JP2010056050A - Light source driving circuit, light source device, and display - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a light source driving circuit which can perform light control more appropriately by improving response to a control signal of output voltage and can suppress action of a rush current to a voltage conversion part, to provide a light source device, and to provide a display. <P>SOLUTION: A light source element 20 is connected to an output terminal 13 at one end of the light source element and the other end of the light source element is connected to one end of a resistive element 15. A Zener diode 16 is connected to the output terminal 13 at a cathode of the diode and an anode of the diode is connected to a feedback voltage impression terminal 14. One end of the resistive element 17 is connected to the feedback voltage impression terminal 14 and the other end of the resistive element 17 is connected to a connection point of the light source element 20 and the resistive element 15. A switching transistor 18 is constructed of, for example, a PNP type transistor, and a PWM circuit 19 carries out switching control of the switching transistor 18 by controlling the voltage impressed on a base terminal of the switching transistor 18. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a light source driving circuit, a light source device, and a display device that control brightness of a light source by controlling a current for driving the light source.

  FIG. 7 is a diagram illustrating a configuration example of a light source driving circuit 90 according to a conventional technique. The light source driving circuit 90 is a driving circuit that supplies a direct current to the light source element 99 to emit light, and includes a voltage conversion unit 901 and a resistance element 905. The light source element 99 is configured by an LED (Light Emitting Diode) or the like, and when the applied voltage increases, the flowing current increases and the brightness increases.

  When a DC or AC voltage is input to the input terminal 902, the voltage converter 901 outputs from the output terminal 904 so that the feedback voltage VFB applied to the feedback voltage application terminal 903 is equal to the reference voltage VREF. The voltage VOUT is controlled. The light source element 99 is connected to the ground via a resistance element 905 having one end connected to the output terminal 904 and the other end connected in series. A connection point between the light source element 99 and the resistance element 905 is connected to a feedback voltage application terminal 903. Since the feedback voltage VFB has such a magnitude that the current flowing into the feedback voltage application terminal 903 can be ignored, the current value of the current IL flowing through the light source element 99 and the resistance value R1 of the resistance element 905 are multiplied. It becomes the voltage of the value.

  For example, in a state before driving the light source element 99, no voltage is applied to the light source element 99, the current value of the current IL flowing through the light source element 99 is “0”, and the feedback voltage VFB is also “0”. When driving of the light source element 99 is started, the output voltage VOUT gradually increases, the current IL flowing through the light source element 99 also increases, and the feedback voltage VFB also increases. When the feedback voltage VFB becomes equal to the reference voltage VREF, the increase of the output voltage VOUT stops.

  FIG. 8 is a diagram illustrating a configuration example of the light source driving circuit 91 according to the related art that can adjust the brightness. The light source drive circuit 91 has a configuration in which a switch 906 and a switch control unit 907 are added to the light source drive circuit 90 shown in FIG.

  The switch 906 is provided to intermittently connect a DC or AC voltage input from the input terminal 902 to the voltage conversion unit 901. Examples of such a switch 906 include a semiconductor switch element, a relay, and a mechanical switch. The switch control unit 907 performs intermittent control of the switch 906. When a semiconductor switch element is used as the switch 906, the switch control unit 907 is configured by, for example, a PWM (Pulse Width Modulation) circuit. The PWM circuit generates a pulse width modulation (PWM) signal and supplies it to the switch 906. In the dimming by PWM, the on period and off period of the switch 906 are controlled by changing the ratio (duty) of the on period to the period of the PWM signal, and the period in which the light source element 99 emits light and the period in which light is not emitted are controlled. It is done by doing.

  In addition to such a configuration, the input voltage to the voltage conversion unit 901 is not interrupted, and a terminal for controlling the intermittent operation is provided in the voltage conversion unit 901, and the brightness adjustment is performed even in a configuration in which a PWM signal is input thereto. Is possible.

  Regardless of the configuration, the dimming is performed by stopping the operation of the voltage conversion unit 901 and lowering the output voltage VOUT during a period in which the light emission of the light source element 99 is turned off.

  As examples of conventional techniques for performing light control of a light source element by PMW control, there are light source devices described in Patent Document 1 and Patent Document 2. In the light emitting diode driving device described in Patent Document 1, when providing a constant voltage necessary for constant current driving to light emitting diodes connected in series, the constant voltage is equal to or lower than the input voltage. Output.

