Patent Publication Number: US-2005140611-A1

Title: Pixel driving circuit

Description:
FIELD OF THE INVENTION  
      The present invention relates to a driving circuit, and more particularly to a pixel driving circuit.  
     BACKGROUND OF THE INVENTION  
      A light emitting diode (LED) is a current driving device. It can emit light by the combination of an electron and an electron hole. With the advances of small size, energy conservation, high contrast ratio and fast response, the light-emitting diode has become the most important illumination device in the next generation of technology.  
       FIG. 1  illustrates the typical drive circuit  100  for driving a light emitting diode. A pixel region is defined by a cross-connected scan line  102  and the data line  104 . A power supply line  106  is arranged in parallel with the data line  104 . It is noted that the power supply line  106  is also arranged in parallel with the scan line  102 . A switching device  108 , a driving device  110 , a storage capacitor  112  and a light emitting diode  114  are located in the pixel region.  
      The gate electrode, source electrode and the drain electrode of the switching device  108  are respectively connected with the scan line  102 , data line  104  and the gate electrode of the driving device  110 . The drain electrode and the source electrode of the driving device  110  are respectively connected with the power supply line  106  and the light emitting diode  114 . The storage capacitor  112  is connected between the gate electrode and the source electrode of the driving device  110 .  
      When the scan line  102  is selected by the gate driving apparatus (not shown in the figure), the switching device  108  is turned on. The data transmitted by the data line  104  is stored in the storage capacitor  112  through the switching device  108 . When the switching device  108  is turned off, the data is maintained in the storage capacitor  112  until the switching device is turned on again.  
      The storage capacitor  112  can maintain the voltage applied to the gate electrode and the source electrode of the driving device  110 . Therefore, the drain current of the driving device  110  is controlled by the storage capacitor  112 . The drain current is supplied to the light emitting diode  114  through the driving device  110 . In other words, after the scan signal transmitted from the scan line  102  selects the switching device  108 , the storage capacitor  112  is charged by the signal transmitted from the data line  104 . The terminal voltage of the storage capacitor  110  can control the drain current in the power supply line  106 .  
      The current flowing through the light emitting diode  114  is controlled by the driving device  110 . The brightness of the light emitting diode  114  is related to the current flowing through the light emitting diode  114 . Therefore, the brightness of the light emitting diode  114  is also controlled by the driving device  110 . In other words, the drain current of the driving device  110  is determined if a predetermined signal is stored in the storage capacitor  112  through the data line  104 . Then, the current for driving the light emitting diode  114  is determined. Therefore, the brightness of the light emitting diode is also determined.  FIG. 2  illustrates another typical drive circuit for driving the light emitting diode. Comparing  FIGS. 1 and 2 , the main difference is the position in which the light emitting diode  114  is connected.  
      However, it is impossible to get an identical brightness for the light emitting diodes in the typical drive circuit even though the voltage between the gate electrode and source electrode of the driving device  110  is fixed. The main reason is that the threshold voltage of each light emitting diode is different. Additionally, the brightness of the light emitting diode is also affected by the use time of the light emitting diode. Therefore, a drive circuit that is not affected by the parameters associated with the light emitting diode is needed.  
     SUMMARY OF THE INVENTION  
      Therefore, the main purpose of the present invention is to provide a drive circuit to compensate for the difference between the light emitting diodes for providing an identical brightness.  
      Another object of the present invention is to provide a drive circuit to obtain an identical brightness from the light emitting diodes after a long time of use.  
      Yet another object of the present invention is to provide a drive circuit to provide a steady light output independent of the variation of parameters associated with the light emitting diodes.  
      According to the drive circuit and operation method thereof in the present invention, a sensor is used to sense the brightness of the light emitting diode to generate different induced currents to compensate for the brightness difference between the light emitting diodes. The different induced currents can cause different driving currents to drive corresponding light emitting diodes. Therefore, each light emitting diode has a specific illumination period according to its initial brightness. The different illumination periods can make each light emitting diode illuminate with the same brightness within a time frame. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein:  
       FIG. 1  and  FIG. 2  illustrate typical drive circuits for driving light emitting diodes;  
       FIG. 3  illustrates a schematic diagram of a drive circuit with compensation function according to the present invention;  
       FIG. 4  illustrates a drive circuit with a compensation function according to the first embodiment of the present invention;  
       FIG. 5  and  FIG. 6  illustrate a waveform diagram of an induced current, brightness and driving current for the present invention; and  
       FIG. 7  illustrates a drive circuit with a compensation function according to the second embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
