Data Input Buffer Circuit for Use in a Semiconductor Memory Device

The present invention relates to a data input buffer used in a semiconductor memory device and particularly to a data input buffer which is not affected by the variation of the power source voltage.

In the semiconductor memory device, each pin is equipped with data input buffers which convert the TTL level signal inputted from the outside into a C-MOS level signal usable in the inside of the semiconductor memory device.

In order to satisfactorily buffer address signals and various control signals which are applied from the outside of semiconductor memory chip, the stability of data input buffer operation is required.

In the general data input buffer, an input trip level is set to determine a predetermined logic state from the TTL level signal which enters from the outside. The trip level is determined according to the size of C-MOS transistor which constitutes a buffer. However, variation of power source voltage lowers the reliability of buffer by making the input trip level unstable.

Referring to FIG. 1A the conventional data input buffer circuit is shown, the conventional data input buffer is connected to power source voltage terminal V.sub.cc through P-MOS transistor 1 which is controlled by a buffer enable signal EN. The input voltage V.sub.in, which is applied by a TTL level outside signal, is connected in common to P-MOS transistor 2, N-MOS transistor 4 and gate 5. The potential of sensing node 3 which connects the drain of said P-MOS transistor 2 and the drain of said N-MOS transistor 4 attains to output voltage V.sub.out, a final output of data input buffer, through inverter 6 and the output voltage V.sub.out is supplied to the inside chip.

FIG. 1B shows the voltage variation (10 V) of the source terminal 10 of P-MOS transistor 2 according to the variation of the power source voltage V.sub.cc FIG. 1C shows an input trip margin (or the amount of voltage which varied between source and drain) of P-MOS transistor 2 according to the variation of the power source voltage in the circuit of the FIG. 1A, when the level of the input voltage V.sub.in is inputted is lower than 0.8 V, the P-MOS transistor 2 turns on and a level of output voltage V.sub.out attains to a low state. When the circuit operates, the voltage V10 of the source terminal 10 of the P-MOS transistor 2 is in V.sub.cc -.alpha. condition (.alpha. is a voltage drop by P-MOS transistor) because the buffer enable signal EN is in a low state. When the P-MOS transistor 2 turns on and an electric current flows, the voltage V10 drops. However, the voltage V10 rises from the initial value V.sub.cc -.alpha. when power source voltage V.sub.cc rises, as illustrated in FIG. 1A. Due to a rise voltage V10 of the source terminal 10, a difference of gate-to-source voltage .vertline.V.sub.in -V10.vertline. of P-MOS transistor 2 increases. Therefore, voltage appearing in the sensing node 3 (or input trip margin ITMG) rises according to a rise in power source voltage V.sub.cc, as illustrated in the graph of FIG. 1C. In other words, input level trip margin expands because the difference of gate-to source voltage V.sub.GS =.vertline.V.sub.in -V10.vertline. of P-MOS transistor 2 increases due to a rise in V10 consequent on a rise in power source voltage V.sub.cc.

Consequently, in such a conventional circuit as stated above, output voltage shows a high state even when input level is higher than 0.8 V, not to speak of when the input level is lower than 0.8 V. Even in the case where power source voltage drops abnormally, an input trip margin ITMG of the P-MOS transistor 2 also increases as in the case where the power source voltage rises. In this case, a low state can be outputted if input level is lower than 2.4 V.

For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying diagrammatic drawings, in which:

FIG. 1A shows a circuit diagram of the conventional data input buffer;

FIGS. 1B and 1C illustrate the graphs showing voltage characteristic according to a conventional data input buffer circuit;

FIG. 2 shows a data input buffer circuit diagram according to the present invention;

FIG. 3 shows an internal structural diagram of the reference voltage generation circuit shown in FIG. 2;

FIG. 4A illustrates the graph showing a voltage characteristic according to the present invention; and

FIG. 4B illustrates the graph showing a current characteristic according to the present invention.

It is, therefore, an object of the present invention to provide a data input buffer which has a stable a input trip level regardless of power source voltage variation.

It is another object of the present invention to provide a data input buffer which has a small change in velocity from input to output in response to a variation of power source voltage.

In order to accomplish above mentioned objects, the present invention is characterized in that, in the data input buffer connected to power source voltage, it is equipped with a conductive passage which, connected between power source voltage and a predetermined level sensing node, adjusts the amount of an electric current according to a level of input voltage and an insulation gate field effect transistor with one end of its channel connected to the conductive passage and other end of its channel connected to ground voltage terminal and its gate to which voltage is applied according to the level of the power source voltage.

Referring to FIG. 2, the circuit of P-MOS transistors 11, 13, N-MOS transistors 15, 16 and inverter 19 are formed in the same manner as that of FIG. 1, but the channel of N-MOS transistor 17 with its gate connected to the output terminal of reference voltage generation circuit 18 is formed between the source node 12 of said P-MOS transistor 13 and ground voltage terminal V.sub.ss. The reference voltage generation circuit 18 produces reference voltage V.sub.ref. In the operation of data input buffer according to the present invention, the source node voltage V.sub.12 of the P-MOS transistor 13, the channel current I.sub.17 of the N-MOS transistor 17 and the reference voltage V.sub.ref are an object of important consideration.

