SINGLE OUTPUT TRANSISTOR OUTPUT STAGE FOR INTERFACE APPLICATIONS

Abstract: An interface output stage includes a pull-up circuit and a pull-down circuit connected to a positive power supply signal line having a first voltage, an output signal line having an output voltage and a negative power supply signal line having a second voltage. The pull-up circuit includes a single output transistor and a body snatcher circuit, both interconnected between the positive power supply signal line and the output signal line. The body snatcher circuit ties the substrates of the output transistor and the transistors forming the body snatcher circuit to either the first voltage or the output voltage. The pull-down circuit is designed generally similar to the pull-up circuit to tie substrates of its transistors to either the output voltage or the second voltage.

Description: SINGLE OUTPUT TRANSISTOR OUTPUT STAGE FOR INTERFACE APPLICATIONS BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates generally to the field of digital communications, and particular to communications interface circuitry. 2. Related Art Various communication interface standards have been created for establishing a common communications scheme between computers and peripheral devices. For example, RS-232 serial communication interface standard has been widely used in many applications, including personal computers. Moreover, RS-485 serial communication interface standard has provided a more robust connection to support higher communication speeds and longer cables . Normally, these and other serial communication interface standards were created for and relied upon power supply voltages of approximately five volts (5V) . However, over the past few years, many electronic devices have been designed to utilize lower power supply voltages. As one illustrative example, portable computers have been manufactured with internal power supplies providing approximately 3V. As a result, traditional output stages have been unable to drive various types of serial communication links (e.g., RS-232 and RS-485 communications) at these lower power supply voltages . The reason is that devices, connected to the serial communication link and operating at higher supply voltages, occasionally require an output to be driven above and/or below the power supply rails. Thus, it has been necessary to implement a high swing interface output stage which would allow the output of an electronic device to be appropriately driven. Referring now to Figure 1 and described in U.S. Patent No. 5,414,314 assigned to the Assignee of the present application, an integrated Complementary Metal- Oxide-Semiconductor (CMOS) high swing interface output stage 100 is shown. The interface output stage 100 utilizes three P-channel devices P10, Pll and P15 and three N-channel devices N12 , N13 and N14. Two of the three P-channel devices P10 and Pll are connected in series between a positive power supply terminal 110 and an output of the interface 120 while N-channel devices N12 and N13 are connected in series between the output of the interface 120 and a negative power supply terminal 130. As described, this conventional high swing interface output stage 100 poses a number of disadvantages. One disadvantage is that this output stage 100 utilizes two output transistor pairs (P10,P11) and (N12,N13) in series. These transistors collectively occupy a substantial percentage of die area, and more than one-half of the die size in some cases. Hence, it would be desirable to replace each series of output transistor pairs with a single output transistor in order to save die area . SUMMARY OF THE INVENTION A circuit and methods for reducing the amount of die area utilized by an interface output stage. The interface output stage includes a pull-up circuit and a pull-down circuit connected to a positive power supply signal line, an output signal line and a negative power supply signal line. The pull-up circuit includes a single output transistor interconnected between the positive power supply signal line and the output signal line while the pull-down circuit includes a single output transistor interconnected between the negative power supply signal line and the output signal line. Both the pull-up circuit and the pull-down circuit include a body snatcher circuit is used to provide a higher or lower of two potentials to substrates of the transistors forming the body snatcher circuit and its corresponding output transistor. The circuit and method of operation are described in detail in the following discussion. BRIEF DESCRIPTION OF THE DRAWINGS The features and advantages of the present invention will become apparent from the following detailed description of the present invention in which: Figure 1 is a circuit diagram of a prior high swing interface output stage. Figure 2 is a block diagram of a system utilizing an interface output stage configured in accordance with a described embodiment of the invention. -4- Figure 3 is a circuit diagram illustrating the interface output stage utilized by the system of Figure 2. