Driving circuit for light emitting matrix display using a switched capacitor circuit

Abstract: In a circuit for driving a light emitting element, a switched capacitor circuit including a capacitor is connected between a power supply and the light emitting element. The capacitor in the switched capacitor circuit is charged to a power supply voltage when switches are exchanged to the sides of charging. The power supply is connected in series with the capacitor when the switches in the switched capacitor circuit are exchanged to the side of discharging. The power supply and capacitor connected in series therewith supplies the driving current to the light emitting element. Changing the number of times of exchange of the switches permits the driving current to flow into the light emitting element to be varied. Thus, the circuit for driving a light emitting element with excellent power-supply efficiency is provided which can drive a light-emitting element even when the power supply voltage for driving the light emitting element is lower than the forward voltage thereof.

Description: The present invention relates to a matrix display system having a plurality of light-emitting elements arranged in a matrix at crossing points thereof. US-A-4,772,886 discloses a matrix display system including cathode lines, anode lines and light emitting elements. This system includes also cathode and anode lines driving units and a clocked scanning pulse generating unit. From US-A-5 442 260, a light-emitting element drive circuit for a camera is known comprising a diode and a light imaging element connected in series with each other between a power source and ground, a first switching element switchable between on and off states for driving the light-emitting element, second and third switching elements connected in series with each other between the power source and ground, and a capacitor connected between a junction point of the diode and light-emitting element and a junction point of the second and third switching elements. Since the light-emitting element is driven by a capacitor charge/discharge operation instead of using a booster power source, the consumption of current is reduced and the light-emitting period is shortened. EP-A-0 409 245 discloses a charge transfer device in which two or more voltage boosting stages are connected in tandem in order to raise the voltage multiplication effect of the device. In a light-emitting element such as an LED and an organic EL, the light-emitting luminance varies undesirably when the driving current flowing into the element varies. For this reason, the driving current flowing into the element must be controlled so that it is maintained constant. Figure 7 shows an example of a conventional driving circuit of a light-emitting element. In this driving circuit, a constant current supply 3 intends to change the driving current, which is supplied to the light-emitting element 2 supplied from a power supply 1 for driving it, into a constant current. It should be noted that the light-emitting element 2 emits light when a switch 4 is opened as indicated by solid line, and ceases to emit light when the switch is closed as indicated by dotted line. Fig. 8 shows the second example of the conventional driving circuit. In this configuration, a high resistance 5, which is inserted in series between a light emitting element 2 and a power supply 1 for driving it, intends to control the driving current flowing through the light emitting element 2 to be constant. It should be noted that the light emitting element 2 emits light when a switch 6 is located at a position indicated by solid line and it ceases to emit light when the switch 6 is changed to another position indicated by dotted line. In both driving circuits described above, the driving power supply 1 for a light emitting element 2 is required to have a power supply voltage Vcc which is higher than the forward voltage Vf of the light emitting element 2. This driving circuit, therefore, cannot be used when the power supply voltage Vcc is lower than the forward voltage Vf. Meanwhile, setting the power supply voltage Vcc for a higher value than the forward voltage Vf results in wasteful power consumption corresponding to the difference therebetween, thereby deteriorating efficiency of power supply. This is very problematic in a portable or vehicle-mounted appliance in which a dry cell or battery is used as a power supply for driving the light emitting element. The forward voltage Vf of LED is as low as + 1.5 V to + 2 V and also relatively stable. On the other hand, that of the organic EL is as high as + 6 V to 12 V and also greatly varies in accordance with luminance, temperature and time passage. Where the organic EL is used as a