Column Driver Circuit PWM Control for AMLCD Silicon Area Reduction
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Solution Overview
Problem
Existing active matrix liquid crystal display (AMLCD) driver ICs face challenges with increasing colour depth, leading to high silicon area requirements and image artefacts due to non-ideal voltage accuracy and the need for complex resistor ladder architectures, which are unsuitable for low silicon-area implementations and cascaded IC configurations.
Innovation Solution
The implementation of a column driver circuit with current source circuits for each column, using a mapping function to derive digital values representing time periods for controlling the supply switch, allowing for independent control of each column and reducing the need for multiple ladder taps and common ramp signals, thereby enabling efficient use of silicon area and accurate pixel brightness control.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If resistor ladder architectures are used to implement DAC functions in source driver ICs, then voltage levels for pixel driving can be generated, but silicon area increases significantly with increasing colour depth
Solution Approach 1:
The patent changes the fundamental parameter domain from voltage-domain DAC (resistor ladders) to time-domain PWM (pulse width modulation). By mapping colour depth values to time periods rather than voltage levels, the system achieves high precision pixel voltage control without requiring large resistor ladder structures. The current source circuits integrate current over time periods determined by PWM signals, eliminating the need for multiple resistor taps and reducing silicon area while maintaining accurate pixel brightness control
Solution Approach 2:
The patent replaces the passive resistor ladder architecture with active current source circuits controlled by PWM timing signals. Instead of using resistive voltage division, the system uses controlled current integration over time, substituting a dynamic timing-controlled mechanism for a static resistive network. This substitution enables high colour depth implementation without the area penalty of extensive resistor ladders
2Ease of operation
If multiple ladder taps and common ramp signals are used for driving columns, then pixel voltage levels can be controlled, but device complexity and silicon area requirements increase
Solution Approach 1:
The patent divides the driver circuit into independent current source circuits for each column, where each circuit operates autonomously with its own PWM-controlled timing. This segmentation eliminates the need for shared resistor ladders and common ramp signals across multiple columns. Each column driver receives digital control signals and independently generates the appropriate PWM waveform, simplifying the overall device architecture while maintaining precise control over each column's pixel brightness
Solution Approach 2:
The patent employs periodic PWM signals to control the timing of current integration in each current source circuit. By using periodic timing signals with variable duty cycles, the system achieves precise pixel voltage control through time-based modulation rather than complex voltage division. This periodic action approach replaces the need for multiple ladder taps and simplifies the control mechanism to timing-based operations
3Productivity
If conventional voltage-domain DAC approaches are used, then pixel drive voltages can be generated, but image artefacts occur due to non-ideal voltage accuracy
Solution Approach 1:
The patent replaces voltage-domain DAC with time-domain PWM control, substituting timing-based current integration for direct voltage generation. This substitution achieves superior voltage accuracy because PWM timing can be controlled with high precision using digital counters and clock signals, avoiding the non-idealities of resistive voltage division. The current integration process naturally filters high-frequency switching artifacts, producing clean pixel drive voltages without image artefacts
Solution Approach 2:
The patent introduces an intermediary current integration process between the PWM control signal and the final pixel voltage. Rather than directly generating pixel voltages through resistor ladders, the system uses controlled current sources that integrate current over the PWM-defined time period. This intermediary integration process acts as a natural low-pass filter, smoothing out switching artifacts and achieving accurate pixel voltage levels without image artefacts
Data Source
AI summary
An active matrix display device has a column driver circuit for providing pixel drive signals to columns of pixels, and comprising current source circuits. Each current source circuit has a supply switch (78) for controlling the time during which the current source supplies current to or drains current from the column. A mapping means (74) derives from a pixel drive level a digital value which represents a time period for the control of the supply switch (78) of each current source circuit. The mapping means (74) implements a single mapping function for use in providing the digital values for all current source circuits. Having a current source circuit for each column facilitates the application of inversion patterns. The conversion of pixel drive levels to values representing time is carried out in a shared manner, so that the required area is kept to a minimum.