Abstract:
A delay circuit for an IC device is disclosed, which comprises a charging/discharging circuit, a voltage divider and a comparator. The charging/discharging circuit selectively effects a charging/discharging operation in response to an input sigal to thereby generate a variable output voltage. The voltage divider divides a source voltage of the IC device to provide a reference voltage having a predetermined constant potential. The comparator is coupled to the charging/discharging circuit and voltage divider at its inverting input and non-inverting input, respectively, and compares the output voltage of the charging/discharging circuit with the reference voltage. A switch circuit is provided which performs a switching operation in response to the input signal to thereby electrically disconnect the non-inverting input of the comparator from the voltage divider. A capacitor is provided which retains the reference voltage at the non-inverting input of the comparator while the comparator is electrically disconnected from the voltage divider.

Description:
This application is a continuation of application Ser. No. 275,386, filed on Nov. 23, 1988, now abandoned. 
     Background of the Invention 
     1. Field of the Invention 
     The present invention relates to a delay circuit, and more particularly, to a delay circuit for preferable use in semiconductor integrated circuit (IC) devices, such as dynamic random access memories (dRAM&#39;s). 
     2. Description of the Related Art 
     Generally, IC devices such as dRAM&#39;s include a delay circuit for properly setting the internal timings in order to provide the proper synchronized operations between internal circuit sections. A conventional delay circuit is arranged to include a charging/discharging circuit supplied with an input signal φ1, a reference voltage generator and a voltage comparator for detecting the potential difference between the former two elements. When the level of the input signal φ1 changes, for example, from&#34;L&#34; level to &#34;H&#34; level, the capacitor potential of the charging/discharging circuit varies accordingly. When a node voltage coupled to the variable capacitor potential becomes equal to a constant reference voltage, the output voltage φ2 of the comparator is inverted. Therefore, it may be considered that the input signal φ1 is delayed by the interval (delay time τ between the supply of the input signal and the inversion of the output voltage φ2 of the comparator. Since this delay time τ can be arbitrarily set to the desired value by properly designing the circuit constants of the capacitor and resistors, it can be set to the proper value for each IC device. 
     The conventional delay circuit, however, has a shortcoming such that the delay time τ which should be constant may vary by generation of noise originated in a change in source voltage Vcc. The power lines and ground potential lines of an IC device to which the delay circuit is applied are generally shared by a plurality of internal circuit sections of the IC device, various noises are likely to occur in these lines. The generation of noise changes either one or both of the source voltage Vcc and ground potential Vss in these lines in the form of AC current. In a dRAM, in particular, charging/discharging of a number of bit lines is executed in a short range at the time of data accessing, a potential change in the power lines of the dRAM is prominent. The potential variation in the power lines renders the node voltage of the charging/discharging circuit of the delay circuit unstable, making the inversion timing of the output voltage φ2 of the comparator unstable. This makes significantly difficult to stably maintain the delay time τ at the desired, designed value. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a new and improved delay circuit which can maintain constant a delay time of a semiconductor integrated circuit even when the source voltage of the circuit varies. 
     In accordance with the above object, this invention is addressed to a specific delay circuit for an IC device, which comprises a charging/discharging circuit, a voltage divider and a comparator. The charging/discharging circuit receives an input signal (φ1) and and selectively effects charging/discharging in response to the input signal (φ1) to thereby generate a variable output voltage. The voltage divider receives the source voltage of the IC device and divides it to provide a reference voltage having a predetermined constant potential. The comparator is coupled to the charging/discharging circuit and voltage divider at its first and second inputs, respectively, and compares the output voltage of the charging/discharging circuit with the reference voltage. A switch circuit is provided which receives the input signal (φ1) and performs a switching operation in response to the input signal (φ1) to thereby electrically disconnect the second input of the comparator from the voltage divider. A capacitor is provided which retains the reference voltage at the second input of the comparator while the comparator is electrically disconnected from the voltage divider. 
     This invention and its objects and advantages will become more apparent from the following detailed description of a preferred embodiment. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the detailed description of a preferred embodiment of the invention presented below, reference is made to the accompanying drawings in which: 
