Integrated circuit having reduced sensitivity to voltage transients

Abstract: The invention relates to an input buffer (16) of an integrated circuit having a PMOS transistor (P1) and an NMOS transistor (N1) connected in series between first and second reference voltage terminals (V DD , V SS ). A first filter (26) comprising a capacitor (C1) and a resistor (R1) is coupled between an input terminal (18), the first reference voltage terminal (V DD ) and the control electrode of the PMOS transistor (P1), and a second filter (28) comprising a capacitor (C2) and a resistor (R2) is coupled between the input terminal (18), the second reference voltage terminal (V SS ) and the control electrode of the NMOS transistor (N1). The filters (26, 28) act as low pass filters between the input terminal (18) and the control electrodes of the PMOS and NMOS transistors (P1, N1), and act as high pass filters between the reference voltage terminals (V DD , V SS ) and the control electrodes. By virtue of the filters (26, 28), the input buffer (16) has reduced sensitivity to voltage transients on the reference voltage terminals (V DD , V SS ).

Description: The present invention relates to integrated circuits, and has application to input buffers for very large scale integrated (VLSI) circuits. The trend in the design of integrated circuit chips is for ever smaller geometry sizes and increasingly faster switching times. To further improve system speed, bus widths have been expanded with additional parallel input and output lines. Each output pin on a chip has an output driver which provides a relatively large current in response to a weaker signal generated by the chip. As the load on the bus increases, higher current is provided by the output drivers. Whenever a logic gate on a chip is switched, as for example a logic gate in an output driver, a transient change in the current is created as the load is charged or discharged. Such transient currents create voltage transients in the power or ground path of the chip. The magnitude of voltage transients increases with increased bus width as additional drivers switch at the same time. Similarly, voltage transients increase as a result of the higher current required to drive larger loads. Logic gates on the same chip, which share the power or ground paths of the output drivers, are known to switch logic states in response to such power or ground path voltage transients. This creates problems for both output and input drivers. For example, a ground transient or "bounce" can cause a high input to be erroneously recognized as a low level by an input buffer of a chip. An input buffer may include an inverter with series connected pull-up and pull-down transistors. The pull-down transistor has its gate connected to the high input and its source connected to ground. When the ground plane is steady, the gate-to-source voltage (V GS ) is large enough to keep the pull-down transistor "on" which keeps the output low. However, the inverter shares a common ground plane with the output drivers on the same chip. If the ground plane bounces as a result of one or more of the outputs switching, V GS decreases, turning off the pull-down transistor, and erroneously switching the output of the inverter. A similar result occurs for the pull-up transistor as a result of a voltage transient on the power supply. In the past, attempts to prevent the problems associated with power and ground bounce have concentrated on limiting the magnitude of the voltage transient. A typical method of reducing the voltage transient is to slow the rate at which nodes switch. A disadvantage of this approach is that it increases somewhat the propagation delays through an input/output buffer and pad driver. Furthermore, as chips become faster, input buffers become increasingly sensitive to smaller voltage transients. Further reduction of the magnitude of the voltage transient would considerably increase propagation delays through the input/output buffer and pad driver. US-A 4 908 528 discloses an "add-on" circuitry serving to achieve a reduction in transient-induced noise within an CMOS inverter string by clamping an output signal of an input inverter. This "add-on" circuitry includes an n-type MOS-transistor and a p-type MOS-transistor connected in series between ground and power supply terminals. A capacitor couples a ground transient to the gate of the n-type MOS-transistor. An n-type MOS-transistor provides AC-isolation between the input node and the gate of the first n-type MOS-transistor. A similar structure is shown for the p-type MOS-transistor. It is an object of the present invention to provide an improved integrated circuit which has reduced sensitivity to voltage transients on reference voltage terminals. This object is achieved by the integrated circuit according to claim 1. Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a prior art input buffer as defined in the preamble of claim 1; Figure 2 is an input buffer with an inverter according to one embodiment of the present invention; Figure 3 is an integrated circuit layout of the circuit shown in Figure 2; and Figure 4 is a Schmitt trigger according to another embodiment of the present invention. Figure 1 shows a prior art input buffer 10 with a filter 12 for reducing the sensitivity of the buffer 10 to transients on the input line IN. The buffer 10 includes two CMOS inverters, each having complementary PMOS and NMOS field effect transistors P and N, respectively. The filter 12 includes a resistor R connected between the input line IN and the control electrodes of the transistors P and N (node 14) of the first inverter, and a capacitor C connected in series with the resistor R and a reference voltage terminal V SS . The filter 12 is a low pass filter for transients appearing on the input. Thus, high frequency voltage transients on the input do not appear on the node 14. The typical layout of the input filter 12 as an integrated