Output buffer circuit

Abstract: A buffer circuit particularly suited for driving the output pad (2) of an integrated circuit, includes features which limit the rate of change of current flow in the power supply and ground lines to reduce noise. The circuit features feedback responsive compensation for variations in the capacitive load on the pad. In one form, the circuit includes feedback capacitors (21,22) between the pad (2) and the control gate of output transistors (18,19) pulling the pad (2). The control gates of such pulling output driver transistors (18,19) are, under one operational condition, provided with a voltage which increases at a limited rate during current enablement. The steady-state enablement of the output driver transistor is established later by circuits (36,38;37,39) logically responsive to the voltage on the output pad (2). The output driver transistors (18,19) are subject to rapid disablement in response to a pulling of the control gate by high speed and drive capacity transistors (23,24). In a preferred arrangement the circuit includes tri-state operation capability.

Description: This invention relates to output buffer circuits suitable for use in an integrated circuit chip. The voltage spikes which appear as noise on the ground lines and power supply lines of integrated circuit devices have increased both in prevalence and relative magnitude with the evolution toward smaller integrated circuit dimensions. Investigations have established that the resistance and inductance of the interconnect lines within the integrated circuit chip have become major sources of transient noise voltages. Rapid changes of current to and from the power supply and ground pads contribute significantly to the levels of noise on such corresponding lines throughout the chip. Concurrently, decreases in integrated circuit dimensions constrain designers from materially reducing the power supply and ground line impedance levels by expanding line widths. Attempts to eliminate or suppress noise sources are further complicated by the increases in the integrated circuit device switching speeds and their associated di/dt effects. Integrated circuit power supply and ground line noise signals attributable to resistance and di/dt effects appear most often when input/output (I/O) buffers are used to switch capacitive loads. The significance of this noise source is expected to increase with the use of more CMOS type integrated circuits, in that CMOS integrated circuit loads are predominantly capacitive. Having regard to Article 54(3) EPC, the document EP-A-0 297 932 discloses a transmitter circuit for transmitting a digital data signal over a bus, which includes a MOSFET bus driver transistor having a high gate-to-drain feedback capacitance, the bus driver transistor being driven by a buffer having pull-up and pull-down transistors, current through which is controlled by constant current sources. The document US-A-4 504 779 discloses a drive circuit for varying the power supplied from a DC power supply to a motor, including a MOSFET transistor having a gate-to-drain feedback capacitance which may have an external capacitor connected in parallel therewith. Connected to the gate electrode of the driver MOSFET are a first bipolar transistor acting as a constant current source, a second bipolar transistor acting as a switchable constant current sink, and, via a zener diode, a third bipolar transistor. The drive circuit provides trapezoidal shaped voltage and current waveforms, and reduces undesirable electromagnetic interference (EMI). The document US-A-4 622 482 discloses a driver circuit including a pair of opposite conductivity type MOS transistors operated in push-pull fashion to drive the output voltage in response to a control signal, wherein capacitors are provided to effect slew rate limiting. Also, each transistor is selectively dynamically biased to ensure a substantially linear slew rate. It is an object of the present invention to provide an output buffer circuit which has a high operating efficiency. Therefore, according to the present invention, there is provided an output buffer circuit for an integrated circuit, including: an output node, driven by a first transistor connected to a first side of a power supply; capacitive feedback means connected between said output node and a control electrode of said first transistor; a second transistor connected between said control electrode of said first transistor and said first side of said power supply to relatively rapidly pull said control electrode of said first transistor to the potential of said first side of the power supply responsive to a DATA IN signal first state; switch means connected between a second side of said power supply to slowly pull said control electrode of said first transistor toward a second side of said power supply responsive to a DATA IN signal second state; and logic means responsive to