HDMI Output Stage With Tunable Source Termination and Level Shifting
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Solution Overview
Problem
Conventional DC coupled transmitters face challenges in implementing source termination in high-performance applications like HDMI, particularly with the migration to lower I/O supply voltages, leading to reliability issues, leakage paths, and inability to enable/disable termination safely without violating HDMI specifications.
Innovation Solution
A 'supply-less' transmitter output stage with a tunable source termination and reference voltage generator, which includes a plurality of transmitter termination slices and level-shift-up blocks, allows for safe enabling/disabling of source termination using an internal supply voltage lower than the far-end termination voltage, minimizing junction stress and eliminating leakage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If a lower internal supply voltage (2.5V or 1.8V) is used in the transmitter output stage, then power consumption is reduced and migration to low voltage I/O devices is achieved, but reliability problems occur, leakage paths are created, and the ability to enable/disable termination is lost
Solution Approach 1:
A level-shifting circuit acts as an intermediary between the low-voltage internal supply (2.5V or 1.8V) and the high-voltage far-end termination (3.3V). This circuit translates the internal supply voltage to the pad voltage level, enabling the transmitter output stage to operate reliably at lower voltages while still providing proper termination at the 3.3V interface level, thus reducing power consumption without sacrificing reliability.
2Ease of manufacture
If a dedicated near-end 3.3V supply is provided for the transmitter output stage, then source termination can be easily implemented, but leakage paths still occur when the near-end 3.3V voltage is removed while far-end termination remains engaged
Solution Approach 1:
The termination enable/disable functionality is made dynamic through a control mechanism that adjusts the termination resistance based on the operational state. When termination is disabled, the control circuit adjusts the termination resistance to match the channel impedance, eliminating leakage paths. This dynamic adjustment allows the system to adapt to different operational modes without requiring a dedicated 3.3V supply that would create persistent leakage.
3Device complexity
If no termination is used, then power consumption is reduced and design simplicity is improved, but reflections caused by large impedance mismatch violate HDMI electrical specifications at high data rates
Solution Approach 1:
A tunable termination circuit is implemented that can dynamically adjust its resistance value based on the operating data rate. At high data rates, the termination resistance is adjusted to match the channel impedance, minimizing reflections and ensuring signal integrity. At lower data rates, the termination can be adjusted or disabled, reducing power consumption and design complexity. This dynamic tuning capability allows the system to meet HDMI specifications across different operating conditions.
4Reliability
If a differential fixed termination external to transmitter PHY is used, then reliability and signal integrity are improved, but cost increases significantly making it commercially non-viable
Solution Approach 1:
The termination circuit is merged with the transmitter output stage, integrating both functions into a single unified circuit block. This integration eliminates the need for separate external termination components, significantly reducing cost while maintaining signal integrity. The combined circuit shares common resources such as power supply, control logic, and physical space on the chip, making the solution commercially viable while preserving the reliability benefits of proper termination.
Data Source
AI summary
A “supply-less” transmitter output stage is provided for a transmitter. This transmitter output stage can include a tunable source termination and a reference voltage generator. The tunable source termination can be coupled between a differential pair of the transmitter. The reference voltage generator can advantageously generate reference voltages from a far-end termination. These reference voltages provide a way of translating the internal supply voltage level to the pad voltage level to enable/disable the tunable source termination. Also, it provides a way to minimize leakage and minimize the junction stress of switching transistors in the tunable source termination as well as the transmitter. The dependency between the reference voltages and the far-end termination voltage makes this design more portable to other supply voltages and other technologies specifications other than HDMI.