AND8130/D

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2

If fewer pins are brought out on the cell phone in, the size

of the connector can be made smaller. In order to minimize

the number of pins and reduce size and cost, it behooves the

manufacturer to employ some form of multiplexing or

sharing of pins on the connector. Pins must be capable of

bilateral transmission of either analog of digital signals.

Figure 1. USB − Type “A”

USB − Type “B”

Figure 2. Example of a Dongle

The semiconductor industry has been serving the need for

bilateral switching for years with standard analog switches.

Available analog switches can pass either digital or analog

signals in two directions. More recently, analog switches

have been fabricated in sub−micron silicon gate CMOS and

exhibit resistance in 5−20

Ω range, and can pass signals 50

could be used to provide USB switching. Also, the

frequency response of the switch should not affect overall

performance of the USB transceiver. Several standard

requirements. One additional requirement to meet USB

safety specifications, states that the analog switch must be

capable of a voltage “fault” of 5.25 Volts for 24 hours. One

way to solve this requirement is simply to operate the switch

at a Vcc of 5.0 V, nominal. While this would work, it places

a cost and component burden on the system. The need for 5.0

V in a product like a cell phone is rare today. If not otherwise,

needed it would place a demand for a regulator and

additional voltage from the dc/dc converter on board.

Camera enabled cell phones have very limited space, so the

5.0 V option is not very desirable. In order to save space and

component cost and minimize the number of power

supplies. The analog switch would need to operate from a

supply voltage of approximately 2.7 V and be able to accept

a 5.25 V fault. If the designer could find a switch that could

accept 5.25 V, while operating from 2.7 V, then the 5.0 V

power supply could disappear. Ordinary analog switches

have diodes connected to the both the supply pin and ground

pin, to protect the device against Electrostatic Discharge

(ESD). If the switch were to be operated at 2.7 V and either

USB data line had 5.25 V fault, the input diode connected to

Vcc would be destroyed. This could cause damage, and

could not be tolerated. A new device, the NLAS2066 dual

analog switch has been introduced from ON Semiconductor,

which permits operation at 2.7 V, while its inputs are permit

to be subject to as much as 5.5 V, regardless of the operating

permit use with USB 2.0 Full Speed specification.

Vcc

I/O

I/O

CTRL

Standard

Analog Switch

Figure 3.