      Without limiting the spirit and scope of the present invention, the drive circuit and operation method thereof proposed in the present invention is illustrated with one preferred embodiment. One of ordinary skill in the art, upon acknowledging the embodiment, can apply the drive circuit and operation method of the present invention to various illumination device. Accordingly, it is impossible to obtain an identical brightness between the light emitting diodes driven by a typical drive circuit even though the drive current is fixed. The main reason for the difference in brightness is that the threshold voltage or the use time for each light emitting diode is different. Therefore, the present invention provides a drive circuit and operation method thereof to compensate for the difference between the light emitting diodes independent of the variation of parameters associated with the light emitting diodes, thus obtaining an identical brightness. The present invention is explained by the following detailed embodiments. However, these embodiments do not limit the scope of the present invention.  
       FIG. 3  illustrates a schematic diagram of a drive circuit with compensation function according to the present invention. A voltage control current source  300 , an illumination device  302  and a photon detection circuit  304  are used in the present invention. The voltage control current source  300  can provide a constant current  306  to drive the illumination device  302 . When the photon detection circuit  304  detects a light  308  emitted from the illumination device  302 , a voltage  310  related to the light  308  is generated to control the voltage control current source  300  to change the current  306 . In other words, the photon detection circuit  304  is used as a feedback circuit. The light  308  of the illumination device  302  is sent to the voltage control current source  300  through the photon detection circuit  304 . Then, the voltage control current source  300  can modulate the current according to the light  308 . The modulated current can change the brightness of the illumination device  302 .  
      According to the drive circuit of the present invention, the different brightnesses of the illumination devices  302  can make the photon detection circuit  304  generate different voltages  310  to control the voltage control current source  300 . Then, the voltage control current source  300  can generate a corresponding current according to the voltage to drive the illumination device  302 . Therefore, though the brightnesses of the illumination devices  302  are different because of the different parameter values, the different brightnesses are sent to the voltage control current source  300  through the photon detection circuit  304  to generate different currents to drive the illumination devices  302 . In other words, the different brightnesses of the illumination devices  302  due to the different parameter can be corrected by adding a feedback circuit. The detailed circuit design is described by the following.  
       FIG. 4  illustrates a drive circuit  400  with compensation function according to the first embodiment of the present invention. A pixel region is defined by a cross-connected scan line  402  and the data line  404 . A power supply line  406  is arranged in parallel with the data line  404 . It is noted that the power supply line  406  can also be arranged in parallel with the scan line  402 . A switching device  408 , a driving device  410 , a storage capacitor  412  and a sensor  416  and a light emitting diode  414  are located in the pixel region. The sensor  416  is a device that receives photons to generate current, such as a photodiode or a photoconductor or a transistor made of amorphous-silicon channel with a connected gate and drain or an amorphous silicon layer. A transistor can be used as the switching device  408  or the driving device  410 .  
      The gate electrode, source electrode and the drain electrode of the switching device  408  are, respectively, connected with the scan line  402 , data line  404  and the gate electrode of the driving device  410 . The source electrode and the drain electrode of the driving device  410  are, respectively, connected with a low power source (−Vss) and the light emitting diode  414 . The storage capacitor  412  is connected between the gate electrode and the source electrode of the driving device  410  for controlling the voltage between the gate electrode and the source electrode of the driving device  410 . A sensor  416  is connected in parallel with the two terminals of the storage capacitor  412  for detecting the brightness of the light emitting diode  414 . A photocurrent is generated when the sensor  416  detects the brightness of the light emitted from the light emitting diode  414 . The voltage on the storage capacitor  412  is discharged by photocurrent to change the voltage between the gate electrode and the source electrode of the driving device  410 .  
      When the scan line  402  is selected by the gate driving apparatus (not shown in the figure), the switching device  408  is turned on. The data transmitted by the data line  404  is stored in the storage capacitor  412  through the switching device  408 . When the switching device  408  is turned off, the data is maintained in the storage capacitor  412  until the switching device  408  is turned on again.  
      The storage capacitor  412  can maintain the voltage applied to the gate electrode and the source electrode of the driving device  410 . Therefore, the drain current of the driving device  410  is controlled by the storage capacitor  412 . The drain current is supplied to the light emitting diode  414  through the driving device  410 . An induced current is generated when the sensor  416  detects the brightness of the light emitting diode  414 . The storage capacitor  412  is discharged to reduce its voltage applied to the gate electrode and the source electrode of the driving device  410 . The reduced voltage reduces the drain current of the driving device  410 . Therefore, the brightness of the light emitting diode  414  is also reduced. The reduced brightness generates a lower induced current. The induced current continues to discharge the storage capacitor to reduce the voltage between the gate electrode and the source electrode of the driving device  410  until the drain current is reduced to nearly zero. At this time, the light emitting diode  414  does not emit any light. Therefore, the sensor  416  does not generate any induced current.  
      The capacitance value of the storage capacitor  412  is C and the terminal voltage of the storage capacitor  412  is V 0 . The threshold voltage of the driving device  410  is V T . The induced current with which the sensor  416  detects the brightness of the light emitting diode  414  is I 416 . Then, the induced current I 416  is related to the charge Q stored in the storage capacitor  412 . In a time frame, the relationship between the current I 416  and the charge Q is described by the following: 
 
