Abstract:
The present invention relates generally to a buffer of a drive Integrated Circuit (IC) and, more particularly, to a buffer of a drive IC for driving a spatial light modulator that can meet a desired dynamic slew rate characteristic by controlling current that affects a slew rate. The buffer capable of controlling the slew rate includes a first amplifier connected to the output terminal of the Digital/Analog (D/A) converter of an IC, and adapted to receive output voltage of the D/A converter and inversely amplify the output voltage, a second amplifier adapted to again inversely amplify output voltage that has been inversely amplified by the first amplifier, a charging means connected to the output and input terminals of the second amplifier, and adapted to be charged with the output voltage of the first amplifier, and a current control unit connected to the output terminal of the first amplifier and the input terminal of the second amplifier, and adapted to control the output voltage of the current control unit.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates generally to a buffer of a drive integrated circuit and, more particularly, to a buffer of a drive integrated circuit for driving a spatial light modulator that can meet a desired dynamic slew rate characteristic by controlling current that affects a slew rate.  
         [0003]     2. Description of the Related Art  
         [0004]     Generally, an optical signal processing technology has advantages in that a great amount of data is quickly processed in a parallel manner unlike a conventional digital information processing technology in which it is impossible to process a great amount of data in real time. Studies have been conducted on the design and production of a binary phase filter, an optical logic gate, a light amplifier, an image processing technique, an optical device, and a light modulator using a spatial light modulation theory.  
         [0005]     The spatial light modulator is applied to optical memory, optical display device, printer, optical interconnection and hologram fields, and studies have been conducted to develop a display device employing it.  
         [0006]     The spatial light modulator is embodied by a reflective deformable grating light modulator  10  as shown in  FIG. 1 . The modulator  10  is disclosed in U.S. Pat. No. 5,311,360 by Bloom et al. The modulator  10  includes a plurality of reflective deformable ribbons  18 , which have reflective surface parts, are suspterminaled on an upper part of a silicon substrate  16 , and are spaced apart from each other at regular intervals. An insulating layer  11  is deposited on the silicon substrate  16 . Subsequently, a sacrificial silicon dioxide film  12  and a low-stress silicon nitride film  14  are deposited.  
         [0007]     The nitride film  14  is patterned by the ribbons  18 , and a portion of the silicon dioxide film  12  is etched, thereby maintaining the ribbons  18  on the oxide spacer layer  12  by a nitride frame  20 .  
         [0008]     In order to modulate light having a single wavelength of λ, the modulator is designed so that thicknesses of the ribbon  18  and oxide spacer  12  are each λ/4.  
         [0009]     Limited by a vertical distance (d) between a reflective surface  22  of each ribbon  18  and a reflective surface of the substrate  16 , a grating amplitude of the modulator  10  is controlled by applying voltage between the ribbon  18  (the reflective surface  22  of the ribbon  18  acting as a first electrode) and the substrate  16  (a conductive layer  24  formed on a lower side of the substrate  16  to act as a second electrode).  
         [0010]     In an undeformed state of the light modulator with no voltage application, the grating amplitude is λ/2 while a total round-trip path difference between light beams reflected from the ribbon and substrate is λ. Thus, a phase of reflected light is reinforced.  
         [0011]     Accordingly, in the undeformed state, the modulator  10  acts as a plane mirror when it reflects incident light. In  FIG. 2 , the reference numeral  20  denotes the incident light reflected by the modulator  10  in the undeformed state.  
         [0012]     When proper voltage is applied between the ribbon  18  and substrate  16 , the electrostatic force enables the ribbon  18  to move downward toward the surface of the substrate  16 . At this time, the grating amplitude is changed to λ/4. The total round-trip path difference is a half of a wavelength, and light reflected from the deformed ribbon  18  and light reflected from the substrate  16  are subjected to destructive interference.  
         [0013]     The modulator diffracts incident light  26  using the interference. In  FIG. 3 , reference numerals  28  and  30  denote light beams diffracted in +/−diffractive modes (D+1, D−1) in the deformed state, respectively.  
         [0014]     However, the light modulator by Bloom adopts an electrostatic method to control the position of a micromirror, which is disadvantageous in that operation voltage is relatively high (usually 30 V or so) and the relationship between the applied voltage and displacement is not linear, thus resulting in poor reliability in the control of light.  
         [0015]     In order to solve such problems, “thin-film piezoelectric light modulator and method of manufacturing the same” is disclosed in Korean Pat. No. P2003-077389.  
         [0016]      FIG. 4  is a cut-away view showing a recess type thin-film piezoelectric light modulator according to a conventional technology.  
