LT1785/LT1785A/
LT1791/LT1791A
1
Rev. F
For more information www.analog.com
TYPICAL APPLICATION
DESCRIPTION
60V Fault Protected
RS485/RS422 Transceivers
The LT
®
1785/LT1791 are half-duplex and full-duplex differ-
ential bus transceivers for RS485 and RS422 applications
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 out-
putscontrol EMI emissions and improve data transmis-
sion integrity on improperly terminated lines. Drivers are
specified 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 eliminates
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.
Normal Operation Waveforms at 250kBaud
FEATURES
APPLICATIONS
n Protected from Overvoltage Line Faults to ±60V
n Pin Compatible with LTC485 and LTC491
n High Input Impedance Supports Up to 128 Nodes
n No Damage or Latchup to ESD
n IEC-1000-4-2 Level 4: ±15kV Air Discharge
n IEC-1000-4-2 Level 2: ±4kV Contact Discharge
n Controlled Slew Rates for EMI Emissions Control
n Guaranteed High Receiver Output State for Floating,
Shorted or Inactive Inputs (LT1785A/LT1791A)
n Outputs Assume a High Impedance When Off or
Powered Down
n Drives Low Cost, Low Impedance Cables
n Short-Circuit Protection on All Outputs
n Thermal Shutdown Protection
n Guaranteed Operation to 125°C
n AEC-Q100 Qualified for Automotive Applications
(LT1785#W)
n Industrial Control Data Networks
n CAN Bus Applications
n HVAC Controls
GND1
GND2
RTERM
178591 TA01
RO1
RE1
DE1
DI1
LT1785
RO2
RE2
DE2
DI2
LT1785
VCC1
VCC2
RTERM
TX
TX
RX
RX
RO
178591 TA02
DI
Y-Z
All registered trademarks and trademarks are the property of their respective owners.
Document Feedback
LT1785/LT1785A/
LT1791/LT1791A
2
Rev. F
For more information www.analog.com
ABSOLUTE MAXIMUM RATINGS
(Note 1)
1
2
3
4
8
7
6
5
TOP VIEW
VCC
B
A
GND
N8 PACKAGE
8-LEAD PDIP
S8 PACKAGE
8-LEAD PLASTIC SO
R
D
RO
RE
DE
DI
TJMAX = 150°C, θJA = 130°C/W (N8)
TJMAX = 150°C, θJA = 150°C/W (S8)
TOP VIEW
N PACKAGE
14-LEAD PDIP
S PACKAGE
14-LEAD PLASTIC SO
1
2
3
4
5
6
7
14
13
12
11
10
9
8
NC
RO
RE
DE
DI
GND
GND
VCC
NC
A
B
Z
Y
NC
R
D
TJMAX = 150°C, θJA = 130°C/W (N)
TJMAX = 150°C, θJA = 150°C/W (S)
PIN CONFIGURATION
ORDER INFORMATION
LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE
LT1785CN8#PBF LT1785CN8#TRPBF 1785 8-Lead PDIP 0°C to 70°C
LT1785CS8#PBF LT1785CS8#TRPBF 1785 8-Lead Plastic SO 0°C to 70°C
LT1785IN8#PBF LT1785IN8#TRPBF 1785I 8-Lead PDIP –40°C to 85°C
LT1785IS8#PBF LT1785IS8#TRPBF 1785I 8-Lead Plastic SO –40°C to 85°C
LT1785ACN8#PBF LT1785ACN8#TRPBF 1785A 8-Lead PDIP 0°C to 70°C
LT1785ACS8#PBF LT1785ACS8#TRPBF 1785A 8-Lead Plastic SO 0°C to 70°C
LT1785AIN8#PBF LT1785AIN8#TRPBF 1785AI 8-Lead PDIP –40°C to 85°C
LT1785AIS8#PBF LT1785AIS8#TRPBF 1785AI 8-Lead Plastic SO –40°C to 85°C
LT1785HN8#PBF LT1785HN8#TRPBF 1785H 8-Lead PDIP –40°C to 125°C
LT1785HS8#PBF LT1785HS8#TRPBF 1785H 8-Lead Plastic SO –40°C to 125°C