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT In the description herein, various terms are used to describe certain architecture or characteristics of the present invention. For example, a "signal line" is generally defined as any electrical or light carrying medium such as one or more electrical wires, bus traces, fiber optics, and the like. Moreover, P-channel transistors are designated with a legend having the letter "P" followed by a specific numeral. N-channel transistors are similarly designated but with a legend having the letter "N" . It is contemplated, however, bipolar transistors may be substituted for a P-channel transistor or a N-channel transistor. Referring to Figure 2, an illustration of an exemplary system including a portable computer 200 in communication with a desktop computer 210 over a serial communication signal line 220 is shown. In compliance with RS-232 serial communications standard or any other serial communications standard (e.g., RS-485, RS-422, etc.), serial communication signal line 220 can be used to establish a network connection with one or more other computers (e.g., as desktop computer 210) as shown. Alternatively, serial communication signal line 220 may be used to provide a connection with one or more peripheral devices such as a modem, printer, and various test equipment. -5- Portable computer 200 is adapted with a serial communication port (not shown) which provides a pathway for information transmitted to and received from serial communication signal line 220. Also not shown, a circuit referred to as an "interface output stage" is connected to a transmit (TX) lead (e.g., pin, socket, etc . ) of the serial communication port to impose a voltage on a corresponding TX signal line of serial communication signal line 220. This voltage signals whether or not data is being transmitted from portable computer 200. Referring now to Figure 3, an illustrative embodiment of the interface output stage is shown. The interface output stage 300 is an integrated circuit (IC) output driver used to transmit information through a TX signal line of a serial communication port. The interface output stage 300 comprises a pull-up circuit 310 connected to both a first power supply terminal 320, namely a positive power supply signal line and an output terminal 330. Interface output stage 300 further comprises a pull-down circuit 350 connected to output terminal 330 and a second power supply terminal 340, which is a negative power supply signal line. The operations of pull-up circuit 310 and pull-down circuit 350 are mutually exclusive based on their respective inputs from transmission circuitry (not shown) and each only impose a voltage drop associated with each single Metal-Oxide-Semiconductor (MOS) output transistor. This provides substantial area reduction (e.g., a factor of 4 from a series of output transistors) and a substantial cost savings . Referring still to Figure 3, one embodiment of pull-up circuit 310 includes a first inverter 311, a second inverter 315, a third inverter 316, an output -6- transistor Pi, a first body snatcher circuit 312 formed by transistors P2 and P3 and a first comparison circuit. The first comparison circuit includes (i) a transistor Ql, (ii) a current source 313, and (iii) an impedance element 314. The first body snatcher circuit 312 of pull-up circuit 310 provides a higher of two potentials to both the substrate of output transistor Pi and translators P2 and P3 as well as a supply input of first inverter 311 via signal line 318. In this embodiment, the potentials are the voltages at the first power supply terminal 320 (measured with a positive predetermined voltage such as "Vcc") or the voltage at output terminal 330 (referred to as "Vout"). Transistors PI, P2 and P3 are formed in a well in the substrate of interface output stage 300. A common connection 319 connects substrates of transistors Pi, P2 and P3 together. A base of bipolar transistor Ql is connected to first power supply terminal 320. The emitter of bipolar transistor Ql is connected to impedance element 314 (which is connected to output terminal 330) while the collector is connected to current source 313. Preferably, current source 313 is an active current source providing a constant current flow to avoid a condition where transistors P2 and P3 are both turned-off. Under a normal operating condition, when the voltage at first power supply terminal 320 equals or exceeds the voltage at output terminal 330 (Vcc > Vout) , transistor Ql is off. Current source 313 will hold node 317 low, which turns off transistor P3 through second inverter 315 and turns on transistor P2 through both second and third inverters 315 and 316. The second and third inverters 315 and 316 ensure that appropriate signal voltages are applied to the gates of transistors P3 and P2. As a result, the common substrate associated with transistors Pi, P2 and P3 is connected to the first power supply terminal 320. Also, the supply input of first inverter 311 is set to Vcc to ensure that output transistor Pi is turned-off upon applying a low voltage to a data input of first inverter 311. More specifically, as current flows into the first comparison circuit, a comparison is made at node 317 between the amount of current through transistor Ql and the current provided by current source 313. When the current through transistor Ql sufficiently exceeds the current provided by current source 313, and consequently Vout is forced higher than Vcc by at least a base- emitter voltage (Vbe) of transistor Ql, transistor Ql begins to turn on. As a result, the voltage at node 317 will increase. This turns on transistor P3 via second inverter 315, and