The Design Of A High-Speed Low Supply Current CAN-bus Transceiver

CAN (Controller Area Network) is one of the serial communication networks, which is the system of distributed control and real-time control. It is one of the most widely used field buses in the world. BOSCH Company developed CAN in 1986 as a communication protocol for automobile. Thereafter, CAN was standardized in ISO-11898 and ISO-11519. Today, CAN is widely accepted for its high performance and reliability, and is used in a broad range of fields from industrial automation and ships to medical and industrial equipment. At present in the market, almost all of the CAN chips are manufactured by foreign semiconductor companies. Chinese universities and institutes focus on the applications of CAN bus, compared with lack of the research on CAN chips. In this thesis, a High-speed Low current supply CAN transceiver is proposed, which fully is suitable for ISO-11898 standard and has an autoshutdown function.At first the electric standard specifications of CAN transceiver are introduced. According to the mainly CAN transceivers datasheets, the block diagram of the CAN transceiver in this thesis is presented. Then the sub-circuits are designed in transistor level, and the simulation results show that the proposed CAN transceiver is suitable for ISO-11898 standard.The sub-circuits of CAN transceiver include 2.5V bandgap reference circuit, over-temperature protection circuit, autoshutdown circuit and wake-up circuit. The transmitter consists of chains of cascaded inverters driving circuit, output circuit with over-voltage protection and slope control circuit with adjustable slew rate. The receiver is a hysteresis voltage comparator. This thesis introduces a 2.5V bandgap reference circuit which uses a PTAT (Proportional To Absolute Temperature) current source. Considering VBE of PNP bipolar transistor has a minus temperature coefficient, we can use it to compensate the plus temperature coefficient of PTAT current source. Then the 2.5V bandgap reference circuit with a very small temperature coefficient and an excellent ability of PSRR is achieved. The 2.5V bandgap reference can provide a stable DC bias when CAN bus is in recessive state. It can also provide a stable reference voltage for other circuits.The temperature detection part of the over-temperature protection circuit is a resistor whose current bias is PTAT current source. The thermal oscillation is solved effectively by a positive feedback. The temperatures of over-temperature protection and over-temperature protection's rescission can be precisely set up by the resistance value.To manage power consumption and achieve the goal of low power, the autoshutdown circuit can put the transceiver into shutdown mode after the transceiver has been inactive for a period of time (T_SHDN). In autoshutdown mode, the main parts of transceiver are shut down. The value of an external capacitor (C_SHDN) connected to SHDN determines the threshold of inactivity time. The wake-up circuit can detect the voltage of CAN bus. Once a dominant signal is detected, the wake-up circuit can send a wake-up signal to autoshutdown circuit, and then the whole transceiver will work.In the transmitter part, two chains of cascaded inverters are used to drive two large size power MOS transistors. The driving current of PMOS can reach 810mA,the driving current of NMOS can reach 410mA. The total propagation delay of the driving circuits are approximately 5ns (PMOS) and 6ns (NMOS). The reason of designing two different chains of cascaded inverters is that the area of PMOS is larger than the area of NMOS, and PMOS needs a larger driving current. The output stage of transmitter sends differential signal to CAN-bus. In dominant state, the differential voltage on CAN bus is 2.6V ( V_CANH = 3.75V, V_CANL = 1.15V). In recessive state, the differential voltage on CAN bus is 0V ( V_CANH = V_CANL=2.5V). The output stage of transmitter also has over-voltage function which can protect the transceiver when the voltage on CAN bus is abnormal. Connecting a resistor from RS to ground is to select slope control mode. By regulating the resistance value of RS we can control the value of output signal's slew rate and reduce EMI effect.The receiver circuit is a hysteresis voltage comparator. It can eliminates the differential mode noise interference effectively. When CAN bus is in the dominant state (V_CANH―V_CANL>0.9V), the RXD sends a low logic voltage. When CAN bus is sin a recessive state (V_CANH―V_CANL<0.5V), the RXD sends a high logic voltage.