LT1785/LT1785A/
LT1791/LT1791A
60V Fault Protected
RS485/RS422 Transceivers
Features
n
n
n
n
n
n
n
n
n
n
n
Description
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
n IEC-1000-4-2 Level 4: ±15kV Air Discharge
n 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 (LT1785A/LT1791A)
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
Guaranteed Operation to 125°C
Applications
Industrial Control Data Networks
CAN Bus Applications
n HVAC Controls
n
n
L, LT, LTC, LTM, µModule, Linear Technology and the Linear logo are registered trademarks of
Linear Technology Corporation. All other trademarks are the property of their respective owners.
The LT®1785/LT1791 are half-duplex and full-duplex differential 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 outputscontrol EMI emissions and improve data transmission 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.
Typical Application
Normal Operation Waveforms at 250kBaud
RO1
RE1
DE1
DI1
RX
LT1785
VCC1
RO
RTERM
TX
Y-Z
GND1
RO2
RE2
DE2
DI2
RX
LT1785
TX
DI
VCC2
RTERM
178591 TA02
GND2
178591 TA01
178591fd
For more information www.linear.com/LT1785
1
LT1785/LT1785A/
LT1791/LT1791A
Absolute Maximum Ratings
(Note 1)
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
Pin Configuration
TOP VIEW
TOP VIEW
RO 1
R
RE 2
DI 4
N8 PACKAGE
8-LEAD PDIP
8
7
DE 3
6
D
14 VCC
NC 1
5
R
13 NC
VCC
RO 2
B
RE 3
12 A
A
DE 4
11 B
GND
DI 5
GND 6
S8 PACKAGE
8-LEAD PLASTIC SO
GND 7
TJMAX = 150°C, θJA = 130°C/W (N8)
TJMAX = 150°C, θJA = 150°C/W (S8)
N PACKAGE
14-LEAD PDIP
10 Z
D
9
Y
8
NC
S PACKAGE
14-LEAD PLASTIC SO
TJMAX = 150°C, θJA = 130°C/W (N)
TJMAX = 150°C, θJA = 150°C/W (S)
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
2
178591fd
For more information www.linear.com/LT1785
LT1785/LT1785A/
LT1791/LT1791A
order information
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
LEAD BASED FINISH
TAPE AND REEL
PART MARKING*
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LT1785CN8
LT1785CN8#TR
1785
8-Lead PDIP
0°C to 70°C
LT1785CS8
LT1785CS8#TR
1785
8-Lead Plastic SO
0°C to 70°C
LT1785IN8
LT1785IN8#TR
1785I
8-Lead PDIP
–40°C to 85°C
LT1785IS8
LT1785IS8#TR
1785I
8-Lead Plastic SO
–40°C to 85°C
LT1785ACN8
LT1785ACN8#TR
1785A
8-Lead PDIP
0°C to 70°C
LT1785ACS8
LT1785ACS8#TR
1785A
8-Lead Plastic SO
0°C to 70°C
LT1785AIN8
LT1785AIN8#TR
1785AI
8-Lead PDIP
–40°C to 85°C
LT1785AIS8
LT1785AIS8#TR
1785AI
8-Lead Plastic SO
–40°C to 85°C
LT1785HN8
LT1785HN8#TR
1785H
8-Lead PDIP
–40°C to 125°C
LT1785HS8
LT1785HS8#TR
1785H
8-Lead Plastic SO
–40°C to 125°C
LT1785AHN8
LT1785AHN8#TR
1785AH
8-Lead PDIP
–40°C to 125°C
LT1785AHS8
LT1785AHS8#TR
1785AH
8-Lead Plastic SO
–40°C to 125°C
LT1791CN
LT1791CN#TR
1791
14-Lead PDIP
0°C to 70°C
LT1791CS
LT1791CS#TR
1791
14-Lead Plastic SO
0°C to 70°C
LT1791IN
LT1791IN#TR
1791I
14-Lead PDIP
–40°C to 85°C
LT1791IS
LT1791IS#TR
1791I
14-Lead Plastic SO
–40°C to 85°C
LT1791ACN
LT1791ACN#TR
1791A
14-Lead PDIP
0°C to 70°C
LT1791ACS
LT1791ACS#TR
1791A
14-Lead Plastic SO
0°C to 70°C
LT1791AIN
LT1791AIN#TR
1791AI
14-Lead PDIP
–40°C to 85°C
LT1791AIS
LT1791AIS#TR
1791AI
14-Lead Plastic SO
–40°C to 85°C
LT1791HN
LT1791HN#TR
1791H
14-Lead PDIP
–40°C to 125°C
LT1791HS
LT1791HS#TR
1791H
14-Lead Plastic SO
–40°C to 125°C
LT1791AHN
LT1791AHN#TR
1791AH
14-Lead PDIP
–40°C to 125°C
LT1791AHS
LT1791AHS#TR
1791AH
14-Lead Plastic SO
–40°C to 125°C
Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container.