through second and third inverters 315 and 316, turns off transistor P2. As a result, the common substrate associated with transistors PI, P2 and P3 is now connected to output terminal 330 and Vout is applied to both the common substrate and the supply input of first inverter 311 controlling the gate voltage for output transistor PI. Particularly by using an active current source for current source 313 (i.e., a current source exhibiting a very high source impedance) , the common substrate is prevented from floating and contributing to an increase in overall noise experienced by interface output stage 300. As further shown in Figure 3, pull-down circuit 350 includes a fourth, a fifth and a sixth inverter 351, 355 and 356, an output transistor Nl, a second body snatcher circuit 352 formed by transistors N2 and N3 , and a second comparison circuit including (i) a transistor Q2 , -8- (ii) a current source 353, and (iii) an impedance element 354. Transistors Nl, N2 and N3 are connected to a well and connected to each other through a common connection 359. Second body snatcher circuit 352 of pull-down circuit 350 provides a lower of two potentials to both the substrate of output transistor Nl and a drain of fourth inverter 351 via communication signal line 358. In this embodiment, the potentials are the voltage at the second power supply terminal 340 (referred to as "GND") or Vout. Under normal operating condition when Vout > GND, transistor Q2 remains off. This places a high voltage on node 357 to turn off transistor N2 (based on the output by fifth inverter 355) and to turn on transistor N3 (based on an output by fifth and sixth inverters 355 and 356) . As a result, substrates associated with transistors Nl, N2 and N3 are connected to the second power supply terminal 340; namely, substrates of transistors PI, P2 and P3 are tied to GND. Also, the ground input of fourth inverter 351 is set to GND. As current flows through transistor Q2 of second comparison circuit, a comparison is made at node 357 between the current through transistor Q2 and the current provided through transistor Q2. As a result, when Vout < GND by at least a base-emitter voltage (Vbe) of transistor Q2 , transistor Q2 begins to turn on. When the current through transistor Q2 exceeds the current through current source 353, node 357 will be pulled low. This turns on transistor N2 via fifth inverter 355 and turns off transistor N3 via the fifth and sixth inverters 355 and 356. As a result, the common substrate associated with transistors Nl, N2 and N3 is connected to output signal line 340 and Vout is applied to both the common substrate and the ground input of fourth inverter 351 controlling the gate voltage of output transistor Nl . The specific embodiments set forth above are intended merely to demonstrate one implementation of the invention and should not be viewed as limiting its scope. Other implementations and embodiments of the invention will be readily apparent to those skilled in the art. Therefore, the scope of protection should be construed from the following claims.

Claims: CLAIMS What is claimed is : 1. An interface output stage comprising: a plurality of signal lines including an output terminal and a power supply terminal; a comparison circuit coupled to the output terminal and the power supply terminal; a first transistor having a substrate; and a body snatcher circuit responsive to the comparison circuit by coupling the substrate of the first transistor to one of the output terminal and the power supply terminal . 2. The interface output stage of claim 1, wherein the body snatcher circuit comprises : a second transistor coupled to the power supply terminal, the substrate of the first transistor and the comparison circuit; and a third transistor coupled to the output terminal, the second transistor, the comparison circuit and the substrate of the first transistor. 3. The interface output stage of claim 2, wherein the first transistor is further coupled to the output terminal and the power supply terminal . 4. The interface output stage of claim 1, wherein the power supply terminal is a positive power supply signal line. -11- 5. The interface output stage of claim 1, wherein the power supply terminal is a negative power supply signal line. 6. The interface output stage of claim 1, wherein the comparison circuit comprises : an impedance element; a bipolar transistor including a base, an emitter and a collector, the base coupled to the power supply terminal and the emitter coupled to the impedance element; and a current source coupled to the collector of the bipolar transistor. 7. The interface output stage of claim 6, wherein the collector of the bipolar transistor is coupled to a gate of the second transistor and a gate of the third transistor. 8. The interface output stage of claim 6, wherein the collector of the bipolar transistor is coupled to a gate of the third transistor via at least one inverter and to a gate of the second transistor via at least two inverters . 9. The interface output stage of claim 6, wherein the substrate of the first transistor is coupled to the output terminal when a current through the bipolar transistor exceeds a current provided by the current source. 10. The interface output stage of claim 1, wherein the substrate of the first transistor is coupled to the output terminal when the voltage at the output terminal -12- exceeds the voltage at the power supply terminal by a predetermined voltage. 