Current driving device

The present invention relates to a current driving device used preferably as a driver for displays such as organic EL (Electro-luminescence) panels, LED panels, and the like.

Recently, flat-panel displays have a larger screen and higher definition, while establishing reduction in thickness, weight, and cost. With such backgrounds, it is required for display drivers to improve the uniformity in the displayed image qualities through decreasing variations between output electric currents that are outputted from output terminals. Variations in the electric currents in static actions of current mirrors include variations caused due to fabrication processes of each transistor, variations of gate voltages caused due to resistances of power supply wirings, and the like. Further, variations due to dynamic actions of the current mirrors include variations caused due to injection of electric charges from a display panel or instantaneous fluctuations of power supply, for example. Furthermore, generally, a driver IC is formed in a multi-output structure with a stick-like slim shape, since it is mounted to a frame part of a flat panel. Because of restrictions in the LSI shape, characteristics of transistors disposed therein vary depending on positions of slim-layout elements. Therefore, even if a same gate voltage is applied in a current mirror structure, the output currents from each of current-summing DA converter circuits do not necessarily become the same.

As a method for decreasing such variations, there is a current driving device having a current output part, such as the one shown in FIG. 6A and FIG. 6B , for example (see US 2005/0231241A1, for example). This current output part has a calibration function and a current output function. “Calibration” means to have a reference current value by a reference current source stored in the current output part.

Under a calibration mode, current output switches So 1 and So 2 are set to a nonconductive state, while a current input switch Sw 1 , a calibration switch Sw 2 , and all signal response switches G 1 -Gm are set to a conductive state. Current output terminals T 1 and T 2 are isolated from the outside. Nodes N 1 and N 2 are short-circuited, and drains and gates of Nch transistors QN 1 -QNm are short-circuited to receive electric currents from a reference current source I 1 . Through this, the Nch transistors QN 1 -QNm generate, at the node N 2 , a gate voltage that is necessary for allowing the electric current from the reference current source I 1 to flow through the transistors themselves. A voltage holding capacitance element C 1 is charged to the above-described gate voltage. A voltage V (N 2 ) of the node N 2 corresponds to a voltage that is capable of passing an electric current that is equal to the reference current generated by the reference current source I 1 through the Nch transistors QN 1 -QNm which are all in a conductive state. In conclusion, the current output part A comes to store the reference current generated by the reference current source I 1 .

Under a current output mode, the current input switch Sw 1 and the calibration switch Sw 2 are turned to a nonconductive state, and either one of the current output switch So 1 or So 2 is set to a conductive state while the other is remained in a nonconductive state. The conduction state of the signal response switches G 1 -Gm is determined depending on an input signal supplied from the outside. The voltage holding capacitance element C 1 holds the voltage charged by the calibration action, and continues to supply the electric current to the gate terminals of the Nch transistors QN 1 -QNm. The Nch transistors QN 1 -QNm output an electric current in accordance with the voltage V (N 2 ) of the node N 2 from one (in an conductive state) of the current output terminals T 1 and T 2 , in accordance with the conduction state of the signal response switches G 1 -Gm.

FIG. 7 shows a timing chart of a series of actions regarding calibration and output of the electric current performed in the current-summing DA converter circuit shown in FIG. 6A and FIG. 6B . The conductive state of the switches is shown with “H”, and the nonconductive state thereof is shown with “L”. “V (N 2 )” indicates the voltage V (N 2 ) of the node N 2 .

Before calibration, the current output part A sets the current output witch So 1 to be in a conductive state, and outputs, from the current output terminal T 1 , the output current in accordance with the state of the signal response switches G 1 -Gm which are controlled by the voltage V (N 2 ) of the node N 2 and an input signal. The current output switch So 2 , the current input switch Sw 1 , and the calibration switch Sw 2 are set to be in a nonconductive state.

During the calibration period, the current output switches So 1 , So 2 are set to a nonconductive state, the current input switch Sw 1 and the calibration switch Sw 2 are set to a conductive state, and the signal response switches G 1 -Gm for selecting the electric currents from the Nch transistors QN 1 -QNm are all set to a conductive state. Through this, the Nch transistors QN 1 -QNm are set in a state where only the electric current of the reference current source I 1 is supplied, and the voltage V (N 2 ) of the node N 2 is determined in accordance with the above-described actions.

After completing the above-described calibration, the current input switch Sw 1 and the calibration switch Sw 2 are set to a nonconductive state, so that the voltage holding capacitance element C 1 holds the electric charge. That is, the voltage V (N 2 ) of the node N 2 is being maintained. Thereafter, the current output switch So 2 is set to a conductive state, and the sum of the electric currents selected by the signal response switches G 1 -Gm in accordance with the input signal is outputted from the current output terminal T 2 .