LT1785AHN8#PBF LT1785AHN8#TRPBF 1785AH 8-Lead PDIP –40°C to 125°C
LT1785AHS8#PBF LT1785AHS8#TRPBF 1785AH 8-Lead Plastic SO –40°C to 125°C
LT1791CN#PBF LT1791CN#TRPBF 1791 14-Lead PDIP 0°C to 70°C
LT1791CS#PBF LT1791CS#TRPBF 1791 14-Lead Plastic SO 0°C to 70°C
Supply Voltage (VCC) ................................................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 (VCC + 6V)
Operating Temperature Range
LT1785C/LT1791C/
LT1785AC/LT1791AC .................................... 0°C to 70°C
LT1785I/LT1791I/
LT1785AI/LT1791AI .................................. –40°C to 85°C
LT1785H/LT1791H/
LT1785AH/LT1791AH .............................–40°C to 125°C
Storage Temperature Range ...................–65°C to 150°C
Lead Temperature (Soldering, 10 sec) .................. 300°C
LT1785/LT1785A/
LT1791/LT1791A
3
Rev. F
For more information www.analog.com
LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE
LT1791IN#PBF LT1791IN#TRPBF 1791I 14-Lead PDIP –40°C to 85°C
LT1791IS#PBF LT1791IS#TRPBF 1791I 14-Lead Plastic SO –40°C to 85°C
LT1791ACN#PBF LT1791ACN#TRPBF 1791A 14-Lead PDIP 0°C to 70°C
LT1791ACS#PBF LT1791ACS#TRPBF 1791A 14-Lead Plastic SO 0°C to 70°C
LT1791AIN#PBF LT1791AIN#TRPBF 1791AI 14-Lead PDIP –40°C to 85°C
LT1791AIS#PBF LT1791AIS#TRPBF 1791AI 14-Lead Plastic SO –40°C to 85°C
LT1791HN#PBF LT1791HN#TRPBF 1791H 14-Lead PDIP –40°C to 125°C
LT1791HS#PBF LT1791HS#TRPBF 1791H 14-Lead Plastic SO –40°C to 125°C
LT1791AHN#PBF LT1791AHN#TRPBF 1791AH 14-Lead PDIP –40°C to 125°C
LT1791AHS#PBF LT1791AHS#TRPBF 1791AH 14-Lead Plastic SO –40°C to 125°C
AUTOMOTIVE PRODUCTS**
LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE
LT1785IS8#WPBF LT1785IS8#WTRPBF 1785I 8-Lead Plastic SO –40°C to 85°C
LT1785AIS8#WPBF LT1785AIS8#WTRPBF 1785AI 8-Lead Plastic SO –40°C to 85°C
LT1785HS8#WPBF LT1785HS8#WTRPBF 1785H 8-Lead Plastic SO –40°C to 125°C
LT1785AHS8#WPBF LT1785AHS8#WTRPBF 1785AH 8-Lead Plastic SO –40°C to 125°C
Contact the factory for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container.
Tape and reel specifications. Some packages are available in 500 unit reels through designated sales channels with #TRMPBF suffix.
**Versions of this part are available with controlled manufacturing to support the quality and reliability requirements of automotive applications. These
models are designated with a #W suffix. Only the automotive grade products shown are available for use in automotive applications. Contact your
local Analog Devices account representative for specific product ordering information and to obtain the specific Automotive Reliability reports for
thesemodels.
ORDER INFORMATION
LT1785/LT1785A/
LT1791/LT1791A
4
Rev. F
For more information www.analog.com
DC ELECTRICAL CHARACTERISTICS
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C, VCC = 5V.