display of "car stereo", since a vehicle-mounted battery must be used as a power supply 1 for driving the light emitting element, as the case may be, the light emitting element, if the power supply is not boosted, cannot be driven. SUMMARY OF THE INVENTION It is the object of the present invention to provide a simple-matrix display device having a driving circuit with excellent power-supply efficiency which can drive a light-emitting element even when the power supply voltage for driving the light emitting element is lower than the forward voltage thereof using a switched capacitor circuit, and in which the luminance can be set. This object is attained by the features of claim 1. Preferred embodiments of the invention are subject matter of the dependent claims. A saw-tooth wave generating circuit is preferably connected in series between a capacitor and ground of the switched capacitor circuit. In this structure, the driving current flowing into the light emitting element can be maintained constant which does not exceed the absolute maximum current of the light emitting element. The instantaneous maximum voltage of an output can be set for a lower point which does not exceed the withstand voltage of a drive IC. The number of times of charging/discharging the switched capacitor circuit is varied in accordance with the current flowing through the light emitting element. In this structure, the light-emitting luminance can be changed freely so that in accordance with the area of the light emitting element, the luminance can be set, thus permitting the tone to be represented. The above and other object and features of the present invention will be more apparent from the following description taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a circuit diagram showing a first driving circuit which may be used in the present invention; Fig. 2 is an operation waveform chart of the circuit of Fig. 1; Fig. 3 is a circuit diagram when the circuit of Fig. 1 is configured into an integrated circuit (IC); Fig. 4 is a circuit diagram of another driving circuit which may be used in the present invention; Fig. 5 is a waveform chart of the saw-tooth voltage produced from a saw-tooth wave generating circuit in the aforementioned driving circuit; Fig. 6 is a block diagram showing an example of a simple-matrix display system configured according to the present invention; and Fig. 7 is a circuit diagram of the first prior art; and Fig. 8 is a circuit diagram of the second prior art. DESCRIPTION OF THE PREFERRED EMBODIMENTS Now referring to the drawings, an explanation will be given of several driving circuits for a light emitting element which may be used in the present invention. Fig. 1 shows a first driving circuit. In Fig. 1, like reference numerals refer to like elements in Figs. 7 and 8. As seen from Fig. 1, in this embodiment, a switched capacitor circuit 10 is connected between a light emitting element 2 and a drive power supply 1 therefor. The switched capacitor circuit 10 includes two switches 7, 8 and a capacitor 9 arranged therebetween. Specifically, the anode of the drive power supply 1 is connected to terminal a of the switch 7 and terminal b of the switch 8, respectively. The terminal c of the switch 7 is connected to the anode of the light emitting element 2. A capacitor 9 having predetermined capacitance is connected between the respective terminals c of the switches 7 and 8. Incidentally, the terminal b of the switch 7 is opened and the terminal a of the switch 8 is connected to ground. The switched capacitor circuit 10 appears to serve as an ordinary boosting circuit. But, as described below in detail, the switched capacitor circuit 10 does not serve as the ordinary boosting circuit for supplying 2Vcc twice as large as a power supply voltage Vcc to load, but as a charge injecting circuit for once storing charges for light emission in the capacitor 9 from the drive power supply 1 and injecting the stored charges into the light emitting element 2. An explanation will be given of the operation of the driving circuit according to this embodiment. It is now assumed that Vcc < Vf < 2Vcc, i.e., the forward voltage Vf of the light emitting element 2 is larger than the power supply voltage Vcc, and smaller than twice of the power supply voltage Vcc. Therefore, when the switched capacitor circuit 10 of Fig. 1 is removed and the drive power supply 1 is directly connected to the light emitting element 2, the light emitting element 2 cannot emit light. Now it is assumed that two switches 7 and 8 within the switched capacitor circuit 10 have been exchanged to the respective terminals a as indicated in solid line. In this state (hereinafter referred to as "charge state"), the capacitor 9 is charged to a