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STF201-30.TC Scheda tecnica(PDF) 4 Page - Semtech Corporation |
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STF201-30.TC Scheda tecnica(HTML) 4 Page - Semtech Corporation |
4 / 7 page 4 2004 Semtech Corp. www.semtech.com PROTECTION PRODUCTS PROTECTION PRODUCTS STF201-22 & STF201-30 Figure 1 - STF201 Functional Circuit Diagram Device Connection The STF201 is designed to provide termination, EMI filtering and ESD protection for two USB I/O lines. The equivalent circuit diagram is shown in Figure 1. The device is connected as follows: USB data lines are routed through the STF201 for easy implementation and optimum pc board layout. Pin 1 is connected to the voltage supply line. The input of the D+ line is routed into pin 2 and out of pin 5. The input of the D- line is connected at pin 3 and the output at pin 4. Pin 6 is connected to ground. The ground connection should be made directly to the ground plane for best results. The path length is kept as short as possible to reduce the effects of parasitic inductance in the board traces. USB Port Design Considerations The Universal Serial Bus (USB) specification requires termination and filtering components for proper opera- tion. In addition, an open USB socket is vulnerable to hazardous ESD discharges in excess of 15kV. These discharges can occur on the data lines or the voltage bus. The STF201 is an easily implemented solution designed to meet the termination & EMI filter require- ments of the USB specification revision 1.1. It also provides ESD protection to IEC 61000-4-2, level 4. A simplified USB port is shown in Figure 3. USB line termination is achieved with series resistors on both the D+ and D- lines. These resistors preserve signal integrity by matching the cable impedance to that of the differential driver. 15k Ω pull-down resistors are used to identify a downstream port while an upstream port is identified with a 1.5K Ω pull up resistor on either the D+ (full speed devices) or the D- (low speed de- vices) data line. Capacitors are used to bypass high frequency energy to ground and for edge rate control of the USB signals. TVS diodes are added for ESD protection of both (D+ & D-) data lines and the voltage bus (V BUS). A power distribution switch and voltage regulator provide the power management functions of the port. Semtech provides a complete solution to simplify USB port design (Figure 4). The STF201 & STF202 inte- grate all of the components necessary for line termina- Figure 2 - STF201 Connection Diagram 1 2 34 5 6 tion, bidirectional EMI filtering, and ESD protection on downstream (STF201) or upstream (STF202) ports. The SC5826 is a dual port power switch that provides individual or ganged port switching, fault reporting, and inrush current limiting as required by the USB specifica- tion. The SC5205 ULDO provides a stable voltage to the USB controller. Board Placement & layout Guidelines. Board layout and placement of the STF201 play a critical role in EMI & ESD suppression. Designing a USB hub to meet EMI & ESD immunity requirements requires a combination of optimum component place- ment, trace routing, and good circuit design practices. Some general guidelines are given below: Avoid running D+ & D- signal line traces near high speed clock lines or similar signal lines. Avoid running critical signal lines near board edges. Locate the USB controller chip physically near the USB connectors. Place the STF201 near the USB connector to restrict transient coupling. Minimize the path length between the USB connec- tor and the STF201 as well as between the USB controller and the STF201. Applications Information |
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