FIG. 8 shows a structure of a current driving device that utilizes the current output part A shown in FIG. 6A and FIG. 6B . (n+1)-numbers of current output parts A 0 -An control the operation states under a calibration mode and a current output mode in accordance with control signals supplied from an action control circuit B. A signal input circuit Din supplies signals for controlling the signal response switches G 1 -Gm within the internal structural elements (see FIG. 6A ) of each of the current output parts A 0 -An. Calibration by the reference current source I 1 is performed on a single current output part among the (n+1)-numbers of current output parts A 0 -A 1 by an internal operation of each of the current output parts A 0 -An. Calibration is performed in order from A 0 , to A 1 , A 2 , - - - , and to An. The current output parts that are not under calibration are set to be in the current output mode, and output an electric current to n-numbers of output terminals OUT 1 -OUTn. The electric currents are outputted in order from A 0 to A 1 , A 2 , - - - , and to An.

The reference current, when there are a plurality of current-summing DA converter circuits with the above-described current output parts, can be combined into the electric currents of the same current source. This makes it possible to achieve the uniformity in the display on a panel.

Other related documents are: Japanese Unexamined Patent Publication 2004-219955, Japanese Unexamined Patent Publication 2005-121843, US2004/0251844A1, US7050024B2, US6594606B2, US2006/0158402A1.

With this method, however, the capacity for charging/discharging the voltage holding capacitance element C 1 becomes insufficient when the current value of the reference current source I 1 is very small, so that it is difficult to charge the current of the Nch transistors QN 1 -QNm to accurately meet the value of the current from the reference current source I 1 within the calibration period. FIG. 9 shows the state where the voltage holding capacitance element C 1 is insufficiently charged because the reference current is very small.

Further, when there is a change in the current value of the reference current source I 1 , the reference currents become varied depending on the output terminals with the successive calibration, until the reference currents of all the current output parts A 0 -An are updated.

An object of the present invention therefore is to provide a current driving device which can perform calibration at a high speed and improve the non-uniformity of the output currents even when a reference current is very small and when there is a change in the reference current.

A current driving device of the present invention comprises:

a first voltage supply part for supplying a first voltage;

a first current supply part for supplying a first electric current;

a plurality of output terminals; and

a plurality of current output parts for outputting an electric current in accordance with the first electric current, each of the current output parts comprising a current-voltage converting function, a voltage-current converting function, a voltage holding part, and at least one current output terminal, wherein

the current output part takes three operation modes, i.e. a voltage supply mode, a current supply mode, and a current output mode,

under the voltage supply mode, the current output part receives the first voltage from the first voltage supply part and holds the voltage in the voltage holding part,

under the current supply mode, the current output part receives the first current from the first current supply part, and generates a second voltage by the current-voltage converting function and holds the voltage in the voltage holding part, and

under the current output mode, the current output part outputs an output current according to the voltage held in the voltage holding part by the voltage-current converting function. The voltage supply mode and the current supply mode correspond to the calibration mode.

In the current driving device constituted in the manner described above, the current output part receives a supply of the first voltage from the first voltage supply part under the voltage supply mode, and holds the first voltage in the voltage holding part. Then, under the current supply mode, the current output part receives a supply of the first electric current from the first current supply part, generates the second voltage by the current-voltage converting function, and holds the second voltage in the voltage holding part. This makes it possible to charge the voltage holding part to a prescribed voltage at a higher speed. Then, under the current output mode, the current output part outputs an electric current according to the voltage held in the voltage holding part through the voltage-current converting function. For charging the voltage holding part, it is charged with the first voltage to a value close to the target voltage, and then charged further with a supply of the first electric current. Thus, even if the reference current supplied from the first current supply part is very small, it is possible to speed up the action for supplying the voltage to the voltage holding part until reaching the reference current and also to obtain the reference current accurately.

In the current driving device with the above-described structure, each of the output terminals is connected to the current output terminals provided to the plurality of the current output parts, and each of the plurality of current output parts is connected in parallel to the first current supply part and the first voltage supply part used in common. With this structure, the voltage holding parts provided to all the current output parts can be charged provisionally to the first voltage when it becomes necessary to perform extensive calibrations on all the current output parts, e.g. right after the startup or when there is a change in the first electric current. Therefore, it is possible to suppress unevenness in the display caused due to the change in the first electric current.

Further, in the current driving device with the above-described structure, each of the current output parts actuates the current-voltage converting function under the voltage supply mode. With this structure, for charging the voltage holding part, the voltage obtained by converting the first electric current from the first current supply part can be combined with the supply of the first voltage from the first voltage supply part. Therefore, still higher speed charging can be achieved.

Furthermore, in the current driving device with the above-described structure, each of the current output parts comprises a function of stopping the current-voltage converting function under the voltage supply mode. With this structure, all the electric currents supplied from the first voltage supply part can be utilized for charging the voltage holding part. Thus, the power consumption can be reduced.

Moreover, the current driving device with the above-described structure further comprises a second current supply part for supplying a second electric current that is proportional to the first electric current, and a current-voltage converting part for generating the first voltage by receiving the second electric current, wherein the first voltage supply part controls supplies of the first voltage generated by the current-voltage converting part to the current output parts. With this structure, the first voltage supplied from the first voltage supply part to the current output part is generated from the second electric current by the current-voltage converting part. The second electric current from the second current supply part is proportional to the first electric current, so that the first voltage comes to have a value that corresponds to the first electric current. Therefore, it is possible to generate the first voltage having a value that almost equals to the final target value to be held in the voltage holding part.