Referring to FIG. 3, between power source voltage terminal V.sub.cc and reference voltage node 34, P-MOS transistors 31, 32, 33 with diodes connected are connected in series from the power source voltage terminal V.sub.cc to the reference voltage node 34. Between the reference voltage node 34 and ground voltage terminal V.sub.ss, a first clamping circuit 42 and a second clamping circuit 43 are connected in parallel. The first clamping circuit 42 comprises P-MOS transistors 35, 36 with diodes connected in series from the reference voltage node 34 and an N-MOS transistor 37 with a gate connected to power source terminal V.sub.cc. The second clamping circuit 43 comprises an N-MOS transistor 38 with a gate connected to power source voltage terminal V.sub.cc and N-MOS transistors 39, 40, 41 with diodes connected in series from the source of the N-MOS transistor 38. The P-MOS transistors 31, 32, 33 connected in series between the power source voltage terminal V.sub.cc and the reference voltage node 34 function as a voltage follower which shows the potential of the reference voltage node 34 according to a level of the power source voltage V.sub.cc. The number of those P-MOS transistors is adjustable at need. The voltage follower can be composed of other general resistances or diodes. It is not necessary to use P-MOS transistors positively. Those who have a usual technical knowledge in this field can easily understand that the first and second clamping circuits 42, 43 can also be composed of other elements. Moreover, the clamping operations of the first and second clamping circuits 42, 43 can be enabled or disabled by supplying a predetermined clock instead of power source voltage V.sub.cc to the gates of the N-MOS transistors 37, 38.

The graph of FIG. 4A shows a correlation between power source voltage V.sub.cc and reference voltage V.sub.ref (or gate voltage V.sub.g of N-MOS transistor 17) and the graph of FIG. 4B shows a correlation between a voltage V.sub.12 of the source node 12 of P-MOS transistor 13 and the channel current I.sub.17 of N-MOS transistor 17 which are illustrated in FIG. 2. Then the operation of the present invention will be described by referring to the graphs of the FIGS. 4A, 4B, the FIGS. 2 and 3. For the convenience of description, the threshold voltages of P-MOS and N-MOS transistors applied to the reference voltage generation circuit 18 are valued at 1 as the absolute value. The reference voltage generation circuit 18 supplies the voltage at a level corresponding to a variation of power source voltage to the gate on N-MOS transistor 17 shown in FIG. 2. (Refer to FIG. 4A). The voltage follower 30 composed of P-MOS transistors 31, 32 and 33 as shown FIG. 3 produces a voltage of .vertline.V.sub.cc -3 V.sub.tp .vertline. at the reference voltage node 34. The first clamping circuit 42 clamps the reference voltage V.sub.ref to the extent of 2 V.sub.tp and the second clamping circuit 43 clamps the reference voltage to the extent of 3 V.sub.tn. Supposing that power source voltage V.sub.cc is 5 V, the values of the above are 2 V, 2 V and 3 V respectively.

Accordingly, when the level of the power source voltage lowers from its initial 5 V, the reference voltage node 34 is connected to the present low power source voltage. In this case, the lowered reference voltage V.sub.ref is applied to the gate of N-MOS transistor 17, so that conductivity of N-MOS transistor 17 is reduced and the V.sub.12 voltage is thereby prevented from being reduced further.

On the other hand, when the level of the power source voltage rises above 5 V, the reference voltage node 34 is connected to ground voltage terminal V.sub.ss. through the first clamping circuit 42. In this case, the reference voltage V.sub.ref attains to a level of about 3 V and this level increases a channel current I.sub.17 by improving the conductivity of N-MOS transistor 17, so that a rise in a level of the source node voltage V.sub.12 caused by a rise in the power source voltage is thereby disturbed.

When the power source voltage rises still more and exceeds 6 V or so, the reference voltage node 34 is clamped to a level of about 4 V by the second clamping circuit 43. As it makes the conductivity of N-MOS transistor 17 greater than in the above case, it makes flow more channel current I.sub.17.

Consequently, the source node voltage V.sub.12 of P-MOS transistor 12 as shown in FIG. 2 is maintained constant by adjusting the current driving ability (or conductivity) of said N-MOS transistor 17 according to a variation of power source voltage even in the case where power source voltage V.sub.cc lowers or rises abnormally, as illustrated in FIG. 4B.

Referring to the Table 1, the result of simulation tells that the conventional art makes a difference of 0.35 V in input trip level which varies according to a variation of power source voltage but the present invention makes a difference of 0.18 V.

According to the Table 2, the conventional art makes a difference of 1.0 ns in velocity from input to output between the maximum value and minimum value of power source voltage but the present invention is greatly improved by making a difference of 0.3 ns. Although there can be a little difference in the results of comparison shown in the Table 1 and 2 depending on the embodied circuit formation and simulation environment, improvement effect based on the present invention will still be produced.

As described hereinbefore, the present invention has the effect of securing the operational stability and reliability of data input buffer by seeing to it that input trip level is not affected by a variation of power source voltage in the data input buffer.