 Q=C ( V   0   −V   T )=∫ I   416   dt  
 
      The induced current I 416  is related to the brightness, B LED , of the light emitting diode  414 . That is, that a functional relationship exists between them. Therefore, the relationship can be expressed by the following: 
 
 I   416 =ƒ( B   LED ) 
 
      Therefore, the total brightness of the light emitting diode  414  in a time frame is equal to sum or integration of the gray levels displayed in the time frame. This is described by the following: 
 
Gray level=∫ B   LED   dt  
 
      According to the foregoing description, the brightness generated by the light emitting diode  414  in a time frame is obviously related to the induced current. In other words, the different brightnesses of the light emitting diodes  414  due to different parameter values or different use times can induce different induced currents, I 416 , through the light emitting diode  416 . The different induced currents I 416  generate different driving currents I 414 . The different driving currents I 414  can correct the different brightnesses to an identical brightness of the light emitting diodes  414 .  
      Accordingly, the relation between the Gray level and the B LED  is described by the following: 
 
Gray level= ∫B   LED   dt  and  I   416 =ƒ( B   LED ) 
 
      On the other hand, the relation between the current I 416  and the charge Q is described by the following: 
 
 Q=C ( V   0   −V   T )=∫ I   416   dt  
 
      Therefore, the Gray level is related to the Q. That is related to the voltage V 0  that is the voltage from the data line. In other words, the characteristic of the special LED does not influence the gray level.  
       FIG. 5  and  FIG. 6  illustrate a waveform relation diagram for the induced current I 416  generated by the sensor  416 , brightness B LED  emitted from the light emitting diode  414  and the driving current I 414  flowing through the light emitting diode  414 . The brightness B LED  emitted from the light emitting diode  414  can make the sensor  416  generate an induced current I 416 . The induced current I 416  can control the driving current I 414  flowing through the light emitting diode  414 . Therefore, if the brightness B LED  is reduced, the induced current I 416  is also reduced. The reduction of the current I 416  can reduces the driving current I 414 .  
      Comparing  FIG. 5  with  FIG. 6 , the period T is the frame time. The B LED  is the brightness emitted from the light emitting diode  414 . Therefore, the brightness emitted from the light emitting diode  414  in a frame time T is described in the following: 
 
Brightness=∫ 0   T   B   LED   dt  
 
      If the brightness B LED  emitted from the light emitting diode  414  is increased, a larger induced current I 416  is generated. The larger induced current I 416  can discharge the storage capacitor  412  faster, that makes a larger voltage reduction between the gate electrode and the source electrode of the driving device  410 . Therefore, the current flowing through the light emitting diode  414  also has a larger reduction. Similarly, the drain current for driving the light emitting diode  414  also has a larger reduction. In other words, a larger brightness causes a sharper change in the induced current I 416 , the brightness B LED  and the driving current I 414  through the light emitting diode  414 . Therefore, a shorter time is required to change the brightness of the light emitting diode to dark as shown in  FIG. 5 . Conversely, a smaller brightness causes a smoother change in the induced current I 416 , the brightness B LED  and the driving current I 414  through the light emitting diode  414 . Therefore, a longer time is required to change the brightness of the light emitting diode to dark as shown in the  FIG. 6 .  
      In other words, the different brightnesses of the light emitting diodes  414  due to the different parameter values or the different use times can induce different induced currents I 416 . The different induced currents I 416  generate different driving currents I 414  to drive corresponding light emitting diodes. Therefore, each light emitting diode has a special illumination period according to its initial brightness. The different illumination period within a frame time associated with each light emitting diode results in the same brightness. In other words, the different driving current I 414  can be compensated by a different brightnesses resulting in an identical perceived brightness for the light emitting diodes  414  integrated across a frame time.  
       FIG. 7  illustrates a drive circuit with compensation function according to the second embodiment of the present invention. Comparing  FIG. 4  with  FIG. 7 , the main difference is the connected position of the light emitting diode  414 .  
      According to the drive circuit and operation method thereof in the present invention, a sensor is used to sense the brightness of the light emitting diode to generate different induced currents to compensate for the brightness difference between the light emitting diodes. The different induced currents can cause different driving currents to drive corresponding light emitting diodes. Therefore, each light emitting diode has a special illumination period according to its initial brightness but resulting in the same brightness integrated through frame time.  
      As is understood by a person skilled in the art, the foregoing preferred embodiments of the present invention are illustrative of the present invention rather than limiting of the present invention. It is intended that this description cover various modifications and similar arrangements included within the spirit and scope of the appended claims, the scope of which should be accorded the broadest interpretation so as to encompass all such modifications and similar structure.