Further, in the current driving device with the above-described structure, the current-voltage converting part comprises a switching part for short-circuiting a node at which the first voltage emerges to a reference voltage node. When there is a change in the first electric current or the second electric current, it also takes time to change the second voltage generated by the current-voltage converting part. With this structure, however, the output voltage of the current-voltage converting part can be reset through actuating the switching part. Therefore, it is possible to speed up the change of the second voltage by the current-voltage converting part.

Furthermore, the current driving device with the above-described structure further comprises a second voltage supply part with a larger voltage supply capacity than that of the first voltage supply part, wherein the use of the first voltage supply part and the use of the second voltage supply part are switched in accordance with the number of the current output parts that take the voltage supply mode among the plurality of current output parts. When there are a large number of current output parts, the load of the voltage supply part changes largely depending on how many current output parts are under the voltage supply mode. With this structure, however, it is possible to reduce EMI (electromagnetic interference) and the power consumption by changing the output voltages smoothly through adding the second voltage supply part and switching between the use of the first voltage supply part and the use of the second voltage supply part in accordance with the load.

Alternatively, in the current driving device with the above-described structure, the first voltage supply part is constituted to be capable of changing its voltage supply capacity in accordance with the number of the current output parts that take the voltage supply mode among the plurality of current output parts. With this structure, it is possible to reduce EMI (electromagnetic interference) and the required area because the first voltage supply part is provided with the function of optimizing the voltage supply capacity in accordance with the load.

A display device according to the present invention related to the current driving device described above comprises one of the above-described current driving devices mounted thereon so as to be driven by the current driving device. Displays on the screen can be made uniform with this display device.

A current driving device according to the present invention comprises:

a current input switch for controlling connection/disconnection states with respect to a current supply part; a voltage holding part for holding a reference voltage, which is charged by a flown current; a calibration switch interposed between the current input switch and the voltage holding part; a plurality of voltage-current converting elements for generating an electric current in accordance with the reference voltage held in the voltage holding part; a plurality of signal response switches that are on/off controlled in accordance with inputted signals, the response switches being connected in series to each of the voltage-current converting elements, and each of which being connected in parallel to the calibration switch; a plurality of current output switches, each being interposed between a connection node, which is provided between the current input switch and the calibration switch, and the plurality of current output terminals; and a high-speeding switch for controlling connection/disconnection states of the voltage supply part with respect to the voltage holding part.

The calibration mode comprises two stages, i.e. the voltage supply mode and the current supply mode. In the voltage supply mode, the high-speeding switch is set to be in a conductive state to connect the voltage supply part to the voltage holding part in order to boost up the potential level of the voltage holding part at a high speed. At that time, the current output switches are all set to be in a nonconductive state, and the calibration switch and all the signal response switches are set to be in a conductive state. Then, in the current supply mode, the high-speeding switch is turned to a nonconductive state. The current input switch and the calibration switch are set to be in a conductive state, so that the sum of the current values flown in the voltage-current converting element becomes equal to the reference current value that is supplied from the current supply part. Thus, the voltage holding part comes to hold the voltage that corresponds to passing the current (equivalent to the reference current value) through the voltage-current converting element. That is, the reference current value is stored. As described, the potential of the voltage holding part is raised at a high speed by using the voltage supply part under the voltage supply mode that is the first half part of the calibration mode, and the reference current value by the current supply part is stored accurately in the latter half stage of the calibration mode. Therefore, even when the current value of the reference current source is very small, it becomes possible with the voltage supply part to compensate for the capacity for supplying the voltage to the voltage holding part till reaching the reference current and to complete the calibration at a high speed for allowing the current flowing the voltage-current converting element to meet accurately with the current value of the reference current source.

The present invention is illustrated be way of example and not limitation in the figures of the accompanying drawings, in which like references indicate similar elements and in which:

FIG. 1A is a circuit diagram for showing a structure of a current output part according to a preferred embodiment of the present invention;

FIG. 1B is a block circuit diagram for showing the structure of the current output part according to the preferred embodiment of the present invention;

FIG. 2 is a timing chart for showing actions of the current output part shown in FIG. 1A and FIG. 1B ;

FIG. 3 is a block circuit diagram for showing an overall structure of a current driving device according to the preferred embodiment of the present invention;

FIG. 4 is a circuit diagram for showing a specific structure of a voltage supply source according to the preferred embodiment of the present invention;

FIG. 5 is a block circuit diagram for showing an overall structure of a current driving device according to a modification example of the preferred embodiment of the present invention;

FIG. 6A is a circuit diagram for showing a structure of a current output part according to a conventional technique;

FIG. 6B is a block circuit diagram for showing the structure of the current output part according to the conventional technique;

FIG. 7 is a timing chart for showing actions of the current output part shown in FIG. 6A and FIG. 6B ;

FIG. 8 is a block circuit diagram for showing an overall structure of a current driving device according to the conventional technique; and

FIG. 9 is a timing chart for showing actions of the current output part shown in FIG. 6A and FIG. 6B , when a reference current is very small.

Hereinafter, an embodiment of a current driving device according to the present invention will be described in detail by referring to the accompanying drawings. Same reference numerals are applied to the same or corresponding components within the drawings.

