E-fuses, or electronic fuses, are integrated circuits that can replace larger conventional fuses or other protection devices such as resettable polymeric fuses. Housed in small plastic packages, such as DFN and Flip-chip, they integrate a control circuit and a power switch with low on-resistance, connecting the input port to the load.
When connected in series to the main power rail, an e-fuse operates like a standard fuse with the ability of detecting and quickly reacting to overcurrent and overvoltage conditions. When an overload condition occurs, the device limits the output current to a safe value defined by the user. If the anomalous overload condition persists, the device goes into an open state, disconnecting the load from the power supply. The overload current limit can be programmed by means of an external resistor.
An input overvoltage clamp function is available in the majority of ST’s e-fuses, therefore the load can also be protected against power supply regulation failure leading to input voltage excursion.
At turn-on, during power hot-plug or hot swapping of boards, the e-fuse ramps up the output voltage in a linear and controlled way, which prevents a high inrush current flowing from the power source to the load, consequently avoiding power supply overload. This function is very useful in applications where a single power supply serves several loads connected to the same rail, such as HDD arrays and servers. The startup time is adjustable by changing a soft start capacitor.
Among ST’s e-fuses, certain devices operate in latch-off or auto-retry mode after a fault event. (The fault status is signaled by the e-fuse to the system controller by means of a Fault pin.) The former type can be reset by the user via the enable pin, the latter autonomously try to re-arm after a fault. On STEF01, a fully programmable universal electronic fuse, the mode can be set by the user via the Auto pin.
Unlike conventional circuit protections, the e-fuse offers much faster and accurate intervention, without requiring replacement after actuation. Typical applications for the e-fuses are HDD/SSD arrays and servers, industrial and network hot-swap boards, and devices supplied by separate external AC adapters.
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