For more information on lead free part marking, go to: http://www.linear.com/leadfree/
This product is only offered in trays. For more information go to: http://www.linear.com/packaging/
178591fd
For more information www.linear.com/LT1785
3
LT1785/LT1785A/
LT1791/LT1791A
dc
Electrical Characteristics l denotes the specifications which apply over the full operating
The
temperature range, otherwise specifications are at TA = 25°C, VCC = 5V.
SYMBOL PARAMETER
CONDITIONS
MIN
VOD1
Differential Driver Output Voltage (Unloaded)
IO = 0
l
VOD2
Differential Driver Output Voltage (With Load)
R = 50Ω (RS422), Figure 1
R = 27Ω (RS485), Figure 1
R = 18Ω
l
l
l
VOD
Change in Magnitude of Driver Differential Output
Voltage for Complementary Output States
R = 27Ω or R = 50Ω, Figure 1
l
VOC
Driver Common Mode Output Voltage
R = 27Ω or R = 50Ω, Figure 1
l
∆|VOC|
Change in Magnitude of Driver Common Mode Output
Voltage for Complementary Output States
R = 27Ω or R = 50Ω, Figure 1
l
VIH
Input High Voltage
DI, DE, RE
l
VIL
Input Low Voltage
DI, DE, RE
l
IIN1
Input Current
DI, DE, RE
l
IIN2
Input Current (A, B); (LT1791 or LT1785 with DE = 0V)
VIN = 12V
VIN = –7V
–60V ≤ VIN ≤ 60V
l
l
l
–0.15
–6
l
l
–0.2
–0.2
2.0
1.5
1.2
2
TYP
MAX
4.1
5
2.70
2.45
2.2
2.5
0.15
–0.08
LT1785/LT1791: –7V ≤ VCM ≤ 12V
LT1785A/LT1791A: –7V ≤ VCM ≤ 12V
∆VTH
Receiver Input Hysteresis
–7V < VCM < 12V
VOH
Receiver Output High Voltage
IO = –400µA, VID = 200mV
l
VOL
Receiver Output Low Voltage
IO = 1.6mA, VID = –200mV
l
Three-State (High Impedance) Output Current at
Receiver 0V < VOUT < 6V
RE > 2V or Power Off
l
–1
Receiver Input Resistance (LT1791)
–7V ≤ VCM ≤ 12V
– 60V ≤ VCM ≤ 60V
l
85
125
125
LT1785
–7V ≤ VCM ≤ 12V
l
50
90
4
3
V
0.2
V
V
5
µA
0.3
mA
mA
mA
6
0.2
0
V
V
20
mV
4
V
0.3
RS485 Unit Load
ICC
V
0.8
Differential Input Threshold Voltage for Receiver
ISC
0.2
V
VTH
RIN
V
V
V
V
2
3.5
UNITS
0.5
V
1
µA
kΩ
kΩ
kΩ
0.25
Driver Short-Circuit Current
VOUT = HIGH, Force VO = –7V
VOUT = LOW, Force VO = 12V
l
l
35
35
Driver Output Fault Current
VO = 60V
VO = –60V
l
l
–6
Receiver Short-Circuit Current
0V ≤ VO ≤ VCC
l
Driver Three-State Output Current
–7V ≤ VO ≤ 12V
–60V ≤ VO ≤ 60V
l
l
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
–0.2
–6
5.5
5.5
4.5
0.2
250
250
mA
mA
6
mA
mA
±35
mA
0.3
6
mA
mA
9
9
8
0.3
mA
mA
mA
mA
178591fd
For more information www.linear.com/LT1785
LT1785/LT1785A/
LT1791/LT1791A
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
tPHL
Driver Input to Output
Figures 3, 5
l
700
2000
ns
l
700
2000
ns
tSKEW
Driver Output to Output
Figures 3, 5
tr, tf
Driver Rise or Fall Time
Figures 3, 5
l
tZH
Driver Enable to Output High
Figures 4, 6
tZL
Driver Enable to Output Low
tLZ
Driver Disable Time from Low
tHZ
100
200
ns
800
2000
ns
l
500
3000
ns
Figures 4, 6
l
800
3000
ns
Figures 4, 6
l
200
5000
ns
Driver Disable Time from High
Figures 4, 6
l
800
5000
ns
tPLH
Receiver Input to Output
Figures 3, 7
l
400
900
ns
tPHL
Receiver Input to Output
Figures 3, 7
l
400
900
ns
tSKD
Differential Receiver Skew
tZL
Receiver Enable to Output Low
Figures 2, 8
l
300
200
1000
ns
tZH
Receiver Enable to Output High
Figures 2, 8
l
300
1000
ns
tLZ
Receiver Disable from Low
Figures 2, 8
l
400
1000
ns
tHZ
Receiver Disable from High
Figures 2, 8
l
400
1000
ns
fMAX
Maximum Data Rate
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
l
250
ns
kbps
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.
