Programmable HDMI Termination Resistor With Dynamic Bias Control
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Solution Overview
Problem
Conventional programmable termination resistors in HDMI transmitters face accuracy issues due to changes in supply voltage and output current, leading to inaccuracy in calibrated termination resistance, especially when using low-voltage devices.
Innovation Solution
A termination resistor circuit with parallel branches of 4 resistors and a transistor switch, along with a current source and another switch, provides a stable biasing voltage to maintain constant drain-source resistance, ensuring high accuracy and reliable operation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a fixed bias voltage (Vbias) is used to control the transistor switch, then the device is protected from voltage stress, but the drain-source resistance becomes inaccurate due to variations in common mode voltage
Solution Approach 1:
The patent applies dynamics by transitioning from a fixed bias voltage to a dynamic bias voltage that adapts to changing common mode voltages. The bias voltage is generated proportionally to the common mode voltage, allowing the transistor's drain-source resistance to remain accurate despite variations in operating conditions. This dynamic adjustment resolves the contradiction by maintaining both device protection and measurement precision.
Solution Approach 2:
The patent changes the parameter of bias voltage from fixed to variable, where the bias voltage is now a function of the common mode voltage. By making the bias voltage proportional to the common mode voltage, the transistor operates in a consistent region regardless of supply voltage or output current changes, thereby maintaining accurate termination resistance while protecting the device.
2Ease of operation
If the gate of the transistor is connected to ground to turn it on, then the transistor switch functions correctly, but low-voltage devices cannot tolerate the voltage stress
Solution Approach 1:
The patent changes the gate voltage parameter from ground (0V) to a dynamic voltage level that is safely above ground but still low enough to protect the transistor. The gate voltage is set to common mode voltage minus a threshold voltage, ensuring the transistor turns on correctly without subjecting low-voltage devices to excessive stress, thus maintaining both operational ease and reliability.
3Object-affected harmful factors
If conventional source termination is used at high data rates, then reflection is reduced, but the termination resistance accuracy degrades due to supply voltage variations
Solution Approach 1:
The patent addresses this contradiction by making the termination resistance dynamic rather than fixed. The bias voltage applied to the transistor is changed from a fixed value to one that varies proportionally with the common mode voltage. This ensures that the effective termination resistance remains accurate despite supply voltage variations, while still providing the reflection reduction benefits of source termination at high data rates.
Data Source
AI summary
A supply-less transmitter output termination resistor with high accuracy is presented. This termination resistor can be used for applications with high supply voltage and low voltage devices. The termination resistor is programmable and includes many parallel branches. Each branch can be turned off or on with a switch. The biasing for the switch is in such a way that it keeps the resistance of the switch constant independent of the supply voltage or the output common mode voltage. This will increase the accuracy of the termination resistor. Besides HDMI this technique can be used for many other applications.