LT1483 Linear Technology, LT1483 Datasheet - Page 6

• Manufacturer Part Number: LT1483
• Description: Ultra-Low Power RS485 Low EMI Transceiver with Shutdown
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
SWITCHI G TI E WAVEFOR S
LTC1483
Basic Theory of Operation
Traditionally RS485 transceivers have been designed us-
ing 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
LTC1483 is a CMOS RS485/RS422 transceiver which
features ultra-low power consumption without sacrificing
ESD and latch-up immunity.
The LTC1483 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
LTC1483 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
driven above V
6
A – B
RO
RO
RO
RE
–V
V
U
V
V
OD2
OD2
3V
0V
5V
0V
OH
OL
CC
or below ground, the P+/N-well diode
U U
W
W
Figure 8. Receiver Enable and Disable Times
Figure 7. Receiver Propagation Delays
t
PHL
W
U
1.5V
0V
1.5V
1.5V
1.5V
t
t
ZL(SHDN)
ZH(SHDN)
OUTPUT NORMALLY HIGH
OUTPUT NORMALLY LOW
, t
, t
t
ZL
ZH
Figure 9. LTC1483 Output Stage
r
t
(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.
r
LOGIC
10ns, t
10ns, t
OUTPUT
INPUT
f
f
10ns
10ns
P1
N1
V
CC
t
t
LZ
HZ
t
SD4
SD3
PLH
LTC1483 • F09
OUTPUT
D1
D2
1.5V
0.5V
0.5V
0V
1.5V
Figure 10. Conventional
CMOS Output Stage
LOGIC
LTC1483 • F08
LTC1483 • F07
P1
N1
V
CC
LTC1483 • F10
OUTPUT
D1
D2