JP2012038782A - Light-emitting element driving circuit and display device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To achieve efficient drive control for individual emission colors of LEDs.SOLUTION: Smoothing capacitors CR, CG, and CB corresponding to respective LEDs are provided. Reference voltages Vref are set for the respective LEDs in advance. The reference voltages Vref are compared with results of detecting currents flowing through the respective LEDs, and power is supplied from a DC/DC converter 10 to the LEDs so as to maintain a constant current flowing through each of the LEDs. Since the smoothing capacitors CR, CG, and CB are charged in accordance with the set reference voltages Vref, an excessively large or small current does not flow into the LEDs during switching among the smoothing capacitors CR, CG, and CB. Thus, response characteristics of the LEDs can be further improved, and a supply current can be supplied more stably.

Description

  The present invention relates to a light emitting element driving circuit and a display device.

  In Patent Document 1, a light emitting element selection control means for sequentially selecting and controlling light emitting elements having different rated current values, and a current supplied from a direct current power source to a light emitting element selected by the light emitting element selection control means are predetermined. A drive circuit for a light emitting element, comprising power supply means for converting to an output current value and outputting, and auxiliary capacity connection control means for connecting an auxiliary capacity corresponding to each light emitting element to the light emitting element in parallel at a predetermined timing. Has been.

JP 2007-273666 A

  However, in the control method described in Patent Document 1, a plurality of light emitting elements, for example, a red light emitting diode (R-LED) that emits red light, a green light emitting diode (G-LED) that emits green light, and a blue light emitting element. In the case of sequentially switching these light emitting diodes (hereinafter abbreviated as “LED” as appropriate), since the electric charge still remains in the auxiliary capacitor, the forward voltage of the switched LED The voltage difference exists. This voltage difference causes a waveform change due to an excessive phenomenon such as undershoot or overshoot in the LED drive current. If the drive current exceeds the rated current of the LED due to this waveform change, the LED may be damaged.

  The present invention has been made to solve the above-described problems, and the object thereof is to provide a light-emitting element that does not damage the light-emitting element even when the light-emitting element is sequentially switched when driving the light-emitting element. A drive circuit and a display device using the drive circuit are provided.

In order to solve the above-described problem, a light-emitting element driving circuit according to the present invention includes a switching control unit that controls a plurality of light-emitting elements having different emission colors to be alternatively driven,
A plurality of capacitive elements provided corresponding to the plurality of light emitting elements, respectively, for smoothing a supply voltage to the corresponding light emitting elements;
A capacitance switching unit that controls a capacitive element corresponding to the driven light emitting element among the plurality of capacitive elements to be electrically connected in parallel with the power source;
A current detection unit for detecting a current value flowing in the driven light emitting element;
A supply voltage control unit that controls a voltage supplied to the light emitting element according to a comparison result between a current value detected by the current detection unit and a reference value set for the light emitting element;
It is characterized by including.

  Further, it is desirable that the reference value is set according to the driven light emitting element.

  Furthermore, it is preferable that the capacitance switching unit performs connection control of the capacitive element in conjunction with the drive control of the light emitting element by the switching control unit.

  By the way, after a switch for applying a voltage between both terminals to drive the light emitting element is turned on by the switching control unit, a smoothing capacitor corresponding to the light emitting element is provided in the capacitance switching unit. A switch for connecting in parallel with the power source is turned on, and then the light emitting element emits light when an applied voltage between both terminals of the light emitting element exceeds a threshold voltage of the light emitting element. It is desirable that the threshold voltage is set.

  The capacitance switching unit is provided with a plurality of switch units made up of a pair of MOS transistors, each having a common gate terminal and the anodes of the parasitic diodes connected to each other, corresponding to each of the plurality of capacitive elements. It is desirable that the voltage applied to the gate terminal of each switch unit is changed in conjunction with the drive control of the light emitting element by the switching control unit.

  The current detection unit is preferably provided in common for the plurality of light emitting elements.

  A display device according to the present invention includes the light-emitting element driving circuit described above and a plurality of light-emitting elements that are driven by the light-emitting color and have different emission colors.

  According to the present invention, when driving light emitting elements by sequentially switching the light emitting elements, the possibility of damaging the light emitting elements is low even if they are connected in parallel while switching the smoothing capacitors.