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
VOD1 Differential Driver Output Voltage (Unloaded) IO = 0 l4.1 5 V
VOD2 Differential Driver Output Voltage (With Load) R = 50Ω (RS422), Figure1
R = 27Ω (RS485), Figure1
R = 18Ω
l
l
l
2.0
1.5
1.2
2.70
2.45
2.2
V
V
V
VOD Change in Magnitude of Driver Differential Output
Voltage for Complementary Output States
R = 27Ω or R = 50Ω, Figure1 l0.2 V
VOC Driver Common Mode Output Voltage R = 27Ω or R = 50Ω, Figure1 l2 2.5 3 V
∆|VOC| Change in Magnitude of Driver Common Mode Output
Voltage for Complementary Output States
R = 27Ω or R = 50Ω, Figure1 l0.2 V
VIH Input High Voltage DI, DE, RE l2 V
VIL Input Low Voltage DI, DE, RE l0.8 V
IIN1 Input Current DI, DE, RE l5 µA
IIN2 Input Current (A, B); (LT1791 or LT1785 with DE = 0V) VIN = 12V
VIN = –7V
–60V ≤ VIN ≤ 60V
l
l
l
–0.15
–6
0.15
–0.08
0.3
6
mA
mA
mA
VTH Differential Input Threshold Voltage for Receiver LT1785/LT1791: –7V ≤ VCM ≤ 12V
LT1785A/LT1791A: –7V ≤ VCM ≤ 12V
l
l
–0.2
–0.2
0.2
0
V
V
∆VTH Receiver Input Hysteresis –7V < VCM < 12V 20 mV
VOH Receiver Output High Voltage IO = –400µA, VID = 200mV l3.5 4 V
VOL Receiver Output Low Voltage IO = 1.6mA, VID = –200mV l0.3 0.5 V
Three-State (High Impedance) Output Current at
Receiver 0V < VOUT < 6V
RE > 2V or Power Off l–1 1 µA
RIN Receiver Input Resistance (LT1791) –7V ≤ VCM ≤ 12V
– 60V ≤ VCM ≤ 60V
l85 125
125
kΩ
kΩ
L
T1785 –7V ≤ VCM ≤ 12V l50 90 kΩ
RS485 Unit Load 0.25
ISC Driver Short-Circuit Current VOUT = HIGH, Force VO = –7V
VOUT = LOW, Force VO = 12V
l
l
35
35
250
250
mA
mA
Driver Output Fault Current VO = 60V
VO = –60V
l
l
–6
6 mA
mA
Receiver Short-Circuit Current 0V ≤ VO ≤ VCC l±35 mA
Driver Three-State Output Current –7V ≤ VO ≤ 12V
–60V ≤ VO ≤ 60V
l
l
–0.2
–6
0.3
6
mA
mA
ICC Supply Current No Load, RE = 0V, DE = 5V
No Load, RE = 5V, DE = 5V
No Load, RE = 0V, DE = 0V
No Load, RE = 5V, DE = 0V
l
l
l
l
5.5
5.5
4.5
0.2
9
9
8
0.3
mA
mA
mA
mA
LT1785/LT1785A/
LT1791/LT1791A
5
Rev. F
For more information www.analog.com
SWITCHING CHARACTERISTICS
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C, VCC = 5V.
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
tPLH Driver Input to Output Figures 3, 5 l700 2000 ns
tPHL Driver Input to Output Figures 3, 5 l700 2000 ns
tSKEW Driver Output to Output Figures 3, 5 100 ns
tr, tfDriver Rise or Fall Time Figures 3, 5 l200 800 2000 ns
tZH Driver Enable to Output High Figures 4, 6 l500 3000 ns
tZL Driver Enable to Output Low Figures 4, 6 l800 3000 ns
tLZ Driver Disable Time from Low Figures 4, 6 l200 5000 ns
tHZ Driver Disable Time from High Figures 4, 6 l800 5000 ns
tPLH Receiver Input to Output Figures 3, 7 l400 900 ns
tPHL Receiver Input to Output Figures 3, 7 l400 900 ns
tSKD Differential Receiver Skew 200 ns
tZL Receiver Enable to Output Low Figures 2, 8 l300 1000 ns
tZH Receiver Enable to Output High Figures 2, 8 l300 1000 ns
tLZ Receiver Disable from Low Figures 2, 8 l400 1000 ns
tHZ Receiver Disable from High Figures 2, 8 l400 1000 ns
fMAX Maximum Data Rate l250 kbps
tSHDN Time to Shut Down Figures 2, 6, 8 3 µs
tZH(SHDN) Driver Enable from Shutdown to Output High Figures 2, 6; RE = 5V 12 µs
tZL(SHDN) Driver Enable from Shutdown to Output Low Figures 2, 6; RE = 5V 12 µs
tZH(SHDN) Receiver Enable from Shutdown to Output High Figures 2, 8; DE = 0V 4 µs
tZL(SHDN) Receiver Enable from Shutdown to Output Low Figures 2, 8; DE = 0V 4 µs
Note 1: Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may affect device
reliability and lifetime.
