A data output circuit

Abstract: When low-level data is output from a data output terminal (T1), a plurality of transistors (12A, 12B), whose source-drain paths are inserted in parallel between the data output terminal (T1) and a power source terminal (T3), begin to conduct at successively different time-points. Thus, the potential fluctuations produced on the ground voltage side (V SS side) and power pource voltage side (V DD ) side by the parasitic resistive and inductive components present on the current are dispersed over time, avoiding circuit component malfunction. Further reduction in noise can be achieved by making the gate drive voltage (N2) of the first transistor (12A) which conducts in a switching cycle when low-level data is output, smaller than that of the remaining transistors in the switching cycle, so that its current when coducting becomes smaller.

Description: Background of the Invention 1. Field of the Invention. This invention generally relates to a data output buffer circuit for a semiconductor integrated circuit, and in particular, it relates to a data output buffer circuit suitable for semiconductor memories wherein high access speed is required. 2. Description of the Prior Art. When data is output from semiconductor integrated circuits such as semiconductor memories, the output load must be charged and discharged at high speed. During this charging and discharging, potential fluctuations, i.e. noise, are generated in the power source voltage and the reference voltaged respectively. When low-level data is output in a typical semiconductor integrated circuit (discharging of output load), the potential fluctuations generated in the reference voltage are at least equal to, and usually greater than, the potential fluctuations generated in the power source voltage when high-level data is output (charging of output load). Such fluctuations or noise during these operations are a factor which may cause a malfunction of the semiconductor integrated circuit. The major part of the potential fluctuations generated in the reference voltage during the sudden discharge of the output load consists of the product L.di/dt where di/dt is the rate of increase with time of the discharge current to the reference voltage and L is the parasitic inductive component on the discharge path. Since the discharge of the output load occurs rapidly, this product may take on large values, increasing the danger of circuit malfunction. Figure 1 is the circuit diagram of a conventional data output circuit for a semiconductor integrated circuit such as a semiconductor memory. In the Figure, the area enclosed by broken lines contains the semiconductor circuit components and the associated output buffer. These semiconductor circuit components may include memory cells, address circuitry, sensing amplifiers, and circuitry for driving the output buffers. It is generally understood that output buffers do not comprise a portion of the internal semiconductor components. This convention will be observed in what follows, i.e., the output buffer is considered distinct from the internal semiconductor circuit components. T1 is a data output terminal. T2 is a power source terminal supplied with power source voltage V DD . T3 is a reference terminal supplied with reference voltage V SS . I/O and I/O are internal data buses. Transistor 11 is a data output buffer for high-level output and transistor 12 is a data output buffer for low-level output. AND gates 13 and 14 control the respective data outputs and comprise circuitry for driving the output buffers. Parasitic resistive components 15 and 16 are present in the power source wirings. D.C. power source 17 feeds power source voltage V DD to the integrated circuit. Capacitance 18 stabilizes D.C. power source 17. Load capacitance 19 is driven by the output data of the integrated circuit. Resistive components 20, 21, 22 and inductive components 23, 24, 25 are present in the respective wirings of the external part of the integrated circuit. When such a data output circuit outputs low-level data, internal data bus I/O becomes level "0", and I/O becomes level "1". Subsequently, the internal control signal  out rises to level "1", causing the output signal of AND gate 14 to become level "1". As a result, transistor 12 for low-level output, whose source-drain path is inserted between data output terminal T1 and reference terminal T3, is turned on and conducts. Thus, load capacitance 19 discharges to level "0" through terminal T1. The waveforms of the various signals in this series of operations are shown by the continuous lines in the waveform plot of Figure 2(a). Signal N in Figure 2(a) is the output signal of AND gate 14. When the load capacitance 19 is discharged, a large discharge current I d is generated through transistor 12, and potential fluctuations commonly called overshoot, as described above, occur on the reference voltage side due to resistive components 20, 16, 22 and inductive components 23, 25 present on the current path. When such fluctuations occur on the ground voltage side (V SS side), similar fluctuations also occur on the power source voltage side (V DD side). These potential fluctuations are shown by the continuous lines in the wave form plot of Figure 2(b). Such fluctuations are particularly severe in an integrated circuit having a plurality of data output terminals when a low-level data is output from all terminals. Thus, the possibility of circuit component malfunction becomes very high. Conventionally, however, as shown in Figure 1, only one low-level output transistor is provided for a single data output terminal. In these circumstances, the only option available for controlling the generation of the overshoot is to greatly restrict the current drive capacity of transistor 12, either by decreasing its channel width W, or by slowing down the rate of increase at its gate drive signal. The fluctuations of the reference and power source potentials