Fractional-N Charge Pump DAC Switching for Lower Charge Injection

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Solution Overview

Problem

Charge injection in fractional-N frequency synthesizers introduces non-linearity due to capacitive coupling from digital control lines to analog signal paths, affecting the accuracy and resolution of frequency synthesis.

Innovation Solution

Implementing a fractional-N frequency synthesizer with an error current steering mechanism that includes a voltage-regulated node, a dummy digital-to-analog converter, and a mismatch shaping circuit to counteract timing and magnitude mismatches, along with a sample-and-hold network to reduce charge injection and spurious noise.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Measurement precision

If an analog switch in a current steering digital-to-analog converter turns on and off to switch between error cancellation phase and full-scale phase, then frequency resolution and accuracy are improved, but charge injection from digital control lines to analog signal paths introduces non-linearity and degrades performance

Engineering Contradiction:

Improvefrequency resolutionVSAvoidcharge injection

Core Design Contradiction:

Measurement precisionVSObject-generated harmful factors

Solution Approach 1:

A dummy digital-to-analog converter is introduced as an intermediary component that mirrors the switching behavior of the main converter. This dummy converter provides a complementary switching path that cancels out the charge injection effects from the main converter, thereby maintaining frequency resolution while eliminating the harmful charge injection non-linearity

Inventive Principle:

Principle #24Intermediary (Mediator)

Solution Approach 2:

The patent changes the switching parameters by introducing complementary switching signals that are inverted relative to the main converter. By controlling the dummy converter with opposite phase switching, the charge injection from digital control lines is transformed from a harmful effect into a canceling effect, maintaining accuracy while improving linearity

Inventive Principle:

Principle #35Parameter changes

2Productivity

If feedforward cancellation is used in a fractional-N frequency synthesizer, then high-speed frequency signal generation with high resolution is achieved, but charge injection from digital control lines introduces non-linearity during phase transitions

Engineering Contradiction:

Improvefrequency signal generation speedVSAvoidsignal linearity

Core Design Contradiction:

ProductivityVSManufacturing precision

Solution Approach 1:

The patent converts the harmful charge injection effect into a beneficial cancellation mechanism. By intentionally creating a complementary charge injection path through the dummy converter that is equal in magnitude but opposite in phase, the harmful non-linearity is transformed into a useful error-correcting mechanism that actually improves signal linearity while maintaining high-speed operation

Inventive Principle:

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

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 charge injection and spurious noise, enhancing the accuracy and resolution of frequency synthesis by maintaining charge balance and minimizing the impact of process, voltage, and temperature variations.

Implementation Method 1

a voltage regulator coupled to the voltage-regulated node and an output node of the charge pump

Methodology Applied

Scientific EffectVoltage regulation:

Implementation Method 2

as an analog switch in a current steering digital-to-analog converter of the fractional-N frequency synthesizer turns on and off, a small amount of charge can be capacitively coupled (i.e., injected) from a digital control line to a corresponding analog signal path

Methodology Applied

Scientific EffectCharge injection reduction: Capacitance

Data Source

PatentUS12537532B2Charge injection reduction in a fractional-N frequency synthesizer

Publication Date: 2026.01.27 SILICON LABORATORIES INC

AI summary

A technique for reducing the effects of charge injection when a charge pump in a fractional-N frequency synthesizer switches from an error cancellation phase of error current generation to a full-scale phase of error current generation uses a dummy digital-to-analog converter to steer excess current from the error cancellation phase to a voltage regulated node. As a result, the voltage swing of an inactive branch of a digital-to-analog converter of the charge pump is the same as the voltage swing of the active branch of the charge pump, i.e., the device overlap capacitance charge injection is in phase for the branches of a digital-to-analog converter in the charge pump.