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STF202-22 Datasheet(PDF) 4 Page - Semtech Corporation

Part No. STF202-22
Description  USB Upstream Port Filter & TVS For EMI Filtering and ESD Protection
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Maker  SEMTECH [Semtech Corporation]
Homepage  http://www.semtech.com

STF202-22 Datasheet(HTML) 4 Page - Semtech Corporation

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Objective - March 03, 2000
USB Upstream Port Filter & TVS
For EMI Filtering and ESD Protection
Figure 1 - STF202 Circuit Diagram
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:
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 (V
BUS). 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
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 (V
BUS). 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.
USB Port Design Considerations
The Universal Serial Bus (USB) specification requires
termination and filtering components for proper
operation. 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
& EMI filter requirements 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 devices) 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
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
Figure 2 - STF202 Connection Diagram
(Full Speed Devices)
integrate all of the components necessary for line
termination, 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 specification. The SC5205 ULDO
provides a stable voltage to the USB controller.
Board Placement & Layout Guidelines.
Designing a USB hub to meet EMI & ESD immunity
requirements requires a combination of optimum
component placement, trace routing, and good circuit
design practices. Some general guidelines are given
Avoid running D+ & D- signal line traces near high
speed clock lines or similar signal lines.
Avoid running critical signal lines near board
Locate the USB controller chip near the USB
Place the STF202 near the USB connector to
restrict transient coupling.
Minimize the path length between the USB
connector and the STF202

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