FIG. 1A is a circuit diagram for showing a structure of a current output part A that is mounted to a current driving device according to a preferred embodiment of the present invention, FIG. 1B is a block circuit diagram for showing the structure of the current output part A, FIG. 2 is a timing chart for describing actions of the current output part A, FIG. 3 is a block circuit diagram for showing the overall structure of the current driving device, and FIG. 4 is a circuit diagram for showing an embodiment of a voltage supply source V 1 .

<Current Output Part>

First, the current output part A will be described by referring to FIG. 1A and FIG. 1B . One end of a voltage holding capacitance element C 1 is connected to a ground terminal, and the other end is connected in common to gate terminals of m-numbers of Nch transistors QN 1 -QNm. Each source of the Nch transistors QN 1 -QNm is connected to a ground terminal, and each drain thereof is connected in series to signal response switches G 1 -Gm which are ON/OFF controlled in accordance with input signals. The other ends of the signal response switches G 1 -Gm are connected in common, and are also connected, via a current input switch Sw 1 , to a reference current source I 1 that supplies a constant current value (first electric current) to the current output part A, while being connected to the voltage holding capacitance element C 1 via a calibration switch Sw 2 . A node N 1 for connecting the current input switch Sw 1 , the calibration switch Sw 2 , and the signal response switches G 1 -Gm is connected to current output terminals T 1 and T 2 via current output switches So 1 and So 2 , respectively. The circuit structure that has been described heretofore is the same as the circuit structure of a conventional technique shown in FIG. 6A and FIG. 6B .

In this embodiment, further, a voltage supply source V 1 for supplying a constant voltage (first voltage) to the current output part A is connected, via a high-speeding switch Sw 3 , to a node N 2 that is a connection between the voltage holding capacitance element C 1 and the calibration switch Sw 2 . The voltage supply source V 1 corresponds to a first voltage supply part. The reference current source I 1 corresponds to a first current supply part. The voltage holding capacitance element C 1 corresponds to a voltage holding part. As the voltage holding capacitance element C 1 , a parasitic capacitance provided originally to the gate terminals of the Nch transistors QN 1 -QNm may be used instead.

This current output part A takes three operation states such as a voltage supply mode M 1 , a current supply mode M 2 , and a current output mode M 3 . Among those, the voltage supply mode M 1 and the current supply mode M 2 constitute a calibration mode Mc.

<<Voltage Supply Mode M 1 >>

Now, the voltage supply mode M 1 of the current output part A will be described first.

First, each of the current output switches So 1 and So 2 is set to a nonconductive state, so that the current output terminals T 1 and T 2 are isolated from the node N 1 . Further, the high-speeding switch Sw 3 is set to a conductive state, so that the voltage supply source V 1 and the node N 2 are connected. At this time, the voltage holding capacitance element C 1 is charged to a constant voltage (this corresponds to the first voltage, and will be referred to as a reference voltage hereinafter) that is supplied from the voltage supply source V 1 .

It is also possible that the current supply mode M 2 to be described next is activated simultaneously with the voltage supply mode M 1 . In that case, the current input switch Sw 1 , the calibration switch Sw 2 , and all the signal response switches G 1 -Gm are set to be in a conductive state.

<<Current Supply Mode M 2 >>

Next, the current supply mode M 2 of the current output part A will be described. Under this current supply mode M 2 , a current-voltage converting function is actuated. Thus, the current input switch Sw 1 , the calibration switch Sw 2 , and all the signal response switches G 1 -Gm are set to be in a conductive state. The high-speeding switch Sw 3 is turned to a nonconductive state so that the node N 2 is isolated from the voltage supply source V 1 . The current output switches So 1 and So 2 are remained in the nonconductive sate.

At this time, the drains and gates of the Nch transistors QN 1 -QNm are connected, thereby exhibiting a diode characteristic. Therefore, the Nch transistors QN 1 -QNm have an electric current (reference current) with a constant current value from the reference current source I 1 flown therein as a load. At this time, a gate voltage for passing the electric current from the reference current source I 1 is generated uniquely at the gates of the Nch transistors QN 1 -QNm. Therefore, the voltage holding capacitance element C 1 accumulates the electric charges in accordance with the gate voltages. As described, the voltage holding capacitance element C 1 holds the gate voltage for allowing the electric current with the same current value as that of the reference current to be flown. The current output part A stores the reference current value.

<<Current Output Mode M 3 >>

Next, the current output mode M 3 of the current output part A will be described. Under this current output mode M 3 , a voltage-current converting function is actuated. Thus, the current input switch Sw 1 and the calibration switch Sw 2 are set to be in a nonconductive state, and either the current output switches So 1 or So 2 is turned to a conductive state from a nonconductive state. The output switches So 1 and So 2 are not set to be in a conductive state simultaneously. The high-speeding switch Sw 3 is remained in the nonconductive state. The voltage holding capacitance element C 1 has an accumulation of the electric charges in accordance with the gate voltages for allowing the electric current with the same current value as that of the reference current to be flown under the above-described current supply mode M 2 . A gate voltage according to the electric charges accumulated in the voltage holding capacitance element C 1 is applied to the gates of the Nch transistors QN 1 -QNm. Thus, it is possible to output a sink current having a current value proportional to the reference current in accordance with the conduction states of the current output switches So 1 , So 2 and the signal response switches G 1 -Gm, through connecting a power source to the current output terminal T 1 or the current output terminal T 2 . The output current also depends on the conduction states of the signal response switches G 1 -Gm, which is the sum of the electric currents outputted from the Nch transistors whose signal response switches G 1 -Gm (connected to each transistor) are in a conductive state among the output currents from the Nch transistors QN 1 -QNm. If all the signal response switches G 1 -Gm are in a conductive state, the output current obtained thereby is the maximum output current of the current output parts A under execution of the above-described current supply. The value thereof equals to the reference current.