  The LED drive control device described in Patent Document 2 includes a PWM control unit that outputs a pulse having a width corresponding to a brightness instruction signal, a voltage by a bias resistor connected to a ground terminal for each LED connected in parallel, and a reference voltage. Comparing with a comparator that compares and outputs a pulse based on the comparison result to the PWM control unit. The PWM control unit is driven when either a pulse with a width corresponding to the brightness instruction signal or an output pulse of the comparator is output. Output a pulse.

JP 2007-281417 A JP 2008-77892 A

  When dimming is performed by reducing the output voltage VOUT, the change in the output voltage VOUT at the time of switching from the on period to the off period and at the time of switching from the off period to the on period cannot follow the pulse change of the PWM signal. In the change from the steady operation voltage to the stop-time voltage and the change from the stop-time voltage to the steady operation voltage, a transient state occurs. As a result, the change between the steady operation current of the current IL flowing through the light source element 99 and the current during dimming does not coincide with the pulse change timing of the PWM signal. As a result, the duty ratio of the PWM signal and the current ratio during dimming (the current average value during dimming with respect to the current value during steady operation) do not match.

  In particular, when a light source element is employed in which the applied voltage and the current are not proportional to each other and the current does not flow at all unless an applied voltage equal to or higher than the threshold voltage is applied (for example, an LED), The difference from the current ratio during dimming becomes more prominent.

  When the duty of the PWM signal is small and the ON period is short, no current flows through the light source element when the output voltage VOUT does not rise to the threshold voltage. Even if the duty of the PWM signal is not small, the same phenomenon occurs even when the ON period is short because the cycle itself is short. Further, in the configuration of the light source driving circuit 91, an inrush current may act on the voltage conversion unit 901 when the switch 906 is switched to the ON period.

  As described above, in the conventional dimming technique, the output voltage VOUT controlled by the voltage conversion unit 901 becomes difficult to dimming due to a response delay with respect to the PWM signal, and the voltage due to the occurrence of the inrush current. The conversion unit 901 may be adversely affected.

  An object of the present invention is to provide a light source driving circuit capable of improving responsiveness to a control signal of an output voltage and performing dimming more appropriately and suppressing an inrush current from acting on a voltage conversion unit, A light source device and a display device are provided.

  The light source driving circuit according to the present invention has an input terminal, an output terminal, and a feedback voltage application terminal, and converts an input voltage input from the input terminal into an output voltage output from the output terminal based on a reference voltage. A voltage conversion unit that connects one end to the ground and generates a voltage according to the current, and a light source connection unit that is located between the output terminal and the voltage generation unit and to which the light source element is connected A Zener diode having a cathode connected to the output terminal and an anode connected to the feedback voltage application terminal, and a connection point between the cathode of the Zener diode and the output terminal of the voltage conversion unit and the light source connection unit. Or a switch unit that switches between conduction and non-conduction between the light source connection unit and the ground.

The light source device according to the present invention includes the light source driving circuit and a light source element connected to the light source connection unit.
The display device according to the present invention includes the light source device.

  In the light source driving circuit according to the present invention, by switching the switch unit, conduction between the connection point of the cathode of the Zener diode and the output terminal of the voltage conversion unit and the light source connection unit, or between the light source connection unit and the ground. And non-conduction are controlled, and dimming is possible.

  In this light source drive circuit, the change in the output voltage can be made to sufficiently follow the switching of the switch unit, and the change in the current flowing through the light source element can be matched with the switching timing of the switch unit. That is, according to the light source driving circuit, the response to the control signal of the output voltage can be improved, and the light source element can be dimmed more appropriately.

  In addition, in this light source drive circuit, since it is not necessary to provide the switch unit between the input terminal and the voltage conversion unit, it is possible to suppress the inrush current from acting on the voltage conversion unit. That is, in the light source driving circuit, electromagnetic noise and acoustic noise resulting from the inrush current acting on the voltage conversion unit can be reduced.

  Since the light source device and the display device according to the present invention include the above-described light source driving circuit, the same effects as those of the above-described light source driving circuit can be obtained.

  FIG. 1 is a block diagram showing a circuit configuration of a light source device 1 according to an embodiment of the present invention. The light source device 1 includes a light source driving circuit 10 and a light source element 20. The light source driving circuit 10 includes a voltage conversion unit 11, resistance elements 15 and 17, a Zener diode 16, a switching transistor 18, and a PWM circuit 19.

