ANALOG DEVICES LT 1785 CS8 Datasheet

LT1785/LT1785A/

LT1791/LT1791A

60V Fault Protected

RS485/RS422 Transceivers

FEATURES

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

APPLICATIONS

n

Industrial Control Data Networks

n

CAN Bus Applications

n

HVAC Controls

L, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.

DESCRIPTION

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
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.

TYPICAL APPLICATION

V

RO1

RE1

DE1

DI1

RO2

RE2

DE2

DI2

RX

LT1785

TX

RX

LT1785

TX

CC1

GND1

V

CC2

GND2

R

R

TERM

TERM

Normal Operation Waveforms at 250kBaud

RO

Y-Z

DI

178591 TA02

178591 TA01

178591fc

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 (V

+ 6V)

CC

PIN CONFIGURATION

TOP VIEW

RO

1

2

3

4

R

D

8-LEAD PLASTIC SO

RE

DE

DI

N8 PACKAGE
8-LEAD PDIP

T

= 150°C, θJA = 130°C/W (N8)

JMAX

= 150°C, θJA = 150°C/W (S8)

T

JMAX

V

8

B

7

A

6

GND

5

S8 PACKAGE

CC

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

TOP VIEW

1

NC

2

RO

3

RE

4

DE

5

DI

6

GND

7

GND

N PACKAGE

14-LEAD PDIP

T

= 150°C, θJA = 130°C/W (N)

JMAX

= 150°C, θJA = 150°C/W (S)

T

JMAX

14

V

CC

R

13

NC

12

A

11

B

10

9

8

S PACKAGE

Z

Y

NC

D

14-LEAD PLASTIC SO

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

178591fc

2

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 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:

This product is only offered in trays. For more information go to:

http://www.linear.com/leadfree/

http://www.linear.com/packaging/

178591fc

3

LT1785/LT1785A/
LT1791/LT1791A

DC ELECTRICAL CHARACTERISTICS

The l denotes the specifi cations which apply over the full operating
temperature range, otherwise specifi cations are at T

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

V

V

V

V

∆|V

V

V

I

IN1

I

IN2

V

∆V

V

V

R

I

SC

I

CC

OD1

OD2

OD

OC

IH

IL

TH

TH

OH

OL

IN

Differential Driver Output Voltage (Unloaded) IO = 0

Differential Driver Output Voltage (With Load) R = 50 (RS422), Figure 1

Change in Magnitude of Driver Differential Output
Voltage for Complementary Output States

Driver Common Mode Output Voltage R = 27 or R = 50, Figure 1

| Change in Magnitude of Driver Common Mode Output

OC

Voltage for Complementary Output States
Input High Voltage DI, DE, RE
Input Low Voltage DI, DE, RE
Input Current DI, DE, RE

Input Current (A, B); (LT1791 or LT1785 with DE = 0V) VIN = 12V

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

Receiver Input Hysteresis –7V < VCM < 12V 20 mV

Receiver Output High Voltage IO = –400µA, VID = 200mV

Receiver Output Low Voltage IO = 1.6mA, VID = –200mV

Three-State (High Impedance) Output Current at
Receiver 0V < V

OUT

< 6V

Receiver Input Resistance (LT1791) –7V ≤ VCM ≤ 12V

LT1785 –7V ≤ VCM ≤ 12V

RS485 Unit Load 0.25

Driver Short-Circuit Current V

Driver Output Fault Current VO = 60V

Receiver Short-Circuit Current 0V ≤ V

Driver Three-State Output Current –7V ≤ V

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

= 25°C, VCC = 5V.

A

R = 27 (RS485), Figure 1
R = 18

R = 27 or R = 50, Figure 1

R = 27 or R = 50, Figure 1

V

= –7V

IN

–60V ≤ V

LT1785A/LT1791A: –7V ≤ V

RE > 2V or Power Off

– 60V ≤ V

= HIGH, Force VO = –7V

OUT

V

= LOW, Force VO = 12V

OUT

V

= –60V

O

O

–60V ≤ V

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

≤ 60V

IN

CM

≤ V

≤ 12V

O

≤ 60V

O

≤ 60V

CC

CM

≤ 12V

l

l

2.0

l

1.5

l

1.2

l

l

2 2.5 3 V

l

l

2V

l

l

l
l

–0.15

l

–6

l

–0.2

l

–0.2

l

3.5 4 V

l

l

–1 1 µA

l

85 125

l

50 90 kΩ

l

35

l

35

l
l

–6

l

l

–0.2

l

–6

l
l
l
l

4.1 5 V

2.70

2.45

2.2

0.2 V

0.2 V

0.8 V

5µA

0.15

0.3

–0.08

6

0.2
0

0.3 0.5 V

125

250
250

6mA

±35 mA

0.3
6

5.5

5.5

4.5

0.2

9
9
8

0.3

mA
mA
mA

kΩ
kΩ

mA
mA

mA

mA
mA

mA
mA
mA
mA

V
V
V

V
V

4

178591fc

LT1785/LT1785A/

LT1791/LT1791A

SWITCHING CHARACTERISTICS

The l denotes the specifi cations which apply over the full operating
temperature range, otherwise specifi cations are at T

