Ensuring compact, flexible, and accurate circuit protection that meets IEC and UL safety standards
input current is first regulated at a fixed level for several microseconds (µs) and then automatically reduced to a safe level. Depending on the eFuse being used, the I-limit value can be fixed or programmable. When an overcurrent occurs, the eFuse reduces the input current for a fixed time, usually several milliseconds (ms), and then turns it back on to see if the fault has been cleared.
If the fault is still there, it will again automatically regulate and reduce the current, wait several ms, and restart. The sequence of reducing the current and restarting until the fault is removed is sometimes referred to as ‘hiccup mode’ protection. In the case of a short-circuit condition, the input current rises very rapidly, and the eFuse immediately reduces the input current to a safe level ( Figure 2 ). Soft start limits the inrush current flow when a device is turned on. Without soft start, the only limitations on the current are the relatively low impedances of the printed circuit board (pc board) traces and the components. High inrush currents can damage the power supply circuit or the components. Soft start slowly turns on the eFuse, providing slew rate control and limiting the inrush current ( Figure 3 ). The soft start rate can be fixed or programmable. UVLO and OVP Too much or too little voltage can also result in system malfunctions and possible damage. UVLO in an eFuse prevents the device from operating if the input voltage is lower than a preset threshold. In addition, if the input voltage rises too slowly, or if the power source has a significant internal resistance (like a battery), the voltage can drop as the load current rises, causing
internal resistor. When the voltage falls to a specified value, the eFuse automatically turns on ( Figure 4 ). The OVP threshold can be fixed or programmable. Thermal protection Excessive temperatures can also result in damage or improper functioning, so eFuses include an internal temperature sensor. OTP is typically implemented as a two-stage process. First is the thermal regulation temperature, usually around 125°C, at which point the eFuse limits current flow to try and stop the temperature rise. If the temperature continues to increase, and the device junction temperature exceeds the thermal shutdown threshold (TSHDN)—usually around 140°C— the eFuse turns off. OTP also includes hysteresis, and the eFuse will restart when the internal temperature falls 20°C below the TSHDN ( Figure 5 ). Compact 5-volt eFuses for battery-powered devices Designers of Bluetooth headsets, wearables, tablet PCs, and other adapter-powered devices can turn to the 5-volt, 5 A-rated LS0505EVD22 in a DFN2X2_8L package, and the 5-volt, 4 A-rated LS0504EVT233 in a SOT23_3L package for compact solutions that provide OVP, OCP, and soft start
Current protection and soft start Excessive currents can cause electronic components to exceed their rated operating temperatures, impairing performance and reducing lifetimes. A current protection circuit monitors the current (I), and if it exceeds the ‘I-limit’ set level which is above the rated ‘Iout’ operating current, the
Figure 4: When the input voltage reaches the OVP clamp value, it is prevented from rising any further, and the eFuse turns off the output to protect the system. (Image source: Littelfuse)
Figure 2: eFuses include current limiting with auto-retry to protect against excessive load currents and short-circuit protection. (Image source: Littelfuse)
Figure 5: OTP includes hysteresis that restarts the eFuse once the temperature has dropped by a predetermined amount. (Image source: Littelfuse)
the voltage to repeatedly cross the UVLO threshold. When that happens, the UVLO function can go into oscillation. Using a UVLO circuit with a hysteresis (lag) of about 150 to 300 millivolts (mV) can eliminate oscillations and ensure smooth operation of the UVLO function.
OVP protects the device from being stressed or damaged by excessively high voltages. When an overvoltage condition is detected, the eFuse immediately clamps the voltage to protect the system, then turns off. It also discharges the output capacitors to ground through an
Figure 3: Soft start in an eFuse prevents potentially harmful inrush currents and can be fixed or programmable. (Image source: Littelfuse)
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