Datasheet

ManualsBrandsLINEAR TECHNOLOGY ManualsInterface ICsInterface IC - transceiver RS422, RS485 1/1 PDIP 8

LT1785/LT1785A

LT1791/LT1791A

60V Fault Protected

RS485/RS422 Transceivers

Normal Operation Waveforms at 250kBaud

GND1

GND2

TERM

1785/91 TA01

RO1

RE1

DE1

DI1

LT1785

RO2

RE2

DE2

DI2

LT1785

CC1

CC2

TERM

The LT

1785/LT1791 are half-duplex and full-duplex dif-

ferential bus transceivers for RS485 and RS422 applica-

tions which feature on-chip protection from overvoltage

faults on the data transmission lines. Receiver input and

driver output pins can withstand voltage faults up to ±60V

with respect to ground with no damage to the device.

Faults may occur while the transceiver is active, shut down

or powered off.

Data rates to 250kbaud on networks of up to 128 nodes are

supported. Controlled slew rates on the driver outputs

control EMI emissions and improve data transmission

integrity on improperly terminated lines. Drivers are speci-

fied to operate with inexpensive cables as low as 72Ω

characteristic impedance.

The LT1785A/LT1791A devices have “fail-safe” receiver

inputs to guarantee a receiver output high for shorted,

open or inactive data lines. On-chip ESD protection elimi-

nates need for external protection devices.

The LT1785/LT1785A are available in 8-lead DIP and SO

packages and the LT1791/LT1791A in 14-lead DIP and SO

packages.

■

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

■

Industrial Control Data Networks

■

CAN Bus Applications

■

HVAC Controls

1785/91 TA02

, LTC and LT are registered trademarks of Linear Technology Corporation.

Y-Z

DESCRIPTIO

FEATURES

APPLICATIO S

TYPICAL APPLICATIO

Summary of content (12 pages)

PAGE 1
LT1785/LT1785A LT1791/LT1791A 60V Fault Protected RS485/RS422 Transceivers U FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ DESCRIPTIO 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
PAGE 2
LT1785/LT1785A LT1791/LT1791A W W W AXI U U ABSOLUTE RATI GS (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 ....
PAGE 3
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.
PAGE 4
LT1785/LT1785A LT1791/LT1791A U SWITCHI G CHARACTERISTICS The ● denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25°C, VCC = 5V.
PAGE 5
LT1785/LT1785A LT1791/LT1791A U W TYPICAL PERFORMANCE CHARACTERISTICS Driver Propagation Delay vs Temperature LT1791 Receiver Input Current vs VIN LT1791 Driver Output Leakage DE = 0V 1000 PROPAGATION DELAY (ns) 900 800 LH 700 1mA/DIV 600 200µA/DIV HL 500 400 300 200 – 60V 100 0 –40 60V VOUT 40 0 60 20 TEMPERATURE (°C) –20 80 – 60V 60V VIN 1785/91 G05 1785/91 G06 100 1785/91 G04 LT1785 Input Characteristics Pins A or B; DE = RE = 0V Receiver Propagation Delay vs Differential Input
PAGE 6
LT1785/LT1785A LT1791/LT1791A U U U PIN FUNCTIONS 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.
PAGE 7
LT1785/LT1785A LT1791/LT1791A TEST CIRCUITS 5V DE A S1 A CL1 DI RO RDIFF B CL2 15pF RE VCC 500Ω OUTPUT UNDER TEST B S2 CL 1785/91 F04 1785/91 F03 Figure 3. Driver/Receiver Timing Test Circuit Figure 4.
PAGE 8
LT1785/LT1785A LT1791/LT1791A U W W SWITCHI G TI E WAVEFOR S 5V f = 125kHz, t r ≤ 10ns, t f ≤ 10ns 1.5V DE 1.5V 0V t LZ t ZL(SHDN), t ZL 5V A, B 2.3V OUTPUT NORMALLY LOW 0.5V 2.3V OUTPUT NORMALLY HIGH 0.5V VOL VOH A, B 0V t HZ t ZH(SHDN), t ZH 1785/91 F06 Figure 6. Driver Enable and Disable Times VOH 1.5V RO VOL f = 125kHz, t r ≤ 10ns, t f ≤ 10ns t PHL VOD2 A–B –VOD2 1.5V OUTPUT 0V t PLH 0V INPUT 1785/91 F07 Figure 7. Receiver Propagation Delays 5V 1.
PAGE 9
LT1785/LT1785A LT1791/LT1791A U U W U APPLICATIO S I FOR ATIO 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 the 120VAC line voltage. line. The DE logic input performs a similar function on the driver outputs.
PAGE 10
LT1785/LT1785A LT1791/LT1791A U U W U APPLICATIO S I FOR ATIO A 12 RO RE DE 2 RX 12 A 120Ω B 11 11 B 3 5 DI 4 4 LT1791 LT1791 TX DE 3 RE 10 Z Z 10 DI 5 TX 120Ω Y 9 9 Y 2 RX RO 1785/91 F09 Figure 9. Full-Duplex RS422 U PACKAGE DESCRIPTION Dimensions in inches (millimeters) unless otherwise noted. N8 Package 8-Lead PDIP (Narrow 0.300) (LTC DWG # 05-08-1510) 0.300 – 0.325 (7.620 – 8.255) 0.009 – 0.015 (0.229 – 0.381) ( 0.045 – 0.065 (1.143 – 1.651) 0.130 ± 0.005 (3.302 ± 0.
PAGE 11
LT1785/LT1785A LT1791/LT1791A U PACKAGE DESCRIPTION Dimensions in inches (millimeters) unless otherwise noted. N Package 14-Lead PDIP (Narrow 0.300) (LTC DWG # 05-08-1510) 0.770* (19.558) MAX 14 13 12 11 10 9 8 1 2 3 4 5 6 7 0.255 ± 0.015* (6.477 ± 0.381) 0.130 ± 0.005 (3.302 ± 0.127) 0.300 – 0.325 (7.620 – 8.255) 0.045 – 0.065 (1.143 – 1.651) 0.020 (0.508) MIN 0.065 (1.651) TYP 0.009 – 0.015 (0.229 – 0.381) +0.035 0.325 –0.015 0.005 (0.125) MIN 0.100 (2.
PAGE 12
LT1785/LT1785A LT1791/LT1791A U TYPICAL APPLICATIONS RO RE DE DI 1 RX RT 120Ω B 7 RT 120Ω 7 B RX 2 2 3 4 1 6 A A 6 LT1785 6 A TX 6 A 7 B 7 B LT1785 TX 3 4 RO RE DE DI 1785/91 F10 LT1785 4 3 DI DE LT1785 2 RE 1 RO 4 DI 3 DE 1 2 RE RO Figure 10. Half-Duplex RS485 Network Operation 8 RO RE DE DI 1 RX RAYCHEM POLYSWITCH TR600-150 ×2 47Ω VCC B 7 2 3 4 RT,120Ω LT1785 A 6 1785/91 F11 0.1µF 47Ω 300V CARBON COMPOSITE 5W 5 TX 1.5KE36CA Figure 11.