Abstract:
A controller area network transceiver and a transmission method for a controller area network provides improved symmetry between its differential output signal CANH and CANL such that capacitive imbalance is minimized. The transceiver disclosed herein includes a driver including a non-inverted output that couples to the first output terminal CANH and a inverted output that couples to the second output terminal CANL. A receiver comparator includes a non-inverted input coupled to the first output terminal CANH and a inverted input coupled to the second output terminal CANL. A first and second impedance matching circuit portions capacitively balance the first and second output terminals such that efficient common-mode rejection is enabled by setting the RC time constants formed by each impedance matching circuit and external resistances to be substantially equivalent. This transceiver provides a high performance, simple, and cost effective design which eliminates capacitive imbalance while decreasing required die area.

Description:
FIELD OF THE INVENTION 
   The present invention relates to controller area network transceivers and, more particularly, to a controller area network transceiver having a capacitive balancing circuit for superior receiver common-mode rejection. 
   BACKGROUND OF THE INVENTION 
   A controller area network (CAN) bus, such as that compliant with the ISO 11898 standard, is used in several systems including industrial, automotive, robotic, and motor control systems to provide a serial communication physical layer. The robust CAN bus provides low power requirements, space savings and reduced resources. As shown in  FIG. 1 , a CAN node is comprised of three basic parts: a processor, a network controller, and a transceiver. The transceiver interfaces the single-ended CAN controller with the differential CAN bus. The bus, as shown in  FIG. 2 , includes multiple nodes that transmit messages on-demand by any node whenever the bus is free. The transceiver broadcasts data such that all nodes receive each message sent on the bus, including the node that sent it. The effect of broadcasting of data allows multiple nodes to utilize the data transmitted. 
   Texas Instruments, Inc. ® has introduced CAN transceiver models: SN65HVD230, SN65HVD231, and SN65HVD232, for use in applications employing the CAN serial communication physical layer compatible with the ISO 11898 Standard. As a CAN transceiver, as shown in  FIG. 3   a , each model provides differential transmit capability to the bus and differential receive capability to a CAN controller at speeds of up to 1 Mbps. Designed for operation in especially harsh environments, the CAN transceiver features cross-wire, over voltage and loss of ground protection from −4 V to +16 V, over-temperature protection as well as −2 V to 7 V common-mode range, and withstands transients of ±25 V. 
     FIG. 3   b  represents the timing diagrams of several signals including the signals found at driver node D, receiver node R, differential nodes, CANH and CANL, and voltage output differential V od  of CAN transceiver  10 . As shown, when the driver input D is low, the differential output nodes, CANH and CANL, are high and low respectively. When the driver input D is high, however, transceiver  10  goes into a tri-state mode where the differential output nodes, CANH and CANL, are both tri-state. 
   In general, the input capacitance of the receiver comparator  14  is capacitively balanced without any added circuitry if the transistors that form the input differential pair within the receiver comparator  14  are made the same size. Conventionally, these transistors are made the same size. Driver  12 , however, includes driver outputs that tend to be the source of capacitive imbalance on the receiver input pins. 
   To correct this capacitive imbalance, additional impedance balancing circuitry must be added to the differential nodes, CANH and CANL, in compliance with the ISO 11898 physical layer standard. Driver arbitration problems arise when one driver on the bus is trying to pull a bus line low while another driver is trying to the pull the same bus line high. The ISO 11898 physical layer standard avoids driver arbitration problems by requiring that only an active pull-up device may be connected to the CANH node and only an active pull-down device may be connected to the CANL node. 
   A known CAN transceiver compliant with ISO 11898 physical layer standard, as shown in  FIG. 4 , includes impedance matching circuitry formed using bipolar transistors,  36  and  42 , and blocking Schottky diodes,  38  and  40  connected as shown. Differential node CANH 1  uses a PNP (or PMOS) transistor  36  as an active device, while differential node CANL 1  uses a NPN (or an NMOS) transistor  40  as an active device. A disadvantage of this design is that the upper PNP transistor  36  connected to node CANH 1  must be substantially larger than the lower NPN transistor  42  connected to node CANL 1  to meet output differential voltage requirements of the ISO11898 standard. Since upper PNP transistor  36  has more capacitance than the lower NPN transistor  42  and the driver outputs,  44  and  46 , are indirectly connected in parallel to the receiver inputs, CANH 1  and CANL 1 , through transistors,  36  and  42 , capacitive imbalance still exists on the receiver inputs, CANH 1  and CANL 1 . Conventionally, the CANH 1  node will have a capacitance of 18 pF while the CANL 1  node will have a capacitance of 7 pF, leaving a difference of 11 pF. 
