SEMICONDUCTOR MEMORY DEVICE

Abstract: A semiconductor memory device of multi-bit type which produces plural output signals (O₁, ..., O₈) corresponding to read-out data from one address at a time including memory means for storing data. In a plurality of output buffer stages for producing the output signals, the operation of the output buffer stages is based upon at least a timing signal. Means (151, ..., 158) for operating the output buffer stages have predetermined time differences. The output signals (O₁, ..., O₈) having predetermined time differences are delivered from the output buffer stages.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor memory device. More particularly, it relates to a semiconductor memory device having a multi-bit constitution, in which the output buffer stages are driven by each sequential output of delay control means and sequential time-delayed signals are output by the output buffer stages to prevent a rush current from flowing at the beginning of the data output and to eliminate a malfunction in a memory device and associated circuits thereof. 2. Description of the Related Art In a known semiconductor memory device such as a dynamic RAM having multi-bit output stages constituted by, e.g., 16 K x 4 bits or 8 K x 8 bits, when data are transmitted at the output stage, large capacitive loads in an exterior data bus line are simultaneously charged or discharged immediately after commencing the transmission of a data output. As a result, a large current flows through a line from the positive side power source line to the output terminal or through a line from the output terminal to the ground side power source line during the period in which the output is changed from a low level to a high level, or vice versa. This excessive transient current is likely to cause a fluctuation of the potential at the positive side power source line or at the ground side source line. The fluctuation is due to insufficient capacity of the power source and parasitic resistance of the power source line, and thus causes a malfunction in the semiconductor memory device itself or in its associated circuits. In particular, the fluctuation of the potential along the ground side power source line is undesirable to avoid malfunctions because this line is usually used as a reference potential line which determines the logic levels of signals. This problem is serious in memory devices of multi-bit type which generate plural output data signals at a time in response to address signals for one address. While the large transient current should be avoided to eliminate the malfunction as stated above, the output buffers contained in the device for generating the data outputs should have sufficient ability of driving external lines for fast read-out operation, resulting in undesirable amount of potential fluctuation along the power source lines particularly when all the data outputs at a time have the same polarity. There is described in US Patent 4 635 234 a memory device designed with the aim of overcoming such a drawback. This device comprises memory means for storing data; plural output stages for producing data outputs, respectively; plural output buffer stages for producing the output signals at associated output terminals, respectively; and means for differentiating the transition timings of said output signals from each other. This prior art device uses a number of timing signals generated outside the output buffer stages, which results in a complex and not very versatile memory device. SUMMARY OF THE INVENTION An object of the present invention is to provide a semiconductor memory device including a plurality of output buffer stages to reduce malfunctions of the memory circuit and associated circuits thereof, by eliminating the multiplication of the transient current by those output stages which would otherwise cause a significant rise in the potential level of the power source line. According to the fundamental aspect of the present invention, there is provided a semiconductor memory device of the aforementioned kind, in which the means for differentiating the transition timings of said output signals from each other are included in each of said output stages and comprise : charging means connected to the associated one of the output terminals for providing a high level at the output terminal when it receives a high level from the associated output stage; discharging means connected to the associated output terminal for providing a low level at the ouptut terminal with a predetermined delay time after it receives a low level from the associated output stage; and delay means for determining said predetermined delay time, wherein said predetermined delay times in each of the output buffer stages are different from each other so that discharging timings at the output stages may be differentiated. The output signals having predetermined time differences are delivered from the output buffer stages. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a block diagram of a semiconductor memory device according to an embodiment of the present invention; Fig. 2 shows the transition of the potential level of data output in the data output terminals and the potential level of the ground side power source V SS ; Fig. 3 shows an example of the semiconductor memory device including the structure of Fig. 1; and Fig.4 shows an example of a semiconductor memory device including the structure of Fig.1. DESCRIPTION OF THE PREFERRED EMBODIMENT Figure 1 is a schematic block diagram of a semiconductor memory device according to an embodiment of the present invention. In Fig. 1, 111 denotes a flip-flop circuit, and 121 an output stage in the form of an inverter. BOT(1), BOT(2), ... BOT(n) denote output buffer circuits. In the output buffer circuit BOT(1), 131 denotes an output switching circuit, 141 an inverter, 151 a delay circuit, 161 an inverter and 171 a NOR gate. 181 denotes a data output terminal. Q1, Q2, Q5, Q6 and Q8, Q9 are pairs of transistors each constituting an inverter, respectively; Q3 is an output switching transistor; Q4 a charge discharging switching transistor; Q7 a transistor forming a delay circuit; and Q10, Q11, Q12, and Q13 are transistors forming a NOR gate. CT is a MOS type capacitor forming a delay circuit, and D(OUT)1 to D(OUT)n are data output terminals for each output buffer BOT(1) to BOT(n). Figure 2 is a diagram showing the transition of the potential level of data output in the data output terminals D(OUT)1 to D(OUT)n and the potential level of the ground side power source V SS . In Fig. 2, due to the action of the output buffer circuit BOT(1) as shown in fig. 1, the charges stored by each capacitor C in each exterior circuit are discharged successively, and accordingly, the potential level of data output terminal D(OUT)1 to D(OUT)n falls to "L" with a time shift. As the charge stored in each capacitor does not flow at the same time, it can be seen that only the potential level of the V SS side power source line rises slowly to ΔH. The flip-flop 111 storing data D1 read out from the memory cell array sends an output caused by a trigger pulse  to the output stage 121. Similarly, other flip-flop circuits 112, 113, ... 11n storing data D2, D3, ... Dn send outputs at the same time by the trigger pulse  to the corresponding output stages 122, 123, ... 12n. When the data output from the output stage 121 changes from "L" to "H", each switching transistor Q1 and Q3 is On. Thus, an "H" potential level is output from the output terminals D(OUT)1 to D(OUT)n and each capacitor is charged. In this case the delay action is not effected by the output buffer circuit BOT(1). When the data output from the output stage 121 changes from "H" to "L", the switching transistor Q2 is ON and the transistor Q3 is OFF, and accordingly, the charge charged in a capacitor C does not flow through the transistor Q2 in the output stage 121 to the V SS side power source line. The data output D₀₁ from the output stage 121 is made "H" through the inverter 141 formed by transistors Q5 and Q6, and applied to the delay circuit 151. In the delay circuit 151, the transistor Q7 is a depletion type and acts as a constant current source, rendering a constant current flow by degrees to a MOS capacitor CT and thus a gradual charging thereof. When the MOS capacitor CT is charged to a predetermined voltage level, the inverter 161 formed by transistors Q8 and Q9 operates, and the output of the inverter being "L" is then applied to the NOR gate circuit 171 formed by transistors Q10, Q11, Q12 and Q13. The output D017 of the NOR gate circuit 171 is "H" and the switching transistor Q4 is made ON, and thus a charge stored in the capacitor C is discharged through the switching transistor Q4. In this case, the discharge; i.e., the trailing of the output data, is done with a time shift at each output terminal D(OUT)1 to D(OUT)n. Each delay circuit 151 in each output buffer is provided with a different time constant from each other, leading to difference in timing of turn-on of the transistor Q₄ in each output buffer. Therefore, as can be seen in Fig. 2, the variation ΔH of the potential level of the ground side power source line is small and slow. The bottom part of Fig. 2 shows a comparison between the present invention and the prior art. Due to the time shift discharge, comparatively little peak current flows, and in response, the height ΔH of the rise of ground side source potential V SS is low. Whereas, in the prior art, a rush current flows and the corresponding rise of ground side source potential V SS ′ is abrupt and high. In a similar manner as explained above, read-out data D₁, ... D n are transferred in parallel from the memory cell array to flip-flops, delivered by a trigger pulse  to each inverters 121, ... 12n, then transferred to each of the output buffers BOT(1), BOT(2), ... BOT(n), and the data outputs D(OUT)1, ... D(OUT)n are fetched out. The timing scheme in each buffer circuit in the device of Fig. 1 is as follows. In the device of Fig. 1, assuming that the positive side power source potential level is V CC , when the output of the output stage 121 changes from "H" to "L", the output voltage V in the delay circuit 151 is approximately expressed by the equation, V = V CC (1 - e -t/T) , where T is a time constant in the delay circuit. Next, assuming that the voltage inverted by an inverter is V INV , then the discharge delay time t of the capacitor in the load is expressed by the equation t = -T ℓn(1 - V INV /V CC ). Accordingly, because of the variation of a time constant T of the delay circuit, the time at which a capacitor C is discharged, which capacitor C is located at each of the output terminals D(OUT)1 to D(OUT)n, can be controlled. In order to achieve the above-described operation, the device of Fig. 1 provides the output stages to which the loads including capacitance are connected, and the output buffer circuits connected to the output stages and having the output switching circuits, the inverter circuits, the delay circuits, and