Medium- and high-voltage systems that range from 9 to 72 V often require one or more of the following circuit capabilities: hot-swap control, circuit-breaker fault protection, and inrush current limiting. Without R4, the circuit shown in the figure provides inrush current limiting and a reliable circuit-breaker function for the load (C1 and R2). Yet it contains only a p-channel MOSFET, a hot-swap controller IC, and two optional resistors (R1 and R3). Adding R4 (a low-value resistor) at the MOSFET drain provides an adjustable trip-point and improved accuracy over the operating temperature range.
For hot-swap applications, U1 limits the inrush current based on a typical gate-drive slew rate of 9 V/ms. Inrush current is given by the equation:
I = (C × dV)/dt = C × SR
where C stands for load capacitance, and SR is the slew rate, set by U1 at 9 V/ms (typical). For a load capacitance of 100 µF, the IC limits inrush current to approximately 0.9 A.
U1's circuit-breaker function uses an internal comparator and the MOSFET on-resistance (RDS(ON)) to sense a fault condition. RDS(ON) for Q1 is typically 52 mΩ, and U1 has selectable circuit-breaker (CB) trip points of 300 mV, 400 mV, or 500 mV. At the lowest trip point (300 mV), the CB trip current at TJ = 25°C is typically 5.77 A.
The circuit breaker's voltage-trip value is determined from the equation:
VCB > RDS(ON) × ILOAD(MAX), or VCB/ILOAD(MAX) > RDS(ON).
Suppose the desired limit is 2 A. Using typical values:
300 mV/2 A ~ 150 mΩ > RDS(ON).
Instead of substituting another MOSFET with higher on-resistance, add a resistor of approximately 100 mΩ (R4) in series with Q1. Besides enabling adjustable circuit-breaker levels, R4 provides better circuit-breaker accuracy and improved stability over temperature. For example, RDS(ON) for Q1 is ~ 52 mΩ at TJ = 25°C and ~ 130 mΩ at TJ = 125°C, a change of 150%. If you add a 100-mΩ, 100-ppm/°C resistor (which varies by 0.001 Ω from 25 to 125°C), the combined variance from 25°C (152 mΩ) to 125°C (231 mΩ) is only 79 mΩ, which is 52%.
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