LT1785 Linear Technology, LT1785 Datasheet - Page 8

• Manufacturer Part Number: LT1785
• Description: 60V Fault Protected RS485/RS422 Transceivers
• Manufacturer: Linear Technology

SWITCHI G TI E WAVEFOR S
APPLICATIO S I FOR ATIO
Overvoltage Protection
The LT1785/LT1791 RS485/RS422 transceivers answer
an applications need for overvoltage fault tolerance on
data networks. Industrial installations may encounter
common mode voltages between nodes far greater than
the – 7V to 12V range specified for compliance to RS485
standards. CMOS RS485 transceivers can be damaged by
voltages above their absolute maximum ratings of typi-
LT1785/LT1785A
LT1791/LT1791A
8
A – B
A, B
A, B
RO
RO
RO
RE
DE
–V
V
V
V
V
V
OD2
OD2
OH
5V
0V
5V
OH
0V
5V
0V
5V
0V
OL
U
OL
U U
W
W
Figure 8. Receiver Enable and Disable Times
Figure 6. Driver Enable and Disable Times
t
PHL
Figure 7. Receiver Propagation Delays
W
1.5V
1.5V
0V
U
2.3V
2.3V
1.5V
1.5V
1.5V
t
t
t
t
ZL(SHDN)
ZH(SHDN)
ZL(SHDN)
ZH(SHDN)
f = 125kHz, t
f = 125kHz, t
f = 125kHz, t
OUTPUT NORMALLY HIGH
OUTPUT NORMALLY HIGH
OUTPUT NORMALLY LOW
OUTPUT NORMALLY LOW
, t
, t
, t
, t
ZL
ZL
ZH
ZH
cally – 8V to 12.5V. Replacement of standard RS485
transceiver components with the LT1785 or LT1791
devices eliminates field failures due to overvoltage faults
or the use of costly external protection devices. The limited
overvoltage tolerance of CMOS RS485 transceivers makes
implementation of effective external protection networks
difficult without interfering with proper data network
performance within the – 7V to 12V region of RS485
operation.
r
r
r
10ns, t
OUTPUT
10ns, t
INPUT
10ns, t
f
f
f
10ns
10ns
10ns
t
t
t
t
LZ
HZ
HZ
LZ
t
PLH
1.5V
1.5V
0.5V
0.5V
0.5V
0.5V
0V
1.5V
1785/91 F07
1785/91 F06
1785/91 F08