CAN Bus Driver Transition Control for Low-EMI Signal Switching
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Solution Overview
Problem
Conventional CAN bus driver circuits experience sharp transition points during mode changes, leading to poor EMI performance and asymmetry in output signals due to process variations, necessitating a solution for multiple phase accelerating and smoothing transitions.
Innovation Solution
A transition controlled circuit with multiple phases is integrated into the pre-driver architecture, utilizing a first and second pathway controlled unit with switches and impedance components, along with a switch control circuit to manage auxiliary switches for smooth phase transitions, ensuring low common mode noise and better EMI performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If conventional single-phase bus driver circuits are used, then the circuit structure is simple, but the transition points are sharp leading to poor EMI performance
Solution Approach 1:
The bus driver circuit is divided into multiple phases (first phase, second phase, third phase) with different impedance values. Each phase is controlled by separate control signals (first control signal, second control signal, third control signal) to sequentially adjust the impedance during transitions, thereby smoothing the transitions and reducing EMI without requiring a completely different circuit architecture.
Solution Approach 2:
The circuit employs dynamic impedance adjustment by switching between multiple impedance values during operation. The impedance of the bus driver is dynamically changed based on the transition state (rising or falling edge) to optimize the transition characteristics and reduce sharp transitions that cause EMI.
2Speed
If multi-phase design for accelerating transition is implemented, then the transition speed is improved, but the transition becomes too sharp leading to bad EMI performance
Solution Approach 1:
The bus driver transitions are divided into periodic phases (first phase, second phase, third phase) with different impedance values. During a rising transition, the impedance progresses through these phases sequentially; during a falling transition, the phases are reversed. This periodic phase structure accelerates transitions while smoothing the overall transition profile to reduce EMI.
Solution Approach 2:
The impedance parameter is changed across multiple discrete values during the transition process. By adjusting the impedance through different phases with different impedance values, the transition is accelerated while the gradual parameter change prevents sharp transitions that would generate EMI.
3Speed
If transistors are designed to enter saturation region, then the switching speed is improved, but asymmetry in output signals occurs due to process variation
Solution Approach 1:
The circuit deliberately employs asymmetric impedance values for rising and falling transitions. The first, second, and third impedance values are configured to compensate for the inherent asymmetry caused by transistor saturation and process variations, thereby achieving symmetric output signals despite the asymmetric transition paths.
Solution Approach 2:
The circuit uses separate control signals for rising and falling transitions that are generated based on the detected transition direction. This feedback mechanism allows the circuit to adjust the impedance progression differently for rising vs. falling edges, compensating for process variations and maintaining signal symmetry.
Data Source
AI summary
A bus driver module with controlled circuit is connected to a controller area network bus for generating a high side output or a low side output, comprising a transition controlled circuit and an output driver. The transition controlled circuit comprises a first pathway controlled unit connected in parallel with a second pathway controlled unit for generating a side switching voltage. The output driver is connected in series with the transition controlled circuit and receives the side switching voltage so as to accordingly generate the output bus signal. Each of the first and second pathway controlled unit comprises a plurality of switches and can be activated depending on an input signal. By controlling the switches of the first or second pathway controlled unit to be sequentially turned on and off successively, the side switching voltage is characterized by a smooth phase transition, low common mode noise and better EMI performances.