Linear Technology LT1791A, LT1791, LT1785A, LT1785 Datasheet

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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

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APPLICATIO S

■

Industrial Control Data Networks

■

CAN Bus Applications

■

HVAC Controls

LT1785/LT1785A

LT1791/LT1791A

60V Fault Protected

RS485/RS422 Transceivers

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 APPLICATIO

RO1

RE1
DE1

DI1

RO2

RE2
DE2

DI2

RX

LT1785

TX

RX

LT1785

TX

V

CC1

R

GND1

V

CC2

R

GND2

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TERM

TERM

Normal Operation Waveforms at 250kBaud

RO

Y-Z

DI

1785/91 TA02

1785/91 TA01

1

LT1785/LT1785A

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

V

CC

NC
A
B
Z
Y
NC

R

D

LT1791/LT1791A

A

W

O

LUTEXI TIS

S

A

WUW

U

ARB

G

(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)

WU

/

1
2
3
4

O

RDER I FOR ATIO

ORDER PART

TOP VIEW

R

D

S8 PACKAGE

8-LEAD PLASTIC SO

V

8

CC

B

7

A

6

GND

5

LT1785CN8
LT1785CS8
LT1785IN8
LT1785IS8
LT1785ACN8
LT1785ACS8

NUMBER

PACKAGE

RO
RE
DE

DI

N8 PACKAGE
8-LEAD PDIP

Operating Temperature Range

LT1785C/LT1791C/

LT1785AC/LT1791AC ............................. 0°C to 70°C

LT1785I/LT1791I............................... –40°C to 85°C

Storage Temperature Range ................ –65°C to 150°C

Lead Temperature (Soldering, 10 sec)................. 300°C

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ORDER PART

NUMBER

LT1791CN
LT1791CS
LT1791IN
LT1791IS
LT1791ACN
LT1791ACS

T

= 150°C, θ

JMAX

= 150°C, θ

T

JMAX

Consult factory for Military grade parts.

= 130°C/ W (N8)

JA

= 150°C/ W (S8)

JA

2

S8 PART MARKING

1785
1785I
1785A

T
T

JMAX
JMAX

= 150°C, θ
= 150°C, θ

= 130°C/ W (N)

JA

= 150°C/ W (S)

JA

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 MIN TYP MAX UNITS

V

OD1

V

OD2

∆V

V

OC

∆V

V

IH

V

IL

I

IN1

I

IN2

V

TH

∆V
V

OH

V

OL

R

IN

I

SC

I

CC

OD

TH

Differential Driver Output Voltage (Unloaded) IO = 0 ● 4.1 5 V
Differential Driver Output Voltage (With Load) R = 50Ω (RS422), Figure 1 ● 2.0 2.70 V

R = 27Ω (RS485), Figure 1
R = 18Ω

● 1.5 2.45 V

● 1.2 2.2 V

Change in Magnitude of Driver Differential Output R = 27Ω or R = 50Ω, Figure 1 ● 0.2 V
Voltage for Complementary Output States

Driver Common Mode Output Voltage R = 27Ω or R = 50Ω, Figure 1 ● 2 2.5 3 V

 Change in Magnitude of Driver Common Mode R = 27Ω or R = 50Ω, Figure 1 ● 0.2 V

OC

Output Voltage for Complementary Output States
Input High Voltage DI, DE, RE ● 2V
Input Low Voltage DI, DE, RE ● 0.8 V
Input Current DI, DE, RE ● 5 µA
Input Current (A, B); (LT1791 or LT1785 with VIN = 12V ● 0.15 0.3 mA

DE = 0V) V

= –7V ● –0.15 –0.08 mA

IN

–60V ≤ V

≤ 60V ● –6 6 mA

IN

Differential Input Threshold Voltage for Receiver LT1785/LT1791: –7V ≤ VCM ≤ 12V ● –0.2 0.2 V

