Abstract:
A power-up signal generation circuit includes a control signal generation unit suitable for generating first and second control voltages based on a power-up signal, a level tracing voltage generation unit suitable for generating a level tracing voltage whose voltage level varies based on the first and second control voltages, and a power-up signal generation unit suitable for generating the power-up signal based on the level tracing voltage, and providing a feedback on the power-up signal to the control signal generation unit.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims priority of Korean Patent Application No. 10-2014-0021933, filed on Feb. 25, 2014, which is incorporated herein by reference in its entirety. 
       BACKGROUND 
       [0002]    1. Field 
         [0003]    Various embodiments of the present invention relate to semiconductor design technology, and more particularly, to a power-up signal&#39; generation circuit in a semiconductor device. 
         [0004]    2. Description of the Related Art 
         [0005]    Semiconductor devices, such as a dynamic random access memory (DRAM), generally include a power-up signal generation circuit to secure a stable operation of an internal circuit. When an external power voltage VDD is supplied to a semiconductor device, the external power voltage VDD gradually increases to a target level. 
         [0006]    However, if the external power voltage VDD is directly supplied to the internal circuit of the semiconductor device before the external power voltage VDD reaches the target level a latch-up phenomenon may occur. Thus, the semiconductor device may be damaged. To prevent such phenomenon from occurring, a power-up signal generation circuit is included in the semiconductor device. The power-up signal generation circuit activates a power-up signal when the external power voltage VDD reaches the target level to stably operate the internal circuit and initialize the semiconductor device. 
         [0007]      FIG. 1  is a circuit diagram illustrating a conventional power-up signal generation circuit, and  FIG. 2  shows timing diagrams of an external power voltage VDD and a power-up signal PWRUP in the power-up signal generation circuit shown in  FIG. 1 . 
         [0008]    Referring to  FIG. 1 , the external power voltage VDD supplied to the power-up signal generation circuit is divided by resistors R 11  and R 12 , and the divided voltage is outputted as a level tracing voltage V_LEVEL. The level tracing voltage V_LEVEL has a voltage level that linearly varies according to a level of the external power voltage VDD. 
         [0009]    An NMOS transistor N 11  receives the level tracing voltage V_LEVEL through a gate thereof and is turned on more strongly as the level of the external power voltage VDD becomes higher. As the NMOS transistor N 11  is turned on more strongly, a detection voltage V_DET becomes lower gradually. When the detection voltage V_DET is lower than a certain level, that is, when the external power voltage VDD becomes higher than a target voltage V_TARGET, the power-up signal PWRUP is activated to a logic high level by an inverter. 
         [0010]    Referring to  FIG. 2 , a variation of the power-up signal PWRUP based on variation of the external power voltage VDD is described herein, along with the concerns of the conventional technology for the power-up signal PWRUP. 
         [0011]    In a duration prior to a time “t1” the external power voltage VDD, which is applied to a circuit to turn on the power of a semiconductor device, is gradually increased. However, since the external power voltage VDD has not yet reached the target voltage V_TARGET, the power-up signal PWRUP is in a deactivated state of a logic low level. 
         [0012]    In a duration between the time “t1” and a time “t2”, the external power voltage VDD is increased higher than the target voltage V_TARGET. The NMOS transistor N 11  is strongly turned on to enable the power-up signal PWRUP to a logic high level. 
         [0013]    In a duration between the time “t2” and a time “t3”, the current consumption amount of the semiconductor device is increased and the external power voltage VDD drops. For example, such a voltage drop may occur when a DRAM device performs an active operation “ACT”, When the external power voltage VDD drops lower than the target voltage V_TARGET, the detection voltage V_DET may be raised. When the detection voltage V_DET is raised and then drops, the power-up signal PWRUP is reset “RESET” and as a result the semiconductor device in the middle of an operation may be inadvertently initialized again. 
