Patent Publication Number: US-8994444-B2

Title: Proportional to absolute temperature current generation circuit having higher temperature coefficient, display device including the same, and method thereof

Description:
PRIORITY STATEMENT 
     This application claims the benefit of priority to Korean Patent Application No. 10-2007-0048691, filed on May 18, 2007, in the Korean Intellectual Property Office, the entire contents of which are incorporated herein in their entirety by reference. 
     BACKGROUND 
     1. Field 
     Example embodiments relate to a proportional to absolute temperature (PTAT) current generation circuit, and for example, to a PTAT current generation circuit having a higher temperature coefficient, a display device including the same, and/or a method thereof. 
     2. Description of Related Art 
     Proportional to absolute temperature (PTAT) current generation circuits outputting PTAT current values are generally used with Inverse Proportional to absolute temperature (IPTAT) current generation circuits outputting IPTAT current values in reference bias circuits (e.g., bandgap circuits). PTAT current generation circuits generally use resistance elements to generate reference current. A resistance value of the resistance elements has a temperature coefficient (e.g., a positive temperature coefficient) which increases in proportion to temperature. The temperature coefficient is the relative change of resistance if temperature changes. 
     Because the resistance elements have the temperature coefficient proportional to temperature, the resistance value of the resistance elements increases if the temperature increases. Accordingly, current output from the PTAT current generation circuits may decrease if the temperature increases. For example, the characteristic of the temperature coefficient of the resistance elements may deteriorate the output current characteristics of the PTAT current generation circuits. As display devices have become larger, current consumed in driving units (e.g., source line drivers) for driving display devices increases, and therefore, an amount of heat generated increases. 
     For example, a source line driver precharges a plurality of source lines with a common voltage and transmits digital image data input from a timing controller to a corresponding one of the source lines. Because a precharge time decreases if temperature increases, the temperature of the source line increases. Accordingly, heat generated by a display panel including the source line may cause a malfunction. 
     SUMMARY 
     Example embodiments provide a proportional to absolute temperature (PTAT) current generation circuit having a higher temperature coefficient using a transistor operating in a weak inversion region. 
     Example embodiments provide a display device and/or a method for reducing heat generated by a display panel by controlling precharge time of a plurality of source lines based on a sensed temperature. 
     According to an example embodiment, a proportional to absolute temperature (PTAT) current generation circuit may include a current mirror unit and/or a level control unit. The current mirror unit may be connected between a first power supply voltage, a first node, and a second node. The level control unit may be connected between the first node, the second node, and a second power supply voltage. The level control unit may be configured to control a level of an output current of the current mirror unit based on a voltage level of the first node and a voltage level of the second node. The level control unit comprises a first transistor connected between the first node and the second power supply voltage, at least one second transistor connected between the second node and a third node, the at least one second transistor configured to operate in a weak inversion region, and/or a third transistor connected between the third node and the second power supply voltage. 
     According to an example embodiment, the current mirror unit may include a first current mirror and/or a second current mirror. The first current mirror may include a first transistor pair, which is connected between the first power supply voltage, a fourth node, and/or a fifth node. The first transistor pair may have a common gate. The second current mirror comprising a second transistor pair, which is connected between the first node, the second node, the fourth node, and/or the fifth node. The second current mirror may have a common gate. 
     According to an example embodiment, the PTAT current generation circuit may include an output unit configured to mirror the output current of the current mirror unit and output a mirrored current. 
     According to an example embodiment, the at least one second transistor may be controlled by a bias voltage and/or may be a metal-oxide semiconductor (MOS) transistor having a temperature coefficient inversely proportional to a temperature. 
     According to an example embodiment, the first transistor and the third transistor may be bipolar junction transistors. 
     According to another example embodiment, a display device may include a display panel, a timing controller, and/or a source line driver. The display panel may include a plurality of source lines and a plurality of gate lines. The timing controller may be configured to generate digital image data and a clock signal. The source line driver may be configured to drive the plurality of source lines based on the digital image data and the clock signal. The source line driver may include a digital-to-analog converter, an output buffer, a transmission switch unit, and/or a temperature sensing unit. The digital-to-analog converter may be configured to generate an analog voltage corresponding to the digital image data. The output buffer may be configured to buffer the analog voltage output from the digital-to-analog converter. The transmission switch unit may be configured to precharge each of the plurality of source lines with a precharge voltage in response to the clock signal and transmit an output signal of the output buffer to a corresponding source line of the plurality of source lines. The temperature sensing unit may be configured to sense a temperature, compare the sensed temperature with a reference temperature, and generate a control signal corresponding to a comparison result. The timing controller may control a pulse width of the clock signal based on the control signal. 