Through a series of these actions, it becomes possible to copy the reference current to the current output part A and to output the electric current in accordance with the reference current and an input signal. The electric current can be copied with only the current supply mode M 2 and the current output mode M 3 . However, with the voltage supply mode M 1 , an insufficient charge of the voltage holding capacitance element C 1 can be overcome and the electric current can be copied at a higher speed by charging the voltage of the Node N 1 that is connected to the gate terminals of the Nch transistors QN 1 -QNm to a value close to a target voltage (convergent voltage under the current supply mode M 2 ).

With this embodiment, it is possible to constitute an m-bit current-summing DA converter circuit through multiplying the current capacities of the Nch transistors QN 1 -QNm by 1, 2, 4, - - - , 2 m . Alternatively, the reference current source I 1 may be provided for each bit. Further, the current-summing DA converter circuit illustrated in the embodiment is merely an example, and the signal response switches G 1 -Gm may be omitted for a case where only the equivalent electric current is to be copied, for example.

Now, the current output part A shown in FIG. 1A is illustrated as a six-terminal circuit device as shown in FIG. 1B . In FIG. 1B , the current output part A comprises a reference current input terminal IREF, a voltage input terminal VREF, current output terminals IOUTA, IOUTB, a control signal input terminal CTL, and a signal input terminal DATA. The reference input terminal IREF is connected to the reference current source I 1 , and receives electric currents inputted from the reference current source I 1 under the voltage supply mode M 1 and the current supply mode M 2 . The voltage input terminal VREF is connected to the voltage supply source V 1 , and receives a reference voltage under the voltage supply mode M 1 . When the current output terminal IOUT turns to the current output mode M 3 , the current output terminals IOUT output the electric current stored under the current supply mode M 2 . The control signal input terminal CTL receives inputs of control signals for controlling the operation states, and performs control for switching the current output switches So 1 , So 2 , the current input switch Sw 1 , the calibration switch Sw 2 , the high-speeding switch Sw 3 , and the signal response switches G 1 -Gm within the current output part A. The signal input terminal DATA receives inputs of the input signals of m bits for controlling the output current values, and performs control for switching the signal response switches G 1 -Gm.

Next, actions of the current output part A according to the embodiment that is constituted in the above-described manner will be described by referring to the timing chart of FIG. 2 . FIG. 2 illustrates the above-described three operation modes (the voltage supply mode M 1 , the current supply mode M 2 , and the current output mode M 3 ). The conductive state of the switch is shown with “H”, and the nonconductive state thereof is shown with “L”. Further, “V (N 2 )” indicates the voltage V (N 2 ) of the node N 2 . The voltage V (N 2 ) of the node N 2 according to the embodiment is illustrated with a solid line and the state of the voltage in the case of a conventional technique where the calibration only with the current supply mode M 2 is started simultaneously is illustrated with a broken line.

Before calibration, the current output part A sets the current output switch So 1 to be in a conductive state, and outputs, from the current output terminal T 1 , the output currents according to the states of the signal response switches G 1 -Gm which are controlled by the voltage V (N 2 ) of the node N 2 and the input signals. The current output switch So 2 , the current input switch Sw 1 , the calibration switch Sw 2 , and the high-speeding switch Sw 3 are in a nonconductive state.

During the whole calibration period, the current output switches So 1 , So 2 are set to be in a nonconductive state, and the signal response switches G 1 -Gm for selecting the electric currents from the Nch transistors QN 1 -QNm are all set to be in a conductive state. Details thereof are as follows. Under the voltage supply mode M 1 , the current input switch Sw 1 , the calibration switch Sw 2 , and the high-speeding switch Sw 3 are set to be in a conductive state so as to supply the reference current and the reference voltage to charge the voltage holding capacitance element C 1 from the node N 2 . Under the current supply mode M 2 , the high-speeding switch Sw 3 is turned to a nonconductive state from the states of the switches under the voltage supply mode M 1 . Through this, only the electric current from the reference current source I 1 is to be supplied to the Nch transistors QN 1 -QNm, and the voltage V (N 2 ) of the node N 2 is determined according to the above-described actions. The voltage (N 2 ) of the node N 2 corresponds to a voltage that is capable of passing an electric current that is equivalent to the reference current by the reference current source I 1 through the Nch transistors QN 1 -QNm under such a condition that all the Nch transistors QN 1 -QNm are in a conductive state. AS a result, the current output part A comes to store the reference current by the reference current source I 1 .