It is a figure which shows the main components of one Embodiment of the light emitting element drive circuit used for the display apparatus by this invention. It is a figure which shows the more detailed structure of the light emitting element drive circuit of FIG. It is a wave form diagram which shows operation | movement of the light emitting element drive circuit of FIG. 2, and is a figure which shows operation | movement of a transient state. It is a wave form diagram which shows operation | movement of the light emitting element drive circuit of FIG. 2, and is a figure which shows operation | movement of a steady state. It is a wave form diagram which shows operation | movement of the display apparatus by this invention.

  Embodiments of the present invention will be described below with reference to the drawings. In the drawings referred to in the following description, the same parts as those in the other drawings are denoted by the same reference numerals.

(Drive target of light emitting element drive circuit)
The driving target of the light emitting element driving circuit is a light emitting diode used as a light emitting element of a display device, such as for projection of a projector. In the present embodiment, the light emitting diodes R-LED, G-LED, and B-LED are the objects to be driven. The emission color of the light emitting diode R-LED is red (R), the emission color of the light emitting diode G-LED is green (G), and the emission color of the light emitting diode B-LED is blue (B). Since the light emitting diodes having different emission colors have different forward voltages and rated currents, the drive circuit of this embodiment has a function of flowing an appropriate current to each light emitting diode.

(Configuration example of light emitting element driving circuit)
In FIG. 1, the light emitting element driving circuit according to the present embodiment has an inductor 2 for stabilizing a current due to an input voltage Vin, a diode 3 for preventing a backflow of a current supplied to the light emitting diode, and light emission colors of each other. LED switching control circuit SW for controlling the supply of voltage to a plurality of different light emitting diodes R-LED, G-LED, B-LED (that is, controlling the supplied current), and light emitting diodes R-LED, G- Smoothing capacitor Cout for smoothing the voltage supplied to the LED and B-LED, and electrical connection between the smoothing capacitor Cout and each anode of the light emitting diodes R-LED, G-LED, and B-LED And a capacitor switching circuit SWC for controlling.

  The light emitting element driving circuit according to the present embodiment includes a current detection circuit 5 for detecting a current flowing through the light emitting diodes R-LED, G-LED, and B-LED, and each light emitting diode R-LED, G-LED, The Vref setting unit 4 that generates a reference voltage Vref corresponding to the reference value of the current flowing through the B-LED, and the light emitting diodes R-LED, G, G according to the comparison result between the detection result by the current detection circuit 5 and the reference voltage Vref. -It has the DC / DC converter 10 which controls so that the electric current supplied to LED and B-LED may become an appropriate value, respectively.

  In the figure, the light emitting element driving circuit according to the present embodiment performs field sequential driving of a plurality of light emitting diodes R-LED, G-LED, and B-LED having different emission colors (that is, different forward voltages). As an alternative to these light emitting diodes, it has a function of supplying current. More specifically, the input voltage Vin to this circuit becomes the voltage Vout through the inductor 2 and the diode 3, and this voltage Vout is alternatively applied to the light emitting diodes R-LED, G-LED, and B-LED. Is done. As a result, the light emitting diode in which a current flows emits light in a predetermined color. That is, the light-emitting diodes R-LED, G-LED, and B-LED emit light alternatively to realize field sequential color display.

  In order to selectively emit light from the light emitting diodes R-LED, G-LED, and B-LED, an LED switching control circuit SW and a capacitor switching circuit are generated by an RGB-LED lighting control signal 100 input from a control circuit (not shown). SWC switching control is performed.

(Smoothing capacitor)
Here, the smoothing capacitors Cout are provided with smoothing capacitors CR, CG, and CB corresponding to the light emitting diodes R-LED, G-LED, and B-LED, respectively. That is, a smoothing capacitor CR dedicated to the light emitting diode R-LED, a smoothing capacitor CG dedicated to the light emitting diode G-LED, and a smoothing capacitor CB dedicated to the light emitting diode B-LED are provided in the smoothing capacitor portion Cout. Then, switching control of the capacitor switching circuit SWC is performed so that a smoothing capacitor corresponding to the light emitting diode driven by the switching control of the LED switching control circuit SW is selected. That is, the smoothing capacitor CR is driven when the light emitting diode R-LED is driven, the smoothing capacitor CG is driven when the light emitting diode G-LED is driven, and the smoothing capacitor CB is driven when the light emitting diode B-LED is driven. Switching control is performed by the capacitor switching circuit SWC so as to be selected.

