the transistors TR0, TR1, TR2,..., TRn connected in series to the LED series circuits S1, S2,. Changes. Therefore, in order to compensate for this, the ON time of the ON pulse width PW of the PWM signal output from the main control circuit 11 is changed. Therefore, the brightness of the LEDs of the light emitting circuit 20 can be made constant without using a constant voltage circuit.

  As described above, the backlight dimming control device according to Embodiment 1 of the present invention includes one or more series-connected LED series circuits S0, S1, S2,... That emit light by the power supplied as the power supply voltage + B of the power supply. .., Sn and LED series circuits S0, S1, S2,..., Sn are connected in series, and LED series circuits S0, S1, S2,. Of the transistor TR by controlling the base voltage of the transistor TR connected in series to the specific LED series circuit S0, S1, S2,..., Sn of the light emitting circuit 20 and the emitter-follower connection. A current control circuit 21 for setting an emitter current; and a current control circuit 21 connected in series to determine an energization current to be passed through the current control circuit; .., Sn, and a voltage detection circuit for detecting the voltage supplied to the light emitting circuit 20 from the power supply of the power supply voltage + B. 10 and steps ST3 to ST7 for PWMing currents flowing through the LED series circuits S0, S1, S2,..., Sn of the light emitting circuit 20 based on the voltage value detected by the voltage detection circuit 10. A modulation circuit 23 is provided.

Therefore, when the voltage detection circuit 10 detects the current voltage of the power supply voltage + B by the main control circuit 11, and subtracts the voltage VT stored in the power supply voltage memory MV in the main control circuit 11, the difference is positive or negative. This means that the power supply voltage + B has increased or decreased as compared with the previous setting, so that the voltage drop of the transistor TR0 of the current output circuit 22 becomes higher or lower. This increases or decreases the base voltage of the transistors TR1, TR2,..., TRn connected in series to the LED series circuits S1, S2,. Change. In order to compensate for this, the fluctuation relationship between the fluctuation voltage of the main control circuit 11 and the on-pulse width to be increased or decreased is calculated from a map or by calculation, the on-time of the on-pulse width PW of a specific PWM signal is selected, and its output To change.
Therefore, the brightness of the LED of the light emitting circuit 20 can be set and adjusted to an arbitrary brightness without using a constant voltage circuit, and without being dependent on the supplied voltage, and can be downsized and inexpensive. Backlight dimming control is possible.

Moreover, even in the current between the collector and the emitter of the transistor TR having the emitter-follower connection as the current control circuit 21, the LED series circuit S0 composed of LED01, LED02,. Since it is configured so as to contribute to light emission, it is the least wasteful power usage.
Here, the LED01, LED02,..., LED0m, LED11, LED12,... Of the light emitting circuit 20 connected in series to one or more of the LED series circuits S0, S1, S2,. The resistors R0, R1, ..., Rn connected to LED2m, LEDn1, LEDn2, ..., Dnm are LED11, LED12, ..., LED1m, LED21, DLED22, ..., LED2m, ..., The current value of LEDn1, DLEDn2,..., LEDnm is limited to a predetermined value or less. Therefore, it is possible to use a diode. It can also be omitted when the circuit impedance is large.

  The current control circuit 21 of the backlight dimming control device according to the above embodiment applies the D / A converted output to the base of the transistor TR having an emitter-follower connection, and controls energization as the emitter current of the transistor TR. Therefore, the signal processing of the voltage detection circuit 10 that detects the power supply voltage + B and the modulation circuit 23 that uses the current of the light emitting circuit 20 as a PWM signal can be performed by a single microcomputer. That is, when a microcomputer with analog input / output terminals is used as the main control circuit 11 of this embodiment, the voltage detection circuit 10 and the D / A conversion circuit 13 use the analog input terminals and analog output terminals. can do.