NOR gates. The time-divisional discharge of the capacitor is attained by the output switching circuit including the transistors Q3 and Q4 and the delay circuits. Figures 4 and 3 are block diagrams giving details of the embodiment shown in Fig. 1. Figure 4 is a block diagram of a dynamic RAM (DRAM). The output of the DRAM is done by the multi-bit type buffers for data out. A row address strobe RAS is input to a clock generator No. 1 and a column address strobe CAS is introduced into a clock generator No. 2 through a gate. The outputs of the clock generators No. 1 and No. 2 are sent to address buffers, and another output of the clock generator No. 1 drives a reference control clock and the output is counted by an internal refresh address counter, the count output is then delivered to the address buffer. These are used only in the refresh mode and the explanation is omitted. Two sets of predetermined eight address A₀ to A₇ are applied to the address buffer by the control of the strobe signals RAS and CAS and the two outputs are generated. One output is sent to a column decoder and the other to a row decoder. The address is stored in a 262,144-bit storage which has a format of 64 K words x 4 bits. On the other hand, a write enable signal WE drives a write clock generator and the output is distributed among the data-in buffers No. 1 to No. 4. The output of the clock generator No. 1 is sent to sense amplifiers and the outputs thereof are fed to data-in buffers No. 1 to No. 4. The output of the clock generator No. 2 is sent to a column decoder and data-out buffer No. 1. The output enable signals are delivered to the data-out buffers No. 1 to No. 4. Thus, the outputs of the data-out buffers are fetched through the external pins as the output. In this case, the delay circuits are, respectively connected to the data out buffers. The function of the delay circuits is the same as in Fig. 1. Fig. 3 is a block diagram of an EPROM using output delay circuits with a different time constant, as a second embodiment according to the present invention. An output enable signal OE and a chip enable signal CE are input to an output enable and chip enable circuit and the output is delivered to a data input buffer, a column decoder, a row decoder, and output buffers No. 1 to No. 8. The outputs of a column and row decoder are sent to a memory cell array, and the address is assigned. The output read out from the memory cell array is delivered to output buffers No. 1 to No. 8 and outputs in response to a predetermined time constant are provided. Thus, to begin the operation of data output at a separate time from each output stage, each delay circuit is interposed on the input terminal sides of the output stages of the multi-bit type buffer, and rush current is divided into the currents passing through the number of delay circuits added by one. Accordingly, malfunctioning of the device is prevented.

Claims: 1. A semiconductor memory device of multi-bit type which produces plural output signals corresponding to read-out data from one address when accessed, comprising : memory means for storing data; plural output stages (121, ..., 12n) for producing data outputs, respectively; plural output buffer stages (BOT(1), ..., BOT(n)) for producing the output signals at associated output terminals (D(OUT)1, ..., D(OUT)n), respectively; and means for differentiating the transition timings of said output signals from each other; characterized in that said differentiating means are included in each of said output buffer stages and comprise : charging means (Q3) connected to the associated one of the output terminals for providing a high level at the output terminal when it receives a high level from the associated output stage; discharging means (Q4) connected to the associated output terminal for providing a low level at the ouptut terminal with a predetermined delay time after it receives a low level from the associated output stage; and delay means (151, ..., 15n) for determining said predetermined delay time, wherein said predetermined delay times in each of the output buffer stages are different from each other so that discharging timings at the output stages may be differentiated. 2. A device according to claim 1, wherein said output buffer stages (BOT(1), ..., BOT(n)) comprise an output circuit having switching elements used as said charging and discharging means. 3. A device according to claim 2, wherein switching transistors (Q3) and (Q4) are used as said switching elements. 4. A device according to claim 1, wherein said output buffer stages comprise complementary MOS transistor push-pull inverters as input and output circuits for said delay means (151). 5. A device according to claim 1, wherein said charging means is a switching transistor (Q3) which is connected between the associated output stage (121, ..., 12n) and the associated output terminal (D(OUT)1, ..., D(OUT)n) and is driven by the output of said associated output stage, and said discharging means is a switching transistor (Q4) which is connected between the associated output terminal (D(OUT)1, ..., D(OUT)n) and a fixed voltage source (Vss) and is driven by the output of a NOR gate (171, ..., 17n) receiving the output of the associated output stage (121, ..., 12n) directly at one its inputs and through said delay circuit (151, ..., 15n) at its other input.

Inventors: AKAOGI, TAKAO, TATEMATSU, TAKEO ESUPERANSA DAI-3 TOKAICHIBA 102
Applicants: FUJITSU LIMITED
Application year: 1986
Main classification: G11C7
IPC codes: G11C7