© Semiconductor Components Industries, LLC, 2003

August, 2003 − Rev. 0

1

Publication Order Number:

AND8130/D

AND8130/D

Analog Switch Allows USB

Switching at Low Voltages

Prepared by: Fred Zlotnick

ON Semiconductor

EXECUTIVE SUMMARY

The new NLAS2066 is a dual SPST (Single Pole Single

Throw) switch with over−voltage tolerance (OVT) in it’s

input pins. The OVT feature permits the device to be used

for the purpose of sharing of pins in size constrained

applications. Shared pins permit the designer to have a

smaller connector, saving cost on both male and female

portions.

INTRODUCTION

Our modern world is filled with many new devices that

permit the user to communicate, take photos and short

videos, and listen to music and a whole host of features not

yet on the market. The USB port of a modern computer is the

most convenient, easiest to use, high performance

connection point the consumer can deal with. Although

there are other useful ports, the USB port offers simple

hookup, up to 2.5 watts of power, and speeds from 100

Kbits/sec to 400 Mbits/sec.

A USB port has 4 wires, 2 for bidirectional data, and two

for power. A USB port can supply as much as 500 mA at 5.0

V. Many devices simply plug into the USB port with no need

for an external power supply. Most users are already familiar

with USB peripherals such as printers, pointing devices,

keyboard, and digital still camera. Most peripherals just

“plug and play”. Modern P.C.s take care of the interface with

minimal effort, only requiring the installation of software

drivers. In addition, the number of USB ports can be

increased by adding a hub. USB simply works very well and

is the simplest, most convenient port a consumer can deal

with.

USB 1.1/2.0

USB 2.0 adds a High Speed specification to the original

USB v. 1.1 specification. There are three speeds for USB v.

2.0. The three speed ranges are:

Low Speed:

10−100 Kbits/sec

Full Speed

500K−10 Mbits/sec

High Speed

25−400 Mbits/sec

All USB devices are backward and forward compatible

with both the older 1.1 and 2.0 revisions. If the user had a

device that he recently purchased that is rated as High Speed,

he can still use it with an older PC that only supports the Full

Speed mode.

With its ease of use, and inclusion on nearly all Windows,

Mac and Linux systems built since 1998, it is the ideal

interface for an enhanced feature cell phone. A phone with

a 64 MB card can be updated in a few minutes, even in the

Full Speed mode. Photos, music, address books can all be

updated in just a few seconds. When attempting to add USB

to a device like a cell phone the designer has several issues.

Adding an additional connector adds cost and takes up

space. Adding 4 more pins to the existing connector adds

cost to both the female portion in the phone and all the

various male connectors that need to be supplied, in the form

of “dongles”. One way to simplify the whole system is to

multiplex some of the pins.

MULTIPLEXING PINS

This article will only concern itself with the multiplexing

of the data pins. The power pins provide capability to

re−charge the phone, without need for a wall mounted power

supply. With the close proximity of the wires and the

possibility of a potential fault from the power line to the data

line, specification assumes the data lines can handle a

data is sent differentially as NRZI at a maximum of 10

MBits/sec. The cell phone manufacturer must supply

interface

cables.

The

standard

USB

Type

B,

at

approximately 11 x 11 mm, connector probably is too large

non−standard plug on the one side and a standard USB type

“A” plug on the other. Depending upon the features of the

cell phone, the socket may provide audio, video, RS−232, or

other interfaces.

1. Universal Serial Bus Specification, Revision 2.0 April 26,

2000.

2. A flexible cable with appropriate connectors on both ends.

APPLICATION NOTE

http://onsemi.com