       SUMMARY 
       [0014]    Various embodiments of the present invention are directed to a power-up signal generation circuit: of a semiconductor device capable of stably operating by generating a power-up signal in a state in which a voltage level of an external power voltage sufficiently rises by adjusting a level tracing voltage and increasing a voltage level of a target voltage before a power-up. 
         [0015]    Various embodiments of the present invention are directed to a power-up signal generation circuit of a semiconductor device capable of preventing an initialization of an operating semiconductor device by adjusting a level tracing voltage and decreasing a voltage level of a target voltage after a power-up, 
         [0016]    In accordance with an embodiment of the present invention, a power-up signal generation circuit may include: a control signal generation unit suitable for generating first and second control voltages based on a power-up signal; a level tracing voltage generation unit suitable for generating a level tracing voltage whose voltage level varies based on the first and second control voltages; and a power-up signal generation unit suitable for generating the power-up signal based on the level tracing voltage, and providing a feedback on the power-up signal to the control signal generation unit. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  is a circuit diagram illustrating a conventional power-up signal generation circuit. 
           [0018]      FIG. 2  is a timing diagram of a power-up signal generation circuit shown in  FIG. 1 . 
           [0019]      FIG. 3  is a diagram illustrating a power-up signal generation circuit in accordance with an embodiment of the present invention. 
           [0020]      FIG. 4  is a detailed diagram of a control signal generation unit shown in  FIG. 3 . 
           [0021]      FIG. 5  is a graph illustrating a resistance characteristic of a variable resistor according to a control voltage. 
           [0022]      FIG. 6  is a table showing parameters for describing an operation of the power-up signal generation circuit shown in  FIG. 3 . 
           [0023]      FIG. 7  is a waveform diagram showing voltages varying before and after a power-up in the power-up signal generation circuit shown in  FIG. 3 . 
           [0024]      FIG. 8  is a diagram illustrating a power-up signal generation circuit in accordance with an embodiment of the present invention. 
           [0025]      FIG. 9  is a detailed diagram of a control signal generation unit shown in  FIG. 8 . 
           [0026]      FIG. 10  is a table showing parameters for describing an operation of the power-up signal generation circuit shown in  FIG. 8 . 
           [0027]      FIG. 11  is a waveform diagram illustrating a voltage varying before and after a power-up in the power-up signal generation circuit shown in  FIG. 8 . 
       
    
    
     DETAILED DESCRIPTION 
       [0028]    Exemplary embodiments of the present invention will be described below in more detail with reference to the accompanying drawings. The present invention may, however, be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the present invention to those skilled in the art. The drawings are not necessarily to scale and in some instances, proportions may have been exaggerated to clearly illustrate features of the embodiments. Throughout the disclosure, reference numerals correspond directly to the like numbered parts in the various figures and embodiments of the present invention. It is also noted that in this specification, “connected/coupled” refers to one component not only directly coupling another component but also indirectly coupling another component through an intermediate component. In addition, a singular form may include a plural form as long as it is not specifically mentioned in a sentence. 
         [0029]      FIG. 3  is a diagram illustrating a power-up signal generation circuit in accordance with an embodiment of the present invention. 
         [0030]    Referring to  FIG. 3 , the power-up signal generation circuit may include a control signal generation unit  100 , a level tracing voltage generation unit  200 A and a power-up signal generation unit  400 . 
         [0031]    The control signal generation unit  100  generates first and second control voltages VCTRL 1  and VCTRL 2  based on a power-up signal PWRUP. The control signal generation unit  100  performs an initialization operation. That is, before a power-up the control signal generation unit  100  outputs a ground voltage VS 5  as the first control voltage VCTRL 1  and outputs a supply voltage VDD as the second control voltage VCTRL 2 . Further, after the power-up, the control signal generation unit  100  inverts and outputs voltage levels of the first and second control voltages VCTRL 1  and VCTRL 2  at its output node whenever the power-up signal PWRUP toggles from a logic low level to a logic high level. 
