FEATURES
■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
APPLICATIOUS
■Industrial Control Data Networks
■CAN Bus Applications
■HVAC Controls
LT1785/LT1785A
LT1791/LT1791A
60V Fault Protected
RS485/RS422 Transceivers
DESCRIPTIOU
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 outputs control 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.
, LTC and LT are registered trademarks of Linear Technology Corporation.
TYPICAL APPLICATIOU
Normal Operation Waveforms at 250kBaud
RO1 |
RX |
VCC1 |
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RE1 |
LT1785 |
RTERM |
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DE1 |
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RO |
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DI1 |
TX |
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GND1 |
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Y-Z |
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RO2 |
RX |
VCC2 |
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RE2 |
LT1785 |
DI |
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DE2 |
RTERM |
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DI2 |
TX |
1785/91 TA02 |
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GND2 |
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1785/91 TA01 |
1
LT1785/LT1785A
LT1791/LT1791A
ABSOLUTE
WAXIWUW
RATI GS |
(Note 1) |
U |
Supply Voltage (VCC) .............................................. |
18V |
Operating Temperature Range |
|
Receiver Enable Input Voltage .................... |
– 0.3V to 6V |
LT1785C/LT1791C/ |
0°C to 70°C |
Driver Enable Input Voltage ........................ |
– 0.3V to 6V |
LT1785AC/LT1791AC ............................. |
|
Driver Input Voltage .................................. |
– 0.3V to 18V |
LT1785I/LT1791I ............................... |
– 40°C to 85°C |
Receiver Input Voltage ............................... |
– 60V to 60V |
Storage Temperature Range ................ |
– 65°C to 150°C |
Driver Output Voltage ............................... |
– 60V to 60V |
Lead Temperature (Soldering, 10 sec)................. |
300°C |
Receiver Output Voltage ................ |
– 0.3V to (VCC + 6V) |
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PACKAGE/ORDER IUFORWATIOU
TOP VIEW
RO |
1 |
R |
8 |
VCC |
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RE |
2 |
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7 |
B |
DE |
3 |
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6 |
A |
DI |
4 |
D |
5 |
GND |
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N8 PACKAGE |
S8 PACKAGE |
8-LEAD PDIP |
8-LEAD PLASTIC SO |
TJMAX = 150°C, θJA = 130°C/ W (N8) TJMAX = 150°C, θJA = 150°C/ W (S8)
ORDER PART
NUMBER
LT1785CN8
LT1785CS8
LT1785IN8
LT1785IS8
LT1785ACN8
LT1785ACS8
S8 PART MARKING
1785
1785I
1785A
TOP VIEW
NC |
1 |
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14 |
VCC |
RO |
2 |
R |
13 |
NC |
RE |
3 |
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12 |
A |
DE |
4 |
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11 |
B |
DI |
5 |
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10 |
Z |
GND |
6 |
D |
9 |
Y |
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GND |
7 |
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8 |
NC |
N PACKAGE |
S PACKAGE |
14-LEAD PDIP |
14-LEAD PLASTIC SO |
TJMAX = 150°C, θJA = 130°C/ W (N) TJMAX = 150°C, θJA = 150°C/ W (S)
ORDER PART
NUMBER
LT1791CN
LT1791CS
LT1791IN
LT1791IS
LT1791ACN
LT1791ACS
Consult factory for Military grade parts.
2
LT1785/LT1785A
LT1791/LT1791A
DC ELECTRICAL CHARACTERISTICS
The ● denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25°C, VCC = 5V.