power supply voltage Vcc in a path consisting of the anode of the drive power supply 1, a-c terminal of the switch 7, capacitor 9, c-a terminal of the switch 8 and ground. Assuming that the capacitor 9 has C1 of electrostatic capacitance, the charge of Q1 = C1 x Vcc is stored. After a predetermined time, the two switches 7 and 8 are exchanged to the side of the terminal b as illustrated in dotted line. In this state (hereinafter referred to as "discharge state"), the capacitor 9 is connected in series with the drive power supply 1 in a path consisting of the anode of the drive power supply 1, b-c terminal of the switch 8, light emitting element 2 and ground. Because of this series connection, the potential Vp1 at point P1 on the side of the switch 8 of the capacitor 9 is boosted toward the power supply voltage Vcc. Correspondingly, the potential Vp2 at a position on the side of the light emitting element 2 is boosted toward 2Vcc. In this case, since the light emitting element 2 is connected to the point P2 as load, when the voltage Vp2 at point of P2 reaches the forward voltage Vf of the light emitting element 2, charges start to flow to the light emitting element 2 from the capacitor 9, thus starting light emission of the light emitting element 2. Therefore, the voltage Vp2 is not boosted to exceed the forward voltage Vf of the light emitting element 2. After lapse of a predetermined time, the two switches 7 and 8 are exchanged to the side of the respective terminals a (side of charged state) again. The charges Q flowing towards the light emitting element 2 when the discharging operation is once effected is Q = C1 x Vd where Vd = 2Vcc - Vf The light emitting element 2, as described later in connection with Fig. 6, is arranged in a matrix together with other light emitting elements. The light emitting element 2 is commonly driven by a "matrix system" in which a driving voltage is sequentially applied to the light emitting elements at cross points in the matrix to emit light. Assuming that the driving time T1 assigned for a single light emitting element is Td, the switches 7 and 8 are exchanged N times within the driving time T1 to effect the charging/discharging operation N times, the average value I of the driving current flowing into the light emitting element 2 is expressed by I = (Q x N)/Td = (C1 x Vd x N)/Td As apparent from Equation (2), by variation of the number N of times of exchange of the switches 7 and 8, the average value of the driving current flowing into the light emitting element 2 can be varied. Thus, the luminance of the light emitting element 2 can be altered so that in the driving circuit using the switched capacitor circuit 10, the same luminance can be obtained as in the conventional circuit. Fig. 2 is a waveform chart of the voltage Vp1 at the point of P1 and the voltage Vp2 at the point P2 in Fig. 1. The voltage Vp1 at the point P1 goes back and forth between 0 V (ground) and power supply voltage Vcc. The waveform of the voltage Vp1 is shown in a trapezoid shape, considering the slew rate of the circuit. On the other hand, the voltage Vp2 at the point P2 remains the power supply voltage Vcc during the charged state where the switches 7 and 8 are exchanged to the terminals of a . When the switches 7 and 8 are exchanged to the terminals of b to establish the discharge state, the voltage Vp2 at the point P2 is boosted towards 2Vcc in series to voltage Vp1. When the voltage VP2 reaches the forward voltage Vf of the light emitting element 2, charges start to flow from the capacitor 9 to the light emitting element 2. At this time, boosting of the voltage stops. Simultaneously when discharging starts, the light emitting element 2 starts to emit light. After a predetermined time, the voltage Vp1 at the point P1 reaches the power supply voltage Vcc, the voltage Vp2 at the point P2 is gradually lowered as discharging of the capacitor 9 proceeds. At this time, charges sufficient to cause the light emitting element 2 to emit light are no longer supplied. When the voltage Vp2 at the point P1 reaches the power supply voltage Vcc, the light emitting element 2 substantially ceases to emit light. Discharging of the capacitor 9 further proceeds so that the voltage Vp2 at the point P2 is lowered toward the power supply voltage Vcc. Then, since the amount of discharged charges is small, the light emitting element 2 is not caused to emit light. After a predetermined time, the switches 7 and 8 are exchanged to the terminals a to establish the charge state, the capacitor 9 is charged again by the power supply voltage Vcc of the driving power supply 1 for preparation of a next discharging operation. The above charging/discharging operation is repeated so that the light emitting element 2 emits