Abstract:
A switch circuit is disclosed. The switch circuit may include one or more arrangements of transistors coupled in a cascode configuration. The transistors used to implement the switch circuit may be configured for operation within a first range of voltages. The application in which the switch circuit may be implemented may require conveying signals within a second range of voltages that is greater than the first range of voltages. Thus, the switch circuit may include one or more additional transistors to ensure that a voltage drop between any two terminals of the transistors used in the switch circuit is within the first range of voltages.

Description:
BACKGROUND 
     1. Field of the Invention 
     This invention relates to electronic circuits, and more particularly, to switching circuits used in an interface. 
     2. Description of the Related Art 
     Switching circuits are well known in the electronic arts, and may be utilized in a wide variety of applications. These applications may include various types of analog and digital circuits. Some switching circuits may be used to select a signal from one of a number of different sources while other switching circuits may be used for gating a signal such that is may be conveyed from one location to another. 
     Various types of switching circuits may include one or more transistors. The signals used to activate these transistors may be within a pre-defined range of operating voltages. Similarly, the signals that are switched by operation of the transistors of a switching circuit may also vary within a pre-defined range of operating voltages. As technology has advanced, the size of transistors used in many switching circuits has decreased. Moreover, the operating voltages of many circuits in which switching circuits have also decreased. 
     SUMMARY 
     A switch circuit is disclosed. In various embodiments, the switch circuit may include one or more signal paths each including at least two transistors coupled in a cascode configuration. The transistors used to implement the switch circuit may be configured for operation within a first range of voltages. The application in which the switch circuit may be implemented may utilize signals that have a voltage swing (e.g., difference between logic high and logic low voltages) that is greater than the first range of voltages. Thus, the switch circuit may include one or more additional transistors to ensure that a voltage drop between any two terminals of the transistors used in the switch circuit is within the first range of voltages. 
     In one embodiment, a switch circuit includes a first transistor and a second transistor coupled in a cascode configuration between a first data node and a second data node. The transistors may be rated for operation within a first voltage range (e.g., 0-1.8 volts), while a maximum voltage difference between a first and second data nodes of the circuit may be within a second voltage range. The first transistor may be coupled to receive, on its gate terminal, a nominally fixed voltage during operation. The switch may be activated by the assertion of an enable signal on a gate terminal of the second transistor. A third transistor may be coupled to provide a source-drain path between the gate terminal of the first transistor and an intermediate node to which both the first and second transistors are coupled. The third transistor may become active responsive to a de-assertion of the enable signal. Activation of the third transistor may pull the intermediate node toward the nominally fixed voltage, and may thus ensure that a voltage drop between any two transistors of the switch circuit is within the first range of voltages. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following detailed description makes reference to the accompanying drawings, which are now briefly described. 
         FIG. 1  is a block diagram of one embodiment of an integrated circuit having a serial bus interface; 
         FIG. 2  is a schematic diagram of one embodiment of a switch circuit; 
         FIG. 3  is a block diagram of one embodiment of a computer system; and 
         FIG. 4  is a block diagram of one embodiment of a method for operating a switch circuit. 
     
    
    
     While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the appended claims. The headings used herein are for organizational purposes only and are not meant to be used to limit the scope of the description. As used throughout this application, the word “may” is used in a permissive sense (i.e., meaning having the potential to), rather than the mandatory sense (i.e., meaning must). Similarly, the words “include”, “including”, and “includes” mean including, but not limited to. 
     Various units, circuits, or other components may be described as “configured to” perform a task or tasks. In such contexts, “configured to” is a broad recitation of structure generally meaning “having circuitry that” performs the task or tasks during operation. As such, the unit/circuit/component can be configured to perform the task even when the unit/circuit/component is not currently on. In general, the circuitry that forms the structure corresponding to “configured to” may include hardware circuits. Similarly, various units/circuits/components may be described as performing a task or tasks, for convenience in the description. Such descriptions should be interpreted as including the phrase “configured to.” Reciting a unit/circuit/component that is configured to perform one or more tasks is expressly intended not to invoke 35 U.S.C. §112, paragraph six interpretation for that unit/circuit/component. 
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Turning now to  FIG. 1 , a block diagram of one embodiment of an integrated circuit (IC) having a serial bus interface is shown. In the embodiment shown, IC  10  includes a Universal Serial Bus (USB) interface (i.e. USB port) that includes USB switch unit  12 , transceivers  14 , and USB host controller  16 . A USB peripheral device  18  is coupled to IC  10  via a differential signal path that includes the true (D+) and complementary (D−) data lines of the bus. 