     According to an example embodiment, the timing controller may be configured to increase the pulse width of the clock signal at a second logic level if the temperature sensed by the temperature sensing unit is greater than the reference temperature. 
     According to an example embodiment, the timing controller may be configured to control the pulse width of the clock signal to increase a precharge time of the plurality of source lines if the temperature sensed by the temperature sensing unit is greater than the reference temperature. 
     According to an example embodiment, the transmission switch unit may include at least one common switch and/or at least one output switch. The at least one common switch may be configured to precharge each of the plurality of source lines with the precharge voltage in response to the clock signal. The at least one output switch may be configured to transmit the output signal of the output buffer to the corresponding one of the plurality of source lines in response to the clock signal. The at least one common switch and the at least one output switch may be complementarily switched in response to the clock signal. 
     According to an example embodiment, the temperature sensing unit may include a proportional to absolute temperature (PTAT) current generation circuit and/or a comparator. The proportional to absolute temperature (PTAT) current generation circuit may be configured to generate a current in proportion to the temperature. The comparator may be configured to compare an output voltage of the PTAT current generation circuit with a reference voltage and output the control signal corresponding to the comparison result. 
     According to an example embodiment, a method of driving a display device may include generating digital image data and a clock signal. An analog voltage corresponding to the digital image data may be generated. The analog voltage may be buffered. Each of a plurality of source lines may be precharged with a precharge voltage in response to the clock signal and an output signal may be transmitted to a corresponding source line among the plurality of source line. A temperature may be sensed and/or a pulse width of the clock signal may be controlled based on the sensed temperature. 
     According to an example embodiment, the controlling the pulse width may include generating a voltage in proportion to the temperature, comparing the generated voltage with a reference voltage, and generating a control signal corresponding to the comparison result, and/or controlling the pulse width based on the control signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and/or other aspects and advantages will become more apparent and more readily appreciated from the following detailed description of example embodiments taken in conjunction with the accompanying drawings of which: 
         FIG. 1  illustrates a proportional to absolute temperature (PTAT) current generation circuit according to an example embodiment; 
         FIG. 2  is an example graph illustrating an example temperature coefficient of a second transistor illustrated in  FIG. 1 ; 
         FIG. 3  illustrates a display device according to an example embodiment; 
         FIG. 4  illustrates a temperature sensing unit illustrated in  FIG. 3 ; and 
         FIG. 5  illustrates an output voltage of a source line which varies with temperature sensed by the temperature sensing unit illustrated in  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS 
     Example embodiments will now be described more fully hereinafter with reference to the accompanying drawings. Embodiments may, however, be in many different forms and should not be construed as being limited to the example embodiments set forth herein. Rather, these example embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope to those skilled in the art. In the drawings, the thicknesses of layers and regions may be exaggerated for clarity. 
     It will be understood that when a component is referred to as being “on,” “connected to” or “coupled to” another component, it can be directly on, connected to or coupled to the other component or intervening components may be present. In contrast, when a component is referred to as being “directly on,” “directly connected to” or “directly coupled to” another component, there are no intervening components present. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. 
     It will be understood that, although the terms first, second, third, etc. may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer or section from another element, component, region, layer or section. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of the example embodiments. 
     Spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “upper” and the like, may be used herein for ease of description to describe one component or feature&#39;s relationship to another component(s) or feature(s) as illustrated in the drawings. It will be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. 
     The terminology used herein is for the purpose of describing particular example embodiments only and is not intended to be limiting. As used herein, the singular forms “a,” “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, and/or components. 
     Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which example embodiments belong. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. 
     Reference will now be made to example embodiments, which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like components throughout. 
       FIG. 1  illustrates a proportional to absolute temperature (PTAT) current generation circuit  20  according to an example embodiment.  FIG. 2  is an example graph illustrating an example temperature coefficient of a second transistor illustrated in  FIG. 1 . Referring to  FIGS. 1 and 2 , the PTAT current generation circuit  20  may include a current mirror unit  12 , a level control unit  15 , and/or an output unit  17 . It will be apparent that the PTAT current generation circuit  20  may be used for a reference voltage generation circuit and/or be widely used in semiconductor devices and other electronic devices which require PTAT current generation circuits. 
