Differential Charge Pump Feedback for Extended Control Voltage Range
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Solution Overview
Problem
Charge pumps in phase locked loops (PLLs) face challenges in maintaining a constant current over the entire control voltage range, leading to reduced output impedance and stability issues due to sink transistors being pushed into the triode region, affecting damping and jitter performance.
Innovation Solution
A feedback circuit is implemented to compare the drain voltage of sink transistors with a current reference transistor, adjusting the gate bias voltage to reduce voltage differences and maintain a constant sink current, while a common-mode feedback circuit adjusts the gate bias of current source transistors to match source and sink currents, ensuring high output impedance and extended control voltage range.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If the control voltage range is extended, then the frequency tunability is improved, but the sink transistors are pushed into the triode region causing current variation and reduced output impedance
Solution Approach 1:
A feedback circuit is implemented that compares the drain voltage of sink transistors with a current reference transistor, and adjusts the gate bias voltage of the sink transistors to maintain a constant voltage difference. This feedback mechanism ensures that sink currents remain constant even when control voltage varies over an extended range, preventing transistors from being pushed into the triode region and maintaining high output impedance.
2Adaptability or versatility
If the control voltage range is extended, then the frequency tunability is improved, but the output impedance is reduced due to sink transistors operating in triode region
Solution Approach 1:
The feedback circuit continuously monitors the drain voltage of sink transistors and adjusts their gate bias voltage to maintain them in the saturation region throughout the extended control voltage range. This ensures that the charge pump maintains high output impedance across the entire tuning range, preventing the harmful effect of impedance reduction that would otherwise occur when transistors enter the triode region.
3Reliability
If the gate bias voltage of sink transistors is increased to maintain constant current, then the current constancy is improved, but the voltage headroom is reduced
Solution Approach 1:
The gate bias voltage of sink transistors is made dynamic rather than fixed. The feedback circuit continuously adjusts the gate bias voltage based on the instantaneous control voltage level, ensuring that transistors remain in saturation throughout the extended control voltage range. This dynamic adjustment maintains current constancy while efficiently utilizing the available voltage headroom, avoiding the need for a fixed high bias voltage that would waste headroom.
Data Source
AI summary
One aspect of the present disclosure relates to a method for operating a charge pump. The method includes comparing a drain voltage of a current sink transistor of the charge pump with a drain voltage of a current reference transistor, and adjusting a gate bias voltage of the current sink transistor and the current reference transistor in a direction that reduces a difference between the drain voltage of the current sink transistor and the drain voltage of the current reference transistor. The method also includes comparing a common-mode voltage of a loop filter with a reference voltage, and adjusting a gate bias voltage of a current source transistor of the charge pump in a direction that reduces a difference between the common-mode voltage of the loop filter and the reference voltage.