light at a predetermined luminance. Fig. 3 shows an example in which the switched capacitor 10 of the driving circuit of Fig. 1 is IC-ed (integrated-circuited) using C-MOS's. In this figure, although it appears that the capacitor 9 is not IC-ed, in an actual circuit, the capacitor 9 can be integrally formed on a substrate through an IC process of CMOS. The entire circuit of the switched capacitor circuit 10 is IC-ed into a single chip. The driving circuit is suited to mass production, and is miniaturized. In Fig. 3, the portion indicated by reference numeral 11 denotes a clock-phase inverting circuit for turning ON/OFF the switch 7. Fig. 4 shows another driving circuit which may be used in the present invention. In this driving circuit, a saw-tooth wave generating circuit 12 is connected instead of the switch 8 between the capacitor 9 and ground. In operation, after the switch 8 is exchanged to the terminal b , the saw-tooth wave generating circuit 12 generates a saw-tooth wave voltage as shown in Fig. 5. This saw-tooth wave voltage is used to compensate for the voltage drop of the capacitor 9 due to its discharging, thereby capable of maintaining the instantaneous value i0 flowing into the light emitting element 2 constant during the discharging period. Specifically, assuming that the saw-tooth wave voltage outputted from the saw-tooth generating circuit 12 is Vi, the instantaneous current i0 which starts to flow from the capacitor 9 to the light emitting element 2 in discharging is expressed by i0 = C1 x (dVi/dt) Since the gradient of the saw-tooth wave voltage Vi is constant, (Dvi/dt) in Equation (5) is also constant, and hence i0 becomes constant. Fig. 6 shows an embodiment of a simple-matrix display system configured according to the present invention. In this embodiment, nine light emitting elements 2 1 - 2 9 arranged in a matrix are drive-controlled by a driving circuit according to the present invention. The individual light emitting elements 2 1 - 2 9 are connected at cross-points of cathode scanning lines 15 1 - 15 3 and anode driving lines 16 1 - 16 3 . The cathode scanning lines 15 1 - 15 3 are sequentially scanned by a cathode scanning unit 17, and the anode driving lines 16 1 - 16 3 are simultaneously driven by an anode drive unit 18. The driving circuits 19 1 - 19 3 provided corresponding to the respective anode driving lines 16 1 - 16 3 issue driving pulses simultaneously in parallel. The driving circuits 19 1 - 19 3 have either one of the driving circuits shown in Figs. 1 and 4 in accordance with the specification of a used system. A scanning pulse generating unit 20 serves to generate scanning pulses for sequentially selecting the cathode scanning lines 15 1 - 15 3 at predetermined intervals T on the basis of a clock from a clock generating unit 21. A light-emission data generating unit 22 serves to designate the number N of times of switch exchange for the light emitting element to emit light. A counter 23 serves to produce a switch exchange pulse for each of the light emitting elements to emit light. The clock sent from the clock generating unit 21 is divided by the number N of times of switch exchange from the light-emission data generating unit 22 so that a switch exchange pulse for each of the light emitting elements is generated. These switch exchange pulses are sent as a serial signal to a serial-parallel conversion unit 24. The serial-parallel conversion unit 24 divides the switch exchange pulses sent as a serial signal for each of the light emitting elements, and supplies these pulses in an extended time axis, simultaneously in parallel, to the corresponding driving circuits 19 1 - 19 3 in the anode driving unit 18. An explanation will be given of the operation of the display system of Fig. 6 in connection with a light emitting element 2 5 at a center position of the matrix of the light emitting elements 2 1 - 2 9 is to be emitted. The cathode scanning unit 17 sequentially selects the cathode scanning lines 15 1 - 15 3 at predetermined intervals of T. In this selection, the selected cathode scanning line is connected to 0 V (ground) whereas the other cathode scanning lines not selected are connected to a voltage equal to the power supply voltage Vcc for the driving circuits 19 1 - 19 3 . Now when the cathode scanning line 15 2 is selected, as shown in Fig. 6, the cathode scanning line 15 2 is connected to 0 V (ground) and the other cathode scanning lines 15 1 and 15 3 are connected to Vcc. In this state, as shown in Fig. 6, the counter 23 produces switch exchange pulses (1), (2) and (3) for the respective light emitting elements at intervals of Td = 1/3·T. In this example, since only the light emitting element 2 5 of the above three light emitting elements 2 4 , 