Current Limiting Load Switch with Dynamic Reference Voltage
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Solution Overview
Problem
Existing current limiting load switches experience unacceptably high transitional load current overshoots beyond the preset maximum when the load resistance falls within a specific range, due to a discontinuity in sense current behavior caused by the sense FET transitioning from linear to saturation region.
Innovation Solution
A current limiting load switch with a dynamically generated tracking reference voltage (VRdt) is employed, which automatically adjusts the reference voltage through a VRdt-generator circuit, ensuring smooth transitions and preventing overshoots by compensating for changes in the current ratio of the sense FET operational regions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a fixed reference voltage is used in the feedback loop, then the current limiting function is simple to implement, but transitional load current overshoot occurs when the sense FET transitions from linear to saturation region
Solution Approach 1:
The reference voltage is changed from a fixed value to a dynamically adjustable value that tracks the sense FET's operational state. The tracking reference voltage VRdt is generated to automatically adjust according to the sense FET transition, preventing current overshoot while maintaining circuit functionality.
Solution Approach 2:
A feedback mechanism is implemented where the tracking reference voltage VRdt is continuously adjusted based on the sense FET's operating region. The feedback loop monitors the sense current and reference voltage comparison to maintain stable current limiting during transitions between linear and saturation regions.
2Measurement precision
If the sense FET operates in saturation region, then the current limiting precision is improved, but transitional overshoot occurs during region switching
Solution Approach 1:
The tracking reference voltage VRdt is prepared in advance to match the expected sense FET operational state. By anticipating the transition and having the reference voltage ready to track the new operating point, the system prevents overshoot before it occurs during the region switching.
Solution Approach 2:
The reference voltage parameter is changed from fixed to variable, allowing it to adapt to different sense FET operating regions. This parameter change enables the system to maintain precision in both linear and saturation regions without generating harmful transitional overshoots.
3Reliability
If a dynamic tracking reference voltage is implemented, then transitional current overshoot is prevented, but circuit complexity increases
Solution Approach 1:
An intermediary tracking reference voltage VRdt is introduced between the fixed reference voltage source and the feedback loop comparator. This intermediary element absorbs the complexity of dynamic adjustment while maintaining a relatively simple overall circuit structure by mediating between the fixed reference and the variable operating conditions.
Data Source
AI summary
A current limiting load switch for bridging supply Vss and load with a reference voltage VRdt dynamically generated by a VRdt-generator is proposed. It includes: A pair of power FET and sense FET interconnected in split-current configuration. The FET pair develops a load voltage while limiting load current Iload to a preset maximum Imax. The FET pair is sized to draw device currents Ipower and Is with RATIOI=Is/Ipower<<1. The sense FET high-side terminal is coupled to Vss through a sense resistor Rsense developing a sense voltage Vs=Is×Rsense. A current limiting amplifier with inputs connected to VRdt and Vs and output controlling FET pair closing a current limiting feedback loop. The VRdt-generator dynamically adjusts VRdt concurrent and compensatory with an undesirable effect of changing RATIOI caused by the sense FET operational transition thus eliminating a transitional overshoot of Iload beyond Imax.