LDO Regulator Light Load Stability via AC Feedback Loop
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Solution Overview
Problem
Conventional linear regulators experience a reduction in phase margin at lighter load currents due to a decrease in the frequency of the output pole, which is optimized for high load conditions, leading to instability.
Innovation Solution
An additional AC-coupled feedback loop is introduced between the error amplifier output and the base of the pass transistor, comprising a bipolar feedback transistor connected in parallel with the pass transistor, with a feedback capacitor between the collector of the feedback transistor and the error amplifier output, increasing the frequencies of Pole 1 and Pole 2 to offset the decreased Pole 3 frequency at lighter loads.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If a large output capacitor is used to ensure stability under varying conditions, then stability margin is improved, but the frequency of the output pole decreases at light load causing instability
Solution Approach 1:
The patent employs preliminary frequency compensation through properly selected RC components (Rc and Cc) that pre-establish the desired pole-zero configuration. The compensation network is designed in advance to counteract the frequency shift that occurs at light load conditions. By pre-configuring the compensation parameters, the system maintains adequate phase margin across the full load range without requiring active adjustment during operation.
Data Source
AI summary
A linear regulator contains an additional AC-coupled feedback loop between the output of the error amplifier and the base of the pass transistor that increases the frequency of the pole at the output of the error amplifier at light load currents to at least partially offset the decreased frequency of the output pole at the lighter load currents. Thus, a desired phase margin is preserved. The AC-coupled feedback loop includes a bipolar feedback transistor connected in parallel with the pass transistor. A resistor is connected to the emitter of the feedback transistor to reduce the relative gain of the feedback transistor above light load currents. A feedback capacitor Cfb is connected between the collector of the feedback transistor and the output of the error amplifier. The negative AC feedback increases the pole frequency at the output of the error amplifier and the base of the pass transistor.