178591fd
For more information www.linear.com/LT1785
5
LT1785/LT1785A/
LT1791/LT1791A
Typical Performance Characteristics
Driver Differential Output Voltage
vs Load Resistance
Driver Differential Output Voltage
vs Temperature
3.0
4
2.5
DIFFERENTIAL VOLTAGE (V)
2
1
100
LOAD RESISTANCE (Ω)
10
tPHL
800
2.0
DELAY (ns)
OUTPUT VOLTAGE (V)
1000
TA = 25°C
3
0
Receiver Propagation Delay
vs Temperature
1.5
600
tPLH
400
1.0
200
0.5
R = 27Ω
0
–40 –20
1k
40
20
0
60
TEMPERATURE (°C)
178591 G01
80
0
–40
100
40
20
0
60
TEMPERATURE (°C)
–20
178591 G02
Driver Propagation Delay
vs Temperature
80
100
178591 G03
LT1791 Driver Output Leakage
DE = 0V
LT1791 Receiver Input Current
vs VIN
1000
PROPAGATION DELAY (ns)
900
800
LH
700
600
200µA/DIV
1mA/DIV
HL
500
400
300
200
–60V
100
0
–40
–20
40
0
60
20
TEMPERATURE (°C)
80
VIN
60V
–60V
VIN
178591 G05
60V
178591 G06
100
178591 G04
LT1785 Input Characteristics Pins
A or B; DE = RE = 0V
Receiver Propagation Delay
vs Differential Input Voltage
Supply Current vs Temperature
7
6
700
DRIVER AND
RECEIVER ON
600
5
4
RECEIVER ONLY
DELAY (ns)
ICC (mA)
1mA/DIV
3
2
–60V
VA, VB
60V
400
HL VCM = –7V
LH VCM = –7V
300
LH VCM = 12V
200
1
178591 G07
0
–40
100
STANDBY
–20
40
0
60
20
TEMPERATURE (°C)
80
100
178591 G08
6
HL VCM = 12V
500
0
0
1
3
4
2
VIN DIFFERENTIAL (V)
5
178591 G09
178591fd
For more information www.linear.com/LT1785
LT1785/LT1785A/
LT1791/LT1791A
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 circuitgoes 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 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 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 protected 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 inputthresholds –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).
178591fd
For more information www.linear.com/LT1785
7
LT1785/LT1785A/
LT1791/LT1791A
TEST CIRCUITS
A
R
VOD
R
S1
TEST POINT
RECEIVER
OUTPUT
VOC
1k
CRL
1k
VCC
S2
B
1785/91 F02
1785/91 F01
Figure 1. Driver DC Test Load
Figure 2. Receiver Timing Test Load
5V
DE
DI
A
CL1
RDIFF
B
CL2
A
S1
RO
B
RE
15pF
OUTPUT
UNDER TEST
VCC
500Ω
CL
S2
1785/91 F04
1785/91 F03
Figure 3. Driver/Receiver Timing Test Circuit
8
Figure 4. Driver Timing Test Load
178591fd
For more information www.linear.com/LT1785
LT1785/LT1785A/
LT1791/LT1791A
FUNCTION TABLES
LT1785 Transmitting
LT1791
INPUTS
OUTPUTS
INPUTS
OUTPUTS
RE
DE
DI
A
B
RO
0
1
0
0
1
0
0
0
X
≤ –200mV
Hi-Z
Hi-Z
0
0
1
1
1
0
1
0
0
X
≥ 200mV*
Hi-Z
Hi-Z
1
1
0
X
Hi-Z
Hi-Z
Hi-Z
0
0
X
Open
Hi-Z
Hi-Z
1**
1
1
0
0
1
Hi-Z
0
1
0
≤ –200mV
0
1
0
1
1
1
1
0
Hi-Z
LT1785 Receiving
INPUTS
OUTPUT
RE
DE
DI
A-B
Y
Z
RO
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
RE
DE
DI
A-B
RO
0
1
1
Open
1
0
1**
0
0
X
≤ –200mV
0
1
0
X
X
Hi-Z
Hi-Z
Hi-Z
0
0
X
≥ 200mV*
1
1
1
0
X
0
1
Hi-Z
0
0
X
Open
1**
1
1
1
X
1
0
Hi-Z
1
0
X
X
Hi-Z
* ≥ 0mV for LT1785A
** 1 for LT1785A; 1 or 0 for LT1785
* ≥ 0mV for LT1791A
** 1 for LT1791A; 1 or 0 for LT1791
178591fd
For more information www.linear.com/LT1785
9
