INTEGRATED LOGIC CIRCUIT COMPRISING AN OUTPUT CIRCUIT FOR GENERATING AN INCREASING OUTPUT CURRENT LIMITED IN TIME

Abstract: In integrated logic circuits the problem presents itself that the rapid variation of the (dis-) charging of a data output causes a reverse voltage pulse VL to occur across the inductance formed by the connection wires. The reverse voltage is limited by causing the charge or discharge current Id (for the load capacitances present) to increase linearly to a maximally permissible value. This is done by driving the output field effect transistor TL with a control voltage VC which varies with time according to a square root characteristic.

Description: The invention relates to an integrated logic circuit having an output circuit connected between a first and a second supply terminal, in which output circuit the conductivity channel of an insulated gate output field effect transistor connects a data output to one of the two supply terminals, the logic circuit comprising a control circuit for generating a time-dependent control voltage at the control electrode of the output field effect transistor. Such a logic circuit is known from IBM's Technical Disclosure Bulletin, Vol. 27, No. 1A, June 1984, pp . 13 and 14, in which an output circuit is shown in which the control voltage at the control electrode of the output field effect transistor in the first instance increases slowly and, after switching on an extra field effect transistor, increases rapidly to make the output field effect transistor fully conductive in a short period of time. In spite of this provision the circuit arrangement suffers from the disadvantage that in particular during the rapid increase of the control voltage too high inductive reverse voltage peaks may be formed at the supply terminals on the integrated circuit, which peaks might seriously interfere with the correct operation of the circuit. Although the known circuit has for its object to mitigate this disadvantage, if the data output is loaded with a comparatively large capacitance, this will hardly be discharged when the extra field effect transistor is switched on, which causes a high reverse voltage peak to occur across the inductance formed by the connection wires of the integrated circuit. It is possible to switch on the extra field effect transistor at a later instant but this has for its result that the output voltage becomes more inert, which is highly undesirable. It is an object of the invention to provide an integrated logic circuit in which the value of the reverse voltage peak can be limited to a previously chosen value and be independent of the value of the capacitance connected to the data output, the output circuit charging or discharging the data output at a maximum speed. For that purpose an integrated logic circuit according to the invention is characterized in that the control circuit, at least during an initial period after switching on same, generates a control voltage which varies approximately proportionally to the square rout of time. The invention is based on the recognition of the fact that at least during the initial period of controlling the output field effect transistor, which has to charge or discharge the data output, said output field effect transistor is saturated. This involves that its main current in this initial period is independent of the voltage across the conductivity channel and is approximately in a quadratic relationship with the control voltage at the control electrode. Since the value of the inductive reverse voltage peak is proportional to the variation of the current per unit time, an optimum effect is achieved by causing the control voltage at the control electrode to vary approximately root-like, at least during said initial period, so that the main current increases uniformly from an initial value and, during said increase, causes a chosen limited constant inductive reverse voltage to occur. "Root-like" variation of the voltage is in accordance with the n th root of the time and for convenience of description this variation or increase will hereinafter be referred to as "root-like". As noted hereinbefore, the control voltage for the output field effect transistor has a nonlinear increase during an initial period. In present-day conventional field effect transistors the control voltage may also increase, after the said initial period, according to the root-like curve since in said transistors, the aforementioned relationship between the main current, voltage across the conductivity channel and voltage at the control electrode remains also when the voltage at said control electrode increases (channel length at approximately 2 µm). In "narrow" field effect transistors, however, which are now partly under development and (will be) used (channel length 1 to 1.2 µm or smaller) the control voltage after the initial period will not have to increase "root-like" as a function of the time but more rapidly (for example ≈t 3/4 instead of ≈t ½ ) to reach the end in view; a constant increase of the current as a function of time. Dependent on the transistor geometry, the control voltage should increase according to the three-fourths power of time or ultimately even according to a linear function of time. Of course, the variation of the control voltage as a function of time which thus a) always follows the "root-like" curve or b) increases "root-like" only in the initial period and then more rapidly ( ≈t 3/4 or linearly :t) has consequences for the components for generating said control voltage. The above last case is necessary only with narrow transistors in which the electric field strength E becomes higher than IV/µm. By a correct proportioning and choice of the electric components it is now possible to limit the reverse voltage, for which it should be noted that the value of the capacitance present at the data output is irrelevant. Besides, said method of "root-like" driving of the output transistor can provide the fastest possible charge or discharge