LT1785/LT1785A
LT1791/LT1791A
60V Fault Protected
RS485/RS422 Transceivers
FEATURES
s
s
s
s
DESCRIPTIO
s
s
s
s
s
s
Protected from Overvoltage Line Faults to
±
60V
Pin Compatible with LTC485 and LTC491
High Input Impedance Supports Up to 128 Nodes
No Damage or Latchup to ESD
IEC-1000-4-2 Level 4:
±15kV
Air Discharge
IEC-1000-4-2 Level 2:
±4kV
Contact Discharge
Controlled Slew Rates for EMI Emissions Control
Guaranteed High Receiver Output State for Floating,
Shorted or Inactive Inputs
Outputs Assume a High Impedance When Off or
Powered Down
Drives Low Cost, Low Impedance Cables
Short-Circuit Protection on All Outputs
Thermal Shutdown Protection
The LT
®
1785/LT1791 are half-duplex and full-duplex dif-
ferential bus transceivers for RS485 and RS422 applica-
tions which feature on-chip protection from overvoltage
faults on the data transmission lines. Receiver input and
driver output pins can withstand voltage faults up to
±60V
with respect to ground with no damage to the device.
Faults may occur while the transceiver is active, shut down
or powered off.
Data rates to 250kbaud on networks of up to 128 nodes are
supported. Controlled slew rates on the driver outputs
control EMI emissions and improve data transmission
integrity on improperly terminated lines. Drivers are speci-
fied to operate with inexpensive cables as low as 72Ω
characteristic impedance.
The LT1785A/LT1791A devices have “fail-safe” receiver
inputs to guarantee a receiver output high for shorted,
open or inactive data lines. On-chip ESD protection elimi-
nates need for external protection devices.
The LT1785/LT1785A are available in 8-lead DIP and SO
packages and the LT1791/LT1791A in 14-lead DIP and SO
packages.
, LTC and LT are registered trademarks of Linear Technology Corporation.
APPLICATIO S
s
s
s
Industrial Control Data Networks
CAN Bus Applications
HVAC Controls
TYPICAL APPLICATIO
RO1
RE1
DE1
DI1
RX
LT1785
Normal Operation Waveforms at 250kBaud
V
CC1
R
TERM
RO
TX
GND1
Y-Z
V
CC2
RO2
RE2
DE2
DI2
RX
LT1785
R
TERM
DI
TX
GND2
1785/91 TA01
U
1785/91 TA02
U
U
1
LT1785/LT1785A
LT1791/LT1791A
ABSOLUTE
AXI U
RATI GS
Supply Voltage (V
CC
) .............................................. 18V
Receiver Enable Input Voltage .................... – 0.3V to 6V
Driver Enable Input Voltage ........................ – 0.3V to 6V
Driver Input Voltage .................................. – 0.3V to 18V
Receiver Input Voltage ............................... – 60V to 60V
Driver Output Voltage ............................... – 60V to 60V
Receiver Output Voltage ................ – 0.3V to (V
CC
+ 6V)
PACKAGE/ORDER I FOR ATIO
ORDER PART
NUMBER
TOP VIEW
RO 1
RE 2
DE 3
DI 4
N8 PACKAGE
8-LEAD PDIP
D
R
8
7
6
5
V
CC
B
A
GND
LT1785CN8
LT1785CS8
LT1785IN8
LT1785IS8
LT1785ACN8
LT1785ACS8
S8 PACKAGE
8-LEAD PLASTIC SO
T
JMAX
= 150°C,
θ
JA
= 130°C/ W (N8)
T
JMAX
= 150°C,
θ
JA
= 150°C/ W (S8)
S8 PART MARKING
1785
1785I
1785A
T
JMAX
= 150°C,
θ
JA
= 130°C/ W (N)
T
JMAX
= 150°C,
θ
JA
= 150°C/ W (S)
Consult factory for Military grade parts.
2
U
U
W
W W
U
W
(Note 1)
Operating Temperature Range
LT1785C/LT1791C/
LT1785AC/LT1791AC ............................. 0°C to 70°C
LT1785I/LT1791I ............................... – 40°C to 85°C
Storage Temperature Range ................ – 65°C to 150°C
Lead Temperature (Soldering, 10 sec)................. 300°C
ORDER PART
NUMBER
TOP VIEW
NC 1
RO 2
RE 3
DE 4
DI 5
GND 6
GND 7
N PACKAGE
14-LEAD PDIP
D
R
14 V
CC
13 NC
12 A
11 B
10 Z
9
8
Y
NC
LT1791CN
LT1791CS
LT1791IN
LT1791IS
LT1791ACN
LT1791ACS
S PACKAGE
14-LEAD PLASTIC SO
LT1785/LT1785A
LT1791/LT1791A
DC ELECTRICAL CHARACTERISTICS
The
q
denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25°C, V
CC
= 5V.