         [0032]    Here, “before the power-up” means a duration in which the power-up signal PWRUP is deactivated to a logic low level, and “after the power-up” means a duration in which the power-up signal PWRUP is activated to a logic high level. 
         [0033]    The level tracing voltage generation unit  200 A generates a level tracing voltage V_LEVEL by dividing the supply voltage VDD based on the first and second control voltages VCTRL 1  and VCTRL 2 . The level tracing voltage generation unit  200 A may include first and second variable resistors R 1  and R 2  coupled in series between a supply voltage (VDD) terminal and a ground voltage (VSS) terminal. 
         [0034]    The level tracing voltage V_LEVEL controls a target voltage V_TARGET for determining an activation timing of the power-up signal PWRUP. 
         [0035]    Before the power-up, the power-up signal PWRUP is controlled to be activated after the supply voltage VDD becomes higher than the target voltage V_TARGET. Before the power-up, the target voltage V_TARGET may be set relatively high to activate the power-up signal PWRUP relatively later. After the power-up, the target voltage V_TARGET may be set relatively low so that a reset of the power-up signal PWRUP rarely occurs. In the embodiment of the present invention, the level tracing voltage generation unit  200 A controls the level tracing voltage V_LEVEL. That is, the level tracing voltage generation unit  200 A outputs the level tracing voltage V_LEVEL having a relatively low voltage level at a first node ND 1  before the power-up, and outputs the level tracing voltage V_LEVEL having a relatively high voltage level at the first node ND 1  after the power-up. 
         [0036]    The power-up signal generation unit  400  generates the power-up signal PWRUP based on the level tracing voltage V_LEVEL. A feedback on the power-up signal PWRUP generated from the power-up signal generation unit  400  is given to the control signal generation unit  100 . 
         [0037]    The power-up signal generation unit  400  may include a detection voltage generation section  400 A and a power-up signal output section  400 B. The detection voltage generation section  400 A generates a detection voltage V_DET with a voltage level controlled based on the level tracing voltage V_LEVEL. The power-up signal output section  4006  outputs the power-up signal PWRUP based on the detection voltage V_DET. 
         [0038]    The detection voltage generation section  400 A includes a PMOS transistor P 1  and a NMOS transistor N 1  coupled in series between the supply voltage (VDD) terminal and the ground voltage (VSS) terminal. The PMOS transistor P 1  is coupled between the supply voltage (VDD) terminal and a second node ND 2 , and has a gate receiving the ground voltage VSS. Since the PMOS transistor P 1  is turned on by the ground voltage VSS applied to its gate, an initial voltage level of the second node ND 2  maintains a supply voltage (VDD) level. The NMOS transistor N 1  is coupled between the second node ND 2  and the ground voltage (VSS) terminal, and has a gate receiving the level tracing voltage V_LEVEL. The detection voltage V_DET is outputted through the second node ND 2 . Since the extent of the turn-on of the NMOS transistor N 1  is determined based on the level tracing voltage V_LEVEL applied to its gate, the detection voltage V_DET may have a voltage level controlled by the level tracing voltage V_LEVEL. 
         [0039]    The power-up signal output section  400 B may change a logic level of the power-up signal PWRUP when the detection voltage V_DET has a level higher that a logic threshold value of an inverter. 
         [0040]      FIG. 4  is a detailed diagram of the control signal generation unit  100  shown in  FIG. 3 . 
         [0041]    Referring to  FIG. 4 , the control signal generation unit  100  may include a first edge-triggered flip-flop  110 A, a second edge-triggered flip-flop  110 B and an initialization section  120 . 
         [0042]    The first and second edge-triggered flip-flops  110 A and  1108  generate the first and second control voltages VCTRL 1  and VCTRL 2 , respectively, based on the power-up signal PWRUP. The first and second edge-triggered flip-flops  110 A and  110 B are disabled before the power-up, and invert and output the voltage levels of the first and second control voltages VCTRL 1  and VCTRL 2 , respectively, whenever the power-up signal PWRUP toggles from a logic low level to a logic high level. 
