6

LTC1481

SWITCHI G TI E WAVEFOR S

1.5V

1.5V

1.5V

1.5V

0.5V

0.5V

OUTPUT NORMALLY LOW

OUTPUT NORMALLY HIGH

3V

0V

RE

5V

0V

RO

RO

LTC1481 • F08

APPLICATIO S I FOR ATIO

Basic Theory of Operation

Traditionally, RS485 transceivers have been designed

using bipolar technology because the common-mode

range of the device must extend beyond the supplies and

the device must be immune to ESD damage and latch-up.

Unfortunately, most bipolar devices draw a large amount

of supply current, which is unacceptable for the numerous

applications that require low power consumption. The

LTC1481 is a CMOS RS485/RS422 transceiver which

features ultra-low power consumption without sacrificing

ESD and latch-up immunity.

The LTC1481 uses a proprietary driver output stage,

which allows a common-mode range that extends beyond

the power supplies while virtually eliminating latch-up and

providing excellent ESD protection. Figure 9 shows the

LTC1481 output stage while Figure 10 shows a conven-

tional CMOS output stage.

When the conventional CMOS output stage of Figure 10

enters a high impedance state, both the P-channel (P1)

and the N-channel (N1) are turned off. If the output is then

(D1) or the N+/P-substrate diode (D2) respectively will

turn on and clamp the output to the supply. Thus, the

output stage is no longer in a high impedance state and is

not able to meet the RS485 common-mode range require-

ment. In addition, the large amount of current flowing

through either diode will induce the well-known CMOS

latch-up condition, which could destroy the device.

The LTC1481 output stage of Figure 9 eliminates these

problems by adding two Schottky diodes, SD3 and SD4.

The Schottky diodes are fabricated by a proprietary modi-

fication to the standard N-well CMOS process. When the

output stage is operating normally, the Schottky diodes

are forward biased and have a small voltage drop across

them. When the output is in the high impedance state and

D1 or D2 still turns on, but SD3 or SD4 will reverse bias and

prevent current from flowing into the N-well or the sub-

strate. Thus the high impedance state is maintained even

with the output voltage beyond the supplies. With no

minority carrier current flowing into the N-well or sub-

strate, latch-up is virtually eliminated under power-up or

power-down conditions.

1.5V

RO

A – B

0V

0V

1.5V

OUTPUT

INPUT

LTC1481 • F07

Figure 7. Receiver Propagation Delays

Figure 8. Receiver Enable and Disable Times