the combination of a voltage level from said output node and said DATA IN signal second state to further pull said control electrode of said first transistor to the potential of said second side of said power supply. An output buffer circuit according to the invention has the advantage of adjusting the rate of change of the output buffer current in relation to the magnitude of the capacitive load on the output pad, limiting rapid current changes and thereby constraining associated power supply and ground line di/dt noise levels, and ensuring full pull-up of the output stage following transition. The combination of these advantages provides load responsive feedback sensitivity, reduces current surge noise, and ensures concluding DC voltage levels of appropriate magnitudes. The invention as preferably embodied incorporates capacitive feedback between the output node and the control electrode of the output transistor to adjust the switching rate of the output transistor in relation to the capacitive load. The turn-on current spike in the output transistor is further controlled by a resistive element interposed between the output transistor control gate and the switched source of the control signal. The steady-state voltage on the output node is established using a pull-up transistor driven by a logic gate output signal. The output state of such logic gate is based upon the combination of the input command signal and the level of the output node voltage in its progression toward the final steady-state level. A preferred embodiment of a tri-state configuration using push-pull output transistors, includes a pull-down transistor acting upon the control gate of the output transistor. The pull-down transistor is designed to switch the control gate of the output transistor significantly faster than the control gate is switched during turn-on. This avoids a switching overlap type of short circuit condition when complementary pairs of transistors are used to drive the output. Tri-state operation is enabled through multiple logic gates, which selectively interact with the turn-on and steady-state control signals. Embodiments of the invention will now be described by way of example with reference to the accompanying drawings, in which:- Fig. 1 is a circuit schematically illustrating the basic features of the invention. Fig. 2 is a schematic showing a preferred embodiment of the invention with CMOS field effect transistors and tri-state control; and Figs. 3-10 are plots illustrating the relative performance levels of the preferred embodiment and representative prior art. Buffer circuit 1 in Fig. 1 illustrates in a fundamental form the elements which contribute to the present invention. The circuit is particularly suited to drive an output pad of an integrated circuit in response to signals generated within the integrated circuit. The schematic in Fig. 1 also includes the output pad 2 and a representative capacitive pad load C L . In the context of the schematic in Fig. 1, the objective is to accept high switching rate DATA IN signals and have field effect transistor 3 drive the output pad 2 accordingly while minimizing the di/dt effects on the ground line. The objectives are satisfied through the contribution of three circuit structure arrangements. First, n-channel output driver field effect transistor 3 has a slew rate limiting feedback capacitor 7 connected between its source electrode 4 and its control gate electrode 6. Second, pull-up action on node 9 and its progressive effect on control electrode 6 to enable driver transistor 3 is constrained by the limited output capacity of inverter 12. Consequently, the slew rate of rising voltage on the gate electrode 6 which enables transistor 3 is reduced through the combined effects of limited drive inverter 12, resistor 11, resistor 13 and capacitor 7. Positive transitioning DATA IN signals are used to directly enable relatively large and fast transistor 8. Transistor 8 pulls node 9 and gate electrode 6 of transistor 3 toward ground significantly faster than the rate at which node 9 is able to transition in the positive direction. The third aspect of the invention arises from the logical combination of signals entering OR gate 14. OR gate 14 combines the DATA IN signal and a feedback signal from pad 2, defining therefrom an output signal to drive p-channel transistor 16. Preferably, OR gate 14 enables transistor 16, to pull node 9 to the supply voltage V DD , after the DATA IN signal is low and the voltage on pad 2 has decreased to approximately half the supply voltage. This third aspect of the invention ensures that the concluding or steady state drive to transistor 3 from node 9 is based upon the full voltage of the power supply V DD . A nominal value for feedback capacitor 7 is in the range of 2 picofarads for an output transistor 3 which is designed to drive a nominal capacitive load C L of 50 picofarads and to exhibit a