LT1785/LT1785A/
LT1791/LT1791A
SWITCHING TIME WAVEFORMS
5V
f = 125kHz, tr ≤ 10ns, tf ≤ 10ns
1.5V
DI
1.5V
0V
tPLH
B
A
VO
0V
–VO
1/2 VO
tPHL
VO
tSKEW
1/2 VO
tSKEW
90%
10%
90%
10%
VDIFF = V(A) – V(B)
tr
tf
1785/91 F05
Figure 5. Driver Propagation Delays
5V
f = 125kHz, tr ≤ 10ns, tf ≤ 10ns
1.5V
DE
1.5V
0V
A, B
2.3V
OUTPUT NORMALLY LOW
0.5V
2.3V
OUTPUT NORMALLY HIGH
0.5V
VOL
A, B
tLZ
tZL(SHDN), tZL
5V
VOH
0V
tHZ
tZH(SHDN), tZH
178591 F06
Figure 6. Driver Enable and Disable Times
RO
VOH
VOL
VOD2
A–B
–VOD2
1.5V
tPHL
1.5V
OUTPUT
tPLH
f = 125kHz, tr ≤ 10ns, tf ≤ 10ns
0V
0V
INPUT
178591 F07
Figure 7. Receiver Propagation Delays
5V
RE
1.5V
RO
5V
RO
0V
1.5V
f = 125kHz, tr ≤ 10ns, tf ≤ 10ns
0V
tZL(SHDN), tZL
tLZ
1.5V
OUTPUT NORMALLY LOW
0.5V
1.5V
OUTPUT NORMALLY HIGH
0.5V
tZH(SHDN), tZH
tHZ
178591 F08
Figure 8. Receiver Enable and Disable Times
10
178591fd
For more information www.linear.com/LT1785
LT1785/LT1785A/
LT1791/LT1791A
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 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.
The high overvoltage rating of the LT1785/LT1791 facilitates 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 transceivers. 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.
178591fd
For more information www.linear.com/LT1785
11
LT1785/LT1785A/
LT1791/LT1791A
Applications Information
Slew Limiting for EMI Emissions Control
Data Network Cable Selection and Termination
The LT1785/LT1791 feature controlled driver output slew
rates to control high frequency EMI emissions from equipment and data cables. The slew limiting limits data rate
operation to 250kbaud. Slew limiting also mitigates the
adverse affects of imperfect transmission line termination 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.
Long distance data networks operating at high data transmission 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.
A 12
RO
RE
DE
DI
2
RX
B 11
9 Y
120Ω
10 Z
TX
3
4
LT1791
LT1791
11 B
Z 10
5
TX
Y 9
120Ω
12 A
RX
5
4
3
2
DI
DE
RE
RO
178591 F09
Figure 9. Full-Duplex RS422
12
178591fd
For more information www.linear.com/LT1785
LT1785/LT1785A/
LT1791/LT1791A
package description
N8 Package
8-Lead PDIP (Narrow 0.300)
(LTC DWG # 05-08-1510)
.400*
(10.160)
MAX
8
7
6
5
1
2
3
4
.255 ± .015*
(6.477 ± 0.381)
.300 – .325
(7.620 – 8.255)
.008 – .015
(0.203 – 0.381)
(
+.035
.325 –.015
8.255
+0.889
–0.381
)
.045 – .065
(1.143 – 1.651)
.065
(1.651)
TYP
.100
(2.54)
BSC
.130 ± .005
(3.302 ± 0.127)
.120
(3.048) .020
MIN (0.508)
MIN
.018 ± .003
(0.457 ± 0.076)
N8 1002
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)
178591fd
For more information www.linear.com/LT1785
13
LT1785/LT1785A/
LT1791/LT1791A
package description
S8 Package
8-Lead Plastic Small Outline (Narrow 0.150)
(LTC DWG # 05-08-1610)
.189 – .197
(4.801 – 5.004)
NOTE 3
.045 ±.005
.050 BSC
8
.245
MIN
.160 ±.005
.010 – .020
× 45°
(0.254 – 0.508)
NOTE:
1. DIMENSIONS IN
.053 – .069
(1.346 – 1.752)
0°– 8° TYP
.014 – .019
(0.355 – 0.483)
TYP
