CAN Bus Transmitter Timing for Lower Spurious Emissions
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Solution Overview
Problem
Current CAN bus systems face challenges in meeting stringent emission requirements and robustness against external interferers, particularly at higher data rates, due to mismatched rise and fall times of CANH and CANL signal lines.
Innovation Solution
A CAN domino driver circuit with PMOSFET and NMOSFET legs, along with capacitors and resistors, is used to achieve matching rise and fall times, and a cascading domino driver circuit with adjustable time delays to optimize signal matching and emission performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-generated harmful factors
If conventional CAN bus drivers are used, then basic communication function is provided, but signal matching between CANH and CANL is poor resulting in high spurious emissions
Solution Approach 1:
The driver circuit is divided into separate CANH and CANL legs with independent control. Each leg has its own buffer amplifier and switching elements, allowing independent optimization of rise and fall times for each differential signal line to achieve better signal matching and reduce spurious emissions.
Solution Approach 2:
The circuit uses adjustable resistors and capacitors to independently control the rise time and fall time parameters of the CANH and CANL signals. By changing these time constants, the signal edges can be matched to minimize differential mode radiation and meet emission requirements.
2Productivity
If data rate is increased to improve communication speed, then productivity increases, but emission requirements become more difficult to meet
Solution Approach 1:
The driver circuit incorporates adjustable components that allow dynamic optimization of signal characteristics at different data rates. The rise and fall times can be tuned to maintain signal integrity and minimize emissions regardless of the operating speed, enabling the system to meet emission requirements across a range of data rates.
3Manufacturing precision
If simple driver circuit is used, then device complexity is low, but rise and fall time matching cannot be achieved
Solution Approach 1:
The circuit implements precise control locally at each differential leg with dedicated buffer amplifiers and timing components. This localized quality control allows independent optimization of signal edges without requiring complex global control mechanisms, achieving precise rise and fall time matching through modular design.
Data Source
AI summary
A CAN bus transmitter has an input to receive a transmit data signal, and CANH and CANL outputs coupled to a CAN bus. The CAN bus transmitter comprises a plurality of CAN driver circuits having inputs coupled through delay circuits with their CANH and CANL outputs in common and connected to the CAN bus. Matching of Cgs capacitances between devices of the CANH and CANL legs provides substantially synchronized changes in the CANH and CANL output logic levels upon a change in the input logic level. Variable delaying of the input logic level changes to each of the plurality of CAN driver circuits reduces emission of unwanted signals from the CAN bus.