(LED switching control circuit)
Here, referring to FIG. 2 showing a more detailed configuration of each part of FIG. 1, the LED switching control circuit SW includes an Nch MOS type FET (hereinafter abbreviated as a switch) corresponding to each light emitting diode. It has been. That is, a switch SWR is provided corresponding to the light emitting diode R-LED, a switch SWG corresponding to the light emitting diode G-LED, and a switch SWB corresponding to the light emitting diode B-LED. The anode terminals of the light emitting diodes R-LED, G-LED, and B-LED are connected to the cathode side of the diode 3, that is, the voltage Vout. The cathode terminals of the light emitting diodes R-LED, G-LED, and B-LED are connected to the corresponding drain terminals of the switch SWR, switch SWG, and switch SWB. In addition, the corresponding control signals of the RGB-LED lighting control signal 100 are applied to the gate terminals of the switch SWR, the switch SWG, and the switch SWB, and these source terminals are connected to the current detection resistance element Rsense provided in common. It is connected. Since the resistance element Rsense for current detection is provided in common for each of the light emitting diodes R-LED, G-LED, and B-LED, an increase in circuit scale can be minimized.

  In such a configuration, the switch SWR, the switch SWG, and the switch SWB are alternatively turned on by the RGB-LED lighting control signal 100, and function as switches for flowing current to the corresponding light emitting diodes.

(Capacitor switching circuit)
Referring to FIG. 2, the capacitor switching circuit SWC includes a pair of PchMOS type FETs (hereinafter abbreviated as switches) having a common gate terminal and directly connected drain terminals, corresponding to each light emitting diode. . That is, two switches SWCR-1 and SWCR-2 correspond to the light emitting diode R-LED, and two switches SWCG-1 and SWCG-2 correspond to the light emitting diode G-LED. Correspondingly, two switches SWCB-1 and SWCB-2 are respectively provided.

  The cathode terminal of the diode 3 is connected to the source terminal of the switch SWCR-1, and the source terminal of the switch SWCR-2 is connected to the high potential side of the capacitor CR. The cathode terminal of the diode 3 is connected to the source terminal of the switch SWCG-1, and the source terminal of the switch SWCG-2 is connected to the high potential side of the capacitor CG. The cathode terminal of the diode 3 is connected to the source terminal of the switch SWCB-1, and the source terminal of the switch SWCB-2 is connected to the high potential side of the capacitor CB.

  The gate terminals of the switches SWCR-1 and SWCR-2 constitute the drain terminal of the switch SWR constituting the LED switching control circuit SW, and the gate terminals of the switches SWCG-1 and SWCG-2 constitute the LED switching control circuit SW. The drain terminal of the switch SWG to be connected is connected to the gate terminals of the switches SWCB-1 and SWCB-2, and the drain terminal of the switch SWB constituting the LED switching control circuit SW is connected thereto.

  Then, the switches SWCR-1 and SWCR-2, the switches SWCG-1 and SWCG-2, and the switches SWCB-1 and SWCB-2 are alternatively turned on by the RGB-LED lighting control signal 100. Thereby, the switches SWCR-1 and SWCR-2, SWCG-1 and SWCG-2, SWCB-1 and SWCB-2 are respectively connected to the smoothing capacitors CR, CG and CB provided corresponding to the corresponding light emitting diodes. It functions as a switch for supplying the accumulated charge to the anode of the corresponding light emitting diode.

  Here, since the drains of the PchMOS type FETs constituting the switch are connected to each other, the parasitic diodes D1 and D2 are in a state in which the anodes are connected to each other. For this reason, when the gate voltage of the PchMOS type FET constituting each switch exceeds the threshold value due to the parasitic diodes D1 and D2, the switches SWCR-1 and SWCR-2, and the switches SWCG-1 and SWCG- functioning as a switch unit. 2. Switches SWCB-1 and SWCB-2 transition to a conductive state.