     IC  10  may be one of a number of different types of IC&#39;s, and thus may include various other functional units that are not explicitly shown here. In one embodiment, IC  10  may include one or more processors having one or more execution cores, various levels of cache memory, and so forth. IC  10  may also be a system on a chip (SOIC) in some embodiments, including processors, one or more peripherals, one or more memory controllers, etc. In another embodiment, IC  10  may be part of a computer system chipset, and thus may include the USB interface as well as a number of other bus interfaces (e.g., PCI/PCI-X, Firewire, GPIB, and so forth). Furthermore, various embodiments of IC  10  may be implemented in USB peripherals (e.g., printers, cameras, etc.) and portable devices (e.g., portable music players, phones, personal digital assistants, etc.). In general, IC  10  may be any type of IC in which the switch circuit to be discussed below may be implemented. It is noted that the use of USB in the embodiment of  FIG. 1  is exemplary. The use of other types of buses in conjunction with the switch circuit to be discussed below, both serial and parallel, is possible and contemplated. 
     In the embodiment shown, USB host controller  16  is configured to provide host functionality used in controlling communications over a USB. The functions provided by USB host controller  16  may include recognition of the connection of a peripheral device (e.g., USB peripheral device  18 ) to the USB, establishing communications between host controller  16  and peripheral device  18 , and controlling communications between other devices and/or functional units and USB peripheral device  18 . USB host controller  16  may also be configured to perform these functions for a number of different USB links in addition to the one that is explicitly illustrated in  FIG. 1 . 
     In the embodiment shown, USB host controller  16  is coupled to a pair of transceivers  14 . Each transceiver  14  may include a driver configured to drive signals through USB switch unit  12 , and a receiver coupled to receive signals from USB switch unit  12 . USB switch unit  12  may include first and second switches  20 , which correspond to respective differential signal lines D+ and D−, each of which may allow signals to be conveyed to or from the USB peripheral device when activated. The transfer of signals between USB peripheral device  18  and USB host controller  16  may be prevented when the switches of USB switch unit  12  are deactivated. 
     Turning now to  FIG. 2 , a schematic diagram of one embodiment of a switch circuit is shown. In the embodiment shown, switch circuit  20  may be one of one or more switch circuits that may be implemented in USB switch unit  12  of  FIG. 1 . Moreover, switch circuit  20  may be utilized in any implementation where it may provide a suitable switching function. 
     In the embodiment shown, switch circuit  20  includes two signal paths each including a pair of transistors coupled in a cascode configuration. In this particular embodiment, a signal path of switch circuit  20  includes transistors Q 1  and Q 2  coupled in a cascode configuration. NMOS transistor Q 1  in this embodiment includes a source-drain path coupled between a first data node  21  and a first intermediate node  22 . When operating, a gate terminal of transistor Q 1  may be coupled to receive a nominally fixed voltage (1.8 V in this particular example, although other voltages are possible and contemplated). The nominally fixed voltage may vary somewhat during operation, due to switching noise, power transients, and so forth, although generally it is intended that this voltage remain relatively constant (e.g., 1.8 volts±5%). 
     A second NMOS transistor Q 2  in the embodiment shown includes a source-drain path coupled between the first intermediate node  22  and a second data node  24 . A gate terminal of transistor Q 2  may be coupled to receive a true value of an enable signal, Enable_H. When the enable signal is asserted (high) in this embodiment, a first signal path may be provided between first data node  21  and second data node  24  through the source-drain paths of cascode-coupled transistors Q 1  and Q 2 . 
     In the example shown in  FIG. 2 , the second data node of switch circuit  20  is coupled to an exemplary transceiver  14 , which includes a driver  28  and a receiver  26 , each of which may be coupled to receive respective enable signals. A functional unit to which each transceiver  14  may be coupled (e.g., USB host controller  16  of  FIG. 1 ) may be configured such that only one of the drive enable and receive enable signals is asserted at a given time. Driver  28  may be configured to drive a signal onto second data node  24  when the driven enable signal is asserted. Similarly, receiver  26  may be configured to receive a signal from second data node  24  when the receive enable signal is asserted, and drive the received signal to another circuit responsive thereto. 
     In the embodiment shown, switch circuit  20  includes a second pair of cascode-coupled transistors which provide a signal path that is parallel with respect to the first signal path provided through transistors Q 1  and Q 2 . More particularly, the embodiment shown includes a third NMOS transistor Q 4 , which includes a source-drain path coupled between the first data node  21  and a second intermediate node  23 . When operating, transistor Q 4  may be coupled to receive the same nominally fixed voltage on its gate terminal that may also be received by transistor Q 1 . 
