CAN Transmitter Current Smoothing for Symmetrical Bus Transitions
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Solution Overview
Problem
CAN transmitters emit electromagnetic frequency (EMF) radiation due to asymmetrical voltage transitions on the high and low CAN bus lines during signal changes, which can be exacerbated by mismatched turn-on and turn-off times of current sources and sinks.
Innovation Solution
A CAN transmitter circuit design that includes bias voltage circuits, output control stages, and current source/sink circuits with matched RC time constants and linear resistance/voltage changes, ensuring symmetrical voltage transitions and reduced EMF radiation by controlling the turn-on and turn-off rates of MOSFETs.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If current sources and sinks are used to drive CAN bus lines, then signal transmission capability is improved, but asymmetrical voltage transitions occur causing EMF radiation
Solution Approach 1:
The patent intentionally introduces asymmetry in the form of different RC time constants for current sources and sinks to compensate for inherent circuit asymmetries. By carefully selecting resistor and capacitor values, the voltage transition profiles are balanced to achieve symmetrical rising and falling edges, thereby reducing EMF radiation while maintaining effective signal transmission.
Solution Approach 2:
The patent modifies circuit parameters (resistance and capacitance values) to control the turn-on and turn-off rates of current sources and sinks. By adjusting these parameters, the voltage transition characteristics are optimized to ensure symmetrical transitions, which reduces electromagnetic interference while preserving signal integrity.
2Speed
If current sources turn on and off at different rates, then switching speed is improved, but voltage symmetry is lost increasing EMF radiation
Solution Approach 1:
The patent employs asymmetric RC time constant design where current sources and sinks have deliberately different time constants. This asymmetry compensates for the inherently different switching characteristics of PMOS and NMOS transistors, ensuring that despite different turn-on and turn-off speeds, the overall voltage transitions remain symmetrical.
Solution Approach 2:
The patent uses dynamic RC time constant selection where the time constants are optimized for different switching phases. The circuit adapts the charging and discharging rates dynamically to achieve balanced voltage transitions, allowing fast switching while maintaining voltage symmetry through carefully engineered time constant relationships.
3Power
If multiple current source circuits are used to improve signal drive capability, then power transmission is improved, but device area increases
Solution Approach 1:
The patent divides the current drive function into multiple parallel current source circuits, each contributing to the overall signal drive capability. This segmentation allows the use of smaller individual transistors that can be efficiently laid out in parallel, achieving high drive current while optimizing the use of available device area through systematic arrangement.
Solution Approach 2:
The patent combines multiple current source circuits in parallel to achieve high signal drive capability. By merging the output currents of multiple smaller current sources, the circuit achieves the equivalent performance of a single large current source while benefiting from more efficient area utilization and reduced parasitic effects.
Data Source
AI summary
In described examples, a device includes a bias voltage circuit, an output circuit, and multiple current source circuits. A first plurality of the current source circuits is coupled between a first output of the bias voltage circuit and a first input of the output circuit. A second plurality of the current source circuits is coupled between a second output of the bias voltage circuit and a second input of the output circuit. Each of the current source circuits includes first and second resistors, first and second switches, and a transistor. The first switch is coupled between a gate of the transistor and the bias voltage circuit. The second switch is coupled between the gate of the transistor and a first terminal of the transistor. The second terminal of the transistor is coupled to the output circuit.