  Incidentally, as shown in FIG. 2, in the present embodiment, after any switch in the LED switching control circuit SW is turned on by the RGB-LED lighting control signal 100, the corresponding action is taken in conjunction with the switch. A configuration is adopted in which a pair of switches in the capacitor switching circuit SWC transitions to an on state before exceeding the threshold voltage of the light emitting diode. That is, after the RGB-LED lighting control signal 100 is applied and a switch for applying a voltage between the anode and cathode terminals (between both terminals) of the light emitting diode is turned on, the smoothing capacitor is electrically parallel to the power source. When a switch for connecting to (for example, the switches SWCR-1 and SWCR-2) is turned on and then the voltage between the anode and cathode terminals of the light emitting diode exceeds the threshold voltage, the light emitting diode emits light. In other words, by setting the threshold voltage of the light emitting diode to be larger than the threshold voltage of the parasitic diode of the PchMOS type FET, only the RGB-LED lighting control signal 100 is input without providing a special control circuit. Thus, the desired interlocking operation order as described above can be realized.

  Further, a common control circuit (not shown) is provided for the LED switching control circuit SW and the capacitor switching circuit SWC, and the LED switching control circuit SW is received by the RGB-LED lighting control signal 100 output from the control circuit. Further, switching control of the capacitor switching circuit SWC is performed. By adopting this configuration, the circuit scale can be reduced and the power consumption can be reduced as compared with the case where the LED switching control circuit SW and the capacitor switching circuit SWC are provided with separate control circuits. Further, by adopting this configuration, it is not necessary to prepare a special power source for the LED switching control circuit SW and the capacitor switching circuit SWC, so that the circuit scale can be reduced from this point as well, and the power consumption Can be reduced.

(Current detection circuit)
Referring to FIG. 2, the current detection circuit 5 is provided with a resistance element Rsense. The current Iout flowing through the resistance element Rsense is converted into a voltage, and the voltage value FB is input to the DC / DC converter 10. Here, in the LED switching control circuit SW, the light emitting diode R-LED is turned on when the switch SWR is turned on, the light emitting diode G-LED is turned on when the switch SWG is turned on, and the light emitting diode B-LED is turned on when the switch SWB is turned on. , Each emits light. The current detection circuit 5 detects the current value when the light emitting diodes emit light.

(Vref setting part)
Referring to FIG. 2, the Vref setting unit 4 includes a variable resistance element 41R and a switch 42R provided corresponding to the light emitting diode R-LED, and a variable resistance element 41G provided corresponding to the light emitting diode G-LED. The switch 42G, the variable resistance element 41B provided corresponding to the light emitting diode B-LED, and the switch 42B are provided. One end of each switch 42R, 42G, 42B is connected to the power supply voltage VCC, and the other end is connected to one end of each variable resistance element 41R, 41G, 41B. The other ends of the variable resistance elements 41R, 41G, and 41B are connected to the ground via the resistance element 43.

  In such a configuration, the switches 42R, 42G, and 42B are alternatively turned on by the RGB-LED lighting control signal 100. Accordingly, the switch 42R is turned on during the period in which the light emitting diode R-LED emits light, the switch 42G is turned on in the period in which the light emitting diode G-LED emits light, and the switch 42B is turned on in the period during which the light emitting diode B-LED emits light. . For this reason, if the resistance values of the variable resistance elements 41R, 41G, and 41B are appropriately adjusted, the reference voltage Vref suitable for the characteristics (rated current, etc.) of each light emitting diode can be input to the DC / DC converter 10. it can. The reference voltage Vref can be set individually for red R, green G, and blue B in order to obtain a desired output light amount for the luminescent colors RGB.

  Further, although a method for obtaining the reference voltage Vref by dividing the resistance has been described, it is also conceivable to use a device such as a reference voltage generator or a D / A converter.

That is, it is possible to set a dedicated reference voltage Vref for the light emitting diode R-LED, a dedicated reference voltage Vref for the light emitting diode G-LED, and a dedicated reference voltage Vref for the light emitting diode B-LED. That is, the current setting of each LED can be performed by changing the reference voltage Vref. The setting change of the reference voltage Vref can be realized by, for example, resistance voltage division according to each lighting control signal. The relationship between the “LED current” flowing through each light-emitting diode and the “detection resistance value” that is the resistance value of the resistance element Rsense of the current detection circuit 5 is expressed by Expression (1).
LED current = reference voltage (Vref) / detection resistance value (1)
In general, the resistance values of the variable resistance elements 41R, 41G, and 41B are digitally adjusted by a control circuit (not shown) such as a microcomputer.