LT1785/LT1785A/
LT1791/LT1791A
6
Rev. F
For more information www.analog.com
TYPICAL PERFORMANCE CHARACTERISTICS
LT1785 Input Characteristics Pins
A or B; DE = RE = 0V
Supply Current vs Temperature
Receiver Propagation Delay
vs Differential Input Voltage
Driver Differential Output Voltage
vs Load Resistance
Driver Differential Output Voltage
vs Temperature
Receiver Propagation Delay
vs Temperature
Driver Propagation Delay
vs Temperature
LT1791 Driver Output Leakage
DE = 0V
LT1791 Receiver Input Current
vs VIN
LOAD RESISTANCE (Ω)
10
OUTPUT VOLTAGE (V)
4
3
2
1
0
100 1k
178591 G01
TA = 25°C
TEMPERATURE (°C)
–40
DIFFERENTIAL VOLTAGE (V)
3.0
2.5
2.0
1.5
1.0
0.5
0040 60
178591 G02
–20 20 80 100
R = 27Ω
TEMPERATURE (°C)
–40
DELAY (ns)
1000
800
600
400
200
0
040 60
178591 G03
–20 20 80 100
tPLH
tPHL
TEMPERATURE (°C)
–40
PROPAGATION DELAY (ns)
1000
900
800
700
600
500
400
300
200
100
0
040 60
178591 G04
–20 20 80 100
HL
LH
1mA/DIV
178591 G05
–60V VIN 60V
200µA/DIV
178591 G06
–60V VIN 60V
1mA/DIV
178591 G07
–60V VA, VB60V
TEMPERATURE (°C)
–40
I
CC
(mA)
7
6
5
4
3
2
1
0
040 60
178591 G08
–20 20 80 100
RECEIVER ONLY
STANDBY
DRIVER AND
RECEIVER ON
VIN DIFFERENTIAL (V)
0
DELAY (ns)
700
600
500
400
300
200
100
0
178591 G09
25
13 4
HL VCM = 12V
HL VCM = –7V
LH VCM = –7V
LH VCM = 12V
LT1785/LT1785A/
LT1791/LT1791A
7
Rev. F
For more information www.analog.com
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 tog-
gling between transmit and receive modes of operation.
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 out-
puts (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 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 operation. For CAN bus
operation the DE pin is used for signal input to place the
data bus in dominant or recessive states.
DI: Driver Input. TTL level logic input. A logic high at DI
causes driver output A or Y to a high state, and output B
or Z to a low state. Complementary output states occur for
DI low. For CAN bus applications DI should be tied low.
GND: Ground.
Y: Driver Output. The Y driver output is in phase with the
driver input DI. In the LT1785 driver output Y is internally
connected to receiver input A. The driver output assumes
a high impedance state when DE is low, power is off or
thermal shutdown is activated. The driver output is pro-
tected from shorts between ±60V in both active and high
impedance modes. For CAN applications, output Y is the
CANL output node.
Z: Driver Output. The Z driver output is opposite in phase
to the driver input DI. In the LT1785 driver output Z is
internally connected to receiver input B. The driver output
assumes a high impedance state when DE is low, power
is off or thermal shutdown is activated. The driver output
is protected from shorts between ±60V in both active and
high impedance modes. For CAN applications, output Z is
the CANH output node.
A: Receiver Input. The A receiver input forces a high receiver
output when V(A) ≥ [V(B) + 200mV]. V(A) ≤ [V(B)– 200mV]
forces a receiver output low. Receiver inputs A and B are
protected against voltage faults between ±60V. The high
input impedance allows up to 128 LT1785 or LT1791
transceivers on one RS485 data bus.
