Electronic Design

Hot-Swap Controller Creates A Flexible Current Limiter

Current limiting is often necessary to protect a power supply from short circuits and load surges, which can pull the supply voltage low. ICs containing p-channel MOSFETs and adjustable current limits are available for this purpose. Yet most are limited to 5-V systems with a typical maximum current limit of 2 A and low accuracy (20% to 50%, according to the manufacturers).

With the current-regulation capability of these "hot-swap controllers," it's possible to create a more accurate and versatile current limiter (Fig. 1). The external MOSFET and sense resistor let this device protect a system operating from 3 to 12 V. Connecting the CTIM terminal to ground disables the IC's dual-speed/bi-level feature. This forces the device into startup mode, which regulates the current at the limit preset by the sense resistor \[ILIM = (200 mV)/RSENSE\].

Achievable accuracy is 10% plus the sense-resistor error—a great improvement over the approach mentioned above. This circuit does more than protect against short circuits and current surges. It also limits the inrush current and ramps the supply voltage by limiting the MOSFET's gate-charging current to 100 mA. Driving the ON terminal (pin 8) to ground disconnects the load.

The external MOSFET easily handles normal operation. But a short circuit elevates its power dissipation and die temperature by imposing the full supply voltage across the MOSFET and sense resistor. If this condition exists for an extended period, thermal protection becomes necessary.

Thermal protection can be added by placing an NTC temperature monitor close to the MOSFET (on the back side of the board). IC1's internal precision comparator (accessible at the ON terminal) can then be used to disconnect the load when this monitor detects an excessive temperature. For example, the components needed toprovide a temperature threshold of 85°C when VCCIN = 12 V are R1 (20 k(omega)), and the type-B thermistor R2 (10 k(omega) at 25°C)(Fig. 2).

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