INCHES
(MILLIMETERS)
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)
14
5
.150 – .157
(3.810 – 3.988)
NOTE 3
1
RECOMMENDED SOLDER PAD LAYOUT
.016 – .050
(0.406 – 1.270)
6
.228 – .244
(5.791 – 6.197)
.030 ±.005
TYP
.008 – .010
(0.203 – 0.254)
7
2
3
4
.004 – .010
(0.101 – 0.254)
.050
(1.270)
BSC
SO8 0303
178591fd
For more information www.linear.com/LT1785
LT1785/LT1785A/
LT1791/LT1791A
package description
N Package
14-Lead PDIP (Narrow 0.300)
(LTC DWG # 05-08-1510)
.770*
(19.558)
MAX
14
13
12
11
10
9
8
1
2
3
4
5
6
7
.255 ± .015*
(6.477 ± 0.381)
.300 – .325
(7.620 – 8.255)
.008 – .015
(0.203 – 0.381)
(
+.035
.325 –.015
+0.889
8.255
–0.381
NOTE:
1. DIMENSIONS ARE
)
.045 – .065
(1.143 – 1.651)
.130 ± .005
(3.302 ± 0.127)
.020
(0.508)
MIN
.065
(1.651)
TYP
.120
(3.048)
MIN
.005
(0.127) .100
MIN (2.54)
BSC
.018 ± .003
(0.457 ± 0.076)
N14 1103
INCHES
MILLIMETERS
*THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS.
MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED .010 INCH (0.254mm)
178591fd
For more information www.linear.com/LT1785
15
LT1785/LT1785A/
LT1791/LT1791A
Package Description
S Package
14-Lead Plastic Small Outline (Narrow 0.150)
(LTC DWG # 05-08-1610)
.337 – .344
(8.560 – 8.738)
NOTE 3
.045 ±.005
.050 BSC
14
N
12
10
9
8
.160 ±.005
.228 – .244
(5.791 – 6.197)
1
2
3
N/2
N/2
RECOMMENDED SOLDER PAD LAYOUT
.010 – .020
× 45°
(0.254 – 0.508)
.008 – .010
(0.203 – 0.254)
1
2
3
4
5
.053 – .069
(1.346 – 1.752)
NOTE:
1. DIMENSIONS IN
.014 – .019
(0.355 – 0.483)
TYP
INCHES
(MILLIMETERS)
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)
6
.150 – .157
(3.810 – 3.988)
NOTE 3
7
.004 – .010
(0.101 – 0.254)
0° – 8° TYP
.016 – .050
(0.406 – 1.270)
16
11
N
.245
MIN
.030 ±.005
TYP
13
.050
(1.270)
BSC
S14 0502
178591fd
For more information www.linear.com/LT1785
LT1785/LT1785A/
LT1791/LT1791A
Revision History
(Revision history begins at Rev D)
REV
DATE
DESCRIPTION
D
03/15
Added notes to Function Tables
PAGE NUMBER
9
178591fd
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection
of its circuits
as described
herein will not infringe on existing patent rights.
For more
information
www.linear.com/LT1785
17
LT1785/LT1785A/
LT1791/LT1791A
Typical Application
RO
RE
DE
DI
1
RX
2
LT1785
3
4
RT
120Ω
B 7
RT
120Ω
A 6
6
A
TX
6
A
7
B
7 B
RX
6 A
LT1785
1
2
7
B
TX
3
4
RO
RE
DE
DI
178591 F10
LT1785
4
3
DI
DE
LT1785
2
RE
4
1
DI
RO
3
DE
2
RE
1
RO
Figure 10. Half-Duplex RS485 Network Operation
8
RO
RE
DE
DI
1
RX
2
3
4
LT1785
RAYCHEM
POLYSWITCH
TR600-150
×2
47Ω
VCC
B 7
RT,120Ω
A 6
47Ω
CARBON
COMPOSITE
5W
5
TX
0.1µF
300V
178591 F11
1.5KE36CA
Figure 11. RS485 Network with 120V AC Line Fault Protection
Related Parts
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
18 Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
For more information www.linear.com/LT1785
(408) 432-1900 ● FAX: (408) 434-0507
●
www.linear.com/LT1785
178591fd
LT 0315 REV D • PRINTED IN USA
LINEAR TECHNOLOGY CORPORATION 1998
工商网监
湘ICP备2023018690号