The LT1785A/LT1791A have guaranteed receiver input
thresholds –200mV < VTH < 0. Receiver outputs are
guaranteed to be in a high state for 0V inputs.
B: Receiver Input. The B receiver input forces a high
receiver output when V(A) ≥ [V(B) + 200mV]. When
V(A) ≤ [V(B) – 200mV], the B receiver forces a receiver
output low. Receiver inputs A and B are protected against
voltage faults between ±60V. The high input impedance
allows up to 128 LT1785 or LT1791 transceivers on one
RS485 data bus.
The LT1785A/LT1791A have guaranteed receiver input-
thresholds –200mV < VTH < 0. Receiver outputs are
guaranteed to be in a high state for 0V inputs.
VCC: Positive Supply Input. For RS422 or RS485 operation,
4.75V ≤ VCC ≤ 5.25V. Higher VCC input voltages increase
output drive swing. VCC should be decoupled with a 0.1µF
low ESR capacitor directly at Pin 8 (VCC).
LT1785/LT1785A/
LT1791/LT1791A
8
Rev. F
For more information www.analog.com
TEST CIRCUITS
Figure1. Driver DC Test Load Figure2. Receiver Timing Test Load
Figure3. Driver/Receiver Timing Test Circuit Figure4. Driver Timing Test Load
VOD
A
B
R
RVOC
1785/91 F01
RECEIVER
OUTPUT
CRL 1k
S2
TEST POINT
VCC
1k
1785/91 F02
5V
DE
A
B
DI RDIFF
CL1
CL2
RO
15pF
A
B
RE
1785/91 F03
OUTPUT
UNDER TEST
CL
S1
S2
VCC
500Ω
1785/91 F04
LT1785/LT1785A/
LT1791/LT1791A
9
Rev. F
For more information www.analog.com
LT1785 Transmitting
INPUTS OUTPUTS
RE DE DI A B RO
0 1 0 0 1 0
0 1 1 1 0 1
1 0 X Hi-Z Hi-Z Hi-Z
1 1 0 0 1 Hi-Z
1 1 1 1 0 Hi-Z
LT1785 Receiving
INPUTS OUTPUT
RE DE DI A-B RO
0 0 X ≤ –200mV 0
0 0 X ≥ 200mV* 1
0 0 X Open 1**
1 0 X X Hi-Z
* ≥ 0mV for LT1785A
** 1 for LT1785A; 1 or 0 for LT1785
LT1791
INPUTS OUTPUTS
RE DE DI A-B Y Z RO
0 0 X ≤ –200mV Hi-Z Hi-Z 0
0 0 X ≥ 200mV* Hi-Z Hi-Z 1
0 0 X Open Hi-Z Hi-Z 1**
0 1 0 ≤ –200mV 0 1 0
0 1 0 ≥ 200mV* 0 1 1
0 1 0 Open 0 1 1**
0 1 1 ≤ –200mV 1 0 0
0 1 1 ≥ 200mV* 1 0 1
0 1 1 Open 1 0 1**
1 0 X X Hi-Z Hi-Z Hi-Z
1 1 0 X 0 1 Hi-Z
1 1 1 X 1 0 Hi-Z
* ≥ 0mV for LT1791A
** 1 for LT1791A; 1 or 0 for LT1791
FUNCTION TABLES
LT1785/LT1785A/
LT1791/LT1791A
10
Rev. F
For more information www.analog.com
SWITCHING TIME WAVEFORMS
Figure5. Driver Propagation Delays
DI
5V
1.5V
tPLH
tr
tSKEW
1/2 VO
VO
f = 125kHz, tr ≤ 10ns, tf ≤ 10ns
90%
10%
0V
B
A
VO
–VO
0V 90%
1.5V
tPHL
tSKEW
1/2 VO
10%
tf
VDIFF = V(A) – V(B)
1785/91 F05
Figure6. Driver Enable and Disable Times
1.5V
2.3V
2.3V
tZH(SHDN), tZH
tZL(SHDN), tZL
1.5V
tLZ
0.5V
0.5V
tHZ
OUTPUT NORMALLY LOW
OUTPUT NORMALLY HIGH
5V
0V
DE
5V
VOL
VOH
0V
A, B
A, B
178591 F06
f = 125kHz, tr ≤ 10ns, tf ≤ 10ns
Figure7. Receiver Propagation Delays
1.5V
tPHL
RO
–VOD2
A – B 0V 0V
1.5V
tPLH
OUTPUT
INPUT
VOD2
VOL
VOH
178591 F07
f = 125kHz, tr ≤ 10ns, tf ≤ 10ns
Figure8. Receiver Enable and Disable Times
1.5V
tZL(SHDN), tZL
tZH(SHDN), tZH
1.5V
1.5V
1.5V
tLZ
0.5V
0.5V
tHZ
OUTPUT NORMALLY LOW
OUTPUT NORMALLY HIGH
5V
0V
RE
5V
0V
RO
RO
178591 F08
f = 125kHz, tr ≤ 10ns, tf ≤ 10ns
LT1785/LT1785A/
LT1791/LT1791A
11
Rev. F
For more information www.analog.com
APPLICATIONS INFORMATION
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 typically –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 toler-
ance 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.
