Differential Charge Pump Linearity via Segmented Mirror Circuits
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Solution Overview
Problem
Conventional charge pumps in phase-locked loops suffer from non-linearity due to short switching pulses and unequal current sourcing and sinking, which magnifies errors, especially at high frequency scale factors, leading to increased phase noise and fractional spurs.
Innovation Solution
A differential charge pump with an auxiliary charge pump is introduced, featuring parallel primary and mirror circuits with switches and a unity gain buffer, which steers current during switching to improve linearity and reduce mismatches in up and down currents.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If conventional charge pump design is used, then the circuit is simple, but non-linearity occurs due to short switching pulses and unequal current sourcing and sinking
Solution Approach 1:
The charge pump is divided into two separate circuits: a primary charge pump circuit that sources current and a secondary charge pump circuit that sinks current. This segmentation allows each circuit to be optimized independently, with the primary circuit generating positive current pulses and the secondary circuit generating negative current pulses, thereby improving linearity by eliminating the non-linearity inherent in conventional single-circuit designs where current sourcing and sinking share common components
Solution Approach 2:
The secondary charge pump circuit is designed as a mirror image or copy of the primary charge pump circuit, with corresponding transistors and components replicated to create symmetrical current paths. This copying approach ensures that both circuits have identical electrical characteristics, improving matching and linearity while maintaining circuit simplicity through repetitive design structures
2Speed
If high frequency scale factors are used in PLL, then frequency multiplication is achieved, but non-linearity errors are magnified leading to increased phase noise and fractional spurs
Solution Approach 1:
By segmenting the charge pump into separate primary and secondary circuits with independent current paths, the design achieves better linearity that prevents error magnification during frequency multiplication. The segmented architecture ensures that phase detection errors are not amplified by non-linear current transitions, thereby reducing phase noise and fractional spurs even at high frequency scale factors
Solution Approach 2:
The invention changes the operational parameters of the charge pump by providing separate control over positive and negative current pulses. This parameter separation allows for optimized pulse widths, amplitudes, and timing characteristics that maintain linearity during high-frequency operation, preventing the magnification of non-linearity errors that would otherwise occur during frequency multiplication
Data Source
AI summary
A charge pump and a differential phase locked loop incorporating the charge pump. The charge pump includes a differential charge pump and an auxiliary charge pump. The differential charge pump has differential inputs and primary and mirror outputs. The differential charge pump is responsive to a down signal at the differential inputs to provide a negative current at the primary output and a positive current at the mirror output, and further responsive to an up signal at the differential inputs to provide a positive current at the primary output and a negative current at the mirror output. The auxiliary charge pump has differential inputs and an auxiliary output coupled to the mirror output of the differential charge pump. The differential charge pump is responsive to the down signal at the differential inputs to provide a negative current at the auxiliary output, and responsive to the up signal at the differential inputs to provide a positive current at the auxiliary output.