CAN Transmitter Feedback Biasing for Midpoint Voltage Stability
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Solution Overview
Problem
CAN differential signals can cause common mode voltage imbalances leading to electromagnetic interference (EMI) and electromagnetic emissions (EME), which are undesirable in automotive and safety-critical applications.
Innovation Solution
A CAN transmitter design with a feedback network that biases resistive elements to maintain a desired bus midpoint voltage and resistance matching, using a replica branch and feedback network to control bias voltages and resistances.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If CAN transmitters use simple resistive elements without feedback control, then device complexity is reduced, but common mode voltage imbalances occur causing EMI and EME
Solution Approach 1:
The patent implements a feedback network that monitors the actual midpoint voltage of the CAN bus and compares it to a reference midpoint voltage. The feedback signal adjusts the bias voltages of the resistive elements to maintain proper resistance matching and eliminate common mode voltage imbalances, thereby reducing EMI and EME without requiring complex external compensation circuits.
Solution Approach 2:
The patent uses a replica branch that creates a virtual copy of the output branch with scaled-down resistive elements. This replica allows the feedback network to simulate and control the midpoint voltage conditions without directly manipulating the main output signals, enabling precise control while maintaining circuit simplicity.
2Manufacturing precision
If resistance matching is not maintained, then manufacturing precision requirements are relaxed, but common mode voltage disturbances increase
Solution Approach 1:
The feedback network continuously monitors the midpoint voltage and adjusts the bias conditions of the resistive elements to maintain precise resistance matching. This active feedback compensation ensures that even with process variations and temperature changes, the resistance matching remains within tolerances that prevent common mode voltage disturbances.
Solution Approach 2:
The patent dynamically adjusts the bias voltages applied to the resistive elements based on feedback signals. By changing the bias voltage parameters in response to actual operating conditions, the circuit maintains optimal resistance matching without requiring extremely tight manufacturing tolerances on the resistive elements themselves.
3Manufacturing precision
If feedback network is added to control bias voltages, then resistance matching precision is improved, but device complexity increases
Solution Approach 1:
The feedback network utilizes a replica branch with scaled-down versions of the output branch circuitry. This allows the feedback mechanism to work with reduced voltage and current levels, simplifying the implementation of the feedback network while still achieving precise control of the main output resistive elements.
Solution Approach 2:
The feedback network is designed to serve multiple functions: it controls the bias voltages of the resistive elements, maintains resistance matching, and ensures proper midpoint voltage levels all through a single integrated feedback mechanism, reducing the need for separate control circuits.
Data Source
AI summary
A CAN transmitter includes an output branch, a replica branch including a replica of the output branch, and a feedback network. The output branch includes a first resistive element controlled by a first bias voltage and a second resistive element controlled by a second bias voltage. The replica branch has a feedback node that is replicated at a midpoint between the CANH bus terminal and the CANL bus terminal. The feedback network has a first input coupled to the feedback node, a second input configured to receive a midpoint reference voltage indicative of a desired midpoint voltage between the CANH and CANL terminals, and an output at which the first bias voltage is provided. A resistance controller is coupled to a control terminal of the second resistive element and configured to generate the second bias voltage based on a predetermined reference voltage and a bias current.