The high overvoltage rating of the LT1785/LT1791 facili-
tates easy extension to almost any level. Simple discrete
component networks that limit the receiver input and
driver output voltages to less than ±60V can be added
to the device to extend protection to any desired level.
Figure11 shows a protection network against faults to
the120VAC line voltage.
The LT1785/LT1791 protection is achieved by using a high
voltage bipolar integrated circuit process for the transceiv-
ers. The naturally high breakdown voltages of the bipolar
process provides protection in powered-off and high
impedance conditions. The driver outputs use a foldback
current limit design to protect against overvoltage faults
while still allowing high current output drive.
ESD Protection
The LT1785/LT1791 I/O pins have on-chip ESD protection
circuitry to eliminate field failures caused by discharges to
exposed ports and cables in application environments. The
LT1785 pins A and B and the LT1791 driver output pins Y
and Z are protected to IEC-1000-4-2 level 2. These pins will
survive multiple ESD strikes of ±15kV air discharge or ±4kV
contact discharge. Due to their very high input impedance,
the LT1791 receiver pins are protected to IEC-1000-4-2
level 2, or ±15kV air and ±4kV contact discharges. This
level of ESD protection will guarantee immunity from field
failures in all but the most severe ESD environments. The
LT1791 receiver input ESD tolerance may be increased to
IEC level 4 compliance by adding 2.2k resistors in series
with these pins.
Low Power Shutdown
The LT1785/LT1791 have RE and DE logic inputs to control
the receive and transmit modes of the transceivers. The
RE input allows normal data reception when in the low
state. The receiver output goes to a high impedance state
when RE is high, allowing multiplexing the RO data line.
The DE logic input performs a similar function on the driver
outputs. A high state on DE activates the differential driver
outputs, a low state places both driver outputs in to high
impedance. Tying the RE and DE logic inputs together may
be done to allow one logic signal to toggle the transceiver
from receive to transmit modes. The DE input is used as
the data input in CAN bus applications.
Disabling both the driver and receiver places the device
into a low supply current shutdown mode. An internal
time delay of 3µs minimum prevents entering shutdown
due to small logic skews when a toggle between receive
and transmit is desired. The recovery time from shutdown
mode is typically 12µs. The user must be careful to allow
for this wake-up delay from shutdown mode. To allow full
250kbaud data rate transmission in CAN applications, the
RE pin should be tied low to prevent entering shutdown
mode.
LT1785/LT1785A/
LT1791/LT1791A
12
Rev. F
For more information www.analog.com
APPLICATIONS INFORMATION
Slew Limiting for EMI Emissions Control
The LT1785/LT1791 feature controlled driver output slew
rates to control high frequency EMI emissions from equip-
ment and data cables. The slew limiting limits data rate
operation to 250kbaud. Slew limiting also mitigates the
adverse affects of imperfect transmission line termina-
tion caused by stubs or mismatched cable. In some low
speed, short distance networks, cable termination may
be eliminated completely with no adverse effect on data
transmission.