     A first PMOS transistor Q 5  is also included in the second signal path in this embodiment, having a source-drain path coupled between the second intermediate node  23  and the second data node  24 . A gate terminal of transistor Q 5  may be coupled to receive a complement, Enable_L, of the enable signal noted above. When the enable signal is asserted (and thus Enable_L is low), transistor Q 5  may activate and thus provide a second signal path between first data node  21  and second data node  24 . 
     In the absence of transistors Q 3  and Q 6  as shown in the embodiment of  FIG. 2 , the voltage difference that may exist between the first data node  21  and the second data node  24  may exceed the rated operating voltages (e.g., the maximum allowable voltage difference between any two terminals) for the other transistors of the circuit. Consider an example wherein the voltage signal swing on each of data nodes  21  and  24  may be 3.6 volts (e.g., a logic high is 3.6 volts, a logic low is 0 volts), in a switch circuit that differs from switch circuit  20  in that transistors Q 3  and Q 6  are note included. Consider further that each of the transistors of such a switch circuit may have a rated operating voltage of 1.8 volts, with each of the transistors having a threshold voltage of 0.3 volts. In such an example, a voltage difference of 3.6 volts may exist between data nodes  21  and  24  when the switch circuit is deactivated (e.g., when transistors Q 2  and Q 5  are turned off). However, since this particular example stipulates a threshold voltage of 0.3 volts for each of the transistors of the circuit, intermediate nodes  22  and  23  may charge up to 1.5 volts (since the gate terminals of Q 1  and Q 4  receive 1.8 volts) when switch circuit  20  is inactive. That is, nodes  22  and  23  may only charge to a threshold voltage below the gate voltage of transistors Q 1  and Q 4 , after which Q 1  and Q 4  may stop actively conducting current. 
     Accordingly, if first data node  21  is at a voltage of 3.6 volts and Q 2  is inactive because Enable_H is asserted, a voltage difference of 2.1 volts may exist between the source and drain terminals of each of transistors Q 1  and Q 4  (i.e. 3.6 volts−1.5 volts=2.1 volts). This 2.1 volt difference exceeds the rated operating voltage of 1.8 volts for the transistors considered in this example. Such a voltage excess may cause damage to transistors Q 1  and Q 4  and may even render them (and thus the switch circuit lacking transistors Q 3  and Q 6 ) inoperative. However, switch circuit  20  in the embodiment shown includes a pair of protection devices, transistors Q 3  and Q 6 , that may minimize or eliminate the voltage excess. 
     Transistor Q 3  in the embodiment shown is a PMOS transistor that includes a source-drain path coupled between intermediate node  22  and the nominally fixed voltage at the gate terminal of transistor Q 1 . Similarly, transistor Q 6  in the embodiment shown is also a PMOS transistor having a source-drain path coupled between intermediate node  23  and the nominally fixed voltage at the gate terminal of transistor Q 4 . Transistors Q 3  and Q 6  in the embodiment shown are each coupled to receive the Enable_H signal on their respective gate terminals. Since these transistors are PMOS devices in this embodiment, they are thus configured to activate when the enable signal is de-asserted (e.g., at a logic low, or 0 volts). Accordingly, for the embodiment of switch circuit  20  illustrated in  FIG. 2 , transistors Q 3  and Q 6  will be active when transistors Q 2  and Q 5  are inactive (i.e. when the signal path between data nodes  21  and  24  is blocked). When active, transistors Q 3  and Q 6  of this embodiment will pull intermediate nodes  22  and  23 , respectively, toward the nominally fixed voltage present on the gate terminals of Q 1  and Q 4  (e.g., 1.8 volts in the illustrated embodiment of switch circuit  20 ). Thus, the excessive voltage across the source-drain path of transistors Q 1  and Q 4  may be prevented when switch circuit  20  is inactive. 
     Generally speaking, various embodiments of switch circuit  20  as disclosed herein may include at least one signal path having a pair of transistors coupled in a cascode configuration, wherein one of the transistors is coupled to receive (on its respective gate terminal) an enable signal, while the other one of the transistors may included a gate terminal coupled to receive a nominally fixed voltage. Various embodiments of switch circuit  20  as disclosed herein may also include a protection device coupled to provide a signal path between the nominally fixed voltage and an intermediate node of the cascode configuration that may ensure that a voltage difference between any two terminals of a transistor in the circuit does not exceed its rated voltage when switch circuit  20  is turned off. A second signal path including a second pair of transistors coupled in a cascode configuration, along with the corresponding protection device, may also be included in various embodiments of switch circuit  20 . 
