PLL Start-Up Circuit Using Dual Current Paths for Stable Lock Time
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Conventional phase-locked loop (PLL) circuits face challenges due to varying lock times and frequency issues caused by input signal frequency variations, as well as process, voltage, and temperature (PVT) variations, leading to inconsistent initial frequencies and the need for gating circuits that can produce glitches and fail to start oscillating at certain corners.
Innovation Solution
A PLL circuit design that includes a start-up circuit with dual current paths and a current-controlled oscillator (CCO), eliminating the need for cycle counting and gating circuits by generating an initial current to control the voltage-controlled oscillator (VCO) and ensuring consistent initial frequencies across different PVT corners.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a conventional start-up circuit with cycle-counter is used, then the PLL circuit can generate initial frequency, but the lock time varies significantly with input signal frequency
Solution Approach 1:
The patent changes the control parameter from cycle-count-based voltage generation to input voltage directly-controlled current generation. The start-up circuit generates current proportional to the input voltage, which directly controls the VCO frequency without depending on input signal frequency or cycle counting, thereby achieving consistent lock time across different operating conditions.
2Reliability
If the initial frequency is set high to ensure operation above minimum frequency, then the PLL starts oscillating reliably, but the frequency may exceed minimum operated frequency requiring gating circuits
Solution Approach 1:
The patent applies preliminary action by ensuring the VCO starts oscillating at a frequency guaranteed to be above the minimum operated frequency through the start-up circuit's current control mechanism. This preliminary frequency assurance eliminates the need for subsequent gating circuits to adjust or gate the frequency, as the correct frequency range is achieved from the start.
3Device complexity
If the design is adjusted to keep initial frequency below minimum operated frequency, then gating circuits are eliminated, but the PLL may fail to start oscillating at certain PVT corners
Solution Approach 1:
The patent implements feedback by using the input voltage (which represents the desired operating frequency) to directly control the start-up current magnitude. This feedback mechanism ensures that the VCO is driven to oscillate at the correct frequency from the beginning, adapting to PVT variations automatically without requiring gating circuits or risking failure to start oscillation.
4Reliability
If cycle-counter and start-up circuit are used to generate initial voltage, then the VCO can start oscillating, but the circuit complexity increases and PVT variations cause inconsistent behavior
Solution Approach 1:
The patent extracts and eliminates the complex cycle-counter and voltage-generation components from the conventional start-up circuit. Instead, it retains only the essential current control function, using the input voltage to directly control a current source that drives the VCO. This extraction simplifies the circuit while maintaining reliable VCO startup across PVT corners.
Data Source
AI summary
A circuit includes a voltage-controlled oscillator (VCO), which includes a voltage input node having an input voltage; and a start-up circuit. The start-up circuit includes a first current path and a second current path. The first current path has a first current and is configured so that the first current increases in response to a decrease in the input voltage and decreases in response to an increase in the input voltage. The second current path has a second current and is configured so that the second current decreases in response to the decrease in the input voltage and decreases in response to the increase in the input voltage. The VCO further includes a third current path combining a first proportion of the first current and a second proportion of the second current into a combined current; and a current-controlled oscillator (CCO) including an input receiving the combined current and outputting an AC signal.