in response to these methods are indicated by the broken line in Figure 2(b). However, as shown by the broken lines in Figure 2(a), if the rate of increase of the gate drive signal N of transistor 12 is too slow, the change in the signal waveform of terminal T1 is delayed, impairing the high-speed characteristics of the semiconductor memory. A similar effect occurs if the channel width W of transistor 12 is reduced. Thus the conventional circuit is subject to the drawback that the attempts to prevent malfunction by reducing potential fluctuations in the power source have resulted in increased access time. A partial solution to the problem of supply voltage fluctuation is provided by a buffer circuit described in GB-A-2 177 865 which utilizes two switched output transistors. One of these is switched later than the other because the switching voltage applied to its gate is routed via delay elements. However, the applicants have now found that improved protection against the negative effects of fluctuation are obtained if the maximum switching voltage applied to the gate of the first-switched transistor, at least in an initial period is lower than the corresponding maximum of the other. A buffer circuit according to the preamble portion of the independent claims 1 and 3 is known from document JP-A-61 277 225. Brief Summary of the Invention Accordingly, it is an object of the present invention to provide a data output circuit wherein the potential fluctuations generated in the power source may be reduced without impairing high-speed data access. It is another object of the present invention to provide a data output circuit which is suitable for high speed memory. Thus the present invention provides buffer circuit for a semiconductor integrated circuit, said buffer circuit being arranged to receive a power source voltage via first and second power source terminals, and comprising an input terminal for receiving an input signal from the integrated circuit, and an output terminal for outputting an output signal in response to the input signal, said buffer circuit further comprising: a plurality of transistors having first and second states and being coupled in parallel between said output terminal and said second power source terminal; and transistor switch control means for switching each of said plurality of transistors in sequence during a switching cycle from its first state to its second state, said transistor switch control means including delay means for delaying the switching of at least one of said plurality of transistors to its second state; and transistor voltage control means for at least initially supplying a voltage to the control gate of the transistor which is first switched during the switching cycle, which voltage is lower than the voltage supplied to the control gate(s) of the remaining transistor(s) in the switching cycle, said buffer circuit either being characterised in that said transistor voltage control means comprises two further transistors connected in a series with each other between the first and second power source terminals; and the control gate of said first switched transistor is coupled to a point between the two further transistors; or being characterised in that said transistor voltage control means comprises a further transistor , the control gate of which is coupled to said first power source terminal and the source and drain electrodes of which are coupled between the transistor switch control means and the control gate of the transistor which is first switched during the switching cycle; and that said delay means is connected between the control gate of another of said transistors which is switched after said first transistor and the interconnection of said further transistor and the transistor switch control means. Brief Description of the Drawings A more complete appreciation of the invention and many of the attendant advantages thereof will be readily appreciated as the invention becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: Figure 1 shows the circuit diagram of a conventional circuit. Figure 2 shows waveforms associated with the operation of the conventional circuit. Figure 3 shows the circuit diagram of a first embodiment of the present invention. Figure 4 shows waveforms associated with the operation of the circuit shown in Figure 3. Figure 5 shows a detailed delay circuit used in the circuit shown in Figure 3. Figures 6 and 7 show the circuit diagram of other embodiments according to the present invention. Figure 8 shows waveforms associated with the operation of the circuit shown in Figure 7. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Figure 3 is a circuit diagram showing a construction of this invention applied to the data output circuit of a semiconductor memory. In this Figure, T1 is a data output terminal. T2 is a power source terminal coupled to reference voltage V SS . I/O and I/O are internal data buses. Transistor 11 is a data output buffer for high-level output and transistor 12 is a data output buffer for low-level output. AND gates 13 and 14 control the re respective data outputs and comprise circuitry for driving the output buffers. Parasitic resistive components 15 and 16 are present in the power source wirings. In the circuit of this embodiment, transistor 12 for low-level output comprises two transistors 12A and 12B, one electrode of each being coupled to data output terminal T1, with the other electrode of each being coupled to reference terminal T3. Transistors 31 and 32 are connected in series between power source terminal T2 and reference terminal T3. The output signal of AND gate 14 is fed to the gate of transistor 31 directly and the output signal of AND gate 13 is fed to the control gate of transistor 11 for high-level output in the conventional manner. All the above transistors are N-type. Signal delay circuit 35 applies a predetermined time delay to the output signal of AND gate 14. It may consist, for