of the data output with a permitted (constant) reverse voltage. An integrated logic circuit according to the invention may be further characterized in that the control circuit comprises a capacitive load which is present between the control electrode and the supply terminals and a charge source which is connected between the control electrode and one of the two supply terminals. The charge source may charge or discharge the capacitive load for generating the root-like control voltage. The capacitive load may consist only of the internal capacitance of the control electrode of the output field effect transistor, but it is possible to add an extra capacitance so that the value of the capacitive load is less dependent on the spreads in the process of manufacturing the output field effect transistor. Besides, the capacitive load may be chosen so that the associated charge source can be realised in a simple manner in that it comprises only readily reproducible components. The charge source may comprise a current path through a series arrangement of the conductivity channels of a first and a second insulated gate field effect transistor, the control electrode of the first and the conductivity channel of the second field effect transistor being connected to the control electrode of the output field effect transistor and the conductivity channel of the first field effect transistor being connected to one of the two supply terminals, the control electrode of the second field effect transistor being connected to an energisation input for receiving an energisation signal for the output field effect transistor. After receiving the energisation signal, the said current path through a small number of components can provide a good approach to the optimum current versus voltage characteristic for the charge source. In the absence of the energisation signal the second field effect transistor which is then cut off prevents a further charging or discharging of the said control electrode. The charge source may furthermore comprise a third insulated gate field effect transistor the control electrode of which is connected to the energisation input and the conductivity channel of which is connected in parallel with the series arrangement of the first and the second field effect transistors. The addition of the third field effect transistor provides a better approch to the optimum voltage-current characteristic for the charge source and has the important advantage that the control electrode of the output transistor can be driven to the full supply voltage. The charge source may comprise a plurality of parallel-arranged current paths which have mutually different switch-off voltages. The use of said current paths can provide a close approach to the desired voltage-current characteristic for the charge source. If the output field effect transistor is of the n -type channel the capacitive load may comprise a p -channel insulated gate field effect transistor, which field effect transistor has its main electrodes connected to the supply terminal for receiving the more positive of the two supply voltages and its control electrode connected to the control electrode of the output field effect transistor. All the charge sources are liable to generate too little charge current when the control voltage reaches the level of the most positive supply voltage. This effect can be masked by using the above-mentioned p -channel transistor since the conductivity channel of said transistor which is connected as a capacitor disappears as soon as the difference between the supply voltage and the control voltage reaches the threshold voltage of this transistor and as a result of this its capacitance strongly decreases. As a result of this, relatively more current becomes available for charging the control electrode of the output field effect transistor so that the decrease of the charge current generated by the charge source is compensated for. This measure may also be applied with a p-channel output field effect transistor. The invention will now be described in greater detail with reference to the accompanying figures, in which Fig. 1 shows an equivalent circuit diagram of an integrated circuit according to the invention upon discharging a data output, Figs. 2 a to 2 d show a number of time-voltage diagrams, Fig. 3 shows an equivalent circuit diagram of an integrated circuit according to the invention upon charging a data output, Fig. 4 shows a number of voltage-current diagrams for the charge source Fig. 5 shows a possible construction of the charge source, Fig. 6 shows another possible construction of the charge source, Fig. 7 shows a further possible construction of the charge source, and Fig. 8 shows an integrated logic circuit having a preferred form of driving circuit. All the transistors shown in figs. 1 and 3 are of the N-channel type. Fig. 1 shows an equivalent circuit diagram of an output circuit of an integrated circuit according to the invention upon discharging a data output, in which diagram only the significant components are shown. The integrated circuit 2 is provided with a data output 14, and a first and a second supply terminal VDDI and VSSI which are connected via respective inductive connection wires L1 and L2, to the first and second external supply voltages VDDE and VSSE, respectively. The data output 14 is connected, via a capacitor CL1, to the second external supply voltage VSSE to symbolise the capacitive output load. The output circuit furthermore comprises an internal junction 16, which is connected, via a charge source S1 , to the first supply terminal VDDI and, via a capacitor CT1, to the second supply terminal VSSI. The capacitor CT1 symbolises the capacitive load at the control electrode of the output field effect transistor TL also connected to the junction 16, which load consists of the sum of the internal capacitance of the control electrode and the capacitances between the control electrode and the two supply terminals VDDI and VSSI. The