After completing the calibration described above, first, the current input switch Sw 1 and the calibration switch Sw 2 are turned to a nonconductive state so that the voltage holding capacitance element C 1 comes to hold the electric charge. That is, the voltage V (N 2 ) of the node N 2 is maintained. Thereafter, the current output switch So 2 is turned to a conductive state to output, from the current output terminal T 2 , the sum of the electric currents that are selected by the signal response switches G 1 -Gm based on the input signals.

In the explanations above, the current input switch Sw 1 and the calibration switch Sw 2 are turned to a conductive state to achieve high-speed actions under the voltage supply mode M 1 . However, even if the current switch Sw 1 and the calibration switch Sw 2 are in a nonconductive state, it is also possible to achieve faster convergence compared to the case of the conventional technique, and to reduce the electric current by the amount of the reference current source compared to the above-described case.

Next, the overall structure of the current driving device according to the embodiment will be described by referring to FIG. 3 . This current driving device comprises: n-numbers of output terminals OUT 1 -OUTn; (n+1)-numbers of current output parts A 0 -An; a reference current supply transistor QP 1 that constitutes a first current supply part; a reference current supply transistor QP 2 that constitutes a second current supply part; a voltage supply source V 1 ; an action control circuit B; a signal input circuit Din; current-voltage converting transistors QNx, QPx; a reference current source I 1 ; and a reset switch Sw 4 . Each of the current output parts A 0 -An is the current output part shown in FIG. 1A . A reference current input terminal IREF is connected to the reference current supply transistor QP 1 , and a voltage input terminal VREF is connected to the voltage supply source V 1 . An action control terminal CTL receives signals for controlling the action states, which are inputted from the action control circuit B. A signal input terminal DATA receives signals for controlling the output current values from the signal input circuit Din. Each of current output terminals IOUTA and IOUTB is connected to one of the output terminals OUT 1 -OUTn, and outputs an output current corresponding to the reference current and the input signal DATA to the outside in accordance with the state of the control signal CTL. The reference current supply transistor QP 1 is connected between the IREF terminals of the current output parts A 0 -An and a power supply terminal, and a gate terminal thereof is connected to a node N 11 . The reference current supply transistor QP 2 is connected between a node N 12 and the power supply terminal, and a gate terminal thereof is connected to the node N 11 . The P-channel-type current-voltage converting transistor QPx is connected between the reference current source I 1 and the power supply terminal, and a gate terminal thereof is short-circuited with its drain. The N-channel-type current-voltage converting transistor QNx is connected between the reference current supply transistor QN 2 and a ground terminal, and a gate terminal thereof is short-circuited with its drain. The reset switch Sw 4 is connected between the node N 12 and the ground terminal.

Note here that connections of the current output terminals IOUTA and IOUTB of the current output parts A 0 -An are allocated according to the following rules. The current output terminal IOUTA of the current output part A 0 is not connected to the outside, and the current output terminal IOUTB is connected to the output terminal OUT 1 of the current driving device. The current output terminal IOUTA of the current output part A 1 is connected to the output terminal OUT 1 , and the current output terminal IOUTB is connected to the output terminal OUT 2 . The current output parts thereafter are connected in the same manner, and the current output terminal IOUTA of the current output part An is connected to the output terminal OUTn while the current output terminal IOUTB is not connected to the outside. That is, for the n-numbers of output terminals OUT 1 -OUTn, in the i-th current output part Ai among the (n+1)-numbers of current output parts A 0 -An, the current output terminal IOUTA is connected to the i-th output terminal OUTi, and the current output terminal IOUTB is connected to the (i+1)-th output terminal OUT(i+1). However, the current output terminal IOUTA of the first current output part A 0 and the current output terminal IOUTB of the (n+1)-th current output part An are not connected to the outside.

It is not necessary for the first current output part A 0 to have the current output switch So 1 and the current output terminal T 1 , and for the (n+1)-th current output part An to have the current output switch So 2 and the current output terminal T 2 .

There are one more numbers (n+1) of current output parts prepared for the n-numbers of the output terminals in this case. However, it is also possible to prepare two current output parts for a single output terminal, so that one of the output parts can be used as the current output and the other as the calibration. This makes it possible to supply the output current to the current output terminal at all times. Further, in a case where it is not necessary for all the output terminals to be in the current output state, the number of current output parts may be smaller than the number of output terminals.

The current-voltage converting transistor QPx functions as a current-voltage converting part. Upon receiving the electric current from the reference current source I 1 , the current-voltage converting transistor QPx generates, at the node N 1 , a gate voltage (first voltage) for allowing the electric current that is equivalent to the reference current of the reference current source to flow therethrough. The reference current supply transistors QP 1 and QP 2 whose gate terminals are connected to the node N 11 supply the electric current proportional to the reference current of the reference current source I 1 to the IREF terminals of the current output parts A 0 -An and the current-voltage converting transistor QNx, respectively.

The current-voltage converting transistor QNx receives the electric current that is proportional to the current value of the reference current source I 1 from the reference current supply transistor QP 2 . Like the current-voltage converting transistor QPx, the current-voltage converting transistor QNx itself generates, at the node N 12 , a gate voltage for allowing the electric current supplied from the reference current supply transistor QP 2 to flow therethrough.