     The current mirror unit  12  may be connected between a first power supply voltage VDD, a first node N 1 , and/or a second node N 2 . The current mirror unit  12  may mirror a first current I 11  flowing through the first node N 1  and a second current I 12  flowing through the second node N 2 . The current mirror unit  12  may include a first current mirror  12 - 1  and/or a second current mirror  12 - 2 . 
     The first current mirror  12 - 1  may include a first transistor pair MP 1  and MP 2 , which are connected between the first power supply voltage VDD and a fourth node and a fifth node, respectively. For example, transistor MP 1  of the first transistor pair MP 1  and MP 2  may be connected between the first power supply voltage VDD and the fourth node N 4 , and transistor MP 2  of the first transistor pair MP 1  and MP 2  may be connected between the first power supply voltage VDD and the fifth node N 5 . The first transistor pair MP 1  and MP 2  may have a common gate. The common gate of the first transistor pair MP 1  and MP 2  may be connected with the fifth node N 5 . The first transistor pair MP 1  and MP 2  may have a same channel width (W)/channel length (L) ratio (hereinafter referred to as a “W/L ratio”), but example embodiments are not limited thereto and the first transistor pair MP 1  and MP 2  may have different W/L ratios. 
     The second current mirror  12 - 2  may include a second transistor pair MN 1  and MN 2  which are connected between the first node N 1 , the second node N 2 , the fourth node N 4 , and the fifth node N 5 . For example, transistor MN 1  of the second transistor pair MN 1  and MN 2  may be connected between the fourth node N 4  and the first node N 1 , and transistor MN 2  of the second transistor pair MN 1  and MN 2  may be connected between the fifth node N 5  and the second node N 2 . The second transistor pair MN 1  and MN 2  may have a common gate. The common gate of the second transistor pair MN 1  and MN 2  may be connected with the fourth node N 4 . The second transistor pair MN 1  and MN 2  may have the same W/L ratio, but example embodiments are not limited thereto and the second transistor pair MN 1  and MN 2  may have different W/L ratios. 
     The level control unit  15  may be connected between the first node N 1 , the second node N 2 , and a second power supply voltage VSS, e.g., a ground voltage. The level control unit  15  may control a level of the output currents I 11  and I 12  of the current mirror unit  12  based on a voltage level of the first node N 1  and a voltage level of the second node N 2 . The level control unit  15  may include a first transistor BT 1 , a second transistor MN 3 , and/or a third transistor BT 2 . 
     The first transistor BT 1  may be connected between the first node N 1  and the second power supply voltage VSS. The first transistor BT 1  may be a bipolar junction transistor (BJT) which has an emitter connected with the first node N 1  and a base and a collector which are connected with the second power supply voltage VSS. 
     The second transistor MN 3  may be gated in response to a bias voltage Vb and form a current path between the second node N 2  and a third node N 3 .  FIG. 2  shows an ideal temperature coefficient of the second transistor MN 3 . The second transistor MN 3  may operate in a weak inversion region, e.g., in a triode mode having a temperature coefficient (e.g., a negative temperature coefficient) in inverse proportion to a temperature, as illustrated in  FIG. 2 . The second transistor MN 3  may have a resistance value in inverse proportion to the temperature. Accordingly, the second transistor MN 3  may control the level of the second current I 12  according to the temperature. For example, because the resistance of the second transistor MN 3  decreases as the temperature increases, the second current I 12  and the first current I 11  resulting from mirroring the second current I 12  may be increased. Accordingly, the PTAT current generation circuit  20  may generate an output current I out  in proportion to the temperature, thereby having improved output characteristics. For example, the output current I out  of the PTAT current generation circuit  20  according an example embodiment may have a larger variation with respect to the temperature than that of a conventional PTAT current generation circuits using resistance elements. For example, if the temperature changes from 75° C. to 125° C., an output voltage VPTAT of a conventional PTAT current generation circuit using resistance elements changes from about 1.75 V to about 2 V while the output voltage VPTAT of the PTAT current generation circuit  20  according to an example embodiment may change from about 1 V to about 2 V. 
     The third transistor BT 2  may be connected between the third node N 3  and the second power supply voltage VSS. The third transistor BT 2  may be a BJT which has an emitter connected with the third node N 3  and a base and a collector which are connected with the second power supply voltage VSS. 