     FIG. 1 is a diagram illustrating the circuit configuration of a delay circuit according to one preferred embodiment of this invention; 
     FIG. 2 is a diagram illustrating the internal circuit configuration of a CMOS current mirror type differential amplifier serving as a voltage comparator provided in the delay circuit of FIG. 1; 
     FIG. 3 is a diagram illustrating waveforms of major signals generated in essential sections of the delay circuit of FIG. 1; 
     FIG. 4 is a diagram illustrating waveforms of major signals generated in essential sections of the delay circuit of FIG. 1 in a case where noise is applied to the source voltage and ground potential; 
     FIG. 5 is a diagram illustrating the essential circuit configuration of a modification of the delay circuit of FIG. 1; and 
     FIG. 6 is a diagram illustrating the waveforms of major signals generated in essential sections of the delay circuit of FIG. 5. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to FIG. 1, a delay circuit according to a preferred embodiment of the invention is generally designated by reference numeral &#34;10.&#34; Delay circuit 10 is built in a dRAM (not shown) and includes a charging/discharging circuit 12 which is constituted by two MOS type field effect transistors (hereinafter referred to as &#34;MOSFET&#39;s&#34; or simply as &#34;FET&#39;s&#34;), namely p-channel MOSFET Q1 and n-channel MOSFET Q2, a resistor R1 and a capacitor C1. The common gate node of FET&#39;s Q1 and Q2 is coupled to an input terminal 14 to which an input signal φ1 is supplied. Capacitor C1 is provided between a ground potential Vss and a first signal line 16 coupled to the drain node of FET&#39;s Q1 and Q2. The potential on first signal line 16 is denoted by &#34;v1.&#34; 
     A voltage comparator 18 is coupled at its inverting input to signal line 16 and at its non-inverting input to a second signal line 20 to which a reference voltage generator 22 is coupled. The reference voltage generator 22 may be a voltage divider which is constituted by a series circuit of resistors R2 and R3 provided between the source voltage Vcc and ground potential Vss. This voltage divider performs the ordinary voltage dividing operation to divide the source voltage Vcc applied to one resistor R2 in accordance with the resistor ratio of R2 to R3 and supply the divided voltage to signal line 20 as a reference voltage to be supplied to the non-inverting input of comparator 18. 
     Comparator 18 may be a CMOS current mirror type differential amplifier constituted by five FET&#39;s T1 to T5, as shown in FIG. 2. The gates of FET&#39;s T1 and T2 respectively serve as the non-inverting input and inverting input of comparator 18. FET&#39;s T1 and T2 have sources coupled to the ground potential Vss. The common node of FET&#39;s T2 and T4 serves as the output of comparator 18 from which an output voltage φ2 is produced. FET&#39;s T3 and T4 have their sources coupled together to the source voltage Vcc through FET T5. 
     The present delay circuit 10 is featured in that a switch circuit 24 is provided between resistors R2 and R3 of voltage divider 22, as shown in FIG. 1. More specifically, switch circuit 24 includes a series circuit of two FET&#39;s Q3 and Q4, the former FET being provided between resistor R2 and signal line 20 and the latter between signal line 20 and resistor R3. When these FET&#39;s Q3 and Q4 are rendered conductive, resistors R2 and R3 are electrically coupled to signal line 20, whereby voltage divider 22 performs the ordinary operation to apply a divided voltage to line 20. When FET&#39;s Q3 and Q4 are rendered nonconductive, on the other hand, resistors R2 and R3 are disconnected from line 20, thus rendering line 20 as electrically floating. FET&#39;s Q3 and Q4 have their gates coupled together, and an inverter 26 is coupled between their common gate node N1 and input terminal 14 of delay circuit 10. The voltage at this node N1 is denoted by &#34;v3.&#34; 
     As shown in FIG. 1, an additional capacitor C2 is coupled between line 20 and ground potential Vss. The capacitance of capacitor C2 is preferably set in such a way as to make the ratio of this capacitance to the parasitic capacitance of signal line 20 equal to the ratio of the capacitance of capacitor C1 to the parasitic capacitance of signal line 16. 
     Referring to FIGS. 3 and 4, a description will be given below of the operation modes of thus constituted delay circuit 10. In the following description, the basic operation of delay circuit 10 will be described first referring to FIG. 3, followed by the explanation of the signal delay operation of delay circuit 10, with reference to FIG. 4, in the case where a potential change occurs between the source voltage Vcc and ground potential Vss of a dRAM to which this delay circuit is applied. 
     While the input signal φ1 has a &#34;L&#34; level, p-channel FET Q1 of charging/discharging circuit 12 is rendered conductive and n-channel FET Q2 nonconductive. Therefore, the source voltage Vcc is applied through FET Q1 to capacitor C1 and the corresponding charges are accumulated in the capacitor C1. At this time, the potential on signal line 20 is set to the reference voltage generated by reference voltage generator 22 since inverter 26 of switch circuit 24 generates an output voltage of &#34;H&#34; level and FET&#39;s Q3 and Q4 are rendered conductive in response to this output voltage, thereby coupling the voltage divider constituted by resistors R2 and R3 to line 20. The potential v2 on line 20 is expressed as: 
     
