CAN Transmitter Switched Cascode for Low-Jitter Recessive Transitions
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Solution Overview
Problem
Implementations of CAN transceivers often experience excessive jitter and noise, particularly during transitions from dominant to recessive states in CANH and CANL signals, due to sensitivity to power transients.
Innovation Solution
The implementation of a CAN transceiver with a fast CANL control loop and switched output cascode, where the sources of cascode outputs are short-circuited to a cascode bias voltage during recessive states, reducing sensitivity to transient voltages and achieving high impedance, thereby minimizing jitter and noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional CAN transceiver output stages are used, then the transceiver can operate at standard bus rates, but excessive jitter and noise occur during transitions from dominant to recessive states due to sensitivity to power transients
Solution Approach 1:
The patent extracts the cascode output transistors from the active circuit during recessive states and switches them to a high-impedance state by connecting their sources to a cascode bias voltage. This removal of the sensitive output stage during transitions eliminates the source of jitter and noise while maintaining signal stability.
Solution Approach 2:
The patent dynamically switches the state of the cascode output transistors based on the CAN bus state. During dominant states, the transistors are active for signal transmission; during recessive states, they are switched to high-impedance to eliminate jitter. This dynamic switching resolves the contradiction between needing active outputs for transmission and inactive outputs for stability during transitions.
2Speed
If faster rise and fall times are achieved through aggressive switching, then higher bus rates are supported, but electromagnetic interference increases
Solution Approach 1:
The patent applies preliminary anti-action by proactively switching the cascode transistors to high-impedance state before or during transitions to recessive states. This preemptive action prevents the generation of electromagnetic interference during transitions while maintaining fast rise and fall times during active dominant state transitions, thus supporting higher bus rates without excessive EMI.
3Speed
If the output stage remains active during recessive states to maintain signal readiness, then faster response to dominant transitions is achieved, but sensitivity to power transients increases causing jitter
Solution Approach 1:
The patent prepares the output stage for rapid transition by maintaining the cascode transistor structure in place but switching it to a high-impedance state during recessive periods. This preliminary configuration allows instant activation when a dominant state is required while avoiding the jitter caused by keeping the transistors fully active during recessive states. The high-impedance state serves as a ready-but-quiescent condition.
Data Source
AI summary
A controller area network (CAN) transmitter includes an output stage circuit including a CANH port and a CANL port, and an input stage circuit configured to receive an input signal. The input signal is configured to indicate whether the output stage circuit is to provide dominant or recessive states. The CAN transmitter includes a cascode circuit configured to provide output signals on the output stage circuit to provide dominant or recessive states based on the input signal. The CAN transmitter includes a switch circuit configured to, based upon the input signal, switch the cascode circuit on and off.