Power switch design that meets USB specifications

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Power switch design that meets USB specifications [Copy link]

Introduction
　　The Universal Serial Bus (Universal Serial Bus) makes the connection between PC and external devices simple and fast. With the development of computers and USB-related portable devices, USB will be more widely used. Since USB has the characteristics of plug-and-play, at the moment when the load is abnormal, the power switch will flow several amperes of current, causing damage to the circuit.
　　The USB power switch designed in this paper adopts a bootstrap charge pump to provide a gate drive voltage of 2 times the power supply for the N-type power tube. When the load is abnormal, the overcurrent protection circuit can quickly limit the power tube current to avoid damage to the circuit caused by hot plugging.
　　2 Overall design ideas of USB switch circuit

　　Figure 1 shows the overall design of the USB power switch. VIN is the power input and VOUT is the USB output. When the load is normal, the charge pump generates a sufficiently high gate drive voltage to make NHV1 work in the deep linear region to reduce the conduction loss from the input power supply (VIN) to the load voltage (VOUT). When the power tube current is higher than 1A, Currentsense outputs a high level to the overcurrent protection circuit (Currentlimit); the overcurrent protection circuit feeds back the load voltage to the charge pump and adjusts the charge pump output (VPUMP), so that the working state of the power tube changes from the linear region to the saturation region, limiting the power tube current and achieving the purpose of protecting the power tube. When the load returns to normal, Currentsense outputs a low level and the charge pump works normally.
　　Figure 1 Schematic diagram of USB power switch
　　3 Charge pump design
　　Figure 2 is a circuit schematic diagram of a self-bootstrap (SelfBooST) charge pump. In the figure, is the clock signal to control the charge pump operation. In the initial stage, the charges on the capacitor, C1 and the power tube gate capacitor CGAte are both zero. When it is low level, MP1 is turned on, charging C1, V1 potential rises to the power supply potential, V2 potential increases, and MP2 tube is turned on. Assuming that the gate capacitance is much larger than the capacitor C1, all the charges on V2 are transferred to the gate capacitance CGATE. When it is high level, MN1 is turned on, discharging the left plate of C1, V1 potential drops to the ground potential, V2 potential drops, MP2 tube is cut off, MN2 tube is turned on, and the right plate of capacitor C1 is charged to VIN. At the next low level, V1 potential rises to the power supply potential, V2 potential increases to 2VIN, MP2 tube is turned on, and VPUMP potential rises to 2VIN-VT.
　

　　Figure 2 Schematic diagram of bootstrap charge pump The
　　bootstrap charge pump does not need to provide gate drive voltage for MN2 and MP2, and the control is simple, but the output voltage will have a threshold loss. Figure 3 is the improved charge pump circuit diagram, 1 and 2 are complementary non-overlapping clocks. The secondary charge pump composed of MN2, MN5, MP3, MP2 and capacitor C2 provides gate voltage for MN4 and MP4 to ensure that they are completely turned off and on. When 1 is low, MP1 is turned on and the potential increases. At this time, the potential of V3 is zero, MP4 is turned on, and the charge on V2 is transferred to the gate capacitor CGATE, and the potential of VPUMP increases. When 1 is high, MP2 is turned on to charge C2, the potential of V4 rises to the power supply potential, and the potential of V3 rises accordingly. MP3 is turned on and the potential of VPUMP continues to rise. MN3 is equivalent to a diode and plays a unidirectional conductive role.
　　After the VPUMP voltage rises to VIN+VT, MN3 isolates the path from V3 to the power supply to ensure that the charge of V3 is fully charged into the gate capacitor by MP3. In this way, C1 and C2 charge the gate capacitance of each other, and after several clock cycles, the charge pump output voltage is close to twice the power supply voltage.
　　As the charge pump output voltage increases, the load current provided by the power tube gradually increases, avoiding inrush current on the capacitive load.
　

　　Figure 3 Improved charge pump