nominal DC current conduction capability of 24 milliamps. For these capacities, resistors 11 and 13 nominally range from 300 to 400 ohms, while feedback resistor 17 is nominally 250 ohms in value. The physical dimensions and electrical characteristics of pull-down transistor 8 should be selected to ensure that the pull-down rate of gate electrode 6 is approximately four times the pull-up rate produced by inverter 12. A preferred and fully implemented arrangement of the invention, as first rudimentally depicted in Fig. 1, appears in Fig. 2. The circuit now incorporates a tri-state capability with a selectively operable enable mode responsive to an ENABLE signal. To implement the tri-state capability required of the circuit, output pad 2 is driven by a complementary pair of field effect transistors, with p-channel transistor 18 pulling pad 2 to the power supply node V DD and n-channel transistor 19 pulling pad 2 to ground potential. The control or gate electrodes of each of the output pad driver transistors 18 and 19 are coupled through respective feedback capacitors 21 and 22 to the node represented by pad 2. The gate electrodes of transistors 18 and 19 are themselves individually driven by electrically complementary pulling transistors 23 and 24, which pulling transistors act upon intermediate nodes 26 and 27 in direct response to the DATA IN signal level. Again, intermediate nodes 26 and 27 are separated from the control gate electrodes of driver transistors 18 and 19 by respective resistors 28 and 29. The limited current drive capacity of NOR gate 31, as further constrained by resistors 29 and 33, and feedback capacitor 22, together establish the initial slope of the pad-to-ground current conducted by pull-down driver transistor 19. The current capacity of OR gate 32, interacting with capacitor 21 through resistors 28 and 34, provides a corresponding effect on the pull-up driver transistor 18. In keeping with the concepts introduced with reference to the structure in Fig. 1, resistors 33 and 34 separate respective logic gates 31 and 32 from intermediate nodes 27 and 26 to establish a relative difference in the rate of change of the control gate voltages on the driver transistors 18 and 19 depending on whether the input signal transition is high-to-low or low-to-high. Larger, faster and more directly connected transistors 23 and 24 disable their respective output driver transistors 18 and 19 approximately four times faster than the enablement effectuated by respective gates 37 and 36 to eliminate inadvertent and undesirable simultaneous enablements of both transistors 18 and 19. This factor of four is believed to provide a sufficient margin, between the disablement of one output driver transistor and the enablement of its complement transistor, to offset performance differences attributable to normal fabrication process variations. The steady-state signals on output pad 2 are again guaranteed appropriate by logic gates sensing the state of the output pad voltage. In this embodiment, OR gate 36 and NOR gate 37 acting respectively on p-channel transistor 38 and n-channel transistor 39 to fully pull the respective intermediate nodes 27 and 26 to the appropriate power supply and ground potential levels for complete enablement of the output driver transistors 19 and 18. Again, it is preferred that logic gates 36 and 37 respond to the feedback signal conveyed through resistor 41 from pad 2 when the pad voltage reaches approximately half the power supply voltage. A high level signal on the ENABLE/ line places buffer circuit 42 in Fig. 2 into a tri-state mode of operation, whereupon pad 2 is to be simultaneously disconnected from both the power supply and ground lines. This is accomplished by concurrently disabling both the output driver transistors 18 and 19. The ENABLE/ state is conveyed to the gates which define the initial rate of rise of current, namely, gates 31 and 32, as well as the gates which establish the steady-state voltages on nodes 27 and 26, namely, gates 36 and 37. The ability of the circuit in Fig. 2 to materially reduce ground and power supply line noise spikes attributable to high rates of change of current, di/dt, is evidenced in the succession of performance plots depicted in Figs. 3-10. Each of the figures shows a pad output voltage transition of approximately 5 volts and the noise voltage level on the related supply voltage or ground line. Figs. 3-6 are waveforms for pads loaded at a nominal level of 20 picofarads, while Figs. 7-10 relate to pads having nominal 120 picofarad loads. The waveforms in Fig. 3 are attributable to a high-to-low voltage transition of the pad voltage for the circuit in Fig. 2, while Fig. 4 is the corresponding transition performance of a relatively comparable prior art output pad driver circuit. Similarly, Fig. 5 illustrates the positive going transition and associated noise for the circuit of the present invention, while