SYMBOL |
PARAMETER |
CONDITIONS |
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MIN |
TYP |
MAX |
UNITS |
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VOD1 |
Differential Driver Output Voltage (Unloaded) |
IO = 0 |
● |
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4.1 |
5 |
V |
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VOD2 |
Differential Driver Output Voltage (With Load) |
R = 50Ω (RS422), Figure 1 |
● |
2.0 |
2.70 |
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V |
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R = 27Ω (RS485), Figure 1 |
● |
1.5 |
2.45 |
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V |
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R = 18Ω |
● |
1.2 |
2.2 |
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V |
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VOD |
Change in Magnitude of Driver Differential Output |
R = 27Ω or R = 50Ω, Figure 1 |
● |
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0.2 |
V |
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Voltage for Complementary Output States |
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VOC |
Driver Common Mode Output Voltage |
R = 27Ω or R = 50Ω, Figure 1 |
● |
2 |
2.5 |
3 |
V |
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VOC |
Change in Magnitude of Driver Common Mode |
R = 27Ω or R = 50Ω, Figure 1 |
● |
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0.2 |
V |
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Output Voltage for Complementary Output States |
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VIH |
Input High Voltage |
DI, DE, |
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2 |
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V |
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RE |
● |
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VIL |
Input Low Voltage |
DI, DE, |
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0.8 |
V |
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RE |
● |
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IIN1 |
Input Current |
DI, DE, |
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5 |
µA |
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RE |
● |
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IIN2 |
Input Current (A, B); (LT1791 or LT1785 with |
VIN = 12V |
● |
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0.15 |
0.3 |
mA |
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DE = 0V) |
VIN = – 7V |
● |
– 0.15 |
– 0.08 |
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mA |
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– 60V ≤ VIN ≤ 60V |
● |
– 6 |
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6 |
mA |
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VTH |
Differential Input Threshold Voltage for Receiver |
LT1785/LT1791: – 7V ≤ VCM ≤ 12V |
● |
– 0.2 |
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0.2 |
V |
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LT1785A/LT1791A: – 7V ≤ VCM ≤ 12V |
● |
– 0.2 |
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0 |
V |
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VTH |
Receiver Input Hysteresis |
– 7V < VCM < 12V |
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20 |
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mV |
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VOH |
Receiver Output High Voltage |
IO = – 400µA, VID = 200mV |
● |
3.5 |
4 |
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V |
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VOL |
Receiver Output Low Voltage |
IO = 1.6mA, VID = – 200mV |
● |
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0.3 |
0.5 |
V |
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Three-State (High Impedance) Output Current |
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– 1 |
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1 |
µA |
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RE |
> 2V or Power Off |
● |
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at Receiver 0V < VOUT < 6V |
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RIN |
Receiver Input Resistance (LT1791) |
– 7V ≤ VCM ≤ 12V |
● |
85 |
125 |
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kΩ |
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– 60V ≤ VCM ≤ 60V |
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125 |
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kΩ |
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LT1785 |
– 7V ≤ VCM ≤ 12V |
● |
50 |
90 |
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kΩ |
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RS485 Unit Load |
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0.25 |
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ISC |
Driver Short-Circuit Current |
VOUT = HIGH, Force VO = – 7V |
● |
35 |
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250 |
mA |
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VOUT = LOW, Force VO = 12V |
● |
35 |
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250 |
mA |
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Driver Output Fault Current |
VO = 60V |
● |
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6 |
mA |
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VO = – 60V |
● |
– 6 |
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mA |
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Receiver Short-Circuit Current |
0V ≤ VO ≤ VCC |
● |
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±30 |
mA |
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Driver Three-State Output Current |
–7V ≤ VO ≤ 12V |
● |
– 0.2 |
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0.3 |
mA |
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– 60V ≤ VO ≤ 60V |
● |
– 6 |
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6 |
mA |
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ICC |
Supply Current |
No Load, |
RE |
= 0V, DE = 5V |
● |
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5.5 |
9 |
mA |
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No Load, RE = 5V, DE = 5V |
● |
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5.5 |
9 |
mA |
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No Load, RE = 0V, DE = 0V |
● |
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4.5 |
8 |
mA |
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No Load, RE = 5V, DE = 0V |
● |
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0.2 |
0.3 |
mA |
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3
LT1785/LT1785A
LT1791/LT1791A
SWITCHIUG CHARACTERISTICS
The ● denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25°C, VCC = 5V.