Data Network Cable Selection and Termination
Long distance data networks operating at high data trans-
mission rates should use high quality, low attenuation
cable with well-matched cable terminations. Short distance
networks at low data rates may use much less expensive
PVC cable. These cables have characteristic impedances
as low as 72Ω. The LT1785/LT1791 output drivers are
guaranteed to drive cables as low as 72Ω.
Figure9. Full-Duplex RS422
5
4
3
2
A
B
Z
Y
12
11
10
9
RO
RE
DE
DI
178591 F09
TX
120Ω 5
4
3
2
Y
Z
B
A
9
10
11
12 RO
LT1791LT1791
RE
DE
DI
RX
TX
120Ω
RX
LT1785/LT1785A/
LT1791/LT1791A
13
Rev. F
For more information www.analog.com
PACKAGE DESCRIPTION
N8 REV I 0711
.065
(1.651)
TYP
.045 – .065
(1.143 – 1.651)
.130 ±.005
(3.302 ±0.127)
.020
(0.508)
MIN
.018 ±.003
(0.457 ±0.076)
.120
(3.048)
MIN
.008 – .015
(0.203 – 0.381)
.300 – .325
(7.620 – 8.255)
.325 +.035
–.015
+0.889
–0.381
8.255
( )
1 2 34
87 65
.255 ±.015*
(6.477 ±0.381)
.400*
(10.160)
MAX
NOTE:
1. DIMENSIONS ARE INCHES
MILLIMETERS
*THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS.
MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED .010 INCH (0.254mm)
.100
(2.54)
BSC
N Package
8-Lead PDIP (Narrow .300 Inch)
(Reference LTC DWG # 05-08-1510 Rev I)
LT1785/LT1785A/
LT1791/LT1791A
14
Rev. F
For more information www.analog.com
PACKAGE DESCRIPTION
.016 – .050
(0.406 – 1.270)
.010 – .020
(0.254 – 0.508)× 45°
0°– 8° TYP
.008 – .010
(0.203 – 0.254)
SO8 REV G 0212
.053 – .069
(1.346 – 1.752)
.014 – .019
(0.355 – 0.483)
TYP
.004 – .010
(0.101 – 0.254)
.050
(1.270)
BSC
1234
.150 – .157
(3.810 – 3.988)
NOTE 3
8765
.189 – .197
(4.801 – 5.004)
NOTE 3
.228 – .244
(5.791 – 6.197)
.245
MIN .160 ±.005
RECOMMENDED SOLDER PAD LAYOUT
.045 ±.005
.050 BSC
.030 ±.005
TYP
INCHES
(MILLIMETERS)
NOTE:
1. DIMENSIONS IN
2. DRAWING NOT TO SCALE
3. THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS.
MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED .006" (0.15mm)
4. PIN 1 CAN BE BEVEL EDGE OR A DIMPLE
S8 Package
8-Lead Plastic Small Outline (Narrow .150 Inch)
(Reference LTC DWG # 05-08-1610 Rev G)
LT1785/LT1785A/
LT1791/LT1791A
15
Rev. F
For more information www.analog.com
PACKAGE DESCRIPTION
N14 REV I 0711
.020
(0.508)
MIN
.120
(3.048)
MIN
.130 ±.005
(3.302 ±0.127)
.045 – .065
(1.143 – 1.651)
.065
(1.651)
TYP
.018 ±.003
(0.457 ±0.076)
.005
(0.127)
MIN
.255 ±.015*
(6.477 ±0.381)
.770*
(19.558)
MAX
31 24567
8910
11
1213
14
.008 – .015
(0.203 – 0.381)
.300 – .325
(7.620 – 8.255)
.325 +.035
–.015
+0.889
–0.381
8.255
( )
NOTE:
1. DIMENSIONS ARE INCHES
MILLIMETERS
*THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS.
MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED .010 INCH (0.254mm)
.100
(2.54)
BSC
N Package
14-Lead PDIP (Narrow .300 Inch)
(Reference LTC DWG # 05-08-1510 Rev I)
LT1785/LT1785A/
LT1791/LT1791A
16
Rev. F
For more information www.analog.com
PACKAGE DESCRIPTION
1
N
234
.150 – .157
(3.810 – 3.988)
NOTE 3
14 13
.337 – .344
(8.560 – 8.738)
NOTE 3
.228 – .244
(5.791 – 6.197)
12 11 10 9
567
N/2
8
.016 – .050
(0.406 – 1.270)
.010 – .020
(0.254 – 0.508) × 45
0° – 8° TYP
.008 – .010
(0.203 – 0.254)
S14 REV G 0212
.053 – .069
(1.346 – 1.752)
.014 – .019
(0.355 – 0.483)
TYP
.004 – .010
(0.101 – 0.254)
.050
(1.270)
BSC
.245
MIN
N
1 2 3 N/2
.160 ±.005
RECOMMENDED SOLDER PAD LAYOUT
.045 ±.005
.050 BSC
.030 ±.005
TYP
INCHES
(MILLIMETERS)
NOTE:
1. DIMENSIONS IN
2. DRAWING NOT TO SCALE
3. THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS.
MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED .006" (0.15mm)
4. PIN 1 CAN BE BEVEL EDGE OR A DIMPLE
S Package
14-Lead Plastic Small Outline (Narrow .150 Inch)
(Reference LTC DWG # 05-08-1610 Rev G)
LT1785/LT1785A/
LT1791/LT1791A
17
Rev. F
For more information www.analog.com
Information furnished by Analog Devices is believed to be accurate and reliable. However, no responsibility is assumed by Analog
Devices for its use, nor for any infringements of patents or other rights of third parties that may result from its use. Specifications
subject to change without notice. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices.
REVISION HISTORY
REV DATE DESCRIPTION PAGE NUMBER
D 03/15 Added notes to Function Tables 9
E 12/19 Added Auto Grade
Removed lead based finish part options
1, 3
3
F 03/20 Added Auto Grade for H temp range 3
(Revision history begins at Rev D)
LT1785/LT1785A/
LT1791/LT1791A
18
Rev. F
For more information www.analog.com
ANALOG DEVICES, INC. 1998–2020
www.analog.com
03/20
RELATED PARTS
TYPICAL APPLICATION
Figure10. Half-Duplex RS485 Network Operation
4
3
2
1
A
B7
6
RO
RE
DE
DI
178591 F10
RX
TX
RT
120Ω
RT
120Ω
LT1785
4
3
2
1
A
B
7
6
RO
RE
DE
DI
RX
TX
LT1785
4321
AB
7
6
RO
RE
DE
DI
4321
AB
7
6
RO
RE
DE
DI
LT1785
LT1785
4
3
2
1
A
B7
8VCC
5
6
RO
RE
DE
DI
RX
TX
RT,120Ω
178591 F11
0.1µF
300V
47Ω
1.5KE36CA
47Ω
CARBON
COMPOSITE
5W
LT1785
RAYCHEM
POLYSWITCH
TR600-150
× 2
Figure11. RS485 Network with 120V AC Line Fault Protection
PART NUMBER DESCRIPTION COMMENTS
LTC2854/LTC2855 3.3V 20Mbps RS485/RS422 Transceivers with Integrated
Switchable Termination
Up to ±25kV HBM ESD, 125°C Operation
LTC2856/LTC2857/
LTC2858
5V 20Mbps and Slew Rate Limited 15kV RS485/RS422
Transceivers
±15kV ESD, 125°C Operation
LTC2850/LTC2851/
LTC2852
3.3V 20Mbps RS485/RS422 Transceivers ±15kV ESD, 125°C Operation
LTC2859/LTC2861 20Mbps RS485 Transceivers with Integrated Switchable
Termination
Integrated, Switchable,120Ω Termination Resistor, ±15kV ESD
LTC2862/LTC2863/
LTC2864/LTC2865
±60V Fault Protected 3V to 5.5V RS485/RS422 Transceivers 20Mbps or 250kbps, ±15kV HBM ESD, ±25V Common Mode
Range
LTM2881 Complete 3.3V Isolated RS485/RS422 µModule
®
Transceiver
+ Power
2500VRMS Isolation with Integrated Isolated DC/DC Converter and
Switchable Termination