conductivity channel of the output field effect transistor TL connects the data output 14 to the second supply terminal VSSI. The charge source S1 generates the control voltage VC by charging the capacitor CT1, so that the output field effect transistor TL is conductive and the capacitor CL1 is discharged. The control circuit may comprise an extra transistor T12, which, via its conductivity channel, connects the junction 16 to the second supply terminal VSSI and is connected at its control electrode to an energisation input 18 for receiving an energisation signal. With a high energisation signal at input 18 the transistor T12 will be conductive so that the junction 16 obtains a low voltage and cuts off the output transistor TL. With a low energisation signal at input 18 the transistor T12 is cut off so that the junction 16 can be charged by the source S1 and the output transistor TL passes to the conductive state. The energisation signal can also be applied to the charge source S1 (which application is indicated by a broken line) so as to switch off the source S1 when the energisation signal is high, which results in a reduced power dissipation. The conductivity of the transistor T12 is preferably chosen to be small so as to prevent too abrupt a switch off of the output transistor TL. Figs. 2 a to 2 d show a number of time-voltage diagrams of a switching output circuit. Fig. 2 a shows the root-like increasing control voltage VC. Fig. 2 b shows the discharge current I d identified in fig. 1. Fig. 2c shows the voltage Vo at the data output 14 and fig. 2 d shows the inductive reverse voltage V1 across the connection wire L2 in fig. 1. The time origin to of all four figures has been chosen at the instant the charge source S1 begins to charge the junction 16. With the voltage increasing root-like at the junction 16 the discharge current Id through the output field effect transistor TL increases linearly since the transistor TL is saturated and the current ID through its conductivity channel is associated quadratically with the control voltage VC. With a linear increase of the discharge current Id the capacitor CL1 will be discharged according to a quadratic characteristic. The reverse voltage VL across the connection wire L2 is proportional to the rate of increase of the discharge current Id and will be constant since the discharge current Id increases linearly. The discharge current Id increases to its final value which is achieved at the instant t1. After the instant t1 the capacitor CL is discharged with a constant current Id, the voltage Vo at the data output decreasing linearly and the reverse voltage VL disappearing since the discharge current is substantially constant. The discharge current Id remains constant until the capacitor CL1 has been discharged to such an extent that the output transistor TL reaches its linear range. The discharge current Id will then decrease approximately according to an exponential-like function (not shown in figs. 2 a to 2 d ). It will be obvious that this output circuit, starting from given values of the inductance of the connection wire L2 and a specified maximum value for the reverse voltage VL, permits of obtaining as fast as possible a discharge of the capacitor CL1. Besides, the value of the capacitor CL1 proves to have no influence on the value of the reverse voltage VL. A higher value of the capacitor CL1 only results in the voltage Vo at the data output 14 decreasing more slowly. Fig. 3 shows an equivalent circuit diagram of an output circuit of an integrated circuit according to the invention uponcharging the data output 14. Those components which are shown in the diagram which are identical to those of fig. 1 are denoted by the same references. The output field effect transistor TH connects the first supply terminal VDDI to the data output 14. The control electrode of the output transistor TH and a charge source S3 which is connected at its other end to the internal supply terminal VDDI are connected to the junction 36. The capacitor CT3 and transistor T32 shown have the same functions as the capacitor CT1 and transistor T12 of fig. 1. The control electrode of transistor T32 is connected to a data input 38 which receives the inverted energisation signal. Of course the inverted energisation signal may be used to switch on and off the charge source S3 , the energisation signal switching on the charge source S3 if the inverted energisation signal is low and hence the transistor T32 is cut off. The diagrams shown in fig. 2 also apply to the circuit diagram shown in fig. 3, for which it should be noted that the reverse voltage VL shown in fig. 2 d then represents the reverse voltage VH across the connection wire L1 instead of L2 and that naturally the output voltage Vo increases under the influence of the charge current Id from a low to a high voltage instead of decreasing from a high to a low voltage as is shown in fig. 2 c . The remarks made with reference to the first two figures also apply to the diagram shown in fig. 3. It should be added, however, that the formation of the reverse voltage VH in fig. 3 can also be counteracted by the negative feedback reaction of the voltage Vo at the data output 14 on the output transistor TH. However, this voltage Vo depends on the value of the capacitor CL1 and the negative feedback reaction is weaker as CL1 is larger. It is possible to direct the proportioning of the output circuit to the largest value for the capacitor CL1 falling within the specifications, but it is also possible in designing the intergrated circuit to start from the fact that the data output 14 in practice can be loaded with an arbitrarily large capacitor CL. If it is so loaded the negative feedback reaction is so weak during the growth of the charge current Id that it may be neglected and the proportioning of the output circuit must be directed to a design which assumes that the data output 14 is short-circuited to the second supply voltage VSSE. Fig. 4 shows a number of possible voltage-current