LC Driver Output Circuit Segmentation for Voltage Reliability
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Solution Overview
Problem
Existing output circuits for liquid crystal display devices face challenges in reliably and efficiently switching between positive and negative voltages due to the risk of voltage differences exceeding the withstand voltage of transistor switches, leading to parasitic bipolar transistor activation and potential latch-up issues, especially when using single conductive MOS transistor switches.
Innovation Solution
The proposed output circuit employs a combination of PMOS and NMOS transistor switches with voltage control circuits to manage the back gate voltage, ensuring the voltage difference between the source, drain, and gate terminals remains within the withstand voltage range, thereby preventing parasitic bipolar transistor activation and using CMOS switches for wide voltage range compatibility.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Area of stationary object
If a single conductive MOS transistor switch is used to switch between positive and negative voltages, then the circuit area is reduced, but the voltage difference may exceed the withstand voltage of the transistor, causing parasitic bipolar transistor activation and latch-up issues
Solution Approach 1:
The patent divides the single transistor switch into two separate transistor switches: a first transistor switch for switching positive voltage and a second transistor switch for switching negative voltage. Each transistor only needs to withstand half the total voltage range, preventing parasitic bipolar transistor activation while maintaining compact area through shared control circuitry.
Solution Approach 2:
The patent changes the voltage parameter constraints by using two transistors with different withstand voltage ratings matched to their respective voltage ranges. The first transistor handles positive voltage within its withstand capacity, and the second transistor handles negative voltage within its withstand capacity, eliminating the risk of exceeding individual transistor voltage limits.
2Ease of manufacture
If the withstand voltage of transistor switches is kept low (1/2 of liquid crystal drive voltage range), then elements with low withstand voltage can be used, but complementary switches (P-channel and N-channel combined) are not suitable and single conductive transistor switches must be used
Solution Approach 1:
The patent segments the switching function into two separate transistor switches with different conductive types. The first transistor switch uses a first conductive type (e.g., P-channel) for positive voltage switching, while the second transistor switch uses a second conductive type (e.g., N-channel) for negative voltage switching. This segmentation allows each transistor to operate within safe voltage limits while maintaining manufacturing flexibility.
Solution Approach 2:
The patent employs asymmetric transistor configuration where the first and second transistor switches have different conductive types and are optimized for their respective voltage polarities. This asymmetric design allows each transistor to be manufactured with appropriate characteristics for its specific voltage range, improving both ease of manufacture and reliability.
3Reliability
If polarity inversion driving is implemented to prevent liquid crystal panel deterioration, then display reliability is improved, but the output circuit must handle wide voltage ranges which increases circuit complexity
Solution Approach 1:
The patent segments the output circuit into dedicated positive voltage switching and negative voltage switching paths. Each path uses a transistor switch optimized for its voltage range, simplifying the design of each individual switching path while enabling the overall polarity inversion function required for display reliability.
Solution Approach 2:
The patent uses a shared control circuit that generates control signals for both transistor switches, allowing a single control mechanism to manage both positive and negative voltage switching. This multi-functionality reduces overall circuit complexity despite handling wide voltage ranges for polarity inversion driving.
Data Source
AI summary
The disclosure provides an output circuit, a display driver including the output circuit and a display device. The disclosure includes a PMOS transistor switch that outputs a positive voltage signal from an output terminal when it is turned on, an NMOS transistor switch that outputs a negative voltage signal from the output terminal when it is turned on, and a voltage control circuit that supplies a voltage obtained by shifting the level of a voltage of a source or a drain when the PMOS transistor switch is turned on to a high potential side to a back gate of the PMOS transistor switch and supplies a voltage obtained by shifting the level of a voltage of a source or a drain when the NMOS transistor switch is turned on to a low potential side to a back gate of the NMOS transistor switch.