SYMBOL
V
OD1
V
OD2
PARAMETER
Differential Driver Output Voltage (Unloaded)
Differential Driver Output Voltage (With Load)
CONDITIONS
I
O
= 0
R = 50Ω (RS422), Figure 1
R = 27Ω (RS485), Figure 1
R = 18Ω
R = 27Ω or R = 50Ω, Figure 1
R = 27Ω or R = 50Ω, Figure 1
R = 27Ω or R = 50Ω, Figure 1
DI, DE, RE
DI, DE, RE
DI, DE, RE
V
IN
= 12V
V
IN
= – 7V
– 60V
≤
V
IN
≤
60V
LT1785/LT1791: – 7V
≤
V
CM
≤
12V
LT1785A/LT1791A: – 7V
≤
V
CM
≤
12V
– 7V < V
CM
< 12V
I
O
= – 400µA, V
ID
= 200mV
I
O
= 1.6mA, V
ID
= – 200mV
RE > 2V or Power Off
– 7V
≤
V
CM
≤
12V
– 60V
≤
V
CM
≤
60V
– 7V
≤
V
CM
≤
12V
V
OUT
= HIGH, Force V
O
= – 7V
V
OUT
= LOW, Force V
O
= 12V
V
O
= 60V
V
O
= – 60V
0V
≤
V
O
≤
V
CC
–7V
≤
V
O
≤
12V
– 60V
≤
V
O
≤
60V
No Load, RE = 0V, DE = 5V
No Load, RE = 5V, DE = 5V
No Load, RE = 0V, DE = 0V
No Load, RE = 5V, DE = 0V
q
q
q
q
q
q
q
q
q
q
q
q
q
q
q
q
q
q
q
q
MIN
2.0
1.5
1.2
TYP
4.1
2.70
2.45
2.2
MAX
5
UNITS
V
V
V
V
∆V
OD
V
OC
∆
V
OC
V
IH
V
IL
I
IN1
I
IN2
Change in Magnitude of Driver Differential Output
Voltage for Complementary Output States
Driver Common Mode Output Voltage
Change in Magnitude of Driver Common Mode
Output Voltage for Complementary Output States
Input High Voltage
Input Low Voltage
Input Current
Input Current (A, B); (LT1791 or LT1785 with
DE = 0V)
Differential Input Threshold Voltage for Receiver
Receiver Input Hysteresis
Receiver Output High Voltage
Receiver Output Low Voltage
Three-State (High Impedance) Output Current
at Receiver 0V < V
OUT
< 6V
0.2
2
2.5
3
0.2
2
0.8
5
– 0.15
–6
– 0.2
– 0.2
20
3.5
–1
85
50
35
35
–6
±30
– 0.2
–6
5.5
5.5
4.5
0.2
0.3
6
9
9
8
0.3
125
125
90
0.25
250
250
6
4
0.3
0.5
1
0.15
– 0.08
0.3
6
0.2
0
V
V
V
V
V
µA
mA
mA
mA
V
V
mV
V
V
µA
kΩ
kΩ
kΩ
mA
mA
mA
mA
mA
mA
mA
mA
mA
mA
mA
V
TH
∆V
TH
V
OH
V
OL
R
IN
Receiver Input Resistance (LT1791)
LT1785
RS485 Unit Load
I
SC
Driver Short-Circuit Current
Driver Output Fault Current
Receiver Short-Circuit Current
Driver Three-State Output Current
q
q
q
q
q
q
q
q
q
q
q
I
CC
Supply Current
3
LT1785/LT1785A
LT1791/LT1791A
SWITCHI G CHARACTERISTICS
The
q
denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25°C, V
CC
= 5V.