Fig. 6 illustrate the corresponding performance of the prior art. The effects of a larger capacitive load on the circuit of the present invention under the high-to-low transition condition are illustrated in Fig. 7, while the effects of the corresponding conditions on the prior art circuit are shown in Fig. 8. Fig. 9 illustrates the low-to-high transition for the present invention circuit under heavy capacitive load, while Fig. 10 is representative of the prior art performance. A comparison of the noise levels clearly establishes the improved performance exhibited by the present circuit. For instance, a relative 50 millivolts peak-to-peak of noise is exhibited by the present invention in Fig. 3 compared to more than twice that amount for the prior art in Fig. 4. The 20 picofarad load used for the plots in Figs. 3-6 is attributable to the test oscilloscope, the pad being otherwise unloaded. The 120 picofarad loads used for the test results shown in by Figs. 7-10 involved the addition of 100 picofarad passive loads to pad 2. The importance of the feedback capacitors 21 and 22 within the circuit of Fig. 2 should not be underestimated. Not only do the capacitors contribute to the shaping of the waveforms during the three increments of operation described in detail hereinbefore, but they provide negative feedback suitable to constrain the switching rates for low levels of capacitive load on the output pad 2. Whereas most prior art developments focus solely on limiting the current flow through the driver transistors for heavy capacitive load conditions, the present circuit extends the range of the control into low capacitive loading ranges with high di/dt effects. All high rates of change of voltage on the pad are fed back through the capacitors 21 or 22 to the gate electrodes of the driver transistors. Consequently, the present circuit provides both open loop and closed loop compensation for variations of capacitive load on the pad, a class of load typical for contemporary and advanced CMOS class integrated circuit devices.

Claims: An output buffer circuit for an integrated circuit, including: an output node (2), driven by a first transistor (3,19) connected to a first side of a power supply; capacitive feedback means (7,22) connected between said output node (2) and a control electrode of said first transistor (3,19); a second transistor (8,24) connected between said control electrode of said first transistor (3) and said first side of said power supply to relatively rapidly pull said control electrode of said first transistor (3,19) to the potential of said first side of the power supply responsive to a DATA IN signal first state; switch means (16,38) connected between a second side of said power supply to slowly pull said control electrode of said first transistor (3,19) toward a second side of said power supply responsive to a DATA IN signal second state; and logic means (14,36) responsive to the combination of a voltage level from said output node (2) and said DATA IN signal second state to further pull said control electrode of said first transistor (3,19) to the potential of said second side of said power supply. An output buffer circuit according to claim 1, characterized in that said second transistor (8) is connected to said control electrode of said first transistor (3) through a resistor (11,29). An output buffer circuit according to claim 2, characterized in that said switch means (16,38) is a relatively low current driver operable to pull said control electrode at approximately one fourth the pull rate of said second transistor (8,24). An output buffer circuit according to claim 3, characterized in that said switch means includes a transistor (16,38) which is connected to said second side of said power supply and through said resistor (11,29) to said control electrode of said first transistor (3,19). An output buffer circuit according to claim 4, characterized in that said output node is a pad (2) of an integrated circuit and said transistors (3,8;19,24) are field effect devices. An output buffer circuit according to claim 1, characterized in that said logic means includes an OR gate (14,36). An output buffer circuit according to claim 6, characterized in that said OR gate (14,36) is responsive to an output node voltage level of nominally half the power supply voltage. An output buffer circuit according to claim 1, characterized by a complement circuit (18,21,23,26,28,32,37,39) thereof, sharing a common output node (2) and a common set of DATA IN signals. An output buffer circuit according to claim 8, characterized by disabling means adapted to selectively disable said first transistor (19), said second transistor (24) and said switch means (38).

Inventors: PETTY WILLIAM K, PETTY, WILLIAM K.
Applicants: NCR INT INC, NCR INTERNATIONAL INC.
Application year: 1989
Main classification: H03K19
IPC codes: H03K19, G11C7, H03K5, H01L23, G11C11, H03K17