         v2=Vcc·R3/(R2+R3) . . .                           (1) 
    
     Since the output voltage of charging/discharging circuit 12, i.e., the potential v1 on line 16, is higher than the reference voltage or the potential v2 on line 20, the output voltage of comparator 18 has an &#34;L&#34; level. 
     As shown in FIG. 3, when the input signal φ1 changes its level from the &#34;L&#34; level to &#34;H&#34; level at time t1, p-channel FET Q1 of charging/discharging circuit 12 is rendered nonconductive whereas n-channel FET Q2 is rendered conductive. Consequently, the charges accumulated in capacitor C1 are discharged through FET Q2 and resistor R1. This gradually reduces the potential v1 on signal line 16 (i.e., the output voltage of charging/discharging circuit 12), as shown in FIG. 3. Provided that the ON resistance of FET Q2 is sufficiently smaller than the resistance of resistor R1, the output voltage v1 of charging/discharging circuit 12 may be expressed as: 
     
         v1(t)=Vcc·e.sup.-t/C1·R1 . . .           (2) 
    
     When the voltage v1 gradually decreasing becomes the reference voltage v2 at time t2, the output voltage of comparator 18 changes its level from the &#34;L&#34; level to the &#34;H&#34; level, as shown in FIG. 3. The interval between times t1 and t2, or a delay time τ, may be expressed as: 
     
         τ=C1·R1·1n(1+R2/R3) . . .            (3) 
    
     A change in level of the output voltage φ2 is delayed by the time τ from the point of time at which the potential level of the input signal φ1 has changed; the delay time τ can be arbitrarily set by varying the CR time constant of charging/discharging circuit 12. In other words, the input signal φ1 is delayed by the time τ to be the output voltage φ2 of delay circuit 10. 
     It should be noted that when the input signal φ1 rises to the &#34;H&#34; level, the output voltage v3 of inverter 26 has the &#34;L&#34; level so that FET&#39;s Q3 and Q4 of switch circuit 24 are simultaneously rendered conductive. This electrically disconnects voltage-dividing resistors R2 and R3 of voltage divider 22 from each other. At the same time, line 20 coupled to the non-inverting input of comparator 18 is electrically disconnected or insulated from resistors R2 and R3, so that this line 20 becomes electrically floating while retaining the reference voltage v2 thereon. As the reference voltage v2 is constantly maintained by capacitor C2, the potential on line 20 is set to the reference voltage v2. 
     With such an arrangement, even when a change occurs in the source voltage Vcc or ground potential Vss or both, the potential on line 20 is free of the potential change, thus ensuring constant supply of a stable reference voltage v2 to comparator 18. Accordingly, it is possible to surely prevent the delay time τ from being unstable due to a change in source voltage Vcc. This improves the reliability of the delay time τ. Further, the delay circuit of this invention is designed such that when line 20 is disconnected from voltage-dividing resistors R2 and R3, these resistors are insulated from each other. It is therefore possible to completely cut off a current flowing through voltage divider 22. This can make the series circuit of resistors R2 and R3 completely insensitive with respect to a change in source voltage Vcc as well as can reduce the dissipation power of the voltage divider. 
     A detailed description will now be given of the operation of the present delay circuit 10 in the case where the source voltage Vcc or ground potential Vss varies. Let us consider the case where, as shown in FIG. 4, the source voltage Vcc is changing to have an undesirably-increased potential level Vcc1 as indicated by reference numeral &#34;30&#34; before the input signal φ1 changes from the &#34;L&#34; level to the &#34;H&#34; level at time t1. Under this circumstance, the changed source voltage Vcc1 is applied to capacitor C1 of charging/discharging circuit 12, the potential of capacitor C1 gradually decreases from this voltage Vcc1 at the time the discharging starts at time t1. Once the discharging has started, FET Q1 is rendered nonconductive as mentioned earlier to thereby insulate capacitor C1 from the source voltage Vcc, so that capacitor C1 is no longer adversely influenced by the above voltage noise. The reference voltage v2 on line 20 is also undesirably increased when line 20 is electrically floating at time t1 due to division of the source voltage Vcc varied prior to time t1. The reference voltage v2&#39; in this case is expressed as: 
     