         [0017]     Referring to the drawing, the recess type thin-film piezoelectric light modulator according to the conventional technology includes silicon substrates  401  and elements  410 .  
         [0018]     In this case, the elements  410  may have a predetermined width, and be regularly arranged to constitute the recess type thin-film piezoelectric light modulator. Alternatively, such elements  410  may have different widths, and be alternately arranged to constitute the recess type thin-film piezoelectric light modulator. The elements  410  may be positioned to be spaced apart from each other by a predetermined interval (almost the same as the width of an element  410 ). In this case, micromirror layers formed on the entire top surfaces of the silicon substrates  401  diffract incident light by reflecting the incident light.  
         [0019]     Each of the silicon substrates  401  includes a recess to provide an air space to an element  410 , an insulating layer  402  is deposited on the top surface of the silicon substrate  401 , and both sides of the element  410  are attached to both sides of the silicon substrate  401  outside the recess.  
         [0020]     The element  410  is formed in a bar shape, and both sides thereof are attached to both sides of the silicon substrate  401  outside the recess of the silicon substrate  401 . The element  410  includes a lower support  411  the portion of which positioned above the recess of the silicon substrate  401  can move vertically.  
         [0021]     The element  410  includes a lower electrode layer  412  formed on the left side of the lower support  411  and adapted to provide piezoelectric voltage, a piezoelectric material layer  413  formed on the lower electrode layer  412  and adapted to generate a vertical actuating force through shrinkage and expansion when voltage is applied to both sides thereof, and an upper electrode layer  414  formed on the piezoelectric material layer  413  and adapted to provide piezoelectric voltage to the piezoelectric material layer  413 .  
         [0022]     Furthermore, the element  410  includes a lower electrode layer  412 ′ formed on the right side of the lower support  411  and adapted to provide piezoelectric voltage, a piezoelectric material layer  413 ′ formed on the lower electrode layer  412 ′ and adapted to generate a vertical actuating force through shrinkage and expansion when voltage is applied to both sides thereof, and an upper electrode layer  414 ′ formed on the piezoelectric material layer  413 ′ and adapted to provide piezoelectric voltage to the piezoelectric material layer  413 ′.  
         [0023]     A raised type light modulator different from the above-described recess type light modulator is described in detail in Korean Pat. No. P2003-077389.  
         [0024]     Meanwhile, the drive Integrated Circuit (IC) of a spatial light modulator is used to drive the spatial light modulator. Generally, the slew rate of driving voltage is important to drive the spatial light modulator in light of the characteristics of the spatial light modulator. In this case, the slew rate refers to a maximum variation of output voltage per unit time. That is, when output voltage is plotted against time on a graph, the instantaneous slope (differentiated value with respect to time) of an output voltage curve is the slew rate.  
         [0025]     Generally, the higher the slew rate, the better.  
         [0026]     However, an excessively high slew rate in the drive IC for driving the spatial light modulator causes a large amount of residual vibration in the spatial light modulator, thus increasing fatigue in the spatial light modulator.  
         [0027]     Furthermore, an excessively low slew rate in the drive IC for driving the spatial light modulator can solve the problem of fatigue caused by residual vibration, but is problematic in that it is difficult to obtain a desired number of diffracted beams having different intensities at desired time because the drive IC is driven so slowly. Generally, the spatial light modulator is desired to be driven fast. In the case where the slew rate of the drive IC is too low, it is problematic in that the drive IC cannot be driven at a desired speed.  
       SUMMARY OF THE INVENTION  
       [0028]     Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an object of the present invention is to provide a buffer of a drive IC capable of controlling the slew rate of output voltage that can meet a desired dynamic slew rate characteristic by controlling current that affects the slew rate.  