  The voltage converter 11 includes an input terminal 12, an output terminal 13, and a feedback voltage application terminal 14. The voltage conversion unit 11 converts the DC voltage VIN applied to the input terminal 12 and is based on the feedback voltage VFB applied to the feedback voltage application terminal 14. Thus, the output voltage VOUT output from the output terminal 13 is controlled. That is, the voltage converter 11 compares the reference voltage VREF, which is a predetermined reference voltage, with the feedback voltage VFB applied to the feedback voltage application terminal 14 and outputs the output voltage VOUT so that the feedback voltage VFB is equal to the reference voltage VREF. To control. Specifically, when the feedback voltage VFB is lower than the reference voltage VREF, the output voltage VOUT is increased, and when the feedback voltage VFB is higher than the reference voltage VREF, the output voltage VOUT is decreased. The reference voltage VREF may be generated based on a voltage applied to the input terminal 12, a voltage source that outputs the reference voltage VREF may be used, or may be input from the outside.

  The light source element 20 is a light source in which the flowing current IL decreases when the applied voltage decreases and the brightness of light emission decreases. For example, a light source such as a light emitting diode (LED) or an incandescent light bulb. Consists of. The light source element 20 has one end connected to the output terminal 13 and the other end connected to one end of the resistance element 15. A portion located between the output terminal 13 and the resistance element 15 and connected to the light source element 20 is a light source connection portion.

  The resistance element 15 which is a voltage generation unit is a resistance element having one end connected to the other end of the light source element 20 and the other end connected to the ground (grounded).

  The resistance element 17 that is a voltage drop unit is a resistance element having one end connected to the feedback voltage application terminal 14 and the other end connected to a connection point between the light source element 20 and the resistance element 15.

  The Zener diode 16 has a cathode connected to the output terminal 13 and an anode connected to the feedback voltage application terminal 14. The zener diode 16 has a zener voltage VZ that is higher than the terminal voltage VF of the light source element 20 and is as low as possible during normal operation in which a predetermined current is passed through the light source element 20.

  The switching transistor 18 is composed of, for example, a PNP transistor, the cathode of the Zener diode 16 is connected to the emitter terminal, the one end of the light source element 20 is connected to the collector terminal, and the PWM circuit 19 is connected to the base terminal.

  The PWM circuit 19 controls switching between conduction and non-conduction between the connection point between the cathode of the Zener diode 16 and the output terminal of the voltage conversion unit 11 and the light source element 20 by the switching transistor 18. During a period in which the switching transistor 18 is on, the connection between the cathode of the Zener diode 16 and the output terminal of the voltage conversion unit 11 and the light source element 20 is conducted to cause the current IL to flow through the light source element 20. During the period in which the transistor 18 is off, the connection point between the cathode of the Zener diode 16 and the output terminal of the voltage converter 11 and the light source element 20 become non-conductive, and the current IL does not flow through the light source element 20. The PWM circuit 19 includes a pulse generation circuit, a duty control circuit that changes a duty that is a ratio of the on period to the period of the pulse wave according to a dimming signal from an external control device, and the like. The generated PWM signal is output.

  The resistance value of the resistance element 17 is such that the current flowing through the resistance element 17 when the Zener diode 16 functions and the feedback voltage VFB is equal to the reference voltage VREF is IZ from the output voltage VOUT to the resistance element 17 and the resistance element 15. The voltage obtained by subtracting the voltage drop caused by the flow of the current IZ is the minimum resistance value at which the Zener diode 16 operates effectively, that is, the minimum resistance that generates the specular Zener voltage VZ between the terminals of the Zener diode 16. The resistance value is greater than or equal to the value. In this case, the resistance value of the resistance element 17 is determined in consideration of the current value of the current flowing into the feedback voltage application terminal 14 and the influence of external noise.

  During steady operation, the voltage applied to the Zener diode 16 is a voltage obtained by subtracting the voltage drop due to the resistance element 17 from the inter-terminal voltage VF of the light source element 20, that is, a voltage lower than the Zener voltage VZ. The current flowing through the Zener diode 16 and the resistance element 17 can be suppressed to a level that can be ignored as compared with the current value of the current IL flowing through the light source element 20.