SYMBOL
t
PLH
t
PHL
t
SKEW
t
r
, t
f
t
ZH
t
ZL
t
LZ
t
HZ
t
PLH
t
PHL
t
SKD
t
ZL
t
ZH
t
LZ
t
HZ
f
MAX
t
SHDN
t
ZH(SHDN)
t
ZL(SHDN)
t
ZH(SHDN)
t
ZL(SHDN)
PARAMETER
Driver Input to Output
Driver Input to Output
Driver Output to Output
Driver Rise or Fall Time
Driver Enable to Output High
Driver Enable to Output Low
Driver Disable Time from Low
Driver Disable Time from High
Receiver Input to Output
Receiver Input to Output
Differential Receiver Skew
Receiver Enable to Output Low
Receiver Enable to Output High
Receiver Disable from Low
Receiver Disable from High
Maximum Data Rate
Time to Shut Down
Driver Enable from Shutdown to Output High
Driver Enable from Shutdown to Output Low
Receiver Enable from Shutdown to Output High
Receiver Enable from Shutdown to Output Low
Figures 2, 6, 8
Figures 2, 6; RE = 5V
Figures 2, 6; RE = 5V
Figures 2, 8; DE = 0V
Figures 2, 8; DE = 0V
Figures 2, 8
Figures 2, 8
Figures 2, 8
Figures 2, 8
q
q
q
q
q
Note 1:
Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
TYPICAL PERFORMANCE CHARACTERISTICS
Driver Differential Output Voltage
vs Load Resistance
4
T
A
= 25°C
OUTPUT VOLTAGE (V)
3
DIFFERENTIAL VOLTAGE (V)
2
1.5
1.0
0.5
DELAY (ns)
1
0
10
100
LOAD RESISTANCE (Ω)
1k
1785/91 G01
4
U W
U
CONDITIONS
Figures 3, 5
Figures 3, 5
Figures 3, 5
Figures 3, 5
Figures 4, 6
Figures 4, 6
Figures 4, 6
Figures 4, 6
Figures 3, 7
Figures 3, 7
q
q
q
q
q
q
q
q
q
MIN
TYP
700
700
100
MAX
2000
2000
2000
3000
3000
5000
5000
900
900
1000
1000
1000
1000
UNITS
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
kbps
µs
µs
µs
µs
µs
200
800
500
800
200
800
400
400
200
300
300
400
400
250
3
12
12
4
4
Driver Differential Output Voltage
vs Temperature
3.0
2.5
2.0
600
1000
Receiver Propagation Delay
vs Temperature
800
t
PHL
400
t
PLH
200
R = 27Ω
0
–40 –20
40
20
0
60
TEMPERATURE (°C)
80
100
0
–40
–20
40
0
60
20
TEMPERATURE (°C)
80
100
1785/91 G03
1785/91 G03
LT1785/LT1785A
LT1791/LT1791A
TYPICAL PERFORMANCE CHARACTERISTICS
Driver Propagation Delay
vs Temperature
1000
900
PROPAGATION DELAY (ns)
800
700
600
500
400
300
200
100
0
–40
–20
40
0
60
20
TEMPERATURE (°C)
80
100
LH
HL
LT1785 Input Characteristics
Pins A or B; DE = RE = 0V
7
6
5
I
CC
(mA)
1mA/DIV
4
3
2
DELAY (ns)
– 60V
V
A,
V
B
PIN FUNCTIONS
RO:
Receiver Output. TTL level logic output. If the receiver
is active (RE pin low), RO is high if receiver input A
≥
B by
200mV. If A
≤
B by 200mV, then RO will be low. RO
assumes a high impedance output state when RE is high
or the part is powered off. RO is protected from output
shorts from ground to 6V.
RE:
Receiver Output Enable. TTL level logic input. A logic
low on RE enables normal operation of the receiver output
RO. A logic high level at RE places the receiver output pin
RO into a high impedance state. If receiver enable RE and
driver enable DE are both in the disable state, the circuit
goes to a low power shutdown state. Placing either RE or
DE into its active state brings the circuit out of shutdown.
Shutdown state is not entered until a 3µs delay after both
RE and DE are disabled, allowing for logic skews in
toggling between transmit and receive modes of opera-
tion. For CAN bus applications, RE should be tied low to
prevent the circuit from entering shutdown.
DE:
Driver Output Enable. TTL level logic input. A logic
high on DE enables normal operation of the driver outputs
(Y and Z on LT1791, A and B on LT1785). A logic low level
at DE places the driver output pins into a high impedance
U W
1785/91 G04
LT1791 Driver Output Leakage
DE = 0V
LT1791 Receiver Input Current
vs V
IN
1mA/DIV
200µA/DIV
– 60V
V
OUT
60V
1785/91 G05
– 60V
V
IN
60V
1785/91 G06
Supply Current vs Temperature
700
Receiver Propagation Delay
vs Differential Input Voltage
600
HL V
CM
= –7V
500
HL V
CM
= 12V
LH V
CM
= –7V
LH V
CM
= 12V
DRIVER AND
RECEIVER ON
RECEIVER ONLY
400
300
200
100
60V
1785/91 G07
1
0
–40
STANDBY
–20
40
60
0
20
TEMPERATURE (°C)
80
100
0
0
1
3
4
2
V
IN
DIFFERENTIAL (V)
5
1785/91 G09
1785/91 G08
U
U
U
5
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