         [0043]    The first edge-triggered flip-flop  110 A may include a rising-edge-triggered flip-flop which inverts and outputs the voltage level of the first control voltage VCTRL 1  at its output terminal Q whenever the power-up signal PWRUP toggles from a logic low level to a logic high level. That is, the ground voltage VSS is inputted to the first edge-triggered flip-flop  110 A through its data input terminal D, and the power-up signal PWRUP is inputted to the first edge-triggered flip-flop  110 A through its clock terminal CLK. The second edge-triggered flip-flop  1108  may include a rising-edge-triggered flip-flop which inverts and outputs the voltage level of the second control voltage VCTRL 2  at its output terminal Q whenever the power-up signal PWRUP toggles from a logic low level to a logic high level. That is, the supply voltage VDD is inputted to the second edge-triggered flip-flop  1108  through its data input terminal D, and the power-up signal PWRUP is inputted to the second edge-triggered flip-flop  1108  through its clock terminal CLK. 
         [0044]    The initialization section  120  initializes the first control voltage VCTRL 1  to the ground voltage VSS and the second control voltage VCTRL 2  to the supply voltage VDD before the power-up, that is, before the power-up signal PWRUP reaches a given voltage level, The initialization section  120  includes a PMOS transistor P 2  and an NMOS transistor N 2 . The PMOS transistor P 2  is coupled between the supply voltage (VDD) terminal and the output node of the second edge-triggered flip-flop  110 B, and has a gate receiving the power-up signal PWRUP. The NMOS transistor N 2  is coupled between the output node of the first edge-triggered flip-flop  110 A and the ground voltage (VSS) terminal, and has a gate receiving an inverted signal of the power-up signal PWRUP through an inverter IV 1 . 
         [0045]    Accordingly, the PMOS transistor P 2  and the NMOS transistor N 2  of the initialization section  120  are turned on to initialize the first control voltage VCTRL 1  to the ground voltage VSS and the second control voltage VCTRL 2  to the supply voltage VDD before the power-up, respectively. The PMOS transistor P 2  and the NMOS transistor N 2  are turned off after the power-up. 
         [0046]      FIG. 5  is a graph showing a resistance characteristic of a variable resistor according to a control voltage. 
         [0047]    Referring to  FIG. 5 , a variable resistor “A” has a positive coefficient to the control voltage that is a directly proportional relationship. The resistance of the variable resistor “A” increases as a voltage level of the control voltage changes into a second level VCTRL 2  from a first level VCTRL 1 . A variable resistor “B” has a negative coefficient to the control voltage, that is, an inversely proportional relationship. The resistance of the variable resistor “B”&#39; decreases as the voltage level of the control voltage changes into the second level VCTRL 2  from the first level VCTRL 1 . 
         [0048]    In the embodiment of the present invention, the first and second variable resistors R 1  and R 2  provided in the level tracing voltage generation unit  200 A may be the variable resistor “B” which has the negative coefficient to the control voltage, that is, an inversely proportional relationship. However, the present invention is not limited to this structure, and the first and second variable resistors R 1  and R 2  may be the variable resistor “A” which has the positive coefficient to the control voltage, that is, a directly proportional relationship). 
         [0049]    Hereinafter, an operation of the power-up signal generation circuit of  FIG. 3  will be described with reference to  FIGS. 3 ,  4 ,  6  and  7 . 
         [0050]      FIG. 6  is a table showing parameters for describing an operation of the power-up signal generation circuit of  FIG. 3 , such as VCTRL 2 , R 1 , R 2 , V_LEVEL and V_TARGET.  FIG. 7  is a waveform diagram showing voltages varying before and after the power-up in the power-up signal generation circuit shown in  FIG. 3 . 