Preferably, the current-voltage converting transistor QNx is constituted in such a manner that all the Nch transistors QN 1 -QNm shown in FIG. 1A are connected in parallel. Further, when the values of the electric currents supplied from the reference current supply transistors QP 1 and QP 2 have different values, the current-voltage converting capacity of the current-voltage converting transistor QNx may be set in accordance with a ratio of the current values supplied from the reference current supply transistors QP 1 and QP 2 . That is, if the current value supplied from the reference current supply transistor QP 2 is three times as large as the current value supplied from the reference current supply transistor QP 1 , three current-voltage converting transistors QNx, in each of which all the Nch transistors QN 1 -QNm within the current output part A are connected in parallel, are prepared. Through this, it is possible to generate, at the node N 2 , the voltage that is extremely close to the voltage that is generated at the gates of the Nch transistors QN 1 -QNm by the current output part A under the current supply mode M 2 .

The voltage supply source V 1 supplies the voltage that is generated at the node N 12 by the current-voltage converting transistor QNx in the manner described above to the VREF terminals of the current output parts A 0 -An.

Considering a case of applying the current driving device according the embodiment to an actual display panel, if there are 160 pixels in one line, the number of output terminals becomes 160 (n=160). Thus, 161 current output parts A are required. In a case of RGB colors, there are required three times as many as the number of the current output parts A 0 -An and the number of the signal input circuits Din. Furthermore, when the reference current is controlled individually for RGB, three sets of the reference current supply transistor QP 1 and the voltage supply source V 1 are required among the components of the current driving device shown in FIG. 3 . The action control circuit B may be used in common or may be provided individually.

<Calibration Mode Mc>

The calibration mode Mc of the current driving device shown in FIG. 3 will be described.

The action control circuit B sets one of the current output parts A 0 -An to be under the calibration mode Mc. That is, the mode thereof is shifted from the voltage supply mode M 1 to the current supply mode M 2 , and the other current output parts are set to be under the current output mode M 3 .

First, the current output part A 0 is set to be under the voltage supply mode M 1 . Each of the current output parts A 1 -An is set under such a condition that the electric current is outputted from the current output terminal IOUTA to the output terminals OUT 1 -OUTn. Then, the current output part A 0 is set to be under the current supply mode M 2 . No change is applied to the settings of the actions of the current output parts A 1 -An. Through the above, calibration of the current output part A 0 is performed.

Next, the current output part A 0 has the current output terminal IOUTB connected to the output terminal OUT 1 to set the current output mode M 3 . No change is applied to the connections of the current output parts A 2 -An and the output terminals OUT 2 -OUTn. In this state, the current output part A 1 is shifted from the voltage supply mode M 1 to the current supply mode M 2 to perform calibration.

In the same manner, the current output terminal IOUTA of the current output part before the calibration or the current output terminal IOUTB of the current output part after the calibration is connected to the output terminal so as to perform calibration successively for each of the current output parts.

<Current Output Mode M 3 >

Next, the current output mode M 3 of the current driving device shown in FIG. 3 will be described.

The n-numbers of current output parts A that are not set under the calibration mode Mc are set under the current output mode M 3 , and receive display data signals from the signal input circuit Din according to the output terminals to which each of the current output parts A is connected. The current output part A set under the current output mode M 3 outputs a sink current in accordance with the reference current and the aforementioned display data.

<Refresh>

In a case where the calibration is performed only once, the reference voltage held in the voltage holding capacitance elements C 1 is fluctuated due to leaks in the voltage holding capacitance elements C 1 and in the gates of the Nch transistors QN 1 -QNm within the current output parts A 0 -An. Thus, it is necessary to perform calibration for the current output parts periodically. Therefore, the embodiment provides (n+1)-numbers of current output parts A 0 -An for the n-numbers of output terminals OUT 1 -OUTn, and performs calibration on the current output part that is not in the mode of outputting the electric current.

The voltage close to the target voltage is being maintained after performing the calibration once on all the current output parts A 0 -An, except right after the startup or right after a change in the reference current. Thus, calibration under the voltage supply mode M 1 may be omitted.

<Collective Voltage Supply Mode M 1 ′>

In a case where only the periodic calibration described above and a refresh action are performed, it is not possible to obtain a perfect output unless the necessary voltages are held at the nodes N 2 of each current output part, when it is right after the startup of the current driving device or there is a change in the reference current. Thus, non-uniformity is generated in a panel display. Therefore, there is provided an operation mode where the voltage supply source V 1 collectively supplies a voltage to be generated by the current-voltage converting transistor QNx to the nodes N 2 of all the current output parts A 0 -An at the time of the startup or changing the reference current. Through this, it becomes possible to store a current value that is close to a reference current that is set anew to all the current output parts A 0 -An, so that the non-uniformity in the display caused due to the successive calibration can be improved.

