LT1785/LT1785A/
LT1791/LT1791A
1
178591fc
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
specifi ed 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
L, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
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
IEC-1000-4-2 Level 4: ±15kV Air Discharge
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
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 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
LT1785/LT1785A/
LT1791/LT1791A
2
178591fc
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
178591fc
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 specifi ed with wider operating temperature ranges. *The temperature grade is identifi ed 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/
ORDER INFORMATION
LT1785/LT1785A/
LT1791/LT1791A
4
178591fc
DC ELECTRICAL CHARACTERISTICS
The l denotes the specifi cations which apply over the full operating
temperature range, otherwise specifi cations 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 = 50 (RS422), Figure 1
R = 27 (RS485), Figure 1
R = 18
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 = 27 or R = 50, Figure 1 l0.2 V
VOC Driver Common Mode Output Voltage R = 27 or R = 50, Figure 1 l2 2.5 3 V
∆|VOC| Change in Magnitude of Driver Common Mode Output
Voltage for Complementary Output States
R = 27 or R = 50, Figure 1 l0.2 V
VIH Input High Voltage DI, DE, RE l2V
VIL Input Low Voltage DI, DE, RE l0.8 V
IIN1 Input Current DI, DE, RE l5µA
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
LT1785 –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
6mA
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
178591fc
SWITCHING CHARACTERISTICS
The l denotes the specifi cations which apply over the full operating
temperature range, otherwise specifi cations 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
178591fc
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 ()
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
ICC (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
134
HL VCM = 12V
HL VCM = –7V
LH VCM = –7V
LH VCM = 12V
LT1785/LT1785A/
LT1791/LT1791A
7
178591fc
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
178591fc
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
S1
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 Transmitting
INPUTS OUTPUTS
RE DE DI A B RO
0100 1 0
0111 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
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
FUNCTION TABLES
LT1785/LT1785A/
LT1791/LT1791A
9
178591fc
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),t
ZH
tZL(SHDN),t
ZL
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
10
178591fc
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 specifi ed 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
fi eld 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 diffi cult 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 fi eld 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 fi eld
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
11
178591fc
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 72.
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
12
178591fc
PACKAGE DESCRIPTION
N8 Package
8-Lead PDIP (Narrow 0.300)
(LTC DWG # 05-08-1510)
N8 1002
.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
12 34
87 65
.255 ± .015*
(6.477 ± 0.381)
.400*
(10.160)
MAX
.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
LT1785/LT1785A/
LT1791/LT1791A
13
178591fc
PACKAGE DESCRIPTION
S8 Package
8-Lead Plastic Small Outline (Narrow 0.150)
(LTC DWG # 05-08-1610)
.016 – .050
(0.406 – 1.270)
.010 – .020
(0.254 – 0.508)× 45°
0°– 8° TYP
.008 – .010
(0.203 – 0.254)
SO8 0303
.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)
LT1785/LT1785A/
LT1791/LT1791A
14
178591fc
PACKAGE DESCRIPTION
N Package
14-Lead PDIP (Narrow 0.300)
(LTC DWG # 05-08-1510)
N14 1103
.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
LT1785/LT1785A/
LT1791/LT1791A
15
178591fc
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 representa-
tion that the interconnection of its circuits as described herein will not infringe on existing patent rights.
PACKAGE DESCRIPTION
S Package
14-Lead Plastic Small Outline (Narrow 0.150)
(LTC DWG # 05-08-1610)
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 0502
.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)
LT1785/LT1785A/
LT1791/LT1791A
16
178591fc
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 1998
LT 0409 REV C • PRINTED IN USA
RELATED PARTS
TYPICAL APPLICATION
PART NUMBER DESCRIPTION COMMENTS
LTC485 Low Power RS485 Interface Transceiver ICC = 300µA (Typ)
LTC491 Differential Driver and Receiver Pair ICC = 300µA
LTC1483 Ultralow Power RS485 Low EMI Transceiver Controlled Driver Slew Rate
LTC1485 Differential Bus Transceiver 10Mbaud Operation
LTC1487 Ultralow Power RS485 with Low EMI, Shutdown and High Input Impedance Up to 256 Transceivers on the Bus
LTC1520 50Mbps Precision Quad Line Receiver Channel-to-Channel Skew 400ps (Typ)
LTC1535 Isolated RS485 Full-Duplex Transceiver 2500VRMS Isolation in Surface Mount Package
LTC1685 52Mbps RS485 Half-Duplex Transceiver Propagation Delay Skew 500ps (Typ)
LTC1687 52Mbps RS485 Full-Duplex Transceiver Propagation Delay Skew 500ps (Typ)
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