example, of an RC time-constant circuit having a resistance and capacitance or the signal delay of a gate circuit may be utilized. Figure 5 shows one example of signal delay circuit 35. The operation of a circuit constructed as described above will now be discussed with reference to the waveform of Figure 4(a). In the waveform of Figure 4(a),  out is an internal control signal which is supplied to AND gates 13 and 14. N1 is the output signal of AND gate 14. N2 is the signal at point 34 along the series connection of transistors 31 and 32, i.e. the gate signal of transistor 12A. N3 is the output signal of delay circuit 35. In such a data output circuit, internal data bus I/O becomes level "0" and I/O becomes level "1". Subsequently, the internal control signal  out rises to level "1", causing the output signal N1 of AND gate 14 to rise to level "1'. As a result transistor 31 conducts, and signal N2 rises to a level "1" through transistor 31. The level "1" signal is at a voltage V dd -V t , i.e. lower than the power source voltage V dd by an amount equal to the threshold voltage of transistor 31. This voltage is input to the control gate of transistor 12A as signal N2. Since this voltage V dd -V t is sufficiently higher than the threshold voltage of the N-type MOS transistor 12A, transistor 12A conducts, and load capacitance 19 (shown in Figure 1) which is connected via terminal T1, discharges to level "0". When load capacitance 19 discharges, only a small current flows in transistor 12A. The potential fluctuations or overshoot produced on the ground voltage side (V SS side) and power source voltage side (V DD side) by the parasitic resistive and inductive components present on the current path is greatly reduced in comparison with the conventional case, shown in Figure 4. Output signal N3 of delay circuit 35 rises to level "1" with a predetermined time delay after signal N1 has risen to level "1". The voltage of this signal N3 is sufficiently higher than the threshold voltage of transistor 12B, so that transistor 12B conducts. This creates an additional discharge path for the load capacitance coupled with terminal T1. The increase of the discharge current due to transistor 12B is slight, so the potential fluctuations or overshoot generated on the reference and power source voltage sides by the parasitic resistive and inductive components present along the current path is greatly reduced from the conventional case as shown in Figure 4. Thus when level "0" data is output, the periods for which potential fluctuations are generated on the ground and power source voltage sides are dispersed to periods t1 and t2 as shown in Fig. 4. Furthermore, the values of the discharge currents which start to flow during each period are smaller than in the conventional case, and the extent of the fluctuations produced during the respective periods t1 and t2 are decreased. Malfunction of the circuit components due to power source potential fluctuations may therefore be avoided. Though the access time is somewhat longer than that of the conventional circuit, as shown by the broken line in Fig. 4(a), it can be made sufficiently rapid compared with the prior art technique of decreasing the rate of increase of the gate drive signal as shown by the broken line in Fig. 2(a). In addition, the gate drive voltage of transistor 12A, which conducts first in the switching sequence, is lower than the gate drive voltage of transistor 12B, which starts to conduct next in the sequence, by an amount equal to the threshold voltage of a single transistor. The current drive capability of transistor 12A is thus reduced, but the extent of the potential fluctuations when transistor 12A starts to conduct, and the time period over which they are produced, is diminished. Moreover, after level "0" data is output, the internal control signal  out falls to level "O", and the output signal N1 of AND gate 14 also falls to level "O". As a result, transistor 31 turns off, and signal N2 also falls to level "O". However, when signal N1 reaches level "O", the output signal of inverter 33 rises to level "1", causing transistor 32 to turn on and conduct. Consequently, the gate of transistor 12A discharges to level "O", with the result that transistor 12A does not conduct. Figure 6 is a circuit diagram showing the construction of a data output circuit of a further embodiment of the invention. In the circuit, a voltage lower than the level "1" voltage V DD of the output signal of AND gate 14 by an amount equal to the threshold voltage of one transistor is fed to the gate of transistor 12A. This is accomplished by inserting transistor 36 between the output of AND gate 14 and the control gate of transistor 12A. The results obtained with the circuit of this embodiment are thus the same as those of the circuit of Figure 3. Figure 7 is a circuit diagram showing the construction of a data output circuit of yet a further embodiment of the invention. In this circuit, the output signal of signal delay circuit 35 is coupled to the gate of transistor 12A through a capacitance 37. In such a construction, when the output signal N3 of delay circuit 35 rises to level "1", and transistor 12B conducts, as shown by the waveform of Figure 8, the voltage of the gate signal N2 of transistor 12A is pulled up to V DD or more by means of capacitance 37. Consequently, the current drive capability of transistor 12A, which had been reduced when the gate voltage fell below V DD , is greatly increased. The increase in access time is thus reduced to a minimum. The invention may be modified in various ways. For example, the above description was for the case wherein two output transistors 12 were provided for level "O" output. However, more than two may be used. Through the above embodiments, the potential fluctuations generated in the power source voltage may be reduced without impairing high-speed data access. This is particularly beneficial when used in high speed DRAMs.