circuit does not provide reduced sensitivity to transients on the ground plane V SS . Irrespective of the layout of the filter 12 vis-a-vis the first inverter, the buffer 10 is also sensitive to transients on the reference voltage terminals V DD . Figure 2 is an integrated circuit input buffer 16 having reduced sensitivity to voltage transients on the reference voltage terminals V DD and V SS . The buffer 16 includes two CMOS inverters, a first inverter having complementary PMOS and NMOS field effect transistors P1 and N1, respectively, series connected between reference voltage terminals V DD and V SS , and a second inverter having complementary PMOS and NMOS field effect transistors P2 and N2, respectively, similarly series connected between the reference voltage terminals V DD and V SS . The input of the second inverter is connected to an output terminal 20 of the first inverter. A filter 26 which includes a capacitor C1 and a resistor R1 is connected between an input 18 and the reference voltage terminal V DD . A filter 28 which also includes a capacitor C2 and a resistor R2 is connected between the input 18 and the reference voltage terminal V SS . As will be described more fully, the filters 26 and 28 significantly reduce the sensitivity of the circuit 16 to voltage transients on the voltage terminals V DD and V SS , as well as reducing the sensitivity of the circuit 16 to transients on the input terminal 18. The capacitor C1 is connected between the reference voltage terminal V DD and a node 22, which is connected to the control electrode of the transistors P1. The capacitor C2 is connected between the reference voltage terminal V SS and a node 24 which is connected to the control electrode of the transistor N1. The input terminal 18 is connected to the nodes 22 and 24 through the resistors R1 and R2, respectively. The output terminal 20 of the first inverter is connected to the common point between the transistors P1 and N1 and is further connected to the input of the second inverter (transistors P2 and N2). The sizing and layout of the resistors R1 and R2, the capacitors C1 and C2, and the transistors P1 and P2 are important features of the present invention. The resistor R1 and the capacitor C1 are sized to form a high pass filter between the reference voltage terminal V DD and the node 22, and to form a low pass filter between the input terminal 18 and the node 22. Similarly, the resistor R2 and the capacitor C2 are sized to form a high pass filter between the reference voltage terminal V SS and the node 24, and to form a low pass filter between the input terminal 18 and the node 24. In a preferred embodiment, for V DD = 5 volts, and V SS = 0 volts, the transistor P1 has a length to width ratio of 5.5, the transistor N1 has a length to width ratio of 20.0, the resistor R1 is 1000 ohms, the capacitor C1 is 1 pf, the resistor R2 is 1000 ohms, and the capacitor C2 is 3 pf. The integrated circuit layout of the resistors R1 and R2, the capacitors C1 and C2, and the transistors P1 and P2 is shown in Figure 3. The gate or control electrode 22 of the transistor P1 is tightly coupled to the capacitor C1 and the resistor R1 to minimize parasitic capacitance and resistance on the electrode 22. Similarly, the gate or control electrode 24 of the transistor N1 is tightly coupled to the capacitor C2 and the resistor R2 to minimize parasitic capacitance and resistance on the electrode 24. In a preferred embodiment, the integrated circuit is laid out on a P-type substrate. The PMOS transistor P1 is formed in a P+ region within an N-well, and the NMOS transistor N1 is formed in an N+ region. The capacitor C1 is formed by the self-alignement of polysilicon layer 30 and the adjacent P+ regions, and the capacitor C2 is formed by the self-alignment of a polysilicon layer 32 and the adjacent N+ regions. It will be appreciated that the capacitor C1 is formed by using the gate capacitance of the PMOS transistor with its source and drain connected to V DD , and the capacitor C2 is formed by using the gate capacitance of the NMOS transistor with its source and drain connected to V SS . Parasitic capacitance on the gate electrode 22 is significantly reduced by connecting the electrode 22 to the capacitor C1 by metallization runs 34. Parasitic capacitance on the gate electrode 24 is significantly reduced by connecting the electrode 24 to the capacitor C2 by metallization runs 36. The resistor R1 is formed by a serpentine polysilicon run between the input node 18 and the electrode 22, and the resistor R2 is formed by a serpentine polysilicon run between the input node 18 and the electrode 24. Parasitic resistance on the gate electrode 22 is significantly reduced by the short metallization runs 34, and parasitic resistance on the gate electrode 24 is significantly reduced by the short metallization runs 36. Figure 4 shows an alternative embodiment of the present invention in which the input buffer is a Schmitt trigger 38. The first stage of the Schmitt trigger 38 includes two level sensitive elements: a PMOS transistor P3 and parallel connected transistors N1 and N2. The transistor P3 and the parallel connected transistors N1 and N2 are series connected between the reference voltage terminals V DD and V SS . As with the input buffer 10 shown in Figure 2, the Schmitt trigger 38 includes a capacitor C1 and a resistor R1 connected between the reference voltage terminal V DD and an input 40, and a capacitor C2 and a resistor R2 connected between the reference voltage terminal V SS and the input 40. The common node 42 between the capacitor C1 and the resistor R1 is connected to the control electrode of the transistor P3, and the common node 44 between the capacitor C2 and the resistor R2 is connected to the control electrode of the transistor N1. The layout of the input filters formed by the resistor R1 and the capacitor C1 and the resistor R2 and the capacitor C2 is similar to that shown in Figure 3.

