Differential Driver Current Steering for Faster BiCMOS Switching

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Solution Overview

Problem

Conventional differential drivers in BiCMOS processes have low switching speeds due to transistors Q3 and Q4, which limit operational speed, and are prone to electrostatic discharge and ringing, making them undesirable for integrated circuits.

Innovation Solution

A differential driver circuit utilizing feed-forward resistor-capacitor (RC) networks and NPN transistors with scaled sizes and current sources to improve switching speed and reduce electrostatic discharge risks, featuring a bypass capacitor and current steering to manage voltage and current efficiently.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Speed

If transistors Q3 and Q4 are used in BiCMOS driver circuit, then current steering function is achieved, but switching speed is limited and operational speed decreases

Engineering Contradiction:

Improveswitching speedVSAvoidoperational speed

Core Design Contradiction:

SpeedVSProductivity

Solution Approach 1:

The patent removes transistors Q3 and Q4 from the conventional BiCMOS driver circuit to eliminate the switching speed limitation. The current steering function previously performed by these transistors is achieved through an alternative mechanism using resistors and capacitors, thereby extracting the problematic component while preserving the desired function.

Inventive Principle:

Principle #2Taking out (Extraction)

Solution Approach 2:

The patent replaces the transistor-based current steering mechanism with an RC (resistor-capacitor) network-based mechanism. This substitution eliminates the need for transistor switching, thereby removing the switching speed bottleneck while maintaining the current steering capability through voltage-controlled current paths.

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

2Speed

If bipolar transistors are used to replace Q3 and Q4 for higher switching speeds, then switching speed improves, but output voltage swing is limited by junction diodes

Engineering Contradiction:

Improveswitching speedVSAvoidoutput voltage swing

Core Design Contradiction:

SpeedVSShape

Solution Approach 1:

The patent replaces the bipolar transistor switching mechanism with an RC network-based current steering mechanism. This substitution avoids the junction diode limitation of bipolar transistors, allowing for full output voltage swing while achieving high switching speeds through the voltage-controlled nature of the RC network.

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

3Reliability

If transistors Q3 and Q4 are used for current steering, then current control is achieved, but electrostatic discharge events may damage the transistors

Engineering Contradiction:

Improvedamage resistanceVSAvoidelectrostatic discharge vulnerability

Core Design Contradiction:

ReliabilityVSObject-affected harmful factors

Solution Approach 1:

The patent removes transistors Q3 and Q4 from the circuit to eliminate the electrostatic discharge vulnerability. The current steering function is achieved through the RC network, which does not contain vulnerable transistor junctions that can be damaged by ESD events.

Inventive Principle:

Principle #2Taking out (Extraction)

Solution Approach 2:

The patent introduces resistors and capacitors as intermediary components to perform the current steering function without using vulnerable transistors. These passive components are inherently more resistant to electrostatic discharge, serving as protective intermediaries that maintain circuit functionality while reducing ESD risk.

Inventive Principle:

Principle #24Intermediary (Mediator)

4Stability of the object's composition

If transistors Q3 and Q4 are used in the driver circuit, then current steering capability is provided, but ringing and latch-up occur from positive feedback

Engineering Contradiction:

Improvecircuit stabilityVSAvoidringing and latch-up

Core Design Contradiction:

Stability of the object's compositionVSObject-generated harmful factors

Solution Approach 1:

The patent removes transistors Q3 and Q4 to eliminate the positive feedback mechanism that causes ringing and latch-up. The RC network provides current steering without the regenerative feedback inherent in transistor-based designs, thereby eliminating these harmful oscillatory behaviors.

Inventive Principle:

Principle #2Taking out (Extraction)

Solution Approach 2:

The patent replaces the transistor-based positive feedback current steering mechanism with an RC network-based mechanism. The passive nature of resistors and capacitors eliminates the active regenerative feedback that causes ringing and latch-up, providing inherent circuit stability while maintaining current steering capability.

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

Data Source

PatentUS8049534B2Low-power high-speed differential driver with precision current steering

Publication Date: 2011.11.01 TEXAS INSTRUMENTS INC

AI summary

In bipolar CMOS or BiCMOS process technologies, drivers (such as mixed mode or hybrid mode drivers) using both bipolar and CMOS transistors (i.e., field effect transistors or FETs) may have undesirable properties, such as reduced speed, ringing, latch-up, or lower electrostatic discharge (ESD) performance. Here, a mixed or hybrid mode driver is provided that employs a current steering circuit (instead of voltages driven differential pair(s) as is done with conventional drivers) to generate pull-down currents that precisely match the voltages in the pull-up portions of driver. It increases the speed and produces smaller output common-mode voltage fluctuation over conventional drivers. Thus, the driver provided here can be produced in BiCMOS process technologies without the undesirable effects of conventional drivers.