characteristics for the charge source S . The curve indicated by OPT denotes the theoretically optimum voltage-current curve for the charge source S in combination with the capacitance CT1 or CT3. This graph is calculated by means of the following equations: 1. with: VL the target value for the inductive reverse voltage, L the inductance of the connection wire in question, Id the charge discharge current through the output field effect transistor; 2) with: IS the current generated by the current source S , CT the capacitive load at junction 16 or 36, VC the control voltage at junction 16 or 36; and 3) with: K a conductivity constant of the output field effect transistor and Vt the threshold voltage of the output field effect transistor. Differentiation of equation 3) gives 4) Combination of the expressions 1, 2 and 4 gives the following expression for the current IS supplied by the charge source S as a function of the control voltage at junction 16 or 36: 5) which indicates the threoretically optimum curve OPT in fig. 4. The curve OPT can be approached by using the source S5 shown in fig. 5. The source S5 shown in fig. 5 comprises a current path comprising a series arrangement of the conductivity channels of a first and a second transistor T4 and T6 of the P channel type, the control electrode of the first transistor T4 and the conductivity channel of the second transistor T6 being connected to the internal junction 16 or 36. The conductivity channel of the first transistor T4 is connected to the first supply terminal VDDI and the control electrode of the second transistor T6 receives the energisation signal at the energisation. input 18 or 38. The second transistor T6 serves as a switch so that the charge source S5 only generates a current IS when the output transistor is energised. The current-voltage characteristic associated with said source S5 is shown in fig. 4 by curve S5. Its approach to the curve OPT for small values of the control voltage VC may be said to be good but the current IS is essentially too low for higher values of the control voltage VC. The control voltage VC, moreover, will never become higher than the voltage at the first supply terminal VDDI minus the threshold voltage of the first transistor T4, so that the output field effect transistor can never receive the full voltage at the first supply terminal VDDI at its control electrode. The current source S6 shown in fig. 6 mitigates the above disadvantage in that a third transistor T8 of the P channel type, the control electrode of which is connected to the energisation input 18 or 38, is connected in parallel with the current path. The voltage-current characteristic associated with said current source S6 is indicated in the fig. 4 graph by S6. Its approach to curve OPT for high values of the control voltage VC is already much better. The best approach to the curve OPT is obtained by a charge source having different current paths in parallel, the current paths having mutually different switching off voltages. Fig. 7 shows an example of such a source S7 in which, starting from the source S6 of fig. 6, a further current has been added which comprises a series arrangement of the conductivity channels of a fourth transistor D10 (connected as diode) and a fifth transistor T10, both of the P channel type, The conductivity channel of the fourth transistor D10 connected as a diode in the forward direction is connected to the first supply terminal. The control electrode of the fifth transistor T10 is connected to the internal junction 16 or 36 and its conductivity channel is connected to a junction 10 between the first and the second transistor T4 and T6. The first current path containing transistor T4 has a switch-off voltage of one threshold voltage of a P channel transistor, while the second current path containing the transistors T10 and D10 has a switch-off voltage of two such threshold voltages. In this manner the source S7 , can, for the lower control voltages VC, generate an extra current IS by means of the said second current path. The curve denoted by S7 in the voltage-current characteristics of fig. 4 then gives the best approach to the curve OPT. It will be obvious that in this manner any desired approach to the curve OPT can be obtained, albeit at the expense of an increase in the complexity of the charge source used. Fig. 8 shows the integrated circuit 2 with a source S6 and an output field effect transistor TL for discharging the data output 14. The driving circuit furthermore comprises a transistor T14 of the P channel type connected as a capacitor which is connected by means of its main electrodes to the first supply terminal VDDI and by means of its control electrode to the internal junction 16. The charge sources S5 , S6 and S7 the characteristics of which are shown in fig. 4 generate too low a current IS when the control voltage VC approaches the voltage at the first supply terminal VDDI. This undesired effect can be compensated for by the transistor T14 since its capacitance decreases strongly when the control voltage Vc becomes higher than one threshold voltage below the voltage at the first supply terminal VDDI. The capacitive load at the junction 16 hence decreases, as a result of which the "root-like" charging of the junction 16 can be done in spite of the lower current IS It will be obvious that the invention can also be applied to output field effect transistors of the P channel type in which case the transistors in the charge source S and the transistor T14 connected as a capacitor should be of the N-type and be connected to the second supply terminal VSSI. The transistor T12 or T32 must then be chosen to be of the P-channel type and should connect the junction 16 or 36 to the first supply terminal VDDI. From this it is apparent that the output circuit may either comprise output field effect transistors which are both of the P type or the N type or it may comprise a combination of both types, as is usual in the so-called CMOS circuits.

