Research And Design Of The Key Circuit For Electronic Fuse

Along with the widely used of various high-tech products, the importance of USB is obvious as a bridge, which connects various high-tech products. Dedicated protection chip protects high-tech products from abnormal trouble. This kind of protection chip not only follows the USB power management standard, but also has irreplaceable protective functions. In view of this broad application scenario, it is necessary to study the protection chip, which has fast response, low power consumption.The indicators of electronic fuse closely related with practical application are: conduction impedance; current limiting value and response rate; the range of parasitic capacitance of external load. The value of conduction impedance must meet the operating voltage of external load. Based on USB power management standard, the value of conduction impedance must ensure that the supply voltage of load is greater than4.85V. The value of current limit is set at150%of rated current, the current limit of this design is750mA, and the response rate is lus. The value of parasitic capacitance is related with the soft start time, which is set by SOA map of MOSFET, the soft start time is4ms.Over-current protection circuit combines the sampling of RDSON and SENSEFETs. The pre-comparison circuit is the sampling of RDSON, the comparator in pre-comparison is with offset voltage and hysteresis voltage, which is achieved by adjusting the size of differential pairs and current mirror load, current shunt. The current limit of pre-comparison is501mA, and the hysteresis is258mA. The later sampling circuit is the sampling of SENSEFETs. A positive comparator forces the drain voltage of sampling MOS to follow the change of the output voltage, which improves the accuracy. The later comparator has single-side delay, it can output shutdown signal fast, then keep5us to output start signal, which prolong the life of MOSFET. The current limit of later circuit is750mA, the response rate is937ns.MOSFET model is built as electronic switch, I analysis the effect of driving current on the parasitic capacitance of the model, and choose the best driving current and the soft start time. The driving circuit works normally with the1.1V supply voltage, the function of overvoltage and hysteresis is skillfully realized by a NPN. The output voltage rise smoothly without overshoot during startup, the gate-driving circuit is simple and fast response rate with high reliability.The cascaded simulation of driving circuit and over-current circuit is under0.5μm BCD process model. The simulation shows:the conduction impedance of electronic fuse is300m Ω; the electronic fuse can effectively suppress the surge current with the parasitic capacitance1uF-90uF; electronic fuse can effectively protect the power supply system when the load is shorted, it keep the current of supply voltage within750mA.the electronic fuse effectively protects the power supply system and load system.