Claims: An integrated circuit including first (N1) and second (P1) complementary transistors series connected between first (V SS ) and second (V DD ) reference voltage terminals, a first capacitor (C2) connected between said first reference voltage terminal and a control electrode of said first transistor (N1) an input terminal (18) connected to the control electrodes of said first and second transistors for receiving an input signal, and an output terminal (20) connected to the common point between said first and second transistors for providing an output signal derived by way of said first (N1) and second (P1) complementary transistors from the input signal, characterized by a second capacitor (C1) connected between said second reference voltage terminal and a control electrode of said second transistor (P1), whereby the first capacitor is included in a first filter coupled between the input terminal and the first reference voltage terminal, and the second capacitor is included in a second filter coupled between the input terminal and the second reference voltage terminal, said first and second filters being provided for reducing the sensitivity of the circuit to voltage transients on the input terminal and on the first and second reference voltage terminals. An integrated circuit according to Claim 1, characterized by a first resistor (R2) connected between the control electrode of said first transistor (N1) and said input terminal (18), and a second resistor (R1) connected between the control electrode of said second transistor (P1) and said input terminal. An integrated circuit according to Claim 2, characterized in that said first capacitor (C2) and said first resistor (R2) are sized to form a high pass filter between said first reference voltage terminal (V SS ) and the control electrode of said first transistor (N1), and to form a low pass filter between said input terminal (18) and the control electrode of said first transistor, and said second capacitor (C1 ) and said second resistor (R1) are sized to form a high pass filter between said second reference voltage terminal (V DD ) and the control electrode of said second transistor (P1), and to form a low pass filter between said input terminal (18) and the control electrode of said second transistor. An integrated circuit according to any one of the preceding claims, characterized in that said first (N1) and second (P1) complementary transistors are NMOS and PMOS field effect transistors, respectively. An integrated circuit according to any one of the preceding claims, characterized by an inverter (N2, P2) connected between said first (V SS ) and second (V DD ) reference voltage terminals and having an input connected to said output terminal (20). An integrated circuit according to any one of the preceding claims, characterized in that said first transistor (N1) is connected in parallel with a third transistor (N2) so as to form a bistable circuit. An integrated circuit according to any one of the preceding claims, characterized in that the control electrode (24) of said first transistor (N1) and said first capacitor (C2) are formed on the same doped region, and the control electrode (22) of said second transistor (P1) and said second capacitor (C1) are formed on the same doped region An integrated circuit according to claim 7, characterized in that the control electrode (24) of said first transistor (N1) is connected to said first capacitor (C2) by metallization (36) so as to reduce the parasitic capacitance on the control electrode of said first transistor, and the control electrode (22) of said second transistor (P1) is connected to said second capacitor (C1) by metallization (34) so as to reduce the parasitic capacitance on the control electrode of said second transistor.

Inventors: STEELE DAVID P, STEELE, DAVID P.
Applicants: HYUNDAI ELECTRONICS AMERICA, NCR INT INC, SYMBIOS LOGIC INC, SYMBIOS LOGIC INC.
Application year: 1991
Main classification: H03K19
IPC codes: H03K19, H03K3