  By the way, the temperature characteristics of the light emitting diodes R-LED, G-LED, and B-LED are different from each other. In the present embodiment, the reference voltage Vref can be set appropriately for each light emitting diode. Since the control is performed based on the appropriately set reference voltage Vref, each light emitting diode can be controlled appropriately and efficiently. Further, since the reference voltage Vref can be individually set for the light emitting diodes R-LED, G-LED, and B-LED, it can be controlled so as not to exceed the rated current of each light emitting diode.

(Configuration example of DC / DC converter)
Referring to FIGS. 1 and 2, the DC / DC converter 10 receives the voltage value FB corresponding to the current value detected by the current detection circuit 5 and the reference voltage Vref that is the output of the Vref setting unit 4 as inputs. Perform feedback control. That is, the DC / DC converter 10 outputs a voltage value corresponding to a change in the voltage value FB with respect to the reference voltage Vref.

  The DC / DC converter 10 used in the present embodiment is an error amplifier that outputs a voltage value corresponding to the difference between the reference voltage Vref set in the Vref setting unit 4 as a non-inverting input and the voltage value FB as an inverting input. Error AMP) 11, a sawtooth wave generator 13 that generates a sawtooth voltage synchronized with the oscillation output of the oscillator 12, the sawtooth voltage is a non-inverting input, and the output voltage value of the error amplifier 11 is an inverting input. Of the PWM controller 14, a reset set flip-flop (hereinafter abbreviated as RSFF) 15 which is set at the transition timing of the oscillation output of the oscillator 12 and is reset at the transition timing of the output of the PWM controller 14, A driving circuit 16 that outputs a driving signal according to the output Q, and an on state or a driving signal output from the driving circuit 16 And a, a switch 17 according to the NMOSFET turned off.

  The DC / DC converter 10 includes each light emitting diode R-LED according to the comparison result between the reference voltage Vref set in the Vref setting unit 4 and the voltage value FB corresponding to the current detected by the current detection circuit 5. , G-LED and B-LED operate so as to have a constant current. The DC / DC converter 10 employs any configuration of a step-up type, a step-down type, and a step-up / step-down type as long as the current supplied to each light emitting diode R-LED, G-LED, and B-LED can be kept constant. May be.

  The operation of the DC / DC converter 10 is controlled to be in an on state or an off state by an On / Off signal. This On / Off signal is a PWM control signal and is synchronized with the lighting control signal 100 of the light emitting diodes R-LED, G-LED, and B-LED.

(Operation of light emitting element driving circuit)
Next, the operation of the light emitting element driving circuit according to the present embodiment will be described. In the present embodiment, the voltage output from the DC / DC converter 10 is controlled according to the current flowing through the light emitting diode.

  First, the current value flowing through the light emitting diode is detected by the current detection circuit 5, which is converted into a voltage value FB and output. The output voltage value FB is input to the DC / DC converter 10 together with the reference voltage Vref set in the Vref setting unit 4. In the DC / DC converter 10, the voltage value output from the current detection circuit 5 is compared with the reference voltage Vref set in the Vref setting unit 4, and a voltage value corresponding to the result is output. That is, in the light emitting element driving circuit of the present embodiment, the current value flowing through the light emitting diode is detected, and a voltage value controlled according to the detection result is output from the DC / DC converter 10.

  The input voltage Vin is applied to the anode side of the diode 3 via the inductor L. The cathode side of the diode 3 is applied as a voltage Vout to the capacitor switching circuit SWC and each of the light emitting diodes R-LED, G-LED, and B-LED.

  The light-emitting diodes R-LED, G-LED, and B-LED are alternatively controlled to be turned on by the LED switching control circuit SW in order to realize a field sequential color driving system.

  The LED switching control circuit SW turns on the light emitting diode R-LED, the light emitting diode G-LED, and the light emitting diode B-LED for the light emitting diodes R-LED, G-LED, and B-LED. It is controlled by the RGB-LED lighting control signal 100 so as to be in any one of the states.