         [0029]     In order to accomplish the above object, the present invention provides a buffer capable of controlling a slew rate, including a first amplifier connected to the output terminal of the Digital/Analog (D/A) converter of a drive IC, and adapted to receive the output voltage of the D/A converter and inversely amplify the output voltage, a second amplifier adapted to again inversely amplify output voltage that has been inversely amplified by the first amplifier, a charging means connected to the output and input terminals of the second amplifier, and adapted to be charged with the output voltage of the first amplifier, and a current control unit connected to the output terminal of the first amplifier and the input terminal of the second amplifier, and adapted to control output voltage of the current control unit. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0030]     The above and other objects, features and advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:  
         [0031]      FIG. 1  is a perspective view illustrating a conventional electrostatic type grating light modulator;  
         [0032]      FIG. 2  is a view illustrating the reflection of incident light by the conventional electrostatic type grating light modulator;  
         [0033]      FIG. 3  is a view illustrating the diffraction of incident light by the conventional electrostatic type grating light modulator;  
         [0034]      FIG. 4  is a sectional view showing a conventional diffractive thin-film piezoelectric micromirror having piezoelectric materials and a recess;  
         [0035]      FIG. 5  is a sectional view showing the drive module of a light modulator using a buffer capable of controlling a slew rate in a drive IC according to an embodiment of the present invention;  
         [0036]      FIG. 6  is a view showing the internal construction of the drive IC of  FIG. 5 ;  
         [0037]      FIG. 7  is a view showing the internal construction of the D/A converter of  FIG. 6 ;  
         [0038]      FIG. 8  is a circuit diagram of the D/A converter of  FIG. 6 ;  
         [0039]      FIG. 9  is a view showing the construction of the buffer capable of controlling the slew rate according to the embodiment of the present invention;  
         [0040]      FIG. 10  is a graph showing an example illustrating the specification of  FIG. 10 ; and  
         [0041]      FIG. 11  is a view showing the current control unit of  FIG. 9  according to the embodiment of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0042]     Reference now should be made to the drawings, in which the same reference numerals are used throughout the different drawings to designate the same or similar components.  
         [0043]     A preferred embodiment of the present invention is described in detail with reference to FIGS.  5  to  11  below.  
         [0044]      FIG. 5  is a sectional view showing the drive module of a light modulator using a buffer capable of controlling a slew rate in a drive IC according to an embodiment of the present invention.  
         [0045]     Referring to  FIG. 5 , the active element  510  of a spatial light modulator is formed on a portion of a lower substrate  500 . In this case, a silicon substrate, ceramic substrate or Printed Circuit Board (PCB) can be used as the lower substrate  500 .  
         [0046]     The active element  510  of the spatial light modulator includes all the elements (active regions) of diffractive, reflective and transmissive light modulators used in the optical memory, optical display device, printer, optical interconnection, hologram, and display device fields.  
         [0047]     The active element  510  of the spatial light modulator includes an active region  511  (element of light modulator) formed on a portion of the lower substrate  500  and adapted to move vertically when power is applied, a conductive electrode  512  adapted to supply power to the active region  511 , and a lid glass  513  adapted to seal the active region  511 .  
         [0048]     The lid glass  513  functions to protect the active region  511 , mounted on the lower substrate  500 , from a surrounding environment. The lid glass  513  covers the active region  511 , and a portion of the lid glass  513  corresponding to the active region  511  is made of light transmitting material.  
         [0049]     A drive IC  520  is provided beside the active element  510  formed on the lower substrate  500 . The drive IC  520  provides driving voltage to the active region  511  of the active element  510  in response to an externally applied control signal.  
         [0050]     The electrical connection between the drive IC  520  and the active region  511  of the active element  510  is implemented using conductive electrodes  521   a  and  521   b , and is maintained using a flip-chip connection.  
         [0051]     The operation of the drive module of the light modulator is described in detail with reference to the drawings below.  
         [0052]     The drive IC  520  receives a control signal for generating diffracted light using a corresponding pixel from an external control unit (not shown).  
         [0053]     The drive IC  520  generates driving voltage that enables one or more elements (active regions), which constitute the corresponding pixel, to be moved upward or downward by λ/4 (The number of elements that must move is determined by the number of active regions constituting the corresponding pixel. That is, only a single element is driven if two elements constitute a single pixel, and two elements are driven if four elements constitute a single pixel), and applies the driving voltage to one or more elements (one or more active regions).  
         [0054]     As described above, when the driving voltage is applied to one or more elements constituting the corresponding pixel, one or more elements constituting the corresponding pixel move upward or downward by λ/4 due to the applied driving voltage, and generate a height difference of λ/4 between themselves and neighboring elements so that the pixel corresponding to the active region  511  generates 0 and +/−1st order diffracted light when light is incident on the surface of the pixel.  
         [0055]      FIG. 6  is a view showing the internal construction of the drive IC of  FIG. 5 . Referring to the drawing, the drive IC of  FIG. 5  includes a data control unit  610 , a 60-bit shift register  620 , a data register  630 , a data latch  640 , a D/A converter  660  and an output buffer  670 .  
         [0056]     In the present invention, the data control unit  610  receives six pieces of 8-bit data from an external control unit (not shown). The data control unit  610  transmits the six pieces of 8-bit data, received from the external control unit, to the data register  630  through an internal 48-bit line.  