   Furthermore, capacitive imbalance presents another problem in that it prevents the transceiver from passing a common mode rejection test which is conducted in an effort to make certain that the impedance matching circuitry guards the receiver against common-mode transients such as noise which causes the differential output nodes, CANH 1  and CANL 1 , to be pulled higher than the power supply rail voltage V CC  or lower than ground. 
   Given a common-mode rejection test implementation, as shown in  FIG. 5   a , if during the application of the input differential voltage to the differential output nodes, CANH 1  and CANL 1 , the receiver output node R 1  experiences a change of state from low to high when it should remain low, then transceiver  30  has failed the common-mode rejection test. If, however, during application of the differential applied voltage, the receiver output node R 1  remains low or does not experience a change of state, then transceiver  30  has passed the common-mode rejection test. Failure of the common-mode rejection test stems from the difference between the RC time constant of differential output nodes, CANH and CANL. 
   Conforming with ISO11898 requirements,  FIG. 6  illustrates another approach to eliminate the capacitive imbalance in a transceiver architecture. As shown, driver  102  includes outputs,  104  and  106 , couples to nodes, CANH 2  and CANL 2 , respectively. Circuits  114  and  120  serve as active devices for transceiver  100 . Circuit  114  includes transistor  122  having a gate coupled to power supply V CC , a drain coupled to the gate of transistor  124  and a source coupled to node CANH 2 . Transistor  124  includes a drain coupled to the power supply V CC  and a source couple to node CANH 2 . Schottky diode  126  couples between power supply V CC  and bulk nodes of transistors,  122  and  124 . Circuit  120  includes transistor  140  having a gate coupled to power supply V CC , a drain coupled to a Schottky diode  138 , a source coupled to ground and a bulk coupled to the source. Schottky diode  138  couples between node CANL 2  and the drain of transistor  140 . Redundancy through circuits,  116  and  118 , is incorporated within this design to correct any capacitive imbalance, such that circuit  116  is a replica of circuit  114  and circuit  118  is a replica of circuit  120 . 
   Dummy devices,  116  and  118 , are placed on nodes CANH 2  and CANL 2  to mimic the capacitance of the active driver on the corresponding node opposite each respective node. Thereby, differential output nodes CANH 2  and CANL 2  are capacitively balanced. The disadvantage of this architecture is that the dummy devices,  116  and  118 , are designed to be as large as the active devices,  114  and  120 , to perfectly balance the receiver input capacitance. The capacitance, however, does not need to be perfectly balanced; rather, the RC time constant is formed by external resistors in a receiver common-mode rejection test implementation and the capacitance of nodes, CANH 2  and CANL 2 , must be balanced. 
   Furthermore, in order to remain price competitive, minimization of the CAN transceiver die size is an important design criteria. Removing dummy devices,  116  and  118 , from the CAN transceiver architecture would suffice to lower die area and would not result in any performance drawbacks; however, it will cause transceiver  100  to fail common-mode rejection test. 
   Therefore, a need exists for a CAN transceiver that provides improved impedance matching between differential output signals, CANH and CANL, while the requirements of die area are decreased. The impedance matching circuit portions must connect to differential output nodes, CANH and CANL, and capacitively balanced differential output nodes, CANH and CANL, such that the RC time constants formed with external test resistors are equivalent. As a result, the improved transceiver must provide superior common-mode rejection, passing the aforementioned common-mode rejection test. 
   SUMMARY OF THE INVENTION 
   To address the above-discussed deficiencies of CAN transceiver, the present invention teaches a CAN transceiver that eliminates capacitive imbalance within transceiver terminals CANH and CANL while preserving the space requirement on the die to a minimum. This CAN transceiver includes impedance matching sufficient to warrant a 100% pass rate during the common-mode rejection test, rendering superior common-mode rejection. A transceiver in accordance with the present invention includes a driver, having an input, a non-inverted and an inverted output, to generate a first and second output signal at the respective non-inverted and inverted outputs. The non-inverted output couples to the first output terminal CANH and the inverted output couples to the second output terminal CANL. A receiver comparator includes a non-inverted input coupled to the first output terminal and a inverted input coupled to the second output terminal. 
   A first and second impedance matching circuit portion capacitively balance the first and second output terminals such that efficient common-mode rejection is enabled. The first impedance matching circuit portion couples between the non-inverted output of the driver and the non-inverted input of the receiver comparator to receive the first output signal. This first impedance matching circuit portion provides a first capacitance at the first output terminal CANH. Under common-mode rejection testing, the first impedance matching circuit portion connects with a first external resistor to supply a first RC time constant. The second impedance matching circuit portion couples between the inverted output of the driver and the inverted input of the receiver comparator to receive the second output signal. This second impedance matching circuit portion provides a second capacitance at the first output terminal CANL. Under common-mode rejection testing, the second impedance matching circuit portion connects with a second external resistor to supply a second RC time constant. The first and second impedance matching circuit portions are designed such that the first and second capacitance are substantially equivalent. As a result, the first and second RC time constants are substantially equivalent, rendering superior common-mode rejection. 
