Microchip USB3346 Handleiding

Microchip Niet gecategoriseerd USB3346

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 2010-2019 Microchip Technology Inc. DS00002971A-page 1
INTRODUCTION
The USB334x provides the physical layer interface (PHY) 2.0. Proper design tech-in systems using Hi-Speed USB
niques must be used in the printed circuit board (PCB) layout to maintain the signal integrity required for 480 Mbps oper-
ation. The USB334x is available in both 24-pin and 32-pin QFN packages; layout guidelines for general QFN packages
are provided in this application note.
Audience
This document is written for a reader that is familiar with hardware design and the USB 2.0 specification. The goal of
this application note is to provide guidance on sensitive areas of the PCB layout.
Overview
The following recommendations are for a four layer PCB layout with a Microchip part. Our recommendations are not the
only way to layout our 24-pin and 32-pin QFN packages. PCB design engineers will have his/her own preference, and
the implementation will be dependent on complexity and density of layout, PCB real estate, number and types of devices
in circuit and the environment that the final product will reside in. For example, the PCB described in this application
note has components on both sides of the board. A four layer board could be realized with components on one side only.
References
The following documents should be referenced when using this application note:
• Microchip USB334x Data Sheets
• Universal Serial Bus Specification Revision 2.0
AN 22.3
USB334x USB Transceiver Layout Guidelines
AN 22.3
DS00002971A-page 2  2010-2019 Microchip Technology Inc.
GENERAL DESIGN GUIDELINES
This section provides guidelines for the sensitive circuits associated with the system application of the USB334x.
ULPI Bus
The layout of the ULPI interface should be analyzed with the timing requirements of the USB Link controller to ensure
that the proper timing is met. Time-of-flight delays must be added for trace lengths, vias, and connectors in the system.
For more information on ULPI timing and system design, refer to Application Note 19.17 - “ULPI Design Guide”.
Controlled Impedance for USB Traces
The USB 2.0 specification requires that the USB DP/DM traces maintain a nominal 90 Ohms differential impedance +/
- 15% (see USB specification Rev 2.0, paragraph 7.1.1.3 for more details). In this design example, the traces are 7 mil
(0.178mm) wide with line spacing of 7 mils (0.178mm). These numbers are derived for 5 mil (0.127mm) distance from
ground reference plane. For different dielectric thickness, copper weight or board stack-up, trace width and spacing will
need to be recalculated. A continuous ground plane is required directly beneath the DP/DM traces and extending at
least 5 times the spacing width to either side of DP/DM lines.
Maintain symmetry between DP/DM lines in regards to shape and length.
Single ended impedance is not as critical as the differential impedance. A range of 42 Ohms to 78 Ohms is acceptable
(equivalently, common mode impedance must be between 21 Ohms and 39 Ohms).
Figure 1 shows DP/DM traces with approximately equal trace length and symmetry. It is important to maintain a con-
ductor width and spacing that provides differential and common mode impedances compliant with the USB specification.
Use 45 degree turns to minimize impedance discontinuities.
FIGURE 1: EXAMPLE OF ROUTING DP/DM FROM QFN PKG TO TYPE B CONNECTOR
 2010-2019 Microchip Technology Inc. DS00002971A-page 3
AN 22.3
Isolation of DP/DM Traces
The DP/DM traces must be isolated from nearby circuitry and signals. Maintain a distance of components to lines that
is greater than or equal to 5 times the distance of the 7 mil (0.178mm) spacing between the traces. For example, if DP/
DM have a 5 mil (0.127mm) gap between them, it is recommended that no other traces be routed within 25mils
(0.635mm) of each trace.
Whenever possible, it is best to avoid:
• Routing differential pairs under components.
• Routing DP/DM lines over the top of other PCB traces on adjacent layers.
For more information on USB 2.0 High Speed system design, refer to Application Note 20.2 -“Optimizing USB 2.0 High
Speed Physical Layer Design”.
Isolated Shielding on the USB Connector
The USB334x fully supports USB On-the-Go (OTG). Figure 2 shows the Mini-AB connector housing is DC isolated but
AC coupled to the device ground. Industry convention is to ground only the host side of the cable shield. This is done
to provide cable shielding while preventing possible ground currents from flowing in the USB cable if there happens to
be a potential difference between the host and device grounds. If DC grounding is required replace C12 with a zero Ohm
resistor.
In OTG applications the shield may be DC grounded at both ends of the cable.
FIGURE 2: CONNECTIONS TO SHIELD OF USB CONNECTOR

Product specificaties

Merk: Microchip
Categorie: Niet gecategoriseerd
Model: USB3346

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