Microchip USB2229 Handleiding

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SMSC AN 14.18APPLICATION NOTERevision 0.96 (06-01-06)

AN 14.18

Design Guidelines for

USB22XX/USB260X

High-Speed SD

1 Introduction

This application note provides information on Printed Circuit Board layout for High-Speed Secure-Digital

(SD) media sockets with USB22XX and USB260X.

2 Overview

Successful High-Speed operation of Secure-Digital media with USB22XX and USB260X requires

special consideration for Printed Circuit Board (PCB) layout. This application note describes the

important items to consider for layout of PCB.

3 PCB Layout Guidelines

The guidelines presented are applicable to all SMSC card-reader products that support High-Speed

Secure-Digital operation. Guidelines for both two-layer and four-layer PCBs are presented here.

3.1 Power and Ground Distribution to SD Socket

Ground connection between the card-reader and the media socket is important, both for supply return

current and signal return currents. The ground should be solid, have low impedance and few

constrictions between the card-reader and the socket. The card-reader supplies the power to the socket

in most applications since the power-FET is built-in to the SMSC card-reader.

SD socket VDD supply trace from card-reader should be 20 mils wide, 0.5 oz or thicker for a length

up to 3000 mils.

SD socket ground connection: plane, 0.5 oz or thicker, between socket and card-reader.

Note: No constrictions or cuts allowed in the ground between card-reader and socket.

3.2 Signal Traces to SD Socket

The Secure-Digital interface has a total of seven signals to the card-reader. Six of these signals are

critical for high-speed operation: SD_DAT[0:3], SD_CMD and SD_CLK and require special

considerations.

Signal traces must be above a solid and continuous ground plane along the path from card-reader

to socket.

SD_CLK trace requires two grounded guard-traces, one on each side spaced, at 3 times minimum

spacing.

SD_CLK guard traces must have a via connecting to the ground plane at both ends and every 500

mils along the path.

Product specificaties