   Advantages of this design include but are not limited to a CAN transceiver having a high performance, simple, and cost effective design which eliminate capacitive imbalance to provide superior common-mode rejection. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention and the advantages thereof, reference is now made to the following description taken in conjunction with the accompanying drawing in which like reference numbers indicate like features and wherein: 
       FIG. 1  illustrates a known structure of a controller area network (CAN); 
       FIG. 2  displays a known CAN bus having a multipoint topology; 
       FIGS. 3   a  and  3   b  illustrate a known CAN transceiver architecture and signal characteristics, respectively; 
       FIG. 4  shows an existing CAN transceiver architecture; 
       FIG. 5   a  illustrates the test implementation for receiver input common-mode input voltage rejection test; 
       FIG. 5   b  displays the common-mode input signal V 1 ; 
       FIG. 6  displays a known CAN transceiver architecture; 
       FIG. 7  illustrates a CAN transceiver architecture in accordance with the present invention; 
       FIG. 8   a  shows the timing diagram for the tests results of the 900 mV input differential signal on the known CAN transceiver architecture; 
       FIG. 8   b  displays the timing diagram for the tests results of the 500 mV input differential signal on the known CAN transceiver architecture; 
       FIG. 9   a  shows the timing diagram for the tests results of the 900 mV input differential signal on the CAN transceiver architecture in accordance with the present invention; and 
       FIG. 9   b  displays the timing diagram for the tests results of the 500 mV input differential signal on the CAN transceiver architecture in accordance with the present invention. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   The present invention is best understood by comparison with the prior art. Hence, this detailed description begins with a discussion of known CAN transceiver  30  as shown in  FIG. 4  and the common-mode rejection test as applied to this CAN transceiver. As described known CAN transceiver, as shown in  FIG. 4 , includes impedance matching circuitry formed using bipolar transistors,  36  and  42 , and blocking Schottky diodes,  38  and  40  connected as shown. Transceiver  30  has a capacitive imbalance since upper PNP transistor  36  has more capacitance than the lower NPN transistor  42 . 
   As explained, capacitive imbalance leads to failure of the common mode rejection test which is conducted in an effort to make certain that the impedance matching circuitry guards the receiver against common-mode transients such as noise which causes the differential output nodes CANH 1  and CANL 1  to be pulled higher than the power supply rail voltage or lower than ground. 
   A particular approach to common-mode rejection testing is shown in  FIG. 5   a . The input offset voltage of a CAN receiver is typically 700 mV. Thus, guaranteed trip points are exhibited when the input differential voltage V id  is less than or equal to 500 mV, forcing the receiver output R high, and when the input differential voltage V id  is greater than or equal to 900 mV, forcing receiver output R low. To ensure that the receiver  34  is tested for common-mode signal rejection exactly at these guaranteed trip points, external resistors, R 1  and R 2 , bias the receiver inputs to either 500 or 900 mV while the common-mode signal V 1  is injected between the resistors, R 3  and R 4 . 
   To implement this particular testing scheme, resistors, R 1  and R 2 , are of the same value, 450Ω, while resistors, R 3  and R 4 , are 50Ω for the purpose of biasing input differential voltage V id  equal to 500 mV. In contrast, resistors, R 1  and R 2 , are 227Ω and resistors, R 3  and R 4 , are 50Ω for the purpose of biasing input differential voltage V id  equal to 900 mV. The common-mode signal V 1  may be an alternating voltage such as the sine wave shown in  FIG. 5   b  which extends from approximately −7 V to 12 V. All input signals of the input differential voltage V id  may be supplied by a generator similar to the sine wave shown in  FIG. 5   b  having a frequency of less than or equal to 1.5 MHz. 
   If during the application of the input differential voltage V id , output R experiences a change of state from low to high when it should remain low, then the transceiver has failed the common-mode test. If, however, during application of the differential applied voltage V id , output R remains low or does not experience a change of state, then the transceiver has passed the common-mode test. 
   As shown in  FIGS. 8   a  and  8   b , CAN transceiver  30  of  FIG. 4  fails the common-mode rejection test when the input differential voltage V id  is 500 mV and 900 mV, respectively. In  FIG. 8   a , receiver node R should have remained low yet it spiked high for a short period of time. In  FIG. 8   b , receiver node R should have remained high yet it spiked low for a short period of time. Both transitions are indications of failure of the common-mode rejection test. 
   Failure of the common-mode rejection test stems from the difference between the RC time constant of nodes, CANH and CANL. This RC time constant is formed by external resistors, R 1  and R 2 , and the device node capacitance of nodes CANH and CANL. The difference between the RC time constants of differential nodes, CANH and CANL, causes a slow down in the common-mode input signal V 1  that is greater on one node versus the other. As a result, the input differential voltage V id  shrinks or expands which may lead to data errors if the input differential voltage V id  changes for a substantial amount of time. 