Claims: An integrated logic circuit provided with a first and a second supply terminal for receiving a supply voltage and at least one data output, said logic circuit having an output circuit in which the conductivity channel of an insulated gate output field effect transistor connects the said data output to one of the two supply terminals, the output circuit furthermore comprising a control circuit for generating a time-dependent control voltage at the control electrode of the field effect transistor, characterized in that the control circuit, at least during an initial period after switching on same generates a control voltage which varies at least approximately proportionally to the square root of time. An integrated logic circuit as claimed in Claim 1, characterized in that the control circuit comprises a capacitive load which is present between the control electrode and the supply terminals and a charge source which is connected between the control electrode and one of the two supply terminals. An integrated logic circuit as claimed in Claim 2, characterized in that the charge source comprises a current path through a series arrangement of the conductivity channels of a first and a second insulated gate field effect transistor, the control electrode of the first and the conductivity channel of the second field effect transistor being connected to the control electrode of the output field effect transistor and the conductivity channel of the first field effect transistor being connected to one of the two supply terminals, the control electrode of the second field effect transistor being connected to an energisation input for receiving an energisation signal for the output field effect transistor. An integrated logic circuit as claimed in Claim 3, characterized in that the charge sourcecomprises a third insulated gate field effect transistor the control electrode of which is connected to the energisation input and the conductivity channel of which is connected in parallel with the series arrangement of the first and the second field effect transistor. An integrated logic circuit as claimed in Claim 3 or 4, characterized in that the charge source comprises a plurality of parallel-arranged current paths which have mutually different switch-off voltages. An integrated logic circuit as claimed in Claim 3, 4 or 5, characterized in that the output field effect transistor is of the n -channel type and the field effect transistors in the charge source are of the P-channel type, the charge sourcebeing connected to the supply terminal for receiving the more positive of the two supply voltages. An integrated logic circuit as claimed in Claim 6, characterized in that the capacitive load comprises a P-channel insulated gate field effect transistor, which field effect transistor has its main electrodes connected to the supply terminal for receiving the more positive of the two supply voltages and its control electrode connected to the control electrode of the output field effect transistor. An integrated logic circuit as claimed in Claim 3, 4 or 5, characterized in that the output field effect transistor is of the P-channel type and the field effect transistors in the charge source are of the N-channel type, the charge source being connected to the supply terminal for receiving the more negative of the two supply voltages. An integrated logic circuit as claimed in Claim 8, characterized in that the capacitive load comprises an N channel insulated gate field effect transistor its main electrodes connected to the supply terminal for receiving the more negative of the two supply voltages and its control electrode connected to the control electrode of the output field effect transistor.

Inventors: HARTGRING, CORNELIS, KOOMEN, JOANNES, O'CONNNELL, CORMAC MICHAEL, SALTERS, ROELOF
Applicants: N.V. PHILIPS' GLOEILAMPENFABRIEKEN
Application year: 1987
Main classification: H03K19
IPC codes: H03K19, H03K17, H03K5