Abstract:
The present invention discloses a delay circuit which obtains constant a delay time of delay circuit using an output capacitor by making the resistance of MOS transistor lowest, at the low voltage, middle at the intermediate voltage, and largest at the high voltage, so that the delay time of delay circuit using an output capacitor is kept constant regardless of the change in power source voltage.

Description:
BACKGROUND OF INVENTION 
     1. Field of the Invention 
     The present invention relates to a delay circuit and, in particular, to a delay circuit which can maintain a constant delay time regardless of a change in power source voltage. 
     2. Information Disclosure Statement 
     In general, a current driving ability of a MOS transistor used in the delay circuit varies with the change in the power Source voltage. Therefore, a conventional delay circuit in which a capacitor is connected to an output has a great disadvantage in that the change in the delay is large according to the change in the power source voltage. 
     SUMMARY OF THE INVENTION 
     Therefore, an object of the invention is to provide a delay circuit which can solve the above described disadvantage by making a resistance of the MOS transistor small at low voltage(4≦Vcc&lt;5), medium at medium voltage(5≦Vcc&lt;6), and large at high voltage(Vcc≧6). 
     A delay circuit according to the present invention to accomplish the above described object comprises: 
     a pull-up transistor which is connected between a voltage source and an output terminal, and to a gate electrode of which an input signal is supplied: 
     a voltage detection circuit; 
     a plurality of transistors which are connected in parallel between the output terminal and a node, wherein the transistors have variable resistance according to output signals of the voltage detection circuit; 
     a pull-down transistor which is connected between the node and a ground, and to a gate electrode of which the input signal is supplied; and 
     a capacitor connected between the output terminal and the ground. 
     According to the present invention, the delay time of the delay circuit using the MOS transistor and the output capacitor can be kept constant regardless of the change in the power source voltage. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For fuller understanding of the nature and object the invention, reference should be had to the following detailed description taken in conjunction with the accompanying drawings in which: 
     FIGS. 1A to 1C are detailed circuit diagram of a voltage detection circuit for driving a delay circuit according to the present invention; 
     FIG. 2 shows an equivalent circuit diagram of FIG. 1A; and 
     FIG. 3 shows the delay circuit according to the present invention. 
     Similar reference characters refer to similar parts in the several views of the drawings. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention will be described in detail with reference to the accompanying drawings. 
     FIGS. 1A to 1C are detailed circuit diagrams of a voltage detection circuit for driving a delay circuit according to the present invention. 
     First, the operation of the voltage detection circuit shown in FIG. 1A will be described with reference to FIG. 2. 
     PMOS transistors PA1, PA2 and PA3 are connected between a power source Vcc and a ground terminal Vss. The PMOS transistors PA1, PA2 and PA3 can be considered as resistors R1, R2 and R3 shown in FIG. 2. Therefore, the output voltage K1 can be presented by following equation ##EQU1## 
     If R3&gt;&gt;R1(=R2), then K1=Vcc. The output voltage K1 become same as the power source voltage. 
     The output voltage K1 has same inclination as that of the power source voltage Vcc supplied to the source of PMOS transisitor PA4. If PMOS transistors PA5, PA6 and PA7 have the same dimension (Width/length) and threshold voltage(Vt=0.9 V), they will turn on when the power source voltage Vcc is larger than or equal to, for example, 3.0 V. A first reference voltage VREF1 can be presented by following equation. ##EQU2## 
     That is, if Vcc is 3.0 , the first reference voltage VREF1 becomes Vcc, while if Vcc&gt;3.0 V, then the inclination of the first reference voltage VREF 1 becomes (&#34;0&#34;. That is, if the power source voltage is larger than or equal to 3.0 V, the first reference voltage VREFl maintains to be 3.0 V. 
     When VREF 1 is larger than or equal to 3.0 V, that is, when the power source voltage is larger then or equal to 4.0 V, a transistor PA8 turns on so that a node INA maintains to be the power source voltage Vcc. Otherwise, the node INA maintains 0 volt. 
     The voltage of the node INA is supplied to an output terminal a through two inverters IA1 and IA2. 
     Next, the operation of FIG. 1B will be described. 
     PMCOS transistors PB1, PB2 and PB3 are connected between a power source Vcc and a ground terminal Vss. The PMOS transitors PB1, PB2 and PB3 can be considered as resistors R1, R2 and R3. Therefore, the output voltage K1 can be presented by following equation. ##EQU3## 
     If R3&gt;&gt;R1(=R2), then K1=Vcc. The output voltage K1 become same as the power source voltage. 
