VOLTAGE RESPONSIVE CIRCUIT

Abstract: Voltage responsive circuits particularly for detecting MOS integrated circuit input voltages greater than V cc . Various circuits are proposed each of which employ an MOS transistor (Tr1) connected in series with a capacitor (C1) and arranged between the V cc rail and an output terminal (T1). The input voltage (V in ) is applied to the gate of the MOS transistor (Tr1). The signal and/or impedance available at the output terminal (T1) varies in dependence on whether the input voltage is high or lower than V cc . An a.c. signal input may be applied to lower plate of the capacitor (C1) via a resistor (R1 - Fig. 3) and an a.c. signal detector determines the magnitude of the signal at the output, large correspond­ ing to V in < V cc + V T and small corresponding to V in > V cc + V T (V T is the threshold voltage of Tr1). Alternatively a charging current impedance detector (Figs. 4 and 5) or a clocked circuit (Figs. 6 and 7) may be employed.

Description: VOLTAGE RESPONSIVE CIRCUIT This invention relates to voltage responsive circuits and is particularly, but not exclusively, applicable to overvoltage detector circuits for integrated circuits. According to the present invention there is provided a voltage responsive circuit including a first capacitor one terminal of which is connected to a first circuit terminal via the conductive path of a first field-effect transistor, and the other terminal of which first capacitor is connected to a second circuit terminal, wherein in use an input voltage to be compared with a voltage applied to the first circuit terminal is applied to the gate of the first transistor, the signal and/or impedance available at the second circuit terminal varying in dependence on whether the input voltage is higher or lower than the voltage applied to the first circuit terminal. In one arrangement an a.c. signal is applied to the output terminal and a detector employed to determine whether the output terminal impedance is high or low; low corresponding to the input voltage being greater than the supply voltage plus the threshold voltage of the transistor and high corresponding to the input voltage being lower than the supply voltage plus the threshold voltage. A resistive impedance detector may be employed. In another arrangement capacitor charging current impedance detection, with two non-overlapping clock signals, is employed. In a further arrangement, which has a faster response time than the two above-referred-to arrangements, three clock signals are employed. Embodiments of the invention will now be described with reference to the accompanying drawings, in which: Fig. 1 shows a basic circuit diagram; Fig. 2 shows voltage time variations of the circuit of Fig. 1; Fig. 3 shows a version of the basic circuit of Fig. 1 which uses a resistive impedance detector; Fig. 4 shows a version of the basic circuit of Fig. 1 which uses a charging current impedance detector; Fig. 5 shows the clock signals required for the Fig. 4 circuit; Fig. 6 shows a clocked circuit version of the basic circuit of Fig. 1, and Fig. 7 shows the clock signals and output for the Fig. 6 circuit. A second function (upper case) facility on integrated circuit inputs is sometimes desirable, particularly at testing. For example, it may be required to detect when input voltages are greater than the positive supply rail (Vcc) and thus outside normal specified operating conditions. The basic circuit shown in Fig. 1 comprises an NMOS detector for detecting inputs V. greater than the in positive supply rail Vcc. The circuit comprises a transistor-capacitor combination. An NMOS transistor Trl has its drain D connected to the Vcc s i ply rail, and its source S connected to one plate, hereinafter referred to as the top plate, of a capacitor C1 whose other plate is connected to a terminal T1 to which an a.c. signal is applied. The input voltage Vin to be compared with Vcc is applied to the gate G of transistor Trl. The capacitor C1 is designed to be of low impedance at the frequency of the a.c. signal. If Vin < Vcc + VT' where VT is the threshold voltage of transistor Trl, the transistor will only conduct in the source-follower mode. The capacitor C1 will charge until the top plate is at a minimum voltage of Vi, VT, see Fig. 2 in which the dashed line represents the applied a.c. signal and the solid line the top plate voltage. Once charged no further current can flow through the capacitor as the transistor gate voltage (Vin) is insufficient to turn the transistor Trl on. Terminal T1 then appears high impedance to the a.c. signal. If, however, Vin > Vcc + VT' then the transistor Trl has enough gate drive to conduct for any value of VDS. The capacitor receives charge current, and supplies discharge current, through the transistor. Terminal T1 is then low impedance to the a.c. signal. The actual impedance is governed by the series combination of capacitor and channel conduction at a given Vin. Thus, if some means of a.c. impedance detection is applied to terminal T1, then a > Vcc + VT detector is effected. Using an enhancement mode transistor allows this circuit to detect input signals greater than V cc Fig. 3 illustrates the circuit of an embodiment using a resistive impedance detector. The a.c. signal, having a frequency f, is applied to the bottom plate of capacitor C1 via a resistor R1. 