Hot-Swap Controllers - Current Sensing and Current Limiting

Semiconductor Integrated Circuits (ICs)
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The controller monitors the loop current through a sense resistor (RSENSE). Combining this with an Error Amplifier (EA) forms a closed-loop current limit, controlling fault current within a safe range. Unlike traditional open-loop overcurrent protection, this offers faster response and higher accuracy.

We use the ADI LTC4210 as an example to demonstrate a typical single-channel 5V hot-swappable connector.

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Sensing Principle:RSENSE (0.01Ω) is connected in series in the main path. The controller measures the voltage difference between VIN and VOUT via the SENSE pins (VSENSE = ILOAD × RSENSE).

Current Limit Calculation:The current limit threshold for the LTC4210-3 is ILIMIT = 50mV / RSENSE = 50mV / 0.01Ω = 5A, which matches the maximum 4A VOUT specified in the circuit diagram (with a safety margin).

Operating Logic:When ILOAD approaches 5A, the internal error amplifier adjusts the GATE voltage to clamp the current at around 5A, preventing damage from large transient current surges.

Tech Tip: Kelvin Sensing PCB Layout for RSENSE

Kelvin Sensing is a specialized PCB layout method for high-precision current sense resistors (RSENSE). Its purpose is to minimize the impact of trace resistance (RTRACK) on current sensing accuracy.

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  • Resistance Superposition in the Main Current Path:High current flows from VIN through PCB traces → RSENSE → Load. The main trace itself has a small resistance (RTRACK).
  • Contaminated Sensing Voltage:The voltage actually detected by the controller is VSENSE = ILOAD × (RSENSE + RTRACK), instead of the ideal ILOAD × RSENSE. This leads to an overestimation of the current sensed value and inaccurate current limit/protection thresholds.

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