     If a current flowing in the first transistor BT 1  is M times a current flowing in the third transistor BT 2 , the third transistor BT 2  may be a single transistor having an M-fold current (e.g., a transistor having a W/L ratio M times greater than the first transistor BT 1 ) in order to equalize the first current I 11  and the second current I 12 . If M is an integer, the third transistor BT 2  may be implemented by M first transistors. 
     The output unit  17  may include a fifth transistor MP 3  and a sixth transistor MN 5 . The output unit may convert the output current I out  resulting from mirroring the first current I 11  or the second current I 12  and output the output voltage VPTAT in proportion to the temperature. 
     The fifth transistor MP 3  may be gated with the voltage of the fifth node N 5  and form a current path between the first power supply voltage VDD and a sixth node N 6 , thereby controlling the level of the output current I out . The sixth transistor MN 5  may be gated with the bias voltage Vb and form a current path between the sixth node N 6  and the second power supply voltage VSS. The sixth transistor MN 5  may convert the output current I out  into the output voltage VPTAT and may control the level of the output voltage VPTAT. The sixth transistor MN 5  and the second transistor MN 3  may have a same W/L ratio, but example embodiments are not limited thereto and the sixth transistor MN 5  and the second transistor MN 3  may have different W/L ratios. 
       FIG. 3  illustrates a display device  100  according to an example embodiment.  FIG. 4  illustrates a temperature sensing unit  119  illustrated in  FIG. 3 .  FIG. 5  illustrates an output voltage of a source line which varies with a temperature sensed by the temperature sensing unit  119  illustrated in  FIG. 3 . Referring to  FIGS. 3 through 5 , the display device  100  may include a source line driver  110 , a timing controller  120 , a gate driver  130 , and/or a display panel  140 . 
     The source line driver  110  may receive digital image data DATA and a clock signal CLK from the timing controller  120  and drive a plurality of source lines Y 1 , Y 2 , . . . , Y n  connected with the display panel  140 . The source line driver  110  may include a digital-to-analog converter (DAC)  113 , an output buffer  115 , a transmission switch unit  117 , and/or the temperature sensing unit  119 . 
     The DAC  113  may generate an analog voltage corresponding to the digital image data DATA. The output buffer  115  may buffer the analog voltage output from the DAC  113 . The output buffer  115  may control the slew rate of voltage applied to the source lines Y 1  through Y n  based on a bias voltage, which may be different than the bias voltage Vb, (not shown) generated by a bias voltage generator (not shown). 
     The transmission switch unit  117  may precharge the source lines Y 1  through Y n  with a precharge voltage in response to first switching signals CSW and CSWB and transmit the output signal of the output buffer  115  to a corresponding one of the source lines Y 1  through Y n  in response to second switching signals SW and SWB. The first switching signals CSW and CSWB may have the same phase as the clock signals CLK and may be complementary with the second switching signals SW and SWB. The clock signal CLK may be used as a general reference synchronization signal, but example embodiments are not restricted thereto. 
     The transmission switch unit  117  may include at least one common switch TG 12  and at least one output switch TG 10 . For example, the transmission switch unit may include a common switch TG 12  and an output switch TG 10  for each of the source lines Y 1  through Y n , and the common switches TG 12  may be connected in common and to respective source lines of the source lines Y 1  through Y n . If the first switching signals CSW and CSWB are at a second logic level (e.g., a high level of “1”) based on the clock signal CLK, e.g., if the clock signal CLK is at the second logic level, the at least one common switch TG 12  may be turned on to precharge each of the source lines Y 1  through Y n  with the precharge voltage. If the second switching signals SW and SWB are at the second logic level based on the clock signal CLK, e.g., if the clock signal is at a first logic level (e.g., a low level of “0”), the at least one output switch TG 10  may transmit the output signal of the output buffer  115  to a corresponding one of the source lines Y 1  through Y n . For example, the at least one common switch TG 12  and the at least one output switch TG 10  may be complementarily switched in response to the clock signal CLK. 
     The temperature sensing unit  119  may sense the temperature, compare the sensed temperature with a reference temperature, and/or generate a control signal TS corresponding to the comparison result. The temperature sensing unit  119  may include a start-up circuit  121 , the PTAT current generation circuit  20  according to an example embodiment, and/or a comparator  123 . 
     The start-up circuit  121  may enable an operation of the PTAT current generation circuit  20  in response to a power down signal PWD. The start-up circuit  121  may include ninth through thirteenth transistors MPST 1 , MPST 2 , MNST 1 , MNST 2 , and/or MNST 3 . The power down signal PWD may control the operation of the PTAT current generation circuit  20 . 