         [0057]     In this case, the 60-bit shift register  620  shifts the 48-bit data, transmitted from the data control unit  610 , 60 times, thus allowing a total of 48*60-bit data to be stored in the data register  630 . The reason for this is that a total of 360*8=2880-bit data is required because each channel is driven by 8 bits and 360 channels are used, so that 48-bit data is required to be shifted 60 times to store the 2880-bit data in the data register  630 .  
         [0058]     The data register  630  transmits the stored 48*60-bit data to the data latch  640  at one time when a clock is turned on. The data latch  640  outputs 48*60-bit data, which can drive 360 channels, to a level shifter  650 . The level shifter  650  shifts the level of a data signal, output from the data latch  640 , from 3.3 to 15 V, and outputs the 15 V data signal to the D/A converter  660 .  
         [0059]     Thereafter, the D/A converter  660  receives the 15 V 8-bit data output from the level shifter  650 , converts the 15 V 8-bit data into analog data, and outputs the analog data to the output buffer  670 .  
         [0060]     The output buffer  670  buffers the output voltage of the D/A converter  660 , and provides the buffered output voltage to the corresponding element (the active region) of the spatial light modulator.  
         [0061]      FIG. 7  is a view showing the construction of the D/A converter  660  of  FIG. 6 .  
         [0062]     Referring to  FIG. 7 , the D/A converter  660  of  FIG. 6  includes pads  733  to which reference voltage lines  731  are mechanically connected, a reference voltage generating resistor unit  732  adapted to receive signals through the reference voltage lines  731  and generate subdivided reference voltages, a plurality of voltage level selection circuits  734  connected to the reference voltage generating resistor unit  732 , and output buffers  735  placed behind the voltage level selection circuits  734 , respectively.  
         [0063]     The reference voltage generating resistor unit  732  divides externally input reference voltage into a desired number of reference voltages. The voltage level selection circuits  734  receive 8-bit digital data, and select and output a voltage value corresponding to the 8-bit digital data.  
         [0064]      FIG. 8  is a circuit diagram showing the D/A converter of  FIG. 6  according to an embodiment of the present invention.  FIG. 8  illustrates a process of selecting and outputting a voltage value corresponding to 3-bit digital data when the voltage level selection circuits  734  receive the 3-bit digital data.  
         [0065]     Referring to  FIG. 8 , the D/A converter  660  of  FIG. 6  includes a plurality of series-connected resistors R 1  to R 8  constituting the reference voltage generating resistor unit  732 , and a plurality of tree-shaped transistors b 1  to b 3  and {overscore (b 1 )} to {overscore (b 3 )} constituting the voltage level selection circuits  734 .  
         [0066]     The series-connected resistors R 1  to R 8  of the reference voltage generating resistor unit  732  divide input voltage into eight voltages.  
         [0067]     Furthermore, the tree-shaped transistors b 1  to b 3  and {overscore (b 1 )} to {overscore (b 3 )} select the eight divided input voltage. An example is shown in Table 1 with reference to the drawing below.  
                               TABLE 1                                       Output       Input signal   b1   b2   b3   voltage                   111   1   1   1   V1       110   1   1   0   V2       101   1   0   1   V3       100   1   0   0   V4       011   0   1   1   V5       010   0   1   0   V6       001   0   0   1   V7       000   0   0   0   V8                  
 
         [0068]      FIG. 9  is a view showing the construction of a buffer capable of controlling a slew rate according to an embodiment of the present invention.  
         [0069]     Referring to  FIG. 9 , the buffer capable of controlling a slew rate according to the embodiment of the present invention includes a front-stage amplifier  921 , a current control unit  922 , a back-stage inverting amplifier  923 , and a capacitor  924 .  
         [0070]     The inverting terminal of the front-stage amplifier  921  is connected to the output terminal of a D/A converter  910 , and the non-inverting terminal of the front-stage amplifier  921  is connected to the output terminal of the buffer.  
         [0071]     Since the voltage difference between the inverting terminal and non-inverting terminal of the front-stage amplifier  921  is zero, the output voltage is the same as the voltage input to the inverting terminal. That is, the output voltage from the front-stage amplifier  921  is the same as the input voltage from the D/A converter  910  to the non-inverting terminal.  
         [0072]     Furthermore, voltage is output through the output terminal of the front-stage amplifier  921  with the phase of the input voltage inversed. That is, phase inversion occurs in the front-stage amplifier  921 .  
         [0073]     The back-stage inverting amplifier  923  inverses the phase of input voltage and outputs voltage having an inversed phase. That is, the back-stage inverting amplifier  923  receives the output voltage of the front-stage amplifier  921 , and inverses the phase of the voltage and outputs voltage having an inversed phase.  
