AN-5001 Datasheet, PDF(1/3 Page) Fairchild Semiconductor – LVX Low Voltage Dual Supply CMOS Translating Transceivers

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AN-5001 Datasheet, PDF (1/3 Pages) Fairchild Semiconductor – LVX Low Voltage Dual Supply CMOS Translating Transceivers

AN-5001

Fairchild Semiconductor

Application Note

April 1998

Revised May 1999

Using Fairchildâs LVX Low Voltage

Dual Supply CMOS Translating Transceivers

INTRODUCTION

The Fairchild CMOS Translating Transceivers:

74LVX3245, 74LVX4245, 74LVXC3245, and 74LVXC4245

are true voltage translating devices. This is accomplished

by electrically isolating the A-side from the B-side.

An example of when it is necessary to use a true voltage

translator, is in a system translating between 3 volt logic

and 5 volt CMOS logic. In this case, the translators will

insure a clean, fast and valid VIH (Voltage Input High) sig-

nal in both directions. This translation may be problematic

with other forms of translation due to the VIH required, typi-

cally 3.5V for 5V CMOS.

DEVICE TYPE DESCRIPTIONS

The LVX Translating Transceivers are divided into two

types of devices, Dual Supply Translating Transceivers,

and Dual Supply Configurable Voltage Interface Transceiv-

ers. These devices offer the system designer multiple

options for true voltage level translation in a system.

DUAL SUPPLY TRANSLATORS

The Dual Supply Translator devices (74LVX3245 and

74LVX4245) are designed to interface 3 volt to 5 volt sig-

nals. The Transmit/Receive control pin controls data direc-

tion flow. These devices are designed with the higher

voltage VCC able to swing from 4.5V to 5.5V and the lower

voltage VCC able to swing from 2.7V to 3.6V. The control

pins OE (Output Enable) and T/R (Transmit/Receive) are

The 74LVX3245 and 74LVX4245 provide the same 5V to

3V translation function. The difference between the two

devices is the side of the device the 5V and 3V VCCâs and

their associated I/O (Input/Outputs data) ports are on.

In the case of the 74LVX3245 Figure 1, VCCB and the B

I/O port are the 5V side (4.5V to 5.5V) and the VCCA and

the A I/O port are the 3V side (2.7V to 3.6V).

On the 74LVX4245 Figure 2, VCCA and the A I/O port are

the 5V side (4.5V to 5.5V) and the VCCB and the B I/O are

the 3V side (2.7V to 3.6V).

FIGURE 2. 74LVX4245

CONFIGURABLE VOLTAGE INTERFACE

TRANSCEIVERS

The Configurable Voltage Interface transceivers

(74LVXC3245 and 74LVXC4245) are designed for real

time configurable I/O applications such as PCMCIA (Per-

sonal Computer Memory Card Interface Association).

Configurability simply means the device is designed with

the ability of the âconfigurableâ side of the I/O port, to track,

or follow the VCCB voltage level. This is accomplished by

tying VCCB of the device to the PCMCIA card voltage sup-

ply. The card will always experience full rail data signals,

maximizing interface reliability.

The control pins OE (Output Enable) and T/R (Transmit/

Receive) are powered by the VCCA side of the device.

These devices are also designed to allow the configurable

side (VCCB and B I/O port) to float, unconnected to any

voltage or control source. This is allowed when the OE pin

is driven to a logic high.

Floating VCCB and the B port is useful in applications where

a card or cable may need to be left disconnected. Using a

Configurable Voltage Interface device at this point will elim-

inate false signaling or system damage due to the oscilla-

tions that can occur if standard design CMOS inputs and

power pins are left floating.

The 74LVXC3245 and 74LVXC4245 are designed for dif-

ferent VCCA and A I/O port voltage levels.

FIGURE 1. 74LVX3245

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