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STF202-30.TCT

USB Upstream Port Filter and TVS For EMI Filtering and ESD Protection

Semtech Corporation 

STF202-22 and STF202-30

PROTECTION PRODUCTS

Applications Information

Device Connection

The STF202 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:

1 . Full-Speed Devices: For full-speed devices the pull-

up resistor is connected to the D+ line. Pin 1 is

connected to the voltage supply line (VBUS). The

input of the D+ line is routed into pin 3 and out of

pin 4. The input of the D- line is connected at pin 2

and the output at pin 5. Pin 6 is connected to

ground.

2 . Low-Speed Devices: For low speed devices the

pull-up resistor is connected to the D- line. Pins 1

is connected to the voltage supply line (VBUS). The

input of the D- line is connected at pin 3 with the

output taken at pin 4. The input of the D+ line is

connected to pin 2 and the output is at pin 5. Pin

6 is connected to ground.

Figure 1 - STF202 Circuit Diagram

.

Figure 2 - STF202 Connection Diagram

(Full Speed Devices)

1

6

2

5

3

4

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 may occur on the data lines or the

voltage bus. The STF202 is an easily implemented

solution designed to meet the termination and EMI

filter requirements of the USB specification revision

1.1. It also provides ESD protection to IEC 61000-4-2,

level 4.

Semtech provides a complete solution to simplify USB

port design (Figure 4). The STF201 and STF202

integrate all of the components necessary for line

termination, bidirectional EMI filtering, and ESD protec-

tion 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 specification. 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+ and D-) data lines and the

voltage bus (VBUS). A power distribution switch and

voltage regulator provide the power management

functions of the port.

Board Placement and Layout Guidelines.

Designing a USB hub to meet EMI and ESD immunity

requirements requires a combination of optimum

component placement, trace routing, and good circuit

design practices. Some general guidelines are given

below:

z Avoid running D+ and 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 near the USB

connectors.

z Place the STF202 near the USB connector to

restrict transient coupling.

z Minimize the path length between the USB connec-

tor and the STF202

 2004 Semtech Corp.

4

www.semtech.com

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