Feed-Forward Voltage Regulator for High-Frequency Noise Rejection
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Solution Overview
Problem
Conventional low-dropout voltage regulators face limitations in supply noise rejection across a broad noise frequency bandwidth due to bandwidth limitations of the feedback amplifier, feedback amplifier gain, available area for capacitors, and output resistance, leading to degradation in power supply noise rejection ratio (PSRR).
Innovation Solution
A feed-forward compensated voltage regulator is introduced, featuring a noise cancellation circuit with a variable transconductance feed-forward amplifier that injects a compensation current into the control node based on sensed noise signals, stabilizing the feedback loop and providing additional gain to improve PSRR without degrading bandwidth.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a conventional LDO regulator uses a negative feedback amplifier with limited bandwidth, then the circuit complexity is low, but the power supply noise rejection ratio (PSRR) degrades at frequencies beyond the amplifier bandwidth
Solution Approach 1:
The feed-forward amplifier anticipates and compensates for supply noise before it reaches the output by sensing the noise at the input and injecting a compensating signal through the pass element, preventing the noise from affecting the output voltage in the first place
Solution Approach 2:
The feed-forward amplifier acts as an intermediary that senses supply noise and translates it into a compensating control signal for the pass element, mediating between the noisy power supply and the sensitive output to prevent noise transmission
2Object-affected harmful factors
If a large capacitor is used to suppress high-frequency supply noise, then the PSRR improves at high frequencies, but the available area increases
Solution Approach 1:
The feed-forward amplifier replaces the physical capacitor-based noise filtering mechanism with an electronic signal processing approach, using active circuitry to generate compensating signals that electronically cancel noise without requiring large physical energy storage elements
3Object-affected harmful factors
If the feedback amplifier gain is increased to improve DC supply rejection, then the low-frequency PSRR improves, but the bandwidth of the amplifier must be reduced
Solution Approach 1:
The noise rejection function is segmented into two independent pathways: a high-gain feedback path for DC and low-frequency rejection, and a high-bandwidth feed-forward path for high-frequency rejection, allowing each path to be optimized for its specific frequency range without compromising the other
Data Source
AI summary
A voltage regulator includes a pass element having a control input coupled to a control node and operable to generate an output voltage at an output node, a negative feedback amplifier operable to receive a reference voltage and the output voltage and generate a signal at the control node based on a difference between the reference voltage and the output voltage, and a noise cancellation circuit coupled to the control node and the output node and operable to generate a bias current at the control node based on the output voltage.