The operation of the switching transistor 18 during the on period and the off period will be described.
During the period in which the switching transistor 18 is turned on by the PWM circuit 19, a current IL during steady operation flows through the light source element 20 and emits light with a predetermined amount of light. In such a steady operation, the feedback voltage VFB is a voltage obtained by subtracting the inter-terminal voltage VF of the light source element 20 from the output voltage VOUT, and becomes equal to the reference voltage VREF, so that VOUT becomes a constant voltage (VF + VREF).

  During the period when the switching transistor 18 is turned off by the PWM circuit 19, the light source driving circuit 10 stops the current from flowing to the light source element 20, and the current value of the current flowing to the resistance element 15 becomes “0”. When the current value of the current flowing through the resistance element 15 becomes “0”, the feedback voltage VFB becomes a value lower than the reference voltage VREF, and the voltage conversion unit 11 causes the output voltage VOUT to flow through the light source element 20 with a predetermined current. During normal operation, the voltage is raised to a voltage higher than VOUT output from the voltage converter 11.

  When the voltage applied to the Zener diode 16 becomes the Zener voltage VZ while the output voltage VOUT rises, the feedback voltage VFB is lifted by the rising output voltage VOUT through the Zener diode 16, and VFB = VREF = VOUT. -VZ. When the feedback voltage VFB, that is, the voltage obtained by subtracting the Zener voltage VZ from the output voltage VOUT becomes equal to the reference voltage VREF, the output voltage VOUT stops rising and becomes a constant voltage (VZ + VREF). Therefore, it is possible to prevent the output voltage VOUT from rising during the period in which the switching transistor 18 is turned off.

  At this time, a Zener voltage VZ is generated between the terminals of the Zener diode 16, but the current IZ flowing through the Zener diode 16 is limited by the resistance element 17, so that the power loss due to the Zener diode 16 can be reduced. Therefore, the power consumption of the entire light source device 1 can be reduced, and the Zener diode 16 having a small allowable loss and thus having a small outer shape and an inexpensive price can be used.

FIG. 2 shows a timing chart of the PWM signal, the output voltage VOUT, and the current IL.
2 (1) shows a PWM signal waveform, FIG. 2 (2) shows an output signal waveform, and FIG. 2 (3) shows a current waveform flowing through the light source element 20.

  In the example shown in FIG. 2, the duty of the PWM signal is 50%, and the on period Ton and the off period Toff are set to approximately equal times.

  Since the on period Ton of the PWM signal is substantially equal to the on period of the switching transistor 18, and the off period Toff of the PWM signal is substantially equal to the off period of the switching transistor 18, as described above, in the on period Ton, the output voltage VOUT Is constant at VF + VREF, and the output voltage VOUT is constant at VZ + VREF in the off period Toff. Therefore, when the PWM signal is switched from on to off, the output voltage VOUT is switched from VF + VREF to VZ + VREF, and when the PWM signal is switched from off to on, the output voltage VOUT is switched from VZ + VREF to VF + VREF.

  The change of the output voltage VOUT at the time of the pulse switching of the PWM signal is VZ−VF. Since the zener voltage VZ is higher than the terminal voltage VF of the light source element 20 and is as low as possible, VZ−VF can be made extremely small.

  Conventionally, the change in the output voltage VOUT is from 0 to VF + VREF, and the change in the output voltage VOUT cannot follow the change in the PWM signal. However, in the present invention, by reducing the change in the output voltage VOUT, Can keep up with changes.

  Thereby, the change of the current IL flowing through the light source element 20 can be matched with the switching timing of the PWM signal, and the light source element 20 can be dimmed more appropriately.

  In particular, since the output voltage VOUT of the voltage converter 11 does not cause a delay in response to the PWM signal, the output voltage VOUT is sufficient even when the duty of the PWM signal is small or when the cycle is short. Therefore, the light source element 20 can emit light and can be adjusted to a smaller light quantity, and the range of light control can be expanded.

  Further, when the voltage conversion unit 11 is configured to easily generate an inrush current, such as a step-up switching DC-DC converter, for example, the change in the output voltage is sufficiently followed by the control of the switch control unit. The current can be reduced and the accompanying electromagnetic and acoustic noise can be prevented.

  FIG. 3 is a circuit diagram showing a detailed configuration of the light source device 1. The same components as those shown in FIG. 1 are denoted by the same reference numerals, and description thereof is omitted to avoid duplication.