  The capacitor switching circuit SWC operates in conjunction with the LED switching control circuit SW. That is, in the state where the light emitting diode R-LED is lit, the RGB-LED lighting control signal is connected so that the capacitor CR provided corresponding to the light emitting diode R-LED is connected in parallel to the voltage source of the input voltage Vin. 100. Similarly, in the state where the light emitting diode G-LED is lit, RGB-LED lighting control is performed so that the capacitor CG provided corresponding to the light emitting diode G-LED is connected in parallel to the voltage source of the input voltage Vin. Controlled by signal 100. Further, when the light emitting diode B-LED is turned on, the RGB-LED lighting control signal is connected so that the capacitor CB provided corresponding to the light emitting diode B-LED is connected in parallel to the voltage source of the input voltage Vin. 100.

  In short, while the On / Off signal is on and the DC / DC converter 10 is operating, the LED LEDs are sequentially switched on / off by the operation control signals of the R-LED, G-LED, and B-LED. . Thereby, for example, a projection light source of a projector is created. At the same time, the output voltage of the DC / DC converter 10 is smoothed by sequentially switching the corresponding capacitors CR, CG, CB in accordance with the LED to be lit.

(Waveform example of each part)
3 and 4 are waveform diagrams showing an operation example of the above-described light emitting element driving circuit. These drawings show an operation example when the light emitting diode R-LED is turned on. In each waveform diagram, the expression of voltage or current pulsation is omitted for the sake of drawing.

  FIG. 3 is a waveform diagram showing an operation in a transient state (when power is supplied to the light emitting element driving circuit or when the charging voltage of each capacitor is low) until the capacitor CR of the light emitting element driving circuit is charged. In the figure, the signal DC / DC_ON indicating the ON state of the DC / DC converter 10, the voltage Vout in FIGS. 1 and 2, R_ON indicating the ON state of the switch SWR corresponding to the light emitting diode R-LED, the light emitting diode R SWCR-1 indicating the conduction state of the switch SWCR-1 corresponding to the LED, SWCR-2 indicating the conduction state of the switch SWCR-2 corresponding to the light emitting diode R-LED, and the capacitor CR corresponding to the light emitting diode R-LED A VCR indicating the charging voltage and a current Iout flowing through the light emitting diode R-LED are shown.

  In the same figure, the On / Off signal synchronized with the lighting control signal 100 is in the on state, that is, during the period when the DC / DC converter 10 is operating, the light emitting diode R-LED is obtained by image processing (not shown) of the projector. When lighting is necessary, the R signal of the RGB-LED lighting control signal 100 transitions to a high level (time t0).

  Then, the switch SWR in the LED switching circuit SW corresponding to the light emitting diode R-LED changes from the off state (blocking state) to the on state (conduction state). As a result, the current Iout starts to flow. At this time, the reference voltage Vref to the DC / DC converter 10 is set by the Vref setting unit 4 so that the drive current of the light emitting diode R-LED becomes a desired value. Further, the gate terminals of the switches SWCR-1 and SWCR-2 in the capacitor switching circuit SWC are set to the low level.

  Thereafter, the DC / DC converter 10 starts to operate, and the voltage Vout gradually increases. When the voltage Vout rises and the voltage between the source and gate terminals of the switch SWCR-1 reaches the threshold voltage Vth, the switch SWCR-1 is turned on (time t1), and the capacitor is connected through the parasitic diode D2 of the switch SWCR-2. The CR begins to charge. As a result, the voltage Vout decreases slightly (time t2).

  When the charging voltage of the capacitor CR reaches the threshold voltage Vth2 of the switch SWCR-2, the switch SWCR-2 is turned on (time t3), and the capacitor CR is completely electrically connected to the output voltage Vout of the DC / DC converter 10. Connected in parallel. That is, the capacitor CR is connected in parallel to a voltage source that supplies the input voltage Vin, and functions as a smoothing capacitor. Thereafter, the voltage Vout rises again, and the capacitor CR is charged with an optimum voltage necessary for lighting the R-LED. When the voltage Vout rises and exceeds the threshold voltage of the light emitting diode R-LED, the light emitting diode R-LED emits light.

  The current flowing into the light emitting diode R-LED is detected through the resistance element Rsense, converted into a voltage signal, and input to the error amplifier 11. The error amplifier 11 is compared with a preset reference voltage Vref and fed back to the DC / DC converter 10. As a result, a desired constant current is generated, and a stable projection light source can be obtained.

  As described above, after the switch for applying a voltage between both terminals of the light emitting diode is turned on, the switch for electrically connecting the smoothing capacitor to the power source is turned on, and further, the light emitting diode When the voltage between both terminals exceeds the threshold voltage, the light emitting diode emits light.

