As an overcurrent protection device, fuses, with the changes in demand and technology, have derived some products with their own characteristics on the basis of traditional disposable fuses, including material devices self-recovery fuses, temperature fuses, explosive fuses and semiconductor devices electronic fuses. This chapter focuses on the working characteristics and advantages of electronic fuses. It can be used as a new choice of overcurrent protection scheme. The electronic fuse, despite its name, is actually a semiconductor overcurrent protection chip.
Electronic fuse Advantages:
Compared to traditional disposable fuses and self-restoring fuses, electronic fuses have:
● Reusable and has no attenuation and life times limit
● The response time is us class, usually less than 5us
● In fault mode, it is locked or self-recovery
● With overvoltage protection function
● Undervoltage protection function
● With soft start function
● With anti-reaction function
● With fault indication function
● Integrated small package
Typical applications:
● USB power switch: laptop, mobile phone, set-top box, display and type C adapter
● Hot swap: SSDS, servers, PLCS, automation, battery management, and communication applications
● Load switch: mobile phones, laptops, tablets
● Overvoltage and anti-reverse protection: mobile phone, computer, DC power port
Example of electronic fuse:
Take the actual product LS1205ExD33 as an example to briefly introduce the function of electronic fuse.
Product features:
● Wide voltage range input, 2.7~18V
● Maximum input/output voltage 20V
● LS1205EV: external adjustable overvoltage limit
● LS1205EF: indicates a fault
● Integrated 24 milliohm low impedance protection switch
● External adjustable overcurrent limit 1~5A
● Externally adjustable soft start time
● Internal back-to-back MOSFETs protect against current backfilling
● Short circuit protection
● Overtemperature protection and self-recovery
● Enable the control pin
● DFN3x3 10-pin package
Function block diagram
Expanding as follows, it can be seen that the electronic fuse is actually a relatively simple chip with overcurrent protection as the main protection function, where the LS1205EV integrates the function of regulating overvoltage through the external pull-up and pull-down resistors.
The LS1205EF removes the overvoltage regulation function and adds the fault indication pin.
Pin definition:
● IN: Power input pin
● OUT: power output pin
● EN: Enable pin, On for voltage over 1.29V, Off for voltage below 1.19V
● GND: Ground foot
● SS: Soft start pin, through the external capacitor to change the soft start time
● ILIM: Programmable overcurrent pin, adjusting the current limit by changing the resistance of the ground pin
● VCP: LS1205EV, programmable overvoltage limit pin, set the overvoltage value by pulling the resistor up to the power supply, pulling the resistor down to the ground and hanging
● FAULT: LS1205EF, fault indicator pin, normally on indicates input undervoltage or no voltage, on indicates input overvoltage or overtemperature protection
Typical application circuit:
Detailed function introduction:
Input and output capacitance selection: bypass capacitor 0.1uF to filter out high-frequency interference, 10uF capacitor to increase power stability.
Enable function: There is an on/off 1.29V/1.19V limit, which should be exceeded to ensure its work.
Soft start power: Adjust the soft start time by connecting the external capacitor. If no capacitor is connected, the standard time TSS_INT is 1.7ms. The soft start time corresponding to the external capacitor is TSS=Css*Rint, where Rint is the internal resistor 250kohm.
Output overvoltage regulation (LS1205EV only) : The output voltage can be adjusted by the high and low level of the VCP pin, the high level is set by connecting the resistance to the IN pin, and the output voltage limit is 5.7V; the low level is set by the VCP pin through the resistance ground, the output voltage limit is 3.8V; the output voltage limit is 14.4V when the VCP is suspended. It is recommended to add 0.1uF capacitor decoupling.
Overcurrent limit adjustment: The overcurrent limit value is changed by external resistance adjustment, where Iinlim=10.5/Rilim, and the output current limit is 1A when the resistance is 10.5kohm. To ensure stable operation of the circuit, you are advised to set the current limit to 1.2 to 1.5 times of the maximum working current to prevent system faults caused by mistriggering.
As an overcurrent protection chip, electronic fuse can provide reliable and stable protection for circuit protection, with fault protection and unlimited number of self-recovery protection functions, integrated overvoltage, overcurrent, overtemperature and other functions can simplify electronic engineers circuit design, especially for applications with high reliability and small size package requirements.
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