         [0051]    Referring to  FIGS. 3 and 6 , before the power-up, the resistance of the first variable resistor R 1  is controlled to have a minimum value R1(min) in response to the first control voltage VCTRL 1  having a ground voltage (VSS) level, and the resistance of the second variable resistor R 2  is controlled to have a value [R2(min)−{R2(delta)*VDDH}] smaller than the resistance of the first variable resistor R 1 , in response to the second control voltage VCTRL 2  having a supply voltage (VDD) level. 
         [0052]    As the resistance of the second variable resistor R 2  is smaller than the resistance of the first variable resistor R 1  before the power-up, the level tracing voltage V_LEVEL at the first node ND 1 , which is set to {1−R1/(R1+R2)}*VDD, becomes lower. Since the NMOS transistor N 1  is weakly turned on based on the level tracing voltage V_LEVEL becoming lower, the voltage level of the detection voltage V_DET at the second node ND 2  becomes higher. Accordingly, the power-up signal PWRUP is deactivated to a logic low level in response to the voltage level of the detection voltage V_DET becoming higher, 
         [0053]    That is, referring to FIG,  7 , before the power-up, the level tracing voltage V_LEVEL having a lower voltage level is outputted at the first node ND 1  so that the power-up signal PWRUP is activated after the supply voltage VDD becomes higher than the target voltage V_TARGET. Accordingly, the power-up timing when the supply voltage VDD reaches the voltage level of the target voltage V_TARGET may be adjusted. After the power-up, the resistance of the first variable resistor R 1  is controlled to have a minimum value [R1(min)−{R1(delta)*VDD}] in response to the first control voltage VCTRL 1  having the supply voltage (VDD) level, and the resistance of the second variable resistor R 2  is controlled to have a value R2(min) greater than the resistance of the first variable resistor R 1 , in response to the second control voltage VCTRL 2  having the ground voltage (VSS) level. 
         [0054]    As the resistance of the second variable resistor R 2  is greater than the resistance of the first variable resistor R 1  after the power-up, the level tracing voltage V_LEVEL at the first node ND 1 , which is set to {1−R1/(R1+R2)}*VDD, becomes higher. Since the NMOS transistor N 1  is strongly turned on in response to the level tracing voltage V_LEVEL becoming higher, the voltage level of the detection voltage V_DET at the second node ND 2  becomes lower. Accordingly, the power-up signal PWRUP is activated to a logic high level in response to the voltage level of the detection voltage V_DET becoming lower. 
         [0055]    That is, referring to  FIG. 7 , after the power-up, the level tracing voltage V_LEVEL having a higher voltage level is outputted through the first node ND 1  to control the target voltage V_TARGET to be lower, so that a reset of the power-up signal PWRUP rarely occurs. 
         [0056]      FIG. 8  is a diagram illustrating a power-up signal generation circuit in accordance with an embodiment of the present invention. 
         [0057]    Referring to  FIG. 8 , the power-up signal generation circuit may include the control signal generation unit  100 , a level tracing voltage generation unit  200 B and the power-up signal generation unit  400 . 
         [0058]    The level tracing voltage generation unit  200 B may include a voltage divider having the first and second variable resistors R 1  and R 2  of  FIG. 3  and a compensation section. The compensation section may include a NMS transistor N 3 . 
         [0059]    The control signal generation unit  100  generates first and second control voltages VCTRL 1  and VCTRL 2  in response to a power-up signal PWRUPB. The control signal generation unit  100  performs an initialization operation. That is, the control signal generation unit  100  outputs a supply voltage VDD as the first control voltage VCTRL 1  and outputs a ground voltage VSS as the second control voltage VCTRL 2  before a power-up. Further, after the power-up, the control signal generation unit  100  inverts and outputs voltage levels of the first and second control voltages VCTRL 1  and VCTRL 2  at its output node whenever the power-up signal PWRUPB toggles to a logic low level from a logic high level. 