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

t

PLH

t

PHL

t

SKEW

, t

t

r

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)

Driver Input to Output Figures 3, 5

Driver Input to Output Figures 3, 5

Driver Output to Output Figures 3, 5 100 ns

Driver Rise or Fall Time Figures 3, 5

Driver Enable to Output High Figures 4, 6

Driver Enable to Output Low Figures 4, 6

Driver Disable Time from Low Figures 4, 6

Driver Disable Time from High Figures 4, 6

Receiver Input to Output Figures 3, 7

Receiver Input to Output Figures 3, 7

Differential Receiver Skew 200 ns

Receiver Enable to Output Low Figures 2, 8

Receiver Enable to Output High Figures 2, 8

Receiver Disable from Low Figures 2, 8

Receiver Disable from High Figures 2, 8

Maximum Data Rate

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

= 25°C, VCC = 5V.

A

l

l

l

200 800 2000 ns

l

l

l

l

l

l

l

l

l

l

l

250 kbps

700 2000 ns

700 2000 ns

500 3000 ns

800 3000 ns

200 5000 ns

800 5000 ns

400 900 ns

400 900 ns

300 1000 ns

300 1000 ns

400 1000 ns

400 1000 ns

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.

178591fc

5

LT1785/LT1785A/
LT1791/LT1791A

TYPICAL PERFORMANCE CHARACTERISTICS

Driver Differential Output Voltage
vs Load Resistance

4

TA = 25°C

3

2

OUTPUT VOLTAGE (V)

1

0

10

100 1k

LOAD RESISTANCE ()

Driver Propagation Delay
vs Temperature

1000

900

800

700

600

500

400

300

PROPAGATION DELAY (ns)

200

100

0

–40

–20

LH

HL

40

20

0

TEMPERATURE (°C)

Driver Differential Output Voltage
vs Temperature

3.0

2.5

2.0

1.5

1.0

DIFFERENTIAL VOLTAGE (V)

0.5

R = 27Ω

178591 G01

0

–40

–20

0

TEMPERATURE (°C)

40

20

60

80

178591 G02

100

LT1791 Driver Output Leakage
DE = 0V

1mA/DIV

–60V

80

178591 G04

100

60

V

IN

178591 G05

200µA/DIV

60V

Receiver Propagation Delay
vs Temperature

1000

t

800

600

400

DELAY (ns)

200

0

–40

–20

PHL

t

PLH

40

20

0

TEMPERATURE (°C)

LT1791 Receiver Input Current
vs V

IN

–60V

V

IN

80

178591 G03

178591 G06

100

60V

60

LT1785 Input Characteristics Pins
A or B; DE = RE = 0V Supply Current vs Temperature

7

DRIVER AND

6

RECEIVER ON

5

(mA)

CC

I

4

3

2

1

0

RECEIVER ONLY

STANDBY

–40

–20

20

0

TEMPERATURE (°C)

1mA/DIV

–60V

V

, V

A

B

60V

178591 G07

6

Receiver Propagation Delay
vs Differential Input Voltage

700

600

LH VCM = 12V

0

0

HL VCM = 12V

1

VIN DIFFERENTIAL (V)

500

400

300

DELAY (ns)

200

100

40

60

80

178591 G08

100

HL VCM = –7V

LH VCM = –7V

2

34

178591 G09

178591fc

5

PIN FUNCTIONS

LT1785/LT1785A/

LT1791/LT1791A

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 < V
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 < V
guaranteed to be in a high state for 0V inputs.

: Positive Supply Input. For RS422 or RS485 operation,

V

CC

4.75V ≤ V
output drive swing. V
low ESR capacitor directly at Pin 8 (V

≤ 5.25V. Higher VCC input voltages increase

CC

< 0. Receiver outputs are

TH

< 0. Receiver outputs are

TH

should be decoupled with a 0.1µF

CC

).

CC

178591fc

7

LT1785/LT1785A/
LT1791/LT1791A

TEST CIRCUITS

A

R

V

OD

V

OC

R

B

1785/91 F01

RECEIVER

OUTPUT

TEST POINT

C

RL

1k

Figure 1. Driver DC Test Load Figure 2. Receiver Timing Test Load

5V

DE

DI

A

R

DIFF

B

A

C

L1

B

C

L2

RE

RO

15pF

1785/91 F03

OUTPUT

UNDER TEST

500Ω

C

L

Figure 3. Driver/Receiver Timing Test Circuit Figure 4. Driver Timing Test Load

S1

S2

1k

V

CC

1785/91 F02

S1

V

CC

S2

1785/91 F04

FUNCTION TABLES

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

8

178591fc

SWITCHING TIME WAVEFORMS

LT1785/LT1785A/

LT1791/LT1791A

DI

DE

A, B

A, B

5V

0V

B

A

V

O

0V

–V

O

V

O

1/2 V

1.5V

O

10%

t

r

f = 125kHz, tr ≤ 10ns, tf ≤ 10ns

t

PLH

t

SKEW

90%

V

DIFF

= V(A) – V(B)

1.5V

1/2 V