<Resetting of Reference Voltage>

As described above, each current output part is capable of provisionally holding the current value that is close to a new reference current because of the voltage supply source V 1 . However, the node N 12 for supplying this voltage is charged by a source current from the reference current supply transistor QP 2 . The current-voltage converting transistor QNx is merely a load for the reference current supply QP 2 that supplies the source current. Thus, especially when the reset reference current is very small, it is difficult for the voltages (?) to be converged to a low voltage. Therefore, it is desirable that a reset switch Sw 4 is deposited to provide a function of short-circuiting the node N 12 to a ground potential.

In the above-described embodiment, the reset switch Sw 4 is connected between the ground potential and the current-voltage converting transistor QNx. However, by generating a lowest potential estimated as the potential of the current-voltage converting transistor QNx and connecting the reset switch Sw 4 between the node of the generated potential and the current-voltage converting transistor QNx, the potential of the current-voltage converting transistor QNx can be reset still faster.

There is a large difference in the load from the viewpoint of the voltage supply source V 1 between the case of performing the processing of the voltage supply mode M 1 with the periodic calibration and the case of performing the processing of the collective voltage supply mode M 1 ′ described in this section. If the voltage supply capacity corresponding to the n-numbers of loads is used for charging a single current output part A, a voltage waveform of the voltage supply line is distorted, and problems such as EMI (electromagnetic interference) may be induced. Inversely, the voltage supply capacity corresponding to a single current output part is insufficient for supplying the voltage for all the current output parts. As a countermeasure for such problems, the voltage supply capacity of the voltage supply part may be varied in accordance with the extent of the capacitance to be charged.

As an example, there is such a type that the number of output-stage transistors within the voltage supply source V 1 is varied in accordance with each of the operation modes described above. FIG. 4 shows an example of such case. The voltage supply source V 1 is constituted with: a differential amplifier Ad; Nch transistors QN 21 , QN 22 ; Pch transistors QP 21 , QP 22 ; and switches Sw 5 , Sw 6 , Sw 7 , and Sw 8 . The differential amplifier Ad receives, at its inverting input terminal, a reference voltage generated by a current-voltage converting transistor QNx. The output terminal of the voltage supply source V 1 is connected to a non-inverting input terminal thereof, so that the voltage supply source V 1 as a whole constitutes a voltage follower. Further, current output parts A 0 -An are connected to the output terminal of the power supply source V 1 . Gates of the Nch transistors QN 21 and QN 22 are connected to the output terminal of the differential amplifier Ad, sources thereof are connected to a ground terminal, and drains thereof are connected to the output terminal of the voltage supply source V 1 via the switches Sw 5 and Sw 6 , respectively. A proper bias voltage is applied to gates of the Pch transistors QP 21 , QP 22 from the outside, sources thereof are connected to a supply potential, and drains thereof are connected to the output terminal via the switches Sw 7 and Sw 8 , respectively. When setting only one of the current output parts (for example, the current output part AO) to be under the voltage supply mode M 1 , the switches Sw 5 , Sw 7 are set to be in a conductive state and the switches Sw 6 , Sw 8 are set to be in a nonconductive state so as to provide the optimum state for having only the voltage holding part (the voltage holding capacitance element C 1 of FIG. 1 ) of the current output part A 0 as a load. When setting all the current output parts A 0 -An to be under the voltage supply mode M 1 , all the switches Sw 5 , Sw 6 , Sw 7 , and Sw 8 are set to be in a conductive state to increase the voltage supply capacity so as to be able to charge the voltage holding capacitance elements C 1 provided to all the current output parts. Through this, the voltage supply source V 1 can supply the voltage with a proper voltage supply capacity in accordance with the load.

FIG. 5 is a circuit diagram for showing a structure of a current driving device according to another embodiment of the present invention. In this embodiment, a second voltage supply part having a different voltage supply capacity is provided for each of the operation modes, i.e. the periodic voltage supply mode M 1 and the collective voltage supply mode M 1 ′. That is, in addition to the voltage supply source V 1 as the first voltage supply part that has a proper voltage supply capacity for a single load of the current output part, the embodiment comprises a voltage supply source V 2 as the second voltage supply part that has a proper voltage supply capacity for all the connected current output parts as the loads. Control signals are inputted to the voltage supply sources V 1 and V 2 from an action control circuit B. Each of the voltage supply sources V 1 and V 2 has a node N 12 exhibiting a first voltage as the input, and is switch-controlled depending upon whether the mode is set as the voltage supply mode M 1 for a single current output part or the collective voltage supply mode M 1 ′. Furthermore, by providing an over lapped period when switching the actions of both voltage supply sources, it is possible to suppress a large fluctuation in the output waveform right after switching the modes.

<Effects>

Through the above-described structures, in the current driving device that performs calibration of the current output parts through supplying a reference current, it becomes possible to speed up the calibration action by supplying a voltage that is close to the final target value before the calibration executed by using the reference current. This makes it possible to deal with a case where the reference current is very small and a case where the number of outputs of the current driving device is increased in accordance with an increase in the scale of panel.

In FIG. 1A , the output current of the current output part is a sink current and Nch transistors are used as the structural elements. In a case where the output current is a source current, however, Pch transistors may be used instead.

While the invention has been described and illustrated in detail, it is to be clearly understood that this is intended be way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of this invention being limited only be terms of the following claims.