PWM Soft Start Circuit Using Merged Error Amplifier Inputs
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Solution Overview
Problem
Existing DC-DC converters face challenges with inrush current during startup, which can damage output capacitors, and conventional soft start circuits require additional error amplifier inputs, leading to increased device size and matching issues.
Innovation Solution
A PWM control circuit with internal and external soft start features, using a three-input error amplifier and a comparator circuit, allows for adjustable soft start times without the need for additional error amplifier inputs, reducing device size and mitigating matching problems.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If internal and external soft start circuitry are connected to the closed loop feedback error amplifier circuit with four inputs, then the soft start function is improved, but the device size increases and device matching problems occur
Solution Approach 1:
The patent combines internal and external soft start circuitry to share a common error amplifier input, merging their functions into a unified control mechanism. This allows both soft start modes to operate through the same input node, reducing the total number of required inputs from four to three, thereby decreasing device size and complexity while maintaining full soft start functionality.
Solution Approach 2:
The error amplifier input is designed to accept multiple functions: it can receive signals from internal soft start circuitry, external soft start circuitry, or reference/feedback signals depending on configuration. This multi-functional input node eliminates the need for separate dedicated inputs for each soft start mode, reducing device complexity while preserving all soft start capabilities.
2Adaptability or versatility
If four error amplifier inputs are used to accommodate both internal and external soft start circuitry, then the soft start control is improved, but the input transistors dimensions must be larger leading to increased die size
Solution Approach 1:
The patent merges the input requirements for internal and external soft start circuitry into a single shared error amplifier input node. By combining these functions, the circuit requires only three inputs instead of four, which reduces the number of input transistors needed and consequently decreases the overall die size while maintaining full adaptability for soft start control.
Solution Approach 2:
The patent changes the operational parameters of the error amplifier by configuring it to selectively respond to different input signals based on the soft start mode. The amplifier can be configured to prioritize internal or external soft start signals as needed, allowing versatile control with fewer physical inputs, thereby reducing die size without sacrificing adaptability.
3Reliability
If a large number of error amplifier inputs are provided, then the soft start functionality is improved, but matching difficulty increases due to large device sizes
Solution Approach 1:
The patent combines multiple soft start input requirements into a single shared error amplifier input node. This merging reduces the total number of input transistors from four to three, which improves manufacturing matching by reducing the number of components that require precise matching. The unified input structure ensures consistent performance while maintaining full soft start functionality.
Data Source
AI summary
PWM control circuits and soft start circuitry thereof are presented in which a source follower circuit provides an input to a pulse generator error amplifier during startup according to a lower one of an internal soft start circuit ramp signal and a voltage across and externally connected capacitor, with a current source connected to the source follower to limit the charging current supplied to the externally connected capacitor.