     If the power down signal PWD is at the first logic level (e.g., the low level of “0”), the ninth transistor MPST 1  and the tenth transistor MPST 2 , which are connected in series between the first power supply voltage VDD and a seventh node N 7  and are gated with the power down signal PWD, may form a current path between the first power supply voltage VDD and the seventh node N 7 . The thirteenth transistor MNST 3  may be gated with a voltage of the seventh node N 7  and form a current path between the fifth node N 5  and the second power supply voltage VSS, thereby gating the first transistor pair MP 1  and MP 2  with the voltage of the fifth node N 5 . Accordingly, the PTAT current generation circuit  20  may be enabled and generate the output current I out  in proportion to the temperature. 
     If the power down signal PWD is at the second logic level (e.g., the high level of “1”), the eleventh transistor MNST 1 , which is gated with the power down signal PWD, may form a current path between the seventh node N 7  and the second power supply voltage VSS. The twelfth transistor MNST 2 , which is gated with the voltage of the fourth node N 4 , may form a current path between the seventh node N 7  and the second power supply voltage VSS, thereby lowering a potential of the voltage of the seventh node N 7 . Accordingly, the PTAT current generation circuit  20  may be disabled. The PTAT current generation circuit  20  has been described with reference to  FIGS. 1 and 2 , and therefore, detailed descriptions thereof will be omitted. 
     The comparator  123  may compare the output voltage VPTAT of the PTAT current generation circuit  20  with a reference voltage Vref corresponding to a reference temperature and generate the control signal TS corresponding to the comparison result. For example, the comparator  123  may output the control signal TS at the first logic level (e.g., the low level of “0”) if the output voltage VPTAT of the PTAT current generation circuit  20  is less than the reference voltage Vref and may output the control signal TS at the second logic level (e.g., the high level of “1”) if the output voltage VPTAT of the PTAT current generation circuit  20  is greater than the reference voltage Vref. 
     The timing controller  120  may generate the digital image data DATA and the clock signal CLK and control the pulse width of the clock signal CLK based on the control signal TS generated by the temperature sensing unit  119 . For example, the timing controller  120  may not change the pulse width of the clock signal CLK is the control signal TS is at the first logic level (e.g., the low level of “0”), for example, if the temperature sensed by the temperature sensing unit  119  is less than the reference temperature. The timing controller  120  may increase the pulse width of the clock signal CLK, e.g., at the high level of “1”, if the control signal TS is at the second logic level (e.g., the high level of “1”), for example, if the temperature sensed by the temperature sensing unit  119  is greater than the reference temperature. 
     As illustrated in  FIG. 5 , if the control signal TS is at the second logic level (e.g., the high level of “1”), a first period T 1  of the clock signal CLK increases by a pulse width td. Accordingly, the at least one common switch TG 12  may be turned on during the second period T 2  of the clock signal CLK at the second logic level having the increased pulse width td. Therefore, the second period T 2 , during which each of the source lines Y 1  through Y n  is precharged with a precharge voltage, e.g., VDD=(VDD 1 +, . . . +VDDn)/n, may become greater than the first period T 1  and the at least one output switch TG 10  may transmit the output signal of the output buffer  115  to a corresponding one of the source lines Y 1  through Y n  in response to the clock signal CLK at the first logic level (e.g., the low level of “0”) after the first period T 1  lapses. 
     For example, the timing controller  120  may control the pulse width of the clock signal CLK according to an increase of the temperature to delay the precharge time and the transmission of the output signal, so that malfunctions caused by heat generation of the source line driver  110 , the source lines Y 1  through Y n , and/or the display panel  140  may be reduced. 
     The gate driver  130  may supply voltage to a plurality of gate lines G 1 , G 2 , . . . G n . The display panel  140  may include the gate lines G 1  through G n  and the source lines Y 1  through Y n  and may be driven by the source line driver  110  and/or the gate driver  130  to display images. 
     As described above, example embodiments may improve output characteristics of a PTAT current generation circuit using a transistor operating in a weak inversion region. Example embodiments may reduce malfunctions caused by heat generation of a display device by controlling a precharge time of a plurality of source lines based on a sensed temperature. 
     Although example embodiments have been shown and described in this specification and figures, it would be appreciated by those skilled in the art that changes may be made to the illustrated and/or described example embodiments without departing from their principles and spirit.