         v2&#39;=Vcc1·R3/(R2+R3) . . .                         (4) 
    
     After time t1, voltage-dividing resistors R2 and R3 are insulated from each other to disconnect voltage divider 22 as well as insulate line 20 from these resistors R2 and R3. Accordingly, line 20 is free of the adverse influence of the aforementioned change in source voltage. Even upon occurrence of a change in source voltage Vcc, therefore, the interval between the potential changing time t1 of the input signal φ1 and the potential changing time t2 of the output voltage φ2, i.e., the delay time τ, is kept unchanged. 
     Let us now consider the case where noise 32 is generated in the ground potential Vss after time t1. In this case, as capacitors C1 and C2 are so set as to satisfy the aforementioned relationship, potential changes occurring in both lines 16 and 20 due to the capacitor coupling caused by the application of ground potential noise 32 to these lines 16 and 20 are equal to each other. It is therefore possible to provide delay circuit 10 which is insensitive to a short-range potential change in source voltage Vcc and/or ground potential Vss. This delay circuit 10 is considered particularly suitable for IC devices such as dynamic random access memories. 
     The above embodiment may be modified as shown in FIG. 5 wherein an AND gate 40 is additionally provided between the common gate node N1 of FET&#39;s Q3 and Q4 of switch circuit 24 and inverter 26. AND gate 40 externally receives a control signal φ0 at its first input and receives the input signal φ1 at its second input. As shown in FIG. 6, the control signal φ0 changes its level from the &#34;L&#34; level to the &#34;H&#34; level before the level of the input signal φ1 changes from the &#34;L&#34; level to the &#34;H&#34; level. The time difference between the occurrences of the change in level of the control signal φ0 and the input signal φ1 is denoted by &#34;Td.&#34; Accordingly, the output voltage signal of inverter 26 is permitted to be supplied through AND gate 40 to the common gate node of FET&#39;s Q3 and Q4 only by the time Td. Therefore, the common gate node N1 is supplied with the &#34;H&#34; level voltage only during the time Td and FET&#39;s Q3 and Q4 are rendered nonconductive only during the same time Td. The time Td is set to have such a minimum necessary time interval as to set the voltage v2 divided on line 20 by applying the source voltage Vcc on signal line 20 through voltage divider 22 and charging capacitor C2. 
     With such an arrangement, voltage divider 22 is rendered operative only during a relatively short time, Td, before the input signal φ1 changes its level from the &#34;L&#34; level to the &#34;H&#34; level, and it is rendered inoperative immediately upon elapse of the time Td. It is therefore possible to minimize the occurrence of a leak current or a through current. This can reduce the resistances of voltage-dividing resistors R2 and R3. Accordingly, it is possible to keep signal line 20 at the divided voltage v2 with the reduced impedance until charging/discharging in signal line 16 actually starts. 
     Although the invention has been described with reference to a specific embodiment, it shall be understood by those skilled in the art that numerous modifications may be made which are within the spirit and scope of the inventive contribution. 
     For instance, although capacitors C1 and C2 respectively coupled to first and second signal lines 16 and 20 are both coupled to a low source voltage, i.e., ground potential Vss, in the above embodiments, the delay circuit may be so designed as to permit these capacitors to be coupled to a high source voltage, i.e., source voltage Vcc. Moreover, although a CR delay circuit configuration employing a linear resistor R and a linear capacitor C is disclosed in the foregoing, a MOSFET may replace resistor R1 of charging/discharging circuit 12. In this case, the resultant delay circuit would have the proper source voltage dependent and temperature dependent characteristics.