11. An interface output stage comprising: a plurality of signal lines including an output terminal, a first power supply terminal and a second power supply terminal; a pull-up circuit coupled to the output terminal and the first power supply terminal, the pull-up circuit including a first comparison circuit coupled to the output terminal and the first power supply terminal , a first transistor having a substrate, and a first body snatcher circuit coupled to the first comparison circuit, the first transistor, the output terminal and the first power supply terminal, the first body snatcher circuit coupling either the output terminal or the first power supply terminal to the substrate of the first transistor; and a pull-down circuit coupled to the output terminal and the second power supply terminal, the pull-down circuit including a second comparison circuit coupled to the output terminal and the second power supply terminal , a second transistor having a substrate, and a second body snatcher circuit coupled to the second comparison circuit, the second transistor, the output terminal and the second power supply terminal, the second body snatcher circuit coupling either the output terminal or the second power supply terminal to the substrate of the second transistor. 12. The interface output stage of claim 11, wherein the first power supply terminal is a positive power supply signal line and the second power supply terminal is a negative power supply signal line. 13. The interface output stage of claim 11, wherein the first comparison circuit comprises a transistor in series with a current source, the transistor of the comparison circuit including an emitter coupled to the first output terminal, a base coupled to the first power supply terminal and a collector coupled to the first body snatcher circuit. 14. The interface output stage of claim 13 , wherein the transistor is a bipolar transistor. 15. The interface output stage of claim 11, wherein the first transistor is a Metal-Oxide- Semiconductor (MOS) transistor. 16. The interface output stage of claim 11, wherein the first body snatcher circuit comprises: a third transistor having a substrate interconnected to the substrate of the first transistor; and a fourth transistor having a substrate interconnected to the substrate of the first transistor. 17. The interface output stage of claim 16, wherein each of the first transistor, the third transistor and the fourth transistor are P-channel, Metal-Oxide-Semiconductor (MOS) transistors. -14- 18. The interface output stage of claim 16, wherein the pull-up circuit further comprises a first inverter having an output coupled to a gate of the first transistor and a supply input coupled to a common connection formed by substrates of the first transistor, the third transistor and the fourth transistor. 19. The interface output stage of claim 18, wherein the pull-up circuit further comprises at least two cascaded inverters in which an output from at least one of the at least two cascaded inverters is coupled to a gate of the fourth transistor and an output from the at least two cascaded inverters is coupled to a gate of the third transistor. 20. The interface output stage of claim 19, wherein the third transistor and the fourth transistor are turned on in a mutually exclusive manner. 21. The interface output stage of claim 13, wherein the current source is coupled to the collector of the transistor and a ground reference. 22. The interface output stage of claim 11, wherein the first body snatcher circuit comprises a third transistor and a fourth transistor having a substrate common with the substrate of the first transistor. 23. The interface output stage of claim 22, wherein the second body snatcher circuit comprises a pair of transistors having a substrate common with the substrate of the second transistor. -15- 24. A method for applying a high swing voltage from an interface output stage comprising the steps of: interconnecting substrates of a plurality of transistors of a pull-up circuit of the interface output stage to a first power supply terminal at least when a voltage at the first power supply terminal is equal to a voltage at an output terminal; and interconnecting substrates of the plurality of transistors of the pull-up circuit to the output terminal when the voltage at the output terminal is greater than the voltage at the first power supply terminal by a base-emitter voltage of a first transistor. 25. The method of claim 24 further comprising the steps of: interconnecting substrates of a plurality of transistors of a pull-down circuit of the interface output stage to a second power supply terminal line when the voltage at the output terminal is at least equal to a voltage at the second power supply terminal; and interconnecting substrates of the plurality of transistors of the pull-down circuit to the output terminal when the voltage at the second power supply terminal is greater than the voltage at the output terminal by a base-emitter voltage of a second transistor.

Inventors: BINGHAM DAVID, PARVARANDEH PIROOZ, SHIEH SUI PING, BINGHAM, DAVID, PARVARANDEH, PIROOZ, SHIEH, SUI, PING
Applicants: MAXIM INTEGRATED PRODUCTS, MAXIM INTEGRATED PRODUCTS, INC.
Application year: 1998
Main classification: H03K19
IPC codes: H03K19