  The voltage conversion unit 11 is a step-up DC-DC (direct current-direct current) converter that is realized by a switching type DC-DC converter and has an output voltage higher than an input voltage.

  The voltage conversion unit 11 includes a switching transistor 111, a choke coil 112, capacitors 113 and 114, a diode 115, and a switching control unit 116. The input terminal 11 is connected to the other end of the capacitor 113 whose one end is connected to the ground, the input terminal of the switching control unit 116, and one end of the choke coil 112. The capacitor 113 is a capacitor that smoothes the input voltage VIN.

  The other end of the choke coil 112 is connected in series to one end of the diode 115, and the other end of the diode 115 is connected to the other end of the capacitor 114 whose one end is connected to the ground. The switching transistor 111 is composed of, for example, an NPN transistor, and has a collector terminal connected between the other end of the choke coil 112 and one end of the diode 115, an emitter terminal connected to the ground, and a base terminal connected to the switching control unit 116. Connected to.

  In addition, the switching control unit 116 is connected to the feedback voltage application terminal 14 and controls the switching transistor 111 according to the feedback voltage VFB, so that the voltage conversion unit 11 can control the output voltage VOUT. Specifically, the reference voltage source for generating the reference voltage VREF, the error amplifier for comparing the reference voltage VREF and the feedback voltage VFB, the triangular wave oscillation circuit, the output of the error amplifier and the output of the triangular wave oscillation circuit are compared, and the switching transistor 111 is This is realized by an integrated circuit (IC) in which a comparator, an amplifier and the like for generating a driving signal to be driven are integrated.

  The capacitor 114 is a capacitor that smoothes the output voltage. By appropriately selecting the capacitance value of the smoothing capacitor 114, the electric charge accumulated in the smoothing capacitor 114 is released at the instant when the switching transistor 18 is turned on, and the current IL during normal operation is supplied to the light source element 20. be able to.

  The light source driving circuit 10 further includes a capacitor 117. The capacitor 117 has one end connected to the collector terminal of the switching transistor 18 and the other end connected to the ground. The capacitor 114 smoothes the output voltage and discharges charge when the switching transistor 18 is switched on. However, when the capacitance value necessary for smoothing the output voltage becomes large, the capacitor 114 is switched when the switching transistor 18 is switched on. There is a possibility that the discharge of electric charge becomes excessive and the current flowing through the light source element 20 becomes large. In such a case, by providing the capacitor 115, the current flowing through the light source element 20 can be reduced. When the capacitance value of the capacitor 115 is increased, the current responsiveness is deteriorated. Therefore, it is only necessary to give the capacitor 117 a capacitance value sufficient to absorb an excessive current.

  In many cases, the light source element 20 uses a plurality of elements connected in series. At the time of steady operation, if the current value of the current IL flowing through the light source element 20 is 10 mA, for example, the terminal voltage VF applied between the terminals of the light source element 20 is 20 V, and the reference voltage VREF is 1.0 V, for example, The resistance value R1 of the element 15 is R1 = VREF / IL = 1.0 V / 10 mA = 100Ω.

  The Zener voltage VZ of the Zener diode 16 is higher than the inter-terminal voltage VF, for example, a current of 1 μA and a Zener voltage VZ = 22V. The resistance value R2 of the resistance element 17 is R2 = VREF / IZ = 1.0 V / 1 μA = 1 MΩ when a current of 1 μA flows when the reference voltage VREF = 1.0 V. Since the resistance value R1 of the resistance element 15 is extremely smaller than the resistance value R2 of the resistance element 17, it is ignored in this calculation. The inter-terminal voltage VF is a maximum value including individual differences of the light source elements 20 and changes due to operating temperatures.

  The power consumption after the output terminal 13 of the voltage conversion unit 11 in the ON period Ton, that is, the power consumption not including the voltage conversion unit 11, is mainly due to the light source element 20 and the resistance element 15, and its value IL × VOUT = IL × (VF + VREF) = 10 mA × (20V + 1V) = 210 mW. In this case, since almost no current flows through the resistance element 17, the voltage drop due to the resistance element 17 is set to “0” V.

  In addition, power consumption in the off period Toff is IZ × VOUT = IZ × (VZ + VREF) = 1 μA × (22V + 1V) = 23 μW = 0.023 mW, which is negligibly small compared to power consumption in the on period.