Since the current control circuit 21 of the backlight dimming control device of the above embodiment is a current mirror circuit, it is limited to the number of LEDs arranged in the LED series circuits S0, S1, S2,. The wide surface can be made to have uniform brightness and uniform brightness.
[Embodiment 2]

The current control circuit 21 of the backlight dimming control device of the above embodiment uses the transistor TR as a constant current source and outputs a PWM signal by the modulation circuit 23. However, when the present invention is implemented, The PWM signal can be generated by intermittently modulating the base voltage of the transistor TR of the control circuit 21.
FIG. 8 shows a circuit in that case. FIG. 8 is an overall configuration diagram of the backlight dimming control device according to the second embodiment of the present invention.
The PWM signal of the main control circuit 11 can be implemented simply by setting the output of the D / A conversion circuit 13 of the main control circuit 11 to be intermittent. Other configurations and operations are the same as those of the embodiment shown in FIG.
In this embodiment, the current control circuit 21 can also be used as the modulation circuit 23, and the circuit configuration can be simplified.

  As described above, the backlight dimming control device according to the second embodiment of the present invention includes at least one LED series circuit S0, S1, S2,. .., Sn and LED series circuits S0, S1, S2,..., Sn are connected in series, and LED series circuits S0, S1, S2,. Of the transistor TR by controlling the base voltage of the transistor TR connected in series to the specific LED series circuit S0, S1, S2,..., Sn of the light emitting circuit 20 and the emitter-follower connection. A current control circuit 21 for setting an emitter current; and a current control circuit 21 connected in series to determine an energization current to be passed through the current control circuit; .., Sn, and a voltage detection circuit for detecting the voltage supplied to the light emitting circuit 20 from the power supply of the power supply voltage + B. 10 and the current flowing through each of the LED series circuits S0, S1, S2,..., Sn of the light emitting circuit 20 based on the voltage value detected by the voltage detection circuit 10, and the base of the transistor TR of the current control circuit 21. A modulation circuit 23 including steps ST3 to ST7 that performs PWM by controlling the voltage is provided.

Therefore, when the voltage detection circuit 10 detects the current voltage of the power supply voltage + B by the main control circuit 11, and subtracts the voltage VT stored in the power supply voltage memory MV in the main control circuit 11, the difference is positive or negative. This means that the power supply voltage + B has increased or decreased as compared with the previous setting, so that the voltage drop of the transistor TR0 of the current output circuit 22 becomes higher or lower. This increases or decreases the base voltage of the transistors TR1, TR2,..., TRn connected in series to the LED series circuits S1, S2,. Change. In order to compensate for this, the fluctuation relationship between the fluctuation voltage of the main control circuit 11 and the on-pulse width to be increased or decreased is calculated from a map or by calculation, the on-time of the on-pulse width PW of a specific PWM signal is selected, and its output To change.
Therefore, the brightness of the LED of the light emitting circuit 20 can be set and adjusted to an arbitrary brightness without using a constant voltage circuit, and without being dependent on the supplied voltage, and can be downsized and inexpensive. Backlight dimming control is possible.

  Conventionally, the lowest voltage of the fluctuating power supply voltage is used as the operating voltage, and when the difference from the highest power supply voltage is large, the series resistor R shown in FIG. 1 is connected and consumed there as heat. The desired voltage was obtained by the drop. However, in the embodiment of the present invention, even if the highest voltage of the fluctuating power supply voltage is designed as the operating voltage, the voltage difference of the fluctuation of the power supply voltage can be adjusted by the energization time according to the fluctuation of the power supply voltage. When the forward voltage (forward voltage drop) Vf is greater than Vf, energy loss can be reduced because one or more LEDs can be connected in place of the series resistance. The rated power of the light emitting circuit 20 can also be increased.

  That is, in the conventional case, when the voltage difference between the highest voltage of the power supply voltage and the operating voltage is equivalent to the forward voltage (forward voltage drop) Vf of the light emitting diode, each LED series is designed in accordance with the lowest operating voltage. In this embodiment, an energy loss corresponding to one LED is generated for the circuits S0, S1, S2,..., Sn. The number of LEDs in circuits S0, S1, S2,..., Sn can be increased by one, and the output can be increased. .., Sn LED01, LED02,..., LED0m, LED11, LED12,..., LED2m, LEDn1, LEDn2,. , R1,..., Rn can be reduced in value to a forward voltage (forward voltage drop) Vf or less, and the number of LEDs can be increased accordingly.