SYMBOL |
PARAMETER |
CONDITIONS |
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MIN |
TYP |
MAX |
UNITS |
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tPLH |
Driver Input to Output |
Figures 3, 5 |
● |
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700 |
2000 |
ns |
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tPHL |
Driver Input to Output |
Figures 3, 5 |
● |
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700 |
2000 |
ns |
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tSKEW |
Driver Output to Output |
Figures 3, 5 |
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100 |
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ns |
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tr, tf |
Driver Rise or Fall Time |
Figures 3, 5 |
● |
200 |
800 |
2000 |
ns |
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tZH |
Driver Enable to Output High |
Figures 4, 6 |
● |
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500 |
3000 |
ns |
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tZL |
Driver Enable to Output Low |
Figures 4, 6 |
● |
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800 |
3000 |
ns |
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tLZ |
Driver Disable Time from Low |
Figures 4, 6 |
● |
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200 |
5000 |
ns |
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tHZ |
Driver Disable Time from High |
Figures 4, 6 |
● |
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800 |
5000 |
ns |
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tPLH |
Receiver Input to Output |
Figures 3, 7 |
● |
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400 |
900 |
ns |
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tPHL |
Receiver Input to Output |
Figures 3, 7 |
● |
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400 |
900 |
ns |
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tSKD |
Differential Receiver Skew |
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200 |
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ns |
tZL |
Receiver Enable to Output Low |
Figures 2, 8 |
● |
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300 |
1000 |
ns |
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tZH |
Receiver Enable to Output High |
Figures 2, 8 |
● |
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300 |
1000 |
ns |
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tLZ |
Receiver Disable from Low |
Figures 2, 8 |
● |
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400 |
1000 |
ns |
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tHZ |
Receiver Disable from High |
Figures 2, 8 |
● |
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400 |
1000 |
ns |
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fMAX |
Maximum Data Rate |
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● |
250 |
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kbps |
tSHDN |
Time to Shut Down |
Figures 2, 6, 8 |
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3 |
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s |
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s |
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tZH(SHDN) |
Driver Enable from Shutdown to Output High |
Figures 2, 6; |
RE |
= 5V |
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12 |
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s |
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tZL(SHDN) |
Driver Enable from Shutdown to Output Low |
Figures 2, 6; |
RE |
= 5V |
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12 |
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tZH(SHDN) |
Receiver Enable from Shutdown to Output High |
Figures 2, 8; DE = 0V |
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4 |
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s |
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tZL(SHDN) |
Receiver Enable from Shutdown to Output Low |
Figures 2, 8; DE = 0V |
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4 |
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s |
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Note 1: Absolute Maximum Ratings are those values beyond which the life of a device may be impaired.
TYPICAL PERFORWAUCE CHARACTERISTICS
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Driver Differential Output Voltage |
Driver Differential Output Voltage |
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vs Load Resistance |
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vs Temperature |
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4 |
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3.0 |
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TA = 25°C |
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2.5 |
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(V)VOLTAGEOUTPUT |
3 |
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VOLTAGEDIFFERENTIAL(V) |
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2.0 |
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1.5 |
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1.0 |
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0.5 |
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0 |
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R = 27Ω |
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100 |
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1k |
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10 |
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–40 |
–20 |
0 |
20 |
40 |
60 |
80 |
100 |
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LOAD RESISTANCE (Ω ) |
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TEMPERATURE (°C) |
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1785/91 G01 |
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1785/91 G03 |
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Receiver Propagation Delay |
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vs Temperature |
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1000 |
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800 |
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tPHL |
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(ns) |
600 |
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DELAY |
400 |
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tPLH |
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200 |
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0 |
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0 |
20 |
40 |
60 |
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100 |
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–40 |
–20 |
80 |
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TEMPERATURE (°C) |
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1785/91 G03 |
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4
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