  On the other hand, FIG. 4 is a waveform diagram showing an operation in a state where the capacitor of the light emitting element driving circuit is charged, that is, in a steady state. This figure shows an example in which the R-LED is turned on, as in the case of FIG.

  In FIG. 4, when the On / Off signal synchronized with the lighting control signal 100 is on, that is, during the period when the DC / DC converter 10 is operating, the light emitting diode R-LED is obtained by image processing (not shown) of the projector. When lighting is necessary, the R signal of the RGB-LED lighting control signal 100 transitions to a high level (time t0).

  Then, the switch SWR in the LED switching circuit SW corresponding to the light emitting diode R-LED changes from the off state (blocking state) to the on state (conduction state). As a result, the current Iout starts to flow. At this time, the reference voltage Vref to the DC / DC converter 10 is set by the Vref setting unit 4 so that the drive current of the light emitting diode R-LED becomes a desired value. Further, the gate terminals of the switches SWCR-1 and SWCR-2 in the capacitor switching circuit SWC are set to the low level.

  Thereafter, the DC / DC converter 10 starts to operate, and the voltage Vout gradually increases. When the voltage Vout rises and the voltage between the source and gate terminals of the switch SWCR-1 reaches the threshold voltage Vth, the switch SWCR-1 is turned on (time t2). However, unlike the case of FIG. 3, since the charging voltage of the capacitor CR has already reached the threshold voltage Vth2 of the switch SWCR-2 (time t0), when the switch SWCR-1 is turned on, the capacitor CR becomes It is connected in parallel to a voltage source that supplies the input voltage Vin and functions as a smoothing capacitor. However, when the switch SWCR-1 is turned on, the charging voltage of the capacitor CR may slightly decrease (time t2). However, even if this drop occurs, the current Iout flowing through the light emitting diode R-LED is not affected, and is kept constant.

  By the way, once this circuit starts to operate, the smoothing capacitors CR, CG, and CB are charged and maintained at voltages that respectively light the corresponding LEDs. Therefore, when switching the smoothing capacitor, it is possible to quickly contribute to the current supply to the LED, and the lighting response performance of the LED can be improved.

  Note that when the light emitting diode G-LED and the light emitting diode B-LED are turned on, the operation is the same as described above, and the light emitting diodes R-LED, G-LED, and B-LED operate while being sequentially switched.

(Operation of display device)
FIG. 5 is a waveform diagram showing an operation example of a display device employing the above-described light emitting element driving circuit. In the figure, among the RGB-LED lighting control signal 100, signals R, G, and B for lighting the light emitting diodes R-LED, G-LED, and B-LED, respectively, and the Vref setting unit 4 are set. The reference voltage Vref, the voltage Vout in FIGS. 1 and 2, the period in which the smoothing capacitors CR, CG, CB of the smoothing capacitor unit Cout are used, and the light emitting diodes R-LED, G-LED, B-LED The flowing current Iout is shown.

  In FIG. 5, the pulse widths (high level times) of the signals R, G, and B are different because the lighting periods of the light emitting diodes of the respective colors are different. In the lighting period of the light emitting diode R-LED, the smoothing capacitor CR corresponding to the light emitting diode R-LED is connected in parallel to the voltage source of the input voltage Vin. Similarly, the smoothing capacitor CG corresponding to the light emitting diode G-LED is input during the lighting period of the light emitting diode G-LED, and the smoothing capacitor CB corresponding to the light emitting diode B-LED is input during the lighting period of the light emitting diode B-LED. The voltage Vin is connected in parallel to the voltage source.

  Further, during the lighting period of the light emitting diode R-LED, the reference voltage Vref corresponding to the light emitting diode R-LED is set. Similarly, the reference voltage Vref corresponding to the light emitting diode G-LED is turned on during the lighting period of the light emitting diode G-LED, and the reference voltage Vref corresponding to the light emitting diode B-LED is turned on during the lighting period of the light emitting diode B-LED. Is set. In this example, the light emitting diode B-LED, the light emitting diode R-LED, and the light emitting diode G-LED are arranged in ascending order of the reference voltage Vref. Therefore, the voltage Vout and the current Iout are at levels according to the reference voltage Vref.

