PLL Charge Pump Voltage Tracking for Higher Bandwidth
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Solution Overview
Problem
Phase-locked loops (PLLs) in electronic devices face challenges in maintaining stability and power efficiency due to charge sharing issues between the charge pump and filter, which can reduce bandwidth and increase noise, particularly in power-sensitive applications.
Innovation Solution
Incorporating a buffer between the filter and charge pump to track voltage and adjust current flow, increasing the charge-pump output impedance and thus the locked-loop bandwidth, while using a voltage-tracking function to accommodate a noisier VCO for reduced power consumption.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If charge pump is directly coupled to filter, then circuit simplicity is maintained, but bandwidth is reduced and noise increases
Solution Approach 1:
A buffer circuit is introduced as an intermediary component between the charge pump and the filter. The buffer includes a first transistor with gate coupled to the charge pump output and source coupled to the filter input, acting as a voltage follower that isolates the charge pump from the filter's capacitive load. This intermediary structure increases the output impedance of the charge pump, thereby extending the locked-loop bandwidth without significantly increasing overall circuit complexity.
2Object-affected harmful factors
If VCO noise is reduced, then signal quality improves, but power consumption increases
Solution Approach 1:
The patent converts the harmful effect of VCO noise into a beneficial outcome by using the buffer's voltage-tracking function. The buffer monitors the filter voltage and dynamically adjusts the charge pump current to compensate for noise-induced voltage fluctuations. This allows the system to tolerate higher VCO noise levels while maintaining stable loop operation, thereby enabling the use of lower-power, noisier VCO designs without sacrificing signal quality.
3Speed
If charge pump output impedance is increased, then bandwidth is extended, but stability may be compromised
Solution Approach 1:
The buffer implements a voltage-tracking feedback mechanism where the first transistor's gate is coupled to the charge pump output and its source is coupled to the filter input. This creates a negative feedback loop that automatically adjusts the charge pump current in response to voltage fluctuations at the filter input. The feedback action compensates for the high output impedance effect, maintaining loop stability while preserving the extended bandwidth benefit.
Data Source
AI summary
An apparatus is disclosed for a charge pump with voltage tracking. In an example aspect, the apparatus includes a locked loop having a charge pump, a filter, a second switch, and a buffer. The charge pump includes a first current source, a second current source, and a first switch coupled between the first current source and the second current source. The filter is coupled to the charge pump between the first switch and the second current source. The second switch is coupled to the charge pump between the first current source and the first switch. The buffer is coupled between the filter and the second switch, with the buffer comprising a voltage buffer.