Differential PWM Modulator Using Current Comparators for Noise Immunity

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Solution Overview

Problem

High-frequency PWM modulators in class-D audio power amplifiers face increased disturbances and inefficiencies due to higher switching slopes, leading to a need for improved robustness against noise and reduced current consumption.

Innovation Solution

A fully-differential PWM modulator circuit is introduced, utilizing current comparators instead of voltage comparators, which enhances immunity to common-mode noise and reduces component count, thereby minimizing area and power consumption, and eliminates the need for common-mode feedback.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Loss of energy

If switching frequency is increased in class-D power amplifiers, then LC filter cost is reduced, but disturbances increase due to higher switching slopes

Engineering Contradiction:

ImproveLC filter costVSAvoiddisturbances

Core Design Contradiction:

Loss of energyVSObject-affected harmful factors

Solution Approach 1:

The patent replaces voltage comparators with current comparators in the PWM modulator. This substitution changes the fundamental operating principle from voltage-based comparison to current-based comparison, which inherently provides better immunity to common-mode noise and disturbances while maintaining the high switching frequency operation.

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

Solution Approach 2:

The patent implements a fully differential circuit architecture that changes the signal representation parameters. By using differential currents and voltages instead of single-ended signals, the system achieves better noise rejection and robustness against disturbances while operating at high switching frequencies.

Inventive Principle:

Principle #35Parameter changes

2Object-affected harmful factors

If fully differential circuitry is used, then immunity to common-mode noise is improved, but component count and area increase

Engineering Contradiction:

Improvecommon-mode noise immunityVSAvoidcomponent count

Core Design Contradiction:

Object-affected harmful factorsVSDevice complexity

Solution Approach 1:

The patent combines the differential signal processing and common-mode rejection functions into the core PWM modulation architecture itself. By making the modulator fully differential from the start, the circuit achieves noise immunity without requiring separate common-mode rejection stages, thus reducing overall component count.

Inventive Principle:

Principle #5Merging (Combining)

Solution Approach 2:

The fully differential current comparator serves multiple functions simultaneously: it performs the PWM comparison operation, provides common-mode noise rejection, and enables robust high-frequency operation. This multi-functionality reduces the need for additional dedicated components.

Inventive Principle:

Principle #6Universality (Multi-functionality)

3Object-affected harmful factors

If fully differential circuitry is used, then immunity to common-mode noise is improved, but current consumption increases

Engineering Contradiction:

Improvecommon-mode noise immunityVSAvoidcurrent consumption

Core Design Contradiction:

Object-affected harmful factorsVSUse of energy by moving object

Solution Approach 1:

The patent changes the operating parameters by using current-mode logic throughout the differential circuitry. This current-mode approach, when properly designed with current recycling and efficient biasing, can achieve the same noise immunity as voltage-mode differential circuits while reducing overall power consumption.

Inventive Principle:

Principle #35Parameter changes

4Device complexity

If common-mode feedback is eliminated, then component count is reduced, but control precision may be affected

Engineering Contradiction:

Improvecomponent countVSAvoidcontrol precision

Core Design Contradiction:

Device complexityVSMeasurement precision

Solution Approach 1:

The patent replaces the need for common-mode feedback control with a fully differential current comparator architecture. The differential nature of the circuit inherently rejects common-mode variations, eliminating the need for separate common-mode feedback components while maintaining control precision through the differential comparison mechanism.

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

Data Source

PatentEP3926829A1Modulator circuit, corresponding device and method

Publication Date: 2021.12.22 STMICROELECTRONICS SRL

Figure 1A~1B
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AI summary

A PWM modulator circuit (100) comprises a first half-bridge stage (121) having a first output node (OUTP) and a second half-bridge stage (122) having a second output node (OUTM). The first output node (OUTP) and the second output node (OUTM) are configured to have an electrical load (LD) coupled therebetween to apply thereto a PWM-modulated output signal. The circuit comprises a differential stage (10) having input nodes (InP, InM) configured to receive an input signal (an audio signal for a source S, for instance) applied between the input nodes (InP, InM) and produce a differential control signal (VcP, VcM) for the first half-bridge stage (121) and the second half-bridge stage (122). A current comparator (14) is arranged intermediate the differential stage (10) and the first (121) and second (122) half-bridge stages. The current comparator (14) is configured to produce a PWM-modulated drive signal (Drvin) to drive the half-bridge stages (121, 122) as a function of the input signal applied between the input nodes (InP, InM) in the differential stage (10).