1C2 represents stray circuit capacitances associated with the bottom plate of capacitor C1." For values of Vin < Vcc + VT' the transistor Trl is high impedance. The attenuation of the a.c. signal is determined by WR. by #R.Cstray, where N = 2sf, R is the resistance value of resistor R1 and Astray is the capacitance value of a capacitor C2, and for negligible attenuation R is chosen to be 1/#Cstray). For values of Vin > Vcc T the transistor Trl conducts. If the capacitance value C of capacitor C1 is chosen such that C.) l/(R.#), then the attenuation of the a.c. signal is controlled by the ratio of R to the effective channel resistance of the transistor. The transistor dimensions are thus chosen so that for an appropriate V. > Vcc + VT there is in cc T a large signal attenuation, for example greater than 20dB. Thus if the magnitude of the signal applied to the a.c. signal detector is large, V. < V + V in cc T whereas if the magnitude is small, Vin > Vcc + VT Almost any input waveform with a large component at the fundamental frequency, for example sine, square, triangular, may be used with the circuit of Fig. 3. Fig. 4 illustrates the circuit of an embodiment using a charging current impedance detector. The circuit comprises four enhancement mode NMOS transistors Q1, Q2, Q3 and Q4, two capacitors C3 and C4 and a current source I. The circuit employs non-overlapping clock signals +1 and 42 having a repetition frequency f. Ql to Q4 are assumed to have identical threshold voltages, VT, in this example, although this is not necessary for correct circuit operation. If V. < Vcc initially, then C3 is charged to Vcc V VTr via Q1/Q2, Q4 and I during period P2 of successive clock cycles. This is assumed as the starting condition in the following analysis. If V. < Vcc + VTr during period P1 the top plate of C3 is floating; the bottom plate of C3 is held at Vcc - VT by Q3, thus the top plate is at 2Vcc 2VT; and Q1 is off so that C3 is not discharged. During period P2, C3 is still charged to Vcc - VT SO that no current flows through Q1/Q2, Q4 and I. Hence the output Vout = 0. If, however, Vin > Vcc + VT then during period P1 transistor Q1 will conduct and partially discharge C3, whereupon during period P2 a current, greater than I, now flows through Ql/Q2, Q4, I/C4 to recharge C3. This partially charges C4. So Vout > O. With suitable choice of f, I and the capacitance values C3 and C4 of capacitors C3 and C4, then capacitor C4 will pump up to give a steady state output voltage. Assuming ideal components and that capacitor C4 does not fully discharge via I between pumps, charge analysis of the circuit yields V' out = Vcc - VT - I/(f.C3) as the steady state output voltage, where Vt out is out at the instant when +2 goes low. Capacitor C4 controls the amount of ripple on the output and the response time of the detector. Further analysis yields the relationship n.I = f. (V'out.(C3+C4)-VT.C3) where n is the number of clock cycles taken for V out to recover from V'out to 0 volts when V. changes to in < Vcc + VT. A value of n = 10 gives an acceptable compromise between ripple and response time for many applications. In the embodiment of Fig. 4, therefore, an input voltage greater than Vcc + VT results in the existence of an output voltage (VOUt > 0) and when Vin < Vcc + vT there is no output voltage in cc T (Vout = 0). Fig. 6 shows a clocked circuit for detecting input levels > Vcc + VT. It comprises six NMOS transistors Q5, Q6, Q7, Q8, Q9 and Q10 and a capacitor C5. Transistor Q6 is a depletion mode device whereas Q5 and Q7 to Q10 are enhancement mode devices. All the enhancement thresholds are here assumed to be equal to the value VT, although this is not necessary for correct circuit operation. The three clock signals #1', #2', #3' employed in the circuit of Fig. 6 all have logic levels of either 0 volts or Vcc, and timing as shown in Fig. 7. Referring to Fig. 7, within each clock cycle, the timing relationships generate three distinct periods, P'l, P'2, P'3, corresponding to precharge, sensing, and output circuit functions respectively. Considering the circuit operation in more detail: during precharge period P'1, #1' and #3' 'are 3 high so C5 is charged to Vcc via Q6 and Q8. k and go gogo low at the end of period P'1 before #2' goes high to initiate the sensing period P'2. Q7 now pulls the bottom plate of C5 to Vcc - VTr thus bootstrapping the top plate to 2Vcc - VT If Vin < V cc + VT, Q5 does not conduct and so C5 retains its charge. If Vin > Vcc + VT then CS is partially discharged through Q5. The sensing period is terminated when #2' goes low. 43' now goes high to start the 2 3 output period P'3. The top plate of C5 is clamped to Vcc by Q6. If C5 was not discharged during P'2, the bottom plate is at 0 volts. Otherwise the bottom plate is at > 0 volts. Transistors Q9 and Q10 gate the bottom plate voltage to the output. So if V. < V + VT during period P'2 the in cc T output is 0 volts, whereas if Vin, Vcc + VT during P'2 the output is greater than 0 volts. With idealised components and operating conditions C5 will be discharged to VT during period P'2, so that the output voltage in period P'3 is Vcc - VT. The output pulse may be gated by a suitable clock into a latch or sample and hold circuit to provide a continuously available indication of the input state. The circuit of Fig. 6 has a very fast response time to a change in input level in comparison to the circuits of Figs. 3 and 4. With the clocked circuit of Fig. 6, the response time is controlled by the repetition period of the clocks. New data is available each cycle. The circuits of Figs. 3 and 4 use a data averaging technique and so require many cycles to produce a valid output signal. It will be apparent that although all examples and descriptions given refer to NMOS circuits, the principles involved are easily applied to circuits fabricated in other MOS processes.