For example, in the case of a vehicle, assuming that the power supply voltage is 13.5 [V] and the minimum operating voltage is 10.5 [V] in a steady state, the forward voltage (forward voltage drop) Vf for 3 [V]. LED can be connected.
At this time, the brightness of the light emitting circuit 20 is set by the current i0 of the emitter current of the transistor TR by always controlling the base voltage of the transistor TR in the emitter-follower connection of the current control circuit 21, and the PWM of the modulation circuit 23 The fluctuation of the current i0 based on the fluctuation of the power supply voltage according to the signal is adjusted by adjusting the on-pulse width of a predetermined duty ratio so that the set brightness is constant.

  In this way, the LED series circuits S0, S1, S2,..., Sn composed of a plurality of columns are subjected to analog control of the energization current, and the current and voltage cannot vary. In addition, since only necessary power is supplied, there is no waste in using energy. In particular, by using a voltage supply circuit for controlling the base voltage of the transistor TR connected to the emitter-follower, the output capacity of the constant voltage circuit can be reduced, and the collector-emitter of the transistor TR connected to the emitter-follower is also reduced. .., Sn determines the voltage between the base and emitter of the transistors TR0, TR1, TR2,..., TRn of the LED series circuits S0, S1, S2,. Without dimming, backlight dimming control with less energy loss becomes possible.

FIG. 1 is an electric circuit diagram of a well-known vehicle backlight circuit, in which (a) is a circuit not having a constant voltage circuit, and (b) is a circuit having a constant voltage circuit. is there. FIG. 2 shows a current waveform (a) and a voltage waveform (b) of a backlight circuit for a vehicle. FIG. 3 is an overall configuration diagram of the backlight dimming control device according to Embodiment 1 of the present invention. FIG. 4 is a main part circuit diagram of the backlight dimming control device according to Embodiment 1 of the present invention. FIG. 5A is a characteristic diagram showing the relationship between the voltage and pulse width of the backlight dimming control device according to Embodiment 1 of the present invention, and FIG. 5B is a characteristic showing the relationship between the power supply voltage and the operable voltage range. FIG. FIG. 6 is an explanatory diagram of a PWM signal waveform of the backlight dimming control device according to the first embodiment of the present invention. FIG. 7 is a flowchart of a program executed by the main control circuit of the backlight dimming control device according to Embodiment 1 of the present invention. FIG. 8 is an overall configuration diagram of the backlight dimming control device according to the second embodiment of the present invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Voltage detection circuit 11 Main control circuit 13 D / A conversion circuit 20 Light emission circuit 21 Current control circuit 22 Current output circuit 23 Modulation circuit S0, S1, S2, ..., Sn LED series circuit LED Light emitting diode TR, TR0, TR1, TR2, ..., TRn Transistor FET Switching element

Claims (4)

One or more light emitting diode series circuits connected in series that emit light by power supplied from a power source, and a light emitting circuit connected in series to the light emitting diode series circuit, wherein the light emitting diode series circuit is connected in parallel by one circuit or two or more circuits. When,
A current control circuit for setting an emitter current of the transistor by controlling a base voltage of the transistor connected in series to a specific light-emitting diode series circuit of the light-emitting circuit and having an emitter-follower connection;
A current output circuit connected in series to the current control circuit to determine an energization current to be passed through the current control circuit, and to determine an energization current of each of the light emitting diode series circuits of the light emitting circuit;
A voltage detection circuit for detecting a voltage supplied from the power source to the light emitting circuit;
A backlight dimming control device comprising: a modulation circuit that performs pulse width modulation on a current flowing through each of the light emitting diode series circuits of the light emitting circuit based on a voltage value detected by the voltage detecting circuit.   2. The backlight dimming control according to claim 1, wherein the current control circuit applies a digital / analog converted output to a base of an emitter-follower-connected transistor and sets the output as an emitter current of the transistor. apparatus.   The backlight dimming control device according to claim 1, wherein the current output circuit is a current mirror circuit.   4. The backlight dimming according to claim 1, wherein the current control circuit also serves as the modulation circuit by intermittently connecting a base voltage of a transistor of the current control circuit. 5. Control device.

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