         [0074]     As a result, the phase of the input voltage of the front-stage amplifier  921  is inversed when the input voltage is output from the front-stage amplifier  921 , and the phase of the output voltage of the front-stage amplifier  921  is inversed again when the output voltage passes through the back-stage inverting amplifier  923 . Accordingly, the phase of the voltage output from the back-stage inverting amplifier  923  is finally identical with that of the input voltage of the front-stage amplifier  921 .  
         [0075]     The capacitor  924  is charged with the input voltage of the back-stage inverting amplifier  923  and is discharged from the charged voltage, thus varying the output voltage of an output terminal according to the input voltage.  
         [0076]     In the buffer circuit, a slew rate is generally determined using Equation 1, 
 
 slew   —   rate=dV/dt=I/C   (1) 
 
         [0077]     That is, the slew rate is a rate of voltage variation with respect to time, and is proportional to current and inversely proportional to the capacity of a capacitor. Accordingly, if the current varies, the slew rate varies.  
         [0078]     It can be understood with reference to  FIG. 10  that a specification of driving voltage for driving the active element of the spatial light modulator is represented by the interval between lines a and b.  
         [0079]     In this case, it can be understood that lines c and d deviate from the required driving voltage. Accordingly, current needs to be controlled to move the lines c and d to positions between the lines a and b when the output voltage of the buffer is between the lines c and d.  
         [0080]     For this purpose, in the present invention, a current control unit  922  is placed behind the output terminal of the amplifier  921  in the present invention, thus varying the current input to the back-stage inverting amplifier  923 .  
         [0081]     Accordingly, since the slew rate is proportional to current, the slew rate increases.  
         [0082]      FIG. 11  is a circuit diagram showing the current control unit  922  of  FIG. 9  according to an embodiment of the present invention. The current control unit  922  includes a current source  1110  whose output terminal is connected to an output terminal  1111 , a switching unit  1112 , and a plurality of current suppliers  1120 ,  1122  and  1124  (in this case, MOSFETs are used as the current suppliers  1120 ,  1122  and  1124 ).  
         [0083]     The current source  1110  enables certain current I to flow to the output terminal  1111 . The capacitor C  924  functions as described above. In this case, the output current I is current flowing between the front-stage amplifier  921  and the back-stage inverting amplifier  923 , and current flowing between the input terminal of the back-stage inverting amplifier  923  and the output terminal  1111 . That is, the slew rate is affected by the output current I.  
         [0084]     The switching unit  1112  is composed of, for example, a plurality of MOSFETs  1112   a  and  1112   b . The output terminal of the front-stage amplifier  921  may be connected to the gates of the MOSFETs  1112   a  and  1112   b . As an example, when the output terminal is connected to a gate  1 , the first MOSFET  1112   a  is turned on, the second MOSFET  1112   b  is turned off, and output current I is the sum of current I 1  and current I 2 . As another example, when the output terminal is connected to both gates  1  and  2 , the first and second MOSFETs  1112   a  and  1112   b  are turned on, and output current I is the sum of current I 1 , current I 2  and current I 3  in this case.  
         [0085]     As described above, the MOSFETs  1112   a  and  1112   b , that is, switches, function to turn on the MOSFETs  1122  and  1124  that are current suppliers connected to the source terminals of the MOSFETs  1112   a  and  1112   b , respectively. In this case, each of the MOSFETs  1120 ,  1122  and  1124 , which is a current supplier, has a predetermined current value. For example, the first MOSTFET  1120  has current I 1 , the second MOSFET  1122  has current I 2 , and the third MOSFET  1124  has current I 3 .  
         [0086]     Accordingly, current flowing through the current source  1110  is I 1 +I 2  when the first MOSFET  1112   a  is turned on, and current flowing through the current source  1110  is I 1 +I 2 +I 3  when the second MOSFET  1112   b  is turned on. Meanwhile, although two switches are described for an illustrative purpose, the present invention may have a plurality of switches.  
         [0087]     As described above, the present invention is advantageous in that it is easy to control a slew rate even in the case where it is difficult to control the slew rate in consideration of dispersion of development processes.  
         [0088]     Furthermore, since the present invention can easily control the slew rate, the present invention enables an IC, which is inferior from the aspect of a slew rate, to exhibit desired characteristics by controlling the amount of current, thus achieving high yield.  
         [0089]     Although an embodiment for implementing a buffer capable of controlling the slew rate in a drive IC according to the present invention has been described above, the present invention is not limited thereto and those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.