     It should be noted that the types of transistors, the various voltage levels, and the logic signal assertion levels discussed above are exemplary, and thus are not limiting. Numerous variations utilizing different types of transistors (e.g., PMOS instead of NMOS and vice versa), different voltage levels, different operating voltage ranges, and different logic levels are possible and contemplated. 
     Turning next to  FIG. 3 , a block diagram of one embodiment of a system  30  is shown. In the illustrated embodiment, the system  30  includes at least one instance of an integrated circuit  10  coupled to one or more peripherals  34  and an external memory  32 . A power supply  36  is also provided which supplies the supply voltages to the integrated circuit  38  as well as one or more supply voltages to the memory  32  and/or the peripherals  34 . In some embodiments, more than one instance of the integrated circuit  38  may be included. 
     The external memory  32  may be any desired memory. For example, the memory may include dynamic random access memory (DRAM), static RAM (SRAM), flash memory, or combinations thereof. The DRAM may include synchronous DRAM (SDRAM), double data rate (DDR) SDRAM, DDR2 SDRAM, DDR3 SDRAM, etc. 
     The peripherals  34  may include peripheral  18  shown in  FIG. 1 , and may include any desired circuitry, depending on the type of system  30 . For example, in one embodiment, the system  30  may be a mobile device and the peripherals  34  may include devices for various types of wireless communication, such as wifi, Bluetooth, cellular, global position system, etc. The peripherals  34  may also include additional storage, including RAM storage, solid-state storage, or disk storage. The peripherals  34  may include user interface devices such as a display screen, including touch display screens or multitouch display screens, keyboard or other keys, microphones, speakers, etc. 
     Turning now to  FIG. 4 , a flow diagram of one embodiment of a method for operating a switch circuit is shown. In the embodiment shown, method  40  includes activation of a switch circuit by asserting an enable signal (block  42 ). Using switch circuit  20  as an example, activation thereof may be accomplished by asserting the Enable_H signal at a logic high voltage. The logic high voltage may be received on the gate terminals of transistors Q 2 , Q 3 , and Q 6 . Transistor Q 2  is an NMOS transistor in the embodiment shown in  FIG. 2 , and thus activates responsive to the assertion of the Enable_H signal. Transistors Q 3  and Q 6 , which are PMOS transistors that function as protection devices in the embodiment shown in  FIG. 2 , are deactivated responsive to the assertion of the Enable_H signal. Transistor Q 5  is also a PMOS transistor in the embodiment of  FIG. 2 , and is coupled to receive the signal Enable_L, which is a complement of the Enable_H signal. Thus, when Enable_H transitions high, Enable_L may fall low, and transistor Q 5  may thus activate responsive to the low on its gate terminal. Thus, when transistors Q 2  and Q 5  are both active, two separate signal paths may be provided between second data node  24  and first data node  21 . 
     Deactivation of switch circuit  20  may be performed by de-asserting the enable signal (block  44 ). When Enable_H is de-asserted, it may fall low, while its complement, Enable_L, may transition high. Accordingly, transistors Q 2  and Q 5  may both become inactive, thereby blocking the signal paths between second data node  24  and first data node  22 . 
     Protection devices Q 3  and Q 6  may also be activated in switch circuit  20 , responsive to the de-assertion of the enable signal (block  46 ). When the Enable_H signal falls low, the low may be received on the gate terminals of protection devices Q 3  and Q 6 . These devices may then be activated. Transistor Q 3 , when active, may provide a source-drain path between intermediate node  22  and the nominally fixed voltage (e.g., 1.8 volts in the embodiment of  FIG. 2 ). Transistor Q 6  may similarly provide a source-drain path between intermediate node  23  and the nominally fixed voltage. When transistors Q 3  and Q 6  are active, intermediate nodes  22  and  23  may be pulled up toward the nominally fixed voltage. This may ensure, for example, that a voltage difference between data node  21  and either of intermediate nodes  22  and  23  does not exceed the rated operating voltage range for transistors Q 1  and Q 4 . Thus, for example, if transistors Q 1  and Q 4  are rated for operation in a range of 0-1.8 volts, a voltage of 3.6 volts on data node  21  may not damage these transistors, since intermediate nodes  22  and  23  may be pulled up to a voltage of 1.8 volts (through Q 3  and Q 6 , respectively). Thus, the voltage difference between data node  21  and either of intermediate nodes  22  and  23  may be 1.8 volts, which does not exceed the operating voltage range of transistors Q 1  and Q 4  in this example. 
     Numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. It is intended that the following claims be interpreted to embrace all such variations and modifications.