   To address the deficiencies of known CAN transceivers, the CAN transceiver  150  in accordance with the present invention, shown in  FIG. 7 , provides impedance matching between differential output signals, CANH 3  and CANL 3 , while the requirements of die area are decreased. Driver  152  includes input D 3  and differential outputs,  154  and  156 , couples to nodes, CANH 3  and CANL 3 , respectively. Circuits  158  and  160  serve as active devices or impedance matching circuits for each terminal node CANH 3  and CANL 3 , respectively. Circuit  158  couples between the non-inverting output  154  of driver  152  and the non-inverting input  164  of receiver comparator  162 . Circuit  158  includes transistor  168  having a gate coupled to power supply V CC , a drain coupled to the gate of transistor  170  and a source coupled to node CANH 3 . Transistor  170  includes a drain coupled to the power supply V CC  and a source couple to node CANH 3 . Schottky diode  172  couples between power supply V CC  and bulk nodes of transistors,  168  and  170 . Circuit  160  couples between the inverting output  156  of driver  152  and the inverting input  166  of receiver comparator  162 . Circuit  160  includes transistor  182  having a gate coupled to a pre-drive signal V pd , a drain coupled to a first Schottky diode  180 , a source coupled to ground and a bulk coupled to the source. Schottky diode  180  couples between node CANL 3  and the drain of transistor  182 . A second transistor  176  includes a gate coupled to power supply V CC , a drain coupled to node CANL 3 . A Schottky diode  178  couples between the power supply V CC  and the bulk and source of transistor  176 . A capacitor  174  couples between the source of transistor  176  and ground. 
   In operation the gate of transistor  170  is pulled low to turn transistor  170  on and pull differential output node CANH 3  high, approximately the power supply voltage V CC . When the gate of transistor  182  is high, the differential output node CANL 3  is pulled low, approximately ground. Conversely, when the gate of transistor  170  is high, transistor  170  turns off, pulling differential output node CANH 3  into a tri-state. Accordingly, when the gate of transistor  182  is low, transistor  182  is turned off, pulling differential output node CANL 3  into tri-state. Transistor  168  prevents reverse conduction of transistor  170  in the case where node CANH 3  is pulled to a voltage higher than power supply voltage V CC . In this scenario, transistor  168  pulls the gate of transistor  170  to the same voltage of the drain of transistor  170  to effectively short the transistor  170  when the voltage applied to node CANH 3  is greater than the power supply voltage V CC . Whenever differential output node CANL 3  is above power supply voltage V CC , transistor  176  turns on to enable current to flow across capacitor C 1 . 
   Transceiver  150  provides a low die area solution that passes the common-mode rejection test. A simple but effective capacitance balance is achieved by adding PMOS transistor  176  with backgate blocking diode  178  and capacitor  174 . Theses additional elements balance out the capacitance between nodes, CANH 3  and CANL 3 , by allowing extra capacitance to be seen by the CANL 3  pin when its potential is a threshold voltage V t  above power supply voltage V CC . Accordingly, the impedance matching circuit  160  mimics the capacitance seen on node CANH 3  when its potential is a threshold voltage V t  above power supply voltage V CC . Although the impedance matching circuit  160  does not perfectly balance the capacitance between nodes, CANH 3  and CANL 3 , it provides sufficient capacitive balancing requiring less die area to achieve superior common-mode rejection. Simulations across common mode range of −7 to +12 V reveal the capacitance of the CANH 3  and CANL 3  pins are balanced within 10 pF. 
   The new architecture passes the receiver common-mode rejection test as shown in  FIGS. 9   a  and  9   b .  FIGS. 9   a  and  9   b  illustrate that the CAN transceiver architecture in accordance with the present invention passes the common-mode rejection test when the input differential voltage V id  is 500 mV and 900 mV, respectively. As displayed, during application of the differential applied voltage V id , output R 4  remains low or does not experience a change of state. Thereby, the transceiver has passed the common-mode test. 
   This architecture presents a size savings of approximately 10% over the capacitively balanced architecture shown in  FIG. 6 . Thereby, the transceiver in accordance with the present invention gives a cost and performance advantage over other known alternate architectures. 
   Advantages of this design include but are not limited to a CAN transceiver having a high performance, simple, and cost effective design that eliminates capacitive imbalance to provide superior common-mode rejection. 
   The reader&#39;s attention is directed to all papers and documents which are filed concurrently with this specification and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference. 
   All the features disclosed in this specification (including any accompany claims, abstract and drawings) may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features. 
   The terms and expressions which have been employed in the foregoing specification are used therein as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding equivalents of the features shown and described or portions thereof, it being recognized that the scope of the invention is defined and limited only by the claims which follow.