  In the example shown in FIG. 3, the voltage conversion unit 11 is configured by a step-up switching DC-DC converter. However, a direct-current voltage such as a step-down type or an insulation type DC-DC converter using a transformer is output and output. Any other configuration may be used as long as the DC voltage is feedback-controlled.

  As described above, the voltage converter 11 having the input terminal 12, the output terminal 13, and the feedback voltage application terminal 14 outputs the input voltage input from the input terminal 12 based on the reference voltage VREF from the output terminal 13. It is converted to voltage VOUT. A voltage is generated according to the current by the resistance element 15 which is a voltage generation unit. The light source element 20 is connected by the light source connection part located between the output terminal 13 of the voltage conversion part 11 and the resistance element 15. The Zener diode 16 has a cathode connected to the output terminal 13, an anode connected to the feedback voltage application terminal 14, and a resistance element 17 serving as a voltage drop unit between the feedback voltage application terminal 14 and the light source connection unit. Connected to.

  That is, the output voltage VOUT can be set to VZ + VREF in the off period of the switching transistor 18, and the output voltage VOUT can be set to VF + VREF in the on period of the switching transistor 18. Therefore, it is possible to sufficiently follow the pulse change of the PWM signal by reducing the change of the output voltage VOUT.

  Since the switching transistor 18 in the present invention is provided between the connection point between the voltage conversion unit 11 and the cathode of the Zener diode 16 and the light source connection unit to which the light source element 20 is connected, Inrush current generated by switching to non-conduction does not have to act on the voltage conversion unit 11.

  Furthermore, since the resistance element 15 which is a voltage generation unit generates a voltage proportional to the flowing current, it is possible to generate a voltage proportional to the current value of the current.

  Furthermore, since the voltage generation unit is configured by the resistance element 15, the voltage generation unit can be realized by using a simple circuit element.

  Furthermore, since one end of the resistance element 15 is connected to the light source connection portion and the other end is connected to the ground, a voltage corresponding to the current can be generated at one end connected to the light source connection portion.

  Furthermore, since the resistance element 17 that is a voltage drop unit drops in proportion to the current, a voltage drop that is proportional to the current value of the current can be generated.

  Furthermore, since the voltage drop unit is configured by the resistance element 17, a voltage generation unit can be realized using a simple circuit element.

  The resistance element 17 that is a voltage drop unit is preferable because it exhibits a high effect in reducing power consumption as described above. However, if the light source driving circuit 10 does not require reduction in power consumption, the resistance element 17 has a resistance. The value R2 may be low or may not include the resistance element 17.

  Furthermore, since a smoothing capacitor such as capacitors 113 and 114 connected to at least one of the input terminal 12 and the output terminal 13 of the voltage conversion unit 11 is further provided, fluctuations due to noise in the input voltage or output voltage can be reduced.

  The switching transistor 18 is connected to a connection point between the cathode of the Zener diode 16 and the output terminal of the voltage conversion unit 11 as shown in FIG. 1 and the light source element 20 connected to the light source connection unit. It may be between the light source element 20 and the ground. As long as it is between the light source element 20 and the ground, it may be between the light source element 20 and the resistance element 15, or between the resistance element 15 and the ground if there is no problem with the resistance when the switching transistor 18 is turned on. It may be.

  FIG. 4 is a block diagram showing a circuit configuration of a light source device 1 according to another embodiment of the present invention. A configuration different from the embodiment shown in FIG. 1 is a connection position of the switching transistor 18, and is connected between the other end of the light source element 20 and the one end of the resistance element 15. In the switching transistor 18, the other end of the light source element 20 is connected to an emitter terminal, the one end of the resistance element 15 is connected to a collector terminal, and a PWM circuit 19 is connected to a base terminal.

  FIG. 5 is a block diagram showing a circuit configuration of a light source device 1 according to another embodiment of the present invention. A configuration different from the embodiment shown in FIGS. 1 and 4 is a connection position of the switching transistor 18 and is connected between the other end of the resistance element 15 and the ground. In the switching transistor 18, the other end of the resistance element 15 is connected to the emitter terminal, the ground is connected to the collector terminal, and the PWM circuit 19 is connected to the base terminal.