  Thus, the dedicated reference voltage Vref for the light emitting diode R-LED, the dedicated reference voltage Vref for the light emitting diode G-LED, and the dedicated reference voltage Vref for the light emitting diode B-LED can be appropriately set. It is possible to avoid the inconvenience that an excessive current or an excessive current flows into each light emitting diode, and to control so as not to exceed the rated current. This reduces the possibility of adversely affecting the image quality of the display device or shortening the lifetime of each element.

(Modification)
In the embodiment described above, the anode common configuration is employed for each light emitting diode, but the present invention is not limited to this, and a cathode common configuration may be employed. When the cathode common configuration is employed, a configuration in which the connection between the NMOSFET and the PMOSFET is replaced in FIG. 2 may be used.

  In the above description, the light emission colors of the light emitting diodes are red (R), green (G), and blue (B). However, the red (R) may include the color of the orange to red region. In addition, the green color (G) may include the color of the green to yellow-green region.

(Summary)
In this embodiment, the capacitor switching circuit allows the corresponding smoothing capacitors CR, CG, and CB to be connected independently to the light emitting diodes R-LED, G-LED, and B-LED. There is no flow of overcurrent or undercurrent. Thereby, the response characteristic of LED can be made faster and supply current can be supplied more stably. In addition, it is not necessary to prepare a high voltage power supply separately, and a circuit with a smaller number of parts and a smaller mounting area can be realized.

  Note that the light emitting element driving circuit according to the present embodiment can be used when a field sequential driving method is adopted in a portable projector, a stationary projector, and other display devices.

2 inductor 3 diode 4 Vref setting unit 5 current detection circuit 10 DC / DC converter 11 error amplifier 12 oscillator 13 sawtooth wave generator 14 PWM controller 15 reset set flip-flop 16 drive circuit 17 NMOSFET
41R, 41G, 41B Variable resistance element 42R, 42G, 42B Switch 43 Resistance element R-LED Red light emitting diode G-LED Green light emitting diode B-LED Blue light emitting diode Cout Smoothing capacitor CR, CG, CB Smoothing capacitor D1, D2 Parasitic Diode L Inductor Rsense Resistive element SW LED switching control circuit SWC Capacitor switching circuit

Claims (7)

A switching control unit for controlling a plurality of light emitting elements having different emission colors to be driven alternatively;
A plurality of capacitive elements provided corresponding to the plurality of light emitting elements, and smoothing the supply voltage to the corresponding light emitting elements;
A capacitance switching unit that controls a capacitive element corresponding to the driven light emitting element among the plurality of capacitive elements to be electrically connected in parallel with the power source;
A current detection unit for detecting a current value flowing in the driven light emitting element;
A supply voltage control unit that controls a voltage supplied to the light emitting element according to a comparison result between a current value detected by the current detection unit and a reference value set for the light emitting element;
A light-emitting element driving circuit comprising: In claim 1,
The light emitting element driving circuit, wherein the reference value is set according to the light emitting element to be driven. In claim 1 or claim 2,
The light-emitting element driving circuit, wherein the capacitance switching unit performs connection control of the capacitive element in conjunction with the drive control of the light-emitting element by the switching control unit. In any one of Claim 1 to Claim 3,
After a switch for applying a voltage between both terminals to drive the light emitting element is turned on, a smoothing capacitor corresponding to the light emitting element is electrically parallel to the power source in the capacitance switching unit. The threshold value is set so that the light emitting element emits light when a switch for connecting to the light emitting element is turned on by the switching control unit, and then an applied voltage between both terminals of the light emitting element exceeds a threshold voltage of the light emitting element. A light-emitting element driving circuit, wherein a voltage is set. In any one of Claim 1 to Claim 4,
In the capacitance switching unit, a plurality of switch units each including a pair of MOS transistors having a common gate terminal and in which anodes of parasitic diodes are connected are provided corresponding to the plurality of capacitive elements, A light emitting element driving circuit, wherein a voltage applied to a gate terminal of each switch unit is changed in conjunction with driving control of the light emitting element by the switching control unit. In any one of Claim 1-5,
The light-emitting element drive circuit, wherein the current detection unit is provided in common to the plurality of light-emitting elements. The light emitting element drive circuit according to any one of claims 1 to 6,
A plurality of light emitting elements driven by the light emitting element driving circuit and having different emission colors;
A display device comprising: