LCD PWM Drive Sequencing to Reduce Phase Ripple
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Solution Overview
Problem
Phase ripple in liquid-crystal displays (LCDs) due to rapid voltage alternation causes image blurring, particularly in digitally-driven displays, where liquid crystal elements respond to alternating voltage pulses, leading to undesirable optical ripple.
Innovation Solution
A drive sequence for liquid crystal pulse width modulation (PWM) that evenly distributes pulses of slightly different widths over a time period, using a binary drive sequence to minimize phase ripple while achieving high grayscale granularity and using a low number of bits for encoding, with a method to generate drive sequences that reduce phase ripple and improve grayscale representation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If rapid voltage alternation is used to achieve accurate phase modulation, then phase modulation accuracy is improved, but phase ripple increases causing image blurring
Solution Approach 1:
The patent segments the voltage pulse into multiple sub-pulses with different widths within a single modulation period. Instead of using a single rectangular pulse, the voltage signal is divided into multiple smaller pulses that collectively achieve the desired duty cycle while reducing the peak voltage stress and minimizing phase ripple effects on the liquid crystal elements.
Solution Approach 2:
The patent employs dynamic adjustment of pulse widths and timing parameters to optimize the balance between phase modulation accuracy and ripple minimization. The drive sequence dynamically varies the pulse characteristics based on the required grayscale level and the response characteristics of the liquid crystal elements, allowing adaptive control to achieve low ripple across different operating conditions.
2Device complexity
If longer voltage pulse on/off times are used to simplify control, then device complexity is reduced, but phase ripple increases as liquid crystal element partially responds to each pulse
Solution Approach 1:
The patent segments the control signal into a sequence of shorter pulses rather than using a single long pulse. This segmentation allows the liquid crystal elements to fully respond to each short pulse without causing excessive ripple, while the cumulative effect of multiple pulses achieves the desired average voltage and grayscale level. The control logic remains relatively simple by using standard PWM techniques with adjusted pulse parameters.
3Object-affected harmful factors
If very short voltage pulses are used to minimize phase ripple, then phase ripple is reduced, but the number of pulses required increases affecting encoding complexity
Solution Approach 1:
The patent dynamically adjusts the pulse width and frequency parameters based on the required grayscale level and display conditions. For lower grayscale levels where ripple minimization is most critical, the system uses shorter, more frequent pulses. For higher grayscale levels, longer pulses are used to reduce the total number of switching events. This dynamic adaptation maintains low ripple while controlling encoding complexity through intelligent parameter selection.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution effectively reduces phase ripple in LCDs by evenly distributing pulse widths, improving grayscale representation and reducing power dissipation, while maintaining simplicity and efficiency in pixel circuitry.
Implementation Method 1
The liquid crystal (LC) element responds to this alternating series of pulses as a root mean squared (RMS) average voltage determined by the duty cycle
Implementation Method 2
Phase-modulation applications aim to minimize the amount of phase ripple
Data Source
AI summary
A method is disclosed for controlling a liquid crystal pulse width modulated display. A repetition period includes A group periods, each including B modulation intervals, each modulation interval spanning H unit durations and, except for the final modulation interval of the repetition period, a remainder unit duration. A desired number N of unit duration pulses are distributed into H unit duration pulses for each modulation interval, with remainder desired pulses distributed among the remainder unit durations of the modulation intervals. A drive sequence is generated, including one or more repetitions of the repetition period.