Reduced Swing Differential Pre-Drive Circuit for LCD Timing Controllers

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Solution Overview

Problem

Conventional pre-drive circuits in timing controllers for LCD televisions face limitations in maximum operating frequencies due to rail-to-rail CMOS signaling, leading to glitches in output waveforms caused by capacitive coupling, which are exacerbated by larger display sizes and higher data rates.

Innovation Solution

A differential pre-drive circuit with reduced swing and offset is introduced, utilizing a swing resistor and an offset resistor coupled to an H-bridge of transistors to minimize voltage swing and maintain output transistors in saturation states, enabling higher switching frequencies and cleaner output waveforms.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Strength

If rail-to-rail CMOS signaling is used in the pre-drive circuit, then the output waveform is driven with full voltage swing, but the maximum operating frequency is limited and glitches appear in the output waveform

Engineering Contradiction:

Improveoutput voltage swingVSAvoidmaximum operating frequency

Core Design Contradiction:

StrengthVSSpeed

Solution Approach 1:

The patent changes the voltage swing parameter from rail-to-rail to reduced swing operation. The pre-drive circuit is designed to operate with smaller voltage excursions, keeping the output transistors in the saturation region rather than driving them to rail-to-rail conditions. This parameter change eliminates the charging/discharging delays that limit maximum frequency while maintaining sufficient signal drive capability.

Inventive Principle:

Principle #35Parameter changes

Solution Approach 2:

The patent implements dynamic biasing through the swing resistor that adjusts the operating point based on signal conditions. The circuit dynamically maintains transistors in saturation mode during switching transitions, optimizing both speed and signal integrity. This dynamic operation allows the circuit to achieve higher frequencies without losing the benefits of full voltage swing where needed.

Inventive Principle:

Principle #15Dynamics

2Strength

If rail-to-rail CMOS signaling is used in the pre-drive circuit, then the output waveform is driven with full voltage swing, but glitches appear in the output waveform due to capacitive coupling

Engineering Contradiction:

Improveoutput voltage swingVSAvoidoutput waveform glitches

Core Design Contradiction:

StrengthVSObject-generated harmful factors

Solution Approach 1:

Inventive Principle:

Principle #35Parameter changes

Solution Approach 2:

The patent converts the potential harm of reduced voltage swing into a benefit by using the swing resistor to actively shape the waveform. The reduced swing operation, combined with the swing resistor's impedance, prevents the capacitive coupling effects that cause glitches, while the circuit still delivers adequate signal strength to drive the output stage effectively.

Inventive Principle:

Principle #22Blessing in disguise (Convert harm into benefit)

3Area of stationary object

If larger display sizes are used, then the display format is improved, but transmission distance increases causing clock and data registration instability

Engineering Contradiction:

Improvedisplay sizeVSAvoidclock and data registration stability

Core Design Contradiction:

Area of stationary objectVSReliability

Solution Approach 1:

The patent applies preliminary anti-action by using the reduced swing pre-drive circuit to preemptively prevent signal degradation before it occurs. By keeping transistors in saturation mode and avoiding full rail-to-rail transitions, the circuit pre-conditions the signal to resist degradation over longer transmission distances, thereby maintaining clock and data registration stability in larger display formats.

Inventive Principle:

Principle #9Preliminary anti-action

Data Source

PatentUS8289257B1Reduced swing differential pre-drive circuit

Publication Date: 2012.10.16 NAT SEMICON CORP

AI summary

A circuit for reducing and offsetting the voltage swing of a differential pre-drive circuit. The circuit includes a first H-bridge of transistors receiving a differential pair of input signals. A swing resister is coupled to the H-bridge for reducing a voltage swing of the differential pair of input signals. The reduced swing is generated as a differential pair of output signals. Also, the differential pre-drive circuit includes an offset resistor that is coupled to the H-bridge. The offset resistor acts to offset the differential pair of output signals. As such, the differential pair of output signals having reduced swing and offset as applied to gates of output transistors in an output stage allow the output transistors to remain in the saturation operating state.