         [0060]    Here, “before the power-up” means a duration in which the power-up signal PWRUPB is deactivated to a level corresponding to the supply voltage VDD, and “after the power-up” means a duration that the power-up signal PWRUPB is activated to a logic low level, 
         [0061]    The level tracing voltage generation unit  200 B generates a level tracing voltage V_LEVEL by dividing the supply voltage VDD and the ground voltage VSS in response to the first and second control voltages VCTRL 1  and VCTRL 2 . The level tracing voltage generation unit  200 B may include first and second variable resistors R 1  and R 2  coupled between a supply voltage (VDD) terminal and a ground voltage (VSS) terminal in series. 
         [0062]    The level tracing voltage generation unit  200 B in accordance with the embodiment of the present invention may further include the compensation section that compensates for the level tracing voltage V_LEVEL changing according to the environment condition. The environment condition may vary according to process, voltage and temperature (PVT). 
         [0063]    The compensation section may include the NMOS transistor N 3  which serves as a current source. The NMOS transistor N 3  is coupled between the second variable resistor R 2  and the ground voltage (VSS) terminal, and has a gate receiving an enable signal V_EN, which is outputted at a third node ND 3  between the first variable resistor R 1  and the second variable resistor R 2 . The extent of the turn-on of the NMOS transistor N 3  is controlled in response to a voltage level V_EN of the third node ND 3 . When the voltage level V_EN of the third node ND 3  is lower than a threshold voltage (Vth) of the NMOS transistor N 3 , no current flows through the NMOS transistor N 3 . When the threshold voltage (Vth) of the NMOS transistor N 3  becomes lower, the voltage level V_EN of the third node ND 3  becomes lower and the NMOS transistor N 3  compensates the voltage level V_EN of the third node ND 3 . That is, the NMOS transistor N 3  compensates a process variation. When the threshold voltage (Vth) of the NMOS transistor N 3  becomes lower, a current flowing in the first and second variable resistors R 1  and R 2  is greater, so that the voltage level of the enable signal V_EN is lower. 
         [0064]    The level tracing voltage V_LEVEL is used to control a target voltage V_TARGET for determining an activation timing of the power-up signal PWRUPB. 
         [0065]    In the embodiment of the present invention, before the power-up, the power-up signal PWRUPB may be activated to a logic low level after the supply voltage VDD becomes higher than the target voltage V_TARGET. Before the power-up, the target voltage V_TARGET may be set relatively high so as to activate the power-up signal PWRUPB relatively later. Accordingly, the level tracing voltage generation unit  200 B controls the level tracing voltage V_LEVEL to have a higher voltage level before the power-up, After the power-up, the target voltage V_TARGET may be set relatively low so that a reset of the power-up signal PWRUPB rarely occurs. Accordingly, the level tracing voltage generation unit  200 B controls the level tracing voltage V_LEVEL to have a lower voltage level after the power-up. 
         [0066]    The power-up signal generation unit  400  generates the power-up signal PWRUPB based on the level tracing voltage V_LEV EL. A feedback on the power-up signal PWRUPB generated from the power-up signal generation unit  400  is given to the control signal generation unit  100 . 
         [0067]    The power-up signal generation unit  400  may include the detection voltage generation section  400 A and a power-up signal output section  400 C. The detection voltage generation section  400 A generates a detection voltage V_DET with a voltage level controlled in response to the level tracing voltage V_LEVEL. The power-up signal output section  400 C outputs the power-up signal PWRUPB based on the detection voltage V_DET. 