Claims: A buffer circuit for a semiconductor integrated circuit, said buffer circuit being arranged to receive a power source voltage via first and second power source terminals (T2, T3), and comprising an input terminal for receiving an input signal from the integrated circuit, and an output terminal (T1) for outputting an output signal in response to the input signal, said buffer circuit further comprising: a plurality of transistors (12A, 12B) having first and second states and being coupled in parallel between said output terminal (T1) and said second power source terminal (T3); transistor switch control means for switching each of said plurality of transistors in sequence during a switching cycle from its first state to its second state, said transistor switch control means including delay means (35) for delaying the switching of at least one of said plurality of transistors to its second state; and transistor voltage control means for applying a voltage to the control gate of a transistor (12A) which is first switched during the switching cycle, which voltage reaches an at least initial maximum which is lower than the maximum voltage applied to the control gate(s) of the remaining transistor(s) in the switching cycle; characterised in that said transistor voltage control means comprises two further transistors (31, 32) connected in series with each other between the first and second power source terminals (T2, T3); and that the control gate of said first switched transistor (12A) is coupled to a point between the two further transistors (31, 32). A buffer circuit according to claim 1, further characterised in that a capacitor (37) is coupled between the output of said delay means (35) and the control gate of said transistor (12A) which is first switched during the switching cycle so that when the control gate voltage(s) of the remaining transistor(s) (12B) reache(s) the at least initial maximum value of the control gate voltage of the first-switched transistor, the latter control gate voltage rises to a second higher maximum value. A buffer circuit for a semiconductor integrated circuit, said buffer circuit being arranged to receive a power source voltage via first and second power source terminals (T2, T3), and comprising an input terminal for receiving an input signal from the integrated circuit, and an output terminal (T1) for outputting an output signal in response to the input signal, said buffer circuit further comprising: a plurality of transistors (12A, 12B) having first and second states and being coupled in parallel between said output terminal (T1) and said second power source terminal (T3); transistor switch control means for switching each of said plurality of transistors in sequence during a switching cycle from its first state to its second state, said transistor switch control means including delay means (35) for delaying the switching of at least one of said plurality of transistors to its second state; and transistor voltage control means for applying a voltage to the control gate of a transistor (12A) which is first switched during the switching cycle, which voltage reaches an at least initial maximum which is lower than the maximum voltage applied to the control gate(s) of the remaining transistor(s) in the switching cycle; characterised in that said transistor voltage control means comprises a further transistor (36), the control gate of which is coupled to said first power source terminal (T2) and the source and drain electrodes of which are coupled between the transistor switch control means and the control gate of the transistor (12A) which is first switched during the switching cycle; and that said delay means (35) is connected between the control gate of another (12B) of said transistors which is switched after said first transistor (12A) and the interconnection of said further transistor (36) and the transistor switch control means. A buffer circuit according to any preceding claim, further characterised in that said plurality of transistors comprises two transistors (12A, 12B).

Inventors: OHSHIMA SHIGEO C O PATENT DIVI, TODA HARUKI C O PATENT DIVISIO, OHSHIMA, SHIGEO C/O PATENT DIVISION, TODA, HARUKI C/O PATENT DIVISION
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1988
Main classification: H03K5
IPC codes: H03K19, H03K17