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SBTF201-22.TC

USB Downstream Port Filter & TVS For EMI Filtering and ESD Protection

Semtech Corporation 

STF201-22 & STF201-30

PROTECTION PRODUCTS

Applications Information

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:

z 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.

Figure 1 - STF201 Functional Circuit Diagram

Figure 2 - STF201 Connection Diagram

1

6

2

5

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.

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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.

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. 15kΩ pull-down resistors are

used to identify a downstream port while an upstream

port is identified with a 1.5KΩ 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 ). A power distribution switch and voltage

BUS

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-

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:

z Avoid running D+ & D- signal line traces near high

speed clock lines or similar signal lines.

z Avoid running critical signal lines near board edges.

z Locate the USB controller chip physically near the

USB connectors.

z Place the STF201 near the USB connector to

restrict transient coupling.

z Minimize the path length between the USB connec-

tor and the STF201 as well as between the USB

controller and the STF201.

 2004 Semtech Corp.

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www.semtech.com

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