         [0068]    The detection voltage generation section  400 A includes a PMOS transistor P 4  and an NMOS transistor N 4  coupled between the supply voltage (VDD) terminal and the ground voltage (VSS) terminal in series. The PMOS transistor P 4  is coupled between the supply voltage (VDD) terminal and a fifth node ND 5 , and has a gate for receiving the ground voltage VSS. Since the PMOS transistor P 4  is turned on by the ground voltage VSS applied to its gate, an initial voltage level of the fifth node ND 5  maintains a supply voltage (VDD) level. The NMOS transistor N 4  is coupled between the fifth node ND 5  and the ground voltage (VSS) terminal, and has a gate receiving the level tracing voltage V_LEVEL. The detection voltage V_DET is outputted through the fifth node ND 5 . Since the extent of the turn-on of the NMOS transistor N 4  is determined based on the level tracing voltage V_LEVEL applied to its gate, the detection voltage V_DET may have a voltage level controlled by the level tracing voltage V_LEVEL. 
         [0069]    The power-up signal output section  400 C may change a logic level of the power-up signal PWRUPB when the detection voltage V_DET has a level higher than a logic threshold value of an inverter INV 10 . The power-up signal output section  400 C may include the inverter INV 10  and an inverter INV 11 , which are connected in series, to output the power-up signal PWRUPB. 
         [0070]      FIG. 9  is a detailed diagram of the control signal generation unit  100  shown in  FIG. 8 . 
         [0071]    Referring to  FIG. 9 , the control signal generation unit  100  may include a first edge-triggered flip-flop  910 A, a second edge-triggered flip-flop  910 B and an initialization section  920 . 
         [0072]    The first and second edge-triggered flip-flops  910 A and  910 B generate the first and second control voltages VCTRL 1  and VCTRL 2 , respectively, in response to the power-up signal PWRUPB. The first and second edge-triggered flip-flops  910 A and  910 B are disabled before the power-up, and invert and output the voltage levels of the first and second control voltages VCTRL 1  and VCTRL 2 , respectively, whenever the power-up signal PWRUPB toggles to a logic low level from a logic high level. 
         [0073]    The first edge-triggered flip-flop  910 A may include a falling-edge-triggered flip-flop which inverts and outputs the voltage level of the first control voltage VCTRL 1  at its output terminal Q whenever the power-up signal PWRUPB toggles to a logic low level from a logic high level. That is, the ground voltage VSS is inputted to the first edge-triggered flip-flop  910 A through its data input terminal D, and the power-up signal PWRUPB is inputted to the first edge-triggered flip-flop  910 A through its clock terminal CLK. The second edge-triggered flip-flop  910 B may include a falling-edge-triggered flip-flop which inverts and outputs the voltage level of the second control voltage VCTRL 2  at its output terminal Q whenever the power-up signal PWRUPB toggles from a logic high level to a logic low level. That is, the supply voltage VDD is inputted to the second edge-triggered flip-flop  910 B through its data input terminal D, and the power-up signal PWRUPB is inputted to the second edge-triggered flip-flop  910 B through its clock terminal CLK. 
         [0074]    The initialization section  920  initializes the first control voltage VCTRL 1  to the supply voltage VDD and the second control voltage VCTRL 2  to the ground voltage VSS when the power-up signal PWRUPB is in a deactivated state, that is, before the power-up. The initialization section  920  may include an NMOS transistor N 5  and a PMOS transistor P 5 . The PMOS transistor P 5  is coupled between the supply voltage (VDD) terminal and the output node of the first edge-triggered flip-flop  910 A, and has a gate receiving the power-up signal PWRUPB inverted. The NMOS transistor N 5  is coupled between the output node of the second edge-triggered flip-flop  910 B and the ground voltage (VSS) terminal, and has a gate for receiving an inverted signal of the power-up signal PWRUPB through an inverter IV 2 . 
         [0075]    Accordingly, the PMOS transistor P 5  and the NMOS transistor N 5  of the initialization section  920  are turned on to initialize the first control voltage VCTRL 1  to the supply voltage VDD and the second control voltage VCTRL 2  to the ground voltage VSS before the power-up, respectively. The PMOS transistor P 5  and the NMOS transistor N 5  are turned off after he power-up. 
         [0076]    Hereinafter, an operation of the power-up signal generation circuit of  FIG. 8  will be described with reference to  FIGS. 8 to 11 . 