LT1785A/LT1791A: –7V ≤ V

≤ 12V ● –0.2 0 V

CM

Receiver Input Hysteresis –7V < VCM < 12V 20 mV
Receiver Output High Voltage IO = –400µA, VID = 200mV ● 3.5 4 V
Receiver Output Low Voltage IO = 1.6mA, VID = –200mV ● 0.3 0.5 V
Three-State (High Impedance) Output Current RE > 2V or Power Off ● –1 1 µA

at Receiver 0V < V

OUT

< 6V

Receiver Input Resistance (LT1791) –7V ≤ VCM ≤ 12V ● 85 125 kΩ

– 60V ≤ V

≤ 60V 125 kΩ

CM

LT1785 –7V ≤ VCM ≤ 12V ● 50 90 kΩ
RS485 Unit Load 0.25
Driver Short-Circuit Current V

= HIGH, Force VO = –7V ● 35 250 mA

OUT

= LOW, Force VO = 12V ● 35 250 mA

V

OUT

Driver Output Fault Current VO = 60V ● 6mA

= –60V ● –6 mA

V

O

Receiver Short-Circuit Current 0V ≤ VO ≤ V

CC

● ±30 mA

Driver Three-State Output Current –7V ≤ VO ≤ 12V ● –0.2 0.3 mA

–60V ≤ V

≤ 60V ● –6 6 mA

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Supply Current No Load, RE = 0V, DE = 5V ● 5.5 9 mA

No Load, RE = 5V, DE = 5V
No Load, RE = 0V, DE = 0V
No Load, RE = 5V, DE = 0V

● 5.5 9 mA

● 4.5 8 mA

● 0.2 0.3 mA

3

LT1785/LT1785A

TEMPERATURE (°C)

–40

DELAY (ns)

1000

800

600

400

200

0

0

40

60

1785/91 G03

–20

20

80

100

t

PLH

t

PHL

LT1791/LT1791A

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SWITCHI G CHARACTERISTICS

The ● denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25°C, VCC = 5V.

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

t

PLH

t

PHL

t

SKEW

tr, t

f

t

ZH

t

ZL

t

LZ

t

HZ

t

PLH

t

PHL

t

SKD

t

ZL

t

ZH

t

LZ

t

HZ

f

MAX

t

SHDN

t

ZH(SHDN)

t

ZL(SHDN)

t

ZH(SHDN)

t

ZL(SHDN)

Note 1: Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.

Driver Input to Output Figures 3, 5 ● 700 2000 ns
Driver Input to Output Figures 3, 5 ● 700 2000 ns
Driver Output to Output Figures 3, 5 100 ns
Driver Rise or Fall Time Figures 3, 5 ● 200 800 2000 ns
Driver Enable to Output High Figures 4, 6 ● 500 3000 ns
Driver Enable to Output Low Figures 4, 6 ● 800 3000 ns
Driver Disable Time from Low Figures 4, 6 ● 200 5000 ns
Driver Disable Time from High Figures 4, 6 ● 800 5000 ns
Receiver Input to Output Figures 3, 7 ● 400 900 ns
Receiver Input to Output Figures 3, 7 ● 400 900 ns
Differential Receiver Skew 200 ns
Receiver Enable to Output Low Figures 2, 8 ● 300 1000 ns
Receiver Enable to Output High Figures 2, 8 ● 300 1000 ns
Receiver Disable from Low Figures 2, 8 ● 400 1000 ns
Receiver Disable from High Figures 2, 8 ● 400 1000 ns
Maximum Data Rate ● 250 kbps
Time to Shut Down Figures 2, 6, 8 3 µs
Driver Enable from Shutdown to Output High Figures 2, 6; RE = 5V 12 µs
Driver Enable from Shutdown to Output Low Figures 2, 6; RE = 5V 12 µs
Receiver Enable from Shutdown to Output High Figures 2, 8; DE = 0V 4 µs
Receiver Enable from Shutdown to Output Low Figures 2, 8; DE = 0V 4 µs

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TYPICAL PERFORMANCE CHARACTERISTICS

Driver Differential Output Voltage
vs Load Resistance

4

TA = 25°C

3

2

OUTPUT VOLTAGE (V)

4

1

0

10

LOAD RESISTANCE (Ω)

100 1k

1785/91 G01

Driver Differential Output Voltage
vs Temperature

3.0

2.5

2.0

1.5

1.0

DIFFERENTIAL VOLTAGE (V)

0.5

R = 27Ω

0

–40

–20

0

TEMPERATURE (°C)

40

20

60

80

1785/91 G03

Receiver Propagation Delay
vs Temperature

100