Claims: CLAIMS: 1. A voltage responsive circuit including a first capacitor one terminal of which is connected to a first circuit terminal via the conductive path of a first field-effect transistor, and the other terminal of which first capacitor is connected to a second circuit terminal, characterised in that in use an input voltage to be compared with a voltage applied to the first circuit terminal is applied to the gate of the first transistor, the signal and/or impedance available at the second circuit terminal varying in dependence on whether the input voltage is higher or lower than the voltage applied to the first circuit terminal. 2. A circuit as claimed in claim 1, characterised in that the first transistor is an enhancement mode MOS field effect transistor whose drain is connected to the first circuit terminal. 3. A circuit as claimed in claim 2, characterised in that in use an a.c. signal is applied to the other terminal of the capacitor, the circuit including detector means for determining whether the second circuit terminal impedance is high or low, low corresponding to the input voltage being greater than the voltage applied to the first circuit terminal plus the threshold voltage of the first transistor and high corresponding to the input voltage being lower than the voltage applied to the first circuit terminal plus the threshold voltage of the first transistor. 4. A circuit as claimed in claim 3, characterised in that the detector means is comprised by a resistive impedance detector. 5. A circuit as claimed in claim 4, characterised in that the a.c. signal is applied to the other terminal of the first capacitor via a resistor, wherein the second circuit terminal is earthed, via a second capacitor, and an a.c. signal detector is connected to the second circuit terminal, the magnitude of the signal applied to the a.c. signal detector being high for input voltage values less than the voltage applied to the first circuit terminal plus the threshold voltage of the first transistor, and low for input voltage values greater than the voltage applied to the first circuit terminal plus the threshold voltage of the first transistor. 6. A circuit as claimed in claim 2, characterised in that means are provided for detecting the signal available at the second circuit terminal, which detecting means is comprised by a charging current impedance detector. 7. A circuit as claimed in claim 6, characterised in that a second MOS transistor is connected, to conduct in parallel with the first transistor, between the first circuit terminal and the one terminal of the first capacitor, the gate of the second transistor being connected to the first circuit terminal, that a third MOS transistor is connected, to conduct in parallel with the first and second transistors, between the first circuit terminal and the other terminal of the first capacitor and is connected to conduct in series with a fourth MOS transistor connected to a current source, the second circuit terminal being connected between the fourth transistor and the current source, that in use one clock pulse is applied to the gate of the third transistor and another clock pulse is applied to the gate of the fourth transistor, which clock pulses are non-overlapping, that the current source is connected to a voltage supply and a third capacitor is connected in parallel with the current source, that the second, third and fourth transistors are enhancement mode, and that in use of the circuit there is an output voltage at the second circuit terminal for values of the input voltage greater than the voltage applied to the first circuit terminal plus the threshold voltage of the first transistor and there is no output voltage at the second circuit terminal for values of the input voltage less than the voltage applied to the first circuit terminal plus the threshold voltage of the first transistor. 8. A circuit as claimed in claim 2 and employing first, second and third clock signals, characterised in that a fifth MOS transistor is connected, to conduct in parallel with the first transistor, between the first circuit terminal and the one terminal of the capacitor, that the other terminal of the capacitor is connectable to the second circuit terminal via the conductive path of a sixth MOS transistor whose gate is connected to the gate of the fifth transistor and in use of the circuit applied by the first clock signal, that the second circuit terminal is connected to a voltage supply via the conductive path of a seventh MOS transistor, that eighth and ninth MOS transistors are connected in series between the first circuit terminal and the voltage supply, that the other terminal of the first capacitor is connected to a junction between the eighth and ninth transistors, that the second clock signal is applied to the gates of the seventh and eighth transistor, that the third clock signal is applied to the gate of the ninth transistor, the fifth transistor being depletion mode and the seventh, eighth and ninth being enhancement mode, the clock signals being such that the voltage at the second circuit terminal is zero during an output period corresponding to a first clock signal pulse if, during the previous sensing period corresponding to a second clock signal pulse, the input voltage was less than the voltage applied to the first circuit terminal plus the threshold voltage of the first transistor, and is greater than zero during an output period if, during the preceding sensing period, the input voltage was greater than the voltage applied to the first circuit terminal plus the threshold voltage of the first transistor. 9. A circuit as claimed in any one of the preceding claims and embodied in an integrated circuit, the first circuit terminal corresponding to the Vcc rail thereof. 10. A circuit as claimed in claim 9 as appendant to claim 7 or claim S, characterised in that the voltage supply corresponds to the Vss rail of the integrated circuit. 11. A circuit as claimed in any one of the preceding claims, characterised in that the transistors are n-channel MOS transistors.

Inventors: MONK, TREVOR KENNETH
Applicants: STC PLC
Application year: 1984
Main classification: G01R19
IPC codes: H03K5, G01R19, H03K17