Dual-Mode Differential Driver for Shared LVDS and TMDS Outputs
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Solution Overview
Problem
Existing output drivers for Low Voltage Differential Signaling (LVDS) and Transition Minimized Differential Signaling (TMDS) are incompatible due to differences in common mode voltage and power requirements, leading to leakage current and potential device breakdown when sharing output terminals without modifications.
Innovation Solution
A dual-function driver circuit that includes a current steering circuit with PMOS and NMOS transistors, capable of switching between LVDS and TMDS modes by sharing input terminals and using a selectable current source to manage differential signals, preventing leakage and device breakdown through clamping and well controller mechanisms.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If separate LVDS and TMDS transmitters are used in different chips, then compatibility with respective specifications is achieved, but device complexity and integration are increased
Solution Approach 1:
The patent merges LVDS and TMDS transmitter functions into a single integrated chip. The output driver circuit is designed to support both LVDS and TMDS modes by selectively enabling different transistor pairs (first and second transistors for LVDS, third and fourth transistors for TMDS), eliminating the need for separate chips while maintaining specification compatibility
Solution Approach 2:
The output driver circuit is designed with universal functionality to support both LVDS and TMDS specifications. By incorporating controllable transistor pairs that can be selectively activated, the same circuit structure can adapt to different signaling standards, allowing one chip to perform multiple functions that previously required separate dedicated devices
2Reliability
If LVDS output driver is used for TMDS transmission, then device breakdown occurs due to voltage incompatibility, but using separate drivers increases system complexity
Solution Approach 1:
The output driver employs dynamic configuration through control signals that selectively enable or disable specific transistor pairs based on the operating mode. In LVDS mode, the first and second transistors are enabled while the third and fourth are disabled; in TMDS mode, the configuration is reversed. This dynamic switching prevents voltage incompatibility issues while maintaining a unified circuit structure
Solution Approach 2:
The circuit changes its electrical parameters (which transistors are conductive, current paths, voltage levels) based on the operating mode. By controlling the switching state of different transistor pairs, the driver adapts its electrical characteristics to match either LVDS or TMDS requirements, preventing device breakdown while using a single unified driver circuit
3Speed
If current steering is used to create differential voltage, then high transmission rate is achieved, but leakage current occurs when switching between modes
Solution Approach 1:
The circuit design extracts and isolates the current steering paths for LVDS and TMDS modes into separate, non-overlapping transistor pairs. When switching between modes, one pair is completely disabled while the other is activated, preventing leakage current by ensuring no partial conduction occurs during mode transitions
Data Source
AI summary
Dual-function drivers capable of outputting LVDS or TMDS differential signals by sharing output terminals under differential modes. In the dual-function driver, an input control unit receives a first input signal compliant with a first specification in a first mode and a second input signal compliant with a second specification in a second mode by sharing a pair of input terminals, and a current steering circuit comprises first and second differential pairs. The input control unit enables the first and second differential pairs to output a first differential signal compliant with the first specification through a pair of output terminals during the first mode, and the input control unit disables the first differential pair and enables the second differential pair to output a second differential signal compliant with the second specification on the pair of output terminals during the second mode.