2 5 and 2 6 emits light, the switch exchange pulse is outputted at only the position of the second timing (2) corresponding to the light emitting element 2 5 . The counter 23 divides the clock from the clock generating circuit 21 on the basis of the designated data of the number N of times of switch exchange sent from the light emitting data generating circuit 22 to produce N switch exchange pulses with the driving period Td at the position of the timing (2) and sends these pulses to the serial-parallel conversion unit 24. The serial-parallel conversion unit 24 separates the signals of (1), (2) and (3) sent from the counter 23 as serial signals to extend their time axes to three times 3·Td = T, and sends these time-extended signals (1), (2) and (3) simultaneously in parallel to the corresponding driving circuit 19 1 - 19 3 . In the case of this example, the time-extended switch exchange pulses of (2) are supplied to only the second driving circuit 19 2 . Thus, in response to switch exchange pulses of (2), the driving circuit 19 2 , as described above, exchanges the switches 7 and 8 in the switched capacitor circuit 10 in accordance with the switch exchange pulses of (2). Because of the switch exchange of the switches 7 and 8, the driving circuit 19 2 supplies the driving current corresponding to the number N of times of switch exchange to the anode driving line 16 2 . The driving current flows through the light emitting element 2 5 on the passage consisting of anode driving line 16 2 , light emitting element 2 5 , cathode scanning line 15 2 and 0 V (ground). As a result, the light emitting element 2 5 emits light with the luminance corresponding to the driving current. By the repetition of the above operation at a period T, the light emitting element 2 5 repeats the emission of light. Thus, with the period T set for 30 ms or shorter, because of an afterimage operation, human eyes recognize that the light emitting element 2 5 is emitting light continuously.

Claims: A simple-matrix display system comprising: a matrix for display elements composed of a plurality of cathode scanning lines (15 1 -15 3 ), a plurality of anode drive lines (16 1 -16 3 ) and a plurality of light emitting elements (2 1 -2 3 ) arranged in a matrix at crossing points thereof; a cathode scanning unit (17) for sequentially scanning said cathode scanning lines (15 1 -15 3 ); an anode driving unit (18) for driving said anode driving lines (16 1 -16 3 ) simultaneously with a scanning operation of the cathode scanning unit (17); a clock generating unit (21) for generating clocks; a scanning pulse generating unit (20) for generating scanning pulses for sequentially selecting said cathode scanning lines (15 1 -15 3 ) at prescribed intervals on the basis of the clocks generated from said clock generating unit (21); characterised by a plurality of driving circuits (19 1 -19 3 ), each comprising a light emitting element (2 1 -2 3 ) and a switched capacitor circuit (10) including a capacitor (9) for driving said light emitting element (2 1 -2 3 ), said driving circuits (19 1 -19 3 ) being provided so as to correspond to the anode driving lines (16 1 -16 3 ) within said anode driving unit (18) and supplying driving pulses to the display elements (2 1 -2 3 ) simultaneously in parallel; a light-emission data generating unit (22) for specifying the number (N) of times of switch exchange of the switched capacitor circuit (10) driving the light emitting elements (2 1 -2 3 ) to emit light in the time allotted to the current scan line; a counter (23) for creating a switch exchange pulse for each of the light emitting elements (2 1 -2 3 ) by dividing the clock sent from said clock generating unit (21) by the number (N) of times of switch exchange supplied from said light-emission data generating unit (22); and a serial-parallel conversion unit (24) for converting the switch exchange pulses supplied from said counter (23) as a serial signal, into parallel signals and supplying them to said driving circuits (19 1 -19 3 ) in said anode driving unit (18). A simple-matrix display system according to claim 1, wherein a saw-tooth wave generating circuit (12) is connected in series between the capacitor (9) and ground in said switched capacitor circuit (10). A simple-matrix display system according to claim 1, wherein the number (N) of times of charging/discharging in said switched capacitor circuit (10) is changed in accordance with a driving current to flow into said light emitting element (2 1 -2 3 ).

Inventors: OKUDA YOSHIYUKI, OKUDA, YOSHIYUKI, Okuda, Yoshiyuki, c/o Pioneer Elec. Corp.
Applicants: PIONEER ELECTRONIC CORP, PIONEER ELECTRONIC CORPORATION
Application year: 1996
Main classification: G09G3
IPC codes: H01L33, H05B33, G09G3, H01L51