  FIG. 6 is a cross-sectional view showing a schematic configuration of the display device 30 according to the embodiment of the present invention. The display device 30 includes a light source device 31, a liquid crystal display panel 32, and a housing 33. The light source device 31 includes a plurality of light source units 311, a light guide 312, a light diffuser 313, a reflector 314, a diffuser plate 315, and a prism 316. Each light source unit 311 is configured by the light source device 1 shown in FIG. 1 and arranged on one end face of the light guide 312. The light source device 31 is configured by laminating a reflector 314 that reflects light, a light diffuser 313 that diffuses light, a light guide 312, a diffusion plate 315 that diffuses light, and a prism 316 in this order from the bottom side.

  The liquid crystal display panel 32 is a panel in which a liquid crystal layer made of liquid crystal is formed between two transparent substrates. The light source device 31 is stacked behind the liquid crystal display panel 32 as a backlight, and is housed in the housing 33 together with the liquid crystal display panel 32.

  The light source device 31 and the display device 30 can suppress the inrush current from acting on the voltage conversion unit 11 in addition to performing light control more appropriately. In addition, an increase in power consumption of the light source device 31 or the display device 30 can be suppressed.

  Furthermore, since the light source element 20 is a light emitting diode, a light source according to a conventional technique can be used as the light source element.

It is a block diagram which shows the circuit structure of the light source device 1 which is one Embodiment of this invention. 4 shows a timing chart of a PWM signal, an output voltage VOUT, and a current IL. 2 is a circuit diagram illustrating a detailed configuration of the light source device 1. FIG. It is a block diagram which shows the circuit structure of the light source device 1 which is other embodiment of this invention. It is a block diagram which shows the circuit structure of the light source device 1 which is other embodiment of this invention. It is sectional drawing which shows the schematic structure of the display apparatus 30 which is one Embodiment of this invention. It is a figure which shows the structural example of the light source drive circuit 90 by a prior art. It is a figure which shows the structural example of the light source drive circuit 91 by the prior art which enabled adjustment of the brightness.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1,31 Light source device 10, 90, 91 Light source drive circuit 11,901 Voltage conversion part 12,902 Input terminal 13,904 Output terminal 14,903 Feedback voltage application terminal 15, 17, 905 Resistance element 16,906 Zener diode 18 Switching Transistor 19 PWM circuit 20, 99 Light source element 30 Display device 32 Liquid crystal display panel 33 Housing 111 Switching transistor 112 Choke coil 113, 114, 117 Capacitor 115 Diode 116 Switching control unit 311 Light source unit 312 Light guide 313 Light diffuser 314 Reflection Body 315 Diffuser 316 Prism

Claims (12)

A voltage converter that has an input terminal, an output terminal, and a feedback voltage application terminal, and converts an input voltage input from the input terminal to an output voltage output from the output terminal based on a reference voltage;
One end is connected to the ground and generates a voltage according to the current, and
A light source connection unit, which is located between the output terminal and the voltage generation unit and to which a light source element is connected;
A Zener diode having a cathode connected to the output terminal and an anode connected to the feedback voltage application terminal;
A switch unit that switches between conduction and non-conduction between the connection point of the cathode of the Zener diode and the output terminal of the voltage conversion unit and the light source connection unit, or between the light source connection unit and the ground. A light source drive circuit characterized by the above.   The light source driving circuit according to claim 1, wherein the voltage conversion unit includes a switching type DC-DC converter.   The light source driving circuit according to claim 1, wherein the voltage generation unit generates a voltage proportional to a current.   The light source driving circuit according to claim 1, wherein the voltage generation unit is configured by a resistance element.   5. The light source driving circuit according to claim 4, wherein one end of the resistance element is connected to the light source connection portion, and the other end is connected to a ground.   The light source driving circuit according to claim 1, further comprising a voltage drop unit connected between the feedback voltage application terminal and the light source connection unit.   The light source driving circuit according to claim 6, wherein the voltage drop unit drops a voltage in proportion to a current.   The light source driving circuit according to claim 6, wherein the voltage drop unit includes a resistance element.   The light source driving circuit according to claim 1, further comprising a smoothing capacitor connected to at least one of the input terminal and the output terminal of the voltage conversion unit. A light source driving circuit according to any one of claims 1 to 9,
A light source device connected to the light source connection portion.   The light source device according to claim 10, wherein the light source element is a light emitting diode.   A display device comprising the light source device according to claim 10.

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