         [0077]      FIG. 10  is a table showing parameters for describing an operation of the power-up signal generation circuit shown in  FIG. 8 , such as VCTRL 1 , VCTRL 2 , R 1 , R 2 , V_LEVEL and V_TARGET.  FIG. 11  is a waveform diagram showing voltages varying before and after the power-up in the power-up signal generation circuit shown in  FIG. 8 . 
         [0078]    Referring to  FIGS. 8 and 10 , before the power-up, the resistance of the first variable resistor R 1  is controlled to have a minimum value [R1(min)−{R1(delta)*VDD}] in response to the first control voltage VCTRL 1  having a supply voltage (VDD) level, and the resistance of the second variable resistor R 2  is controlled to have a value R2(min) greater than the resistance of the first variable resistor R 1 , in response to the second control voltage VCTRL 2  having a ground voltage (VSS) level. 
         [0079]    As the resistance of the second variable resistor R 2  is greater than the resistance of the first variable resistor R 1  before the power-up, the level tracing voltage V_LEVEL at the fourth node ND 4  becomes higher. Since the NMOS transistor N 4  is strongly turned on in response to the level tracing voltage V_LEVEL becoming higher, the voltage level of the detection voltage V_DET at the fifth node ND 5  becomes lower. Accordingly, the power-up signal PWRUPB is deactivated to a logic high level in response to the voltage level of the detection voltage V_DET becoming lower. 
         [0080]    That is, referring to  FIG. 11 , before the power-up, the level tracing voltage V_LEVEL having a higher voltage level is outputted at the fourth node ND 4  so that the power-up signal PWRUPB is activated to a logic low level after the supply voltage VDD becomes higher than the target voltage V_TARGET. Accordingly, the power-up timing when the supply voltage VDD reaches the voltage level of the target voltage V_TARGET may be adjusted. 
         [0081]    After the power-up, the resistance of the first variable resistor R 1  is controlled to have a minimum value R1(min) in response to the first control voltage VCTRL 1  having the ground voltage (VSS) level, and the resistance of the second variable resistor R 2  is controlled to have a value [R2(min)−{R2(delta)*VDD}] smaller than the resistance of the first variable resistor R 1 , in response to the second control voltage VCTRL 2  having the supply voltage (VDD) level. 
         [0082]    As the resistance of the second variable resistor R 2  is smaller than the resistance of the first variable resistor R 1  after the power-up, the level tracing voltage V_LEVEL at the fourth node ND 4  becomes lower. Since the NMOS transistor N 4  is weakly turned on in response to the level tracing voltage V_LEVEL becoming lower, the voltage level of the detection voltage V_DET at the fifth node ND 5  becomes higher, Accordingly, the power-up signal PWRUPB is activated to a logic low level in response to the voltage level of the detection voltage V_DET becoming higher. 
         [0083]    That is, referring to  FIG. 11 , after the power-up, the level tracing voltage V_LEVEL having a lower voltage level is outputted through the fourth node ND 4  to control the target voltage V_TARGET to be lower so that a reset of the power-up signal PWRUPB rarely occurs. 
         [0084]    According to the embodiments of the present invention as described above, the power-up signal generation unit may generate a power-up signal when a voltage level of an external power voltage sufficiently rises by adjusting a level tracing voltage and increasing a voltage level of a target voltage before a power-up. Accordingly, the reliability on an overall operation of a semiconductor device may be improved. 
         [0085]    Furthermore, according to the embodiments of the present invention as described above, the power-up signal generation unit may prevent an inadvertent reset of an operating semiconductor device by adjusting a level tracing voltage and decreasing a voltage level of a target voltage after a power-up. 
         [0086]    While the present invention has been described with respect to the specific embodiments, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention as defined in the following claims. 
         [0087]    For instance, positions and kinds of the logic gates and transistors exemplified in the above-described embodiment should be differently implemented according to the polarities of the signals input thereto.