     The output K1 has same inclination as that of the power source voltage Vcc supplied to the source of PMOS transistor PB4. If PMOS transistors PB5, PB6, PB7 and PB8 have the same dimension (Width/length) and threshold voltage(Vt=0.9 V), they will turn on when the power source voltage Vcc is larger than or equal to, for example, 4.0 V. A second reference voltage VREF2 can be presented by following equation. ##EQU4## 
     If R4&gt;&gt;R5(=R6=R7=R8), then ##EQU5## 
     That is, if Vcc is 4.0 V, the second refeencce voltage VREF2 becomes Vcc, while if Vcc&gt;4.0 V, then the inclination of the second reference voltage VREF2 becomes &#34;0&#34;. That is, if the power source voltage is larger than or equal to 4.0 V, the second reference voltage VREF2 maintains to be 4.0 V. 
     When VREF2 is larger than or equal to 4.0 V, that is, when the power source voltage is larger then or equal to 5.0 V, a transistor PB9 turns on so that a node INB maintains to be the power source voltage Vcc. Otherwise, the node INB maintains 0 volt. 
     The voltage of the node INB is supplied to the output terminal B through two inverters IB1 and IB2. 
     Next, the operation of FIG. 1C will be described. 
     PMOS transistors PC1, PC2 and PC3 are connected between a power source Vcc and a ground terminal Vss. The PMOS transistors PC1, PC2 and PC3 can be considered as resistors R1, R2 and R3. Therefore the output voltage K1 can be presented by following equation. ##EQU6## 
     If R3&gt;&gt;R1 (=R2), then K1=Vcc. The output voltage K1 becomes same as the power source voltage. 
     The output K1 has same inclination as that of the power source, voltage Vcc supplied to the source of PMOS transistor PC4. If PMOS transistors PC5, PC6, PC7, PC8 and PC9 have, the same dimension (width/length) and threshold voltage(Vt=0.9 V), they will turn on when the power source voltage Vcc is larger than or equal to, for example, 5.0 V. A third reference voltage VREF3 can be presented by following equation. ##EQU7## 
     If R4&gt;&gt;R5(=R6=R7=R8=R9) then ##EQU8## 
     That is, if Vcc is 5.0 V, the third reference voltage VREF3 becomes Vcc, while if Vcc&gt;5.0 V, then the inclination of the first reference voltage VREF3 becomes &#34;0&#34;. That is, if the power source voltage is larger than or equal to 5.0 V, the third reference voltage VREF3 maintains to be 5. 0 V. 
     When VREF3 is larger than or equal to 5.0 V. that is, when the power source voltage is larger then or equal to 5.0 V, transistor PC10 turns on so that a node INC maintains to be the power source voltage Vcc. Other wise, the node INC maintains 0 volt. 
     The voltage of the node INC is supplied to an output terminal C through two inverters IC1 and IC2. 
     FIG. 3 shows a circuit diagram according to the present invention. 
     A pull up transistor PU1 is connected between a power source Vcc and an output terminal OUT. NMOS transistors ND1, ND2, ND3 are connected in parallel between the output terminal OUT and a node K1. A NMOS transistor ND4 is connected between the node K1 and a ground Vss. An input signal IN is supplied to gate electrodes of the pull-up transistor PU1 and transistor ND4. A capacitor C1 is connected between the output terminal OUT and ground Vss. The signal of the output terminal A of FIG. 1 is input to the gate electrode of the transistor ND3 via two inverters ID1, ID2. The signal of the output terminals A,B of FIGS. 1 and 2, respectively, are input to a NOR gate ID4. An output signals of the NOR gate ID4 is input to the gate electrode of the transistor ND2 via an inverter ID3. The signals of the output terminals A, B, C of FIGS. 1, 2 and 3, respectively are input to a NOR gate ID6. The output signal of the NOR gate ID6 is input, to the gate electrode of a transistor ND1 via an inverter ID5. 
     If the power source voltage Vcc of the delay circuit is 4≦Vcc&lt;5, all of the first through third enable signals EN1, EN2, and EN3 are in condition of the power source voltage Vcc. Therefore, since NMOS transistors ND1, ND2, ND3 are connected in parallel, they have low resistance value. If the power Source voltage Vcc is 5≦Vcc&lt;6, then the first enable signal EN1, becomes O V, and the second and third enable signals EN2, EN3 become Vcc so that NMOS transistor ND3 is turned off while NMOS transistors ND1, ND2 are turned on so as to have intermediate parallel resistance value. If the power source voltage Vcc is 6≦Vcc&lt;7, the first and second enable signal EN1, EN2 become O V, and the third enable signal EN3 becomes Vcc so as to have only the resistance value caused by NMOS transistor ND1. 
     As described above, the present invention has an excellent effect of adjusting the delay time of the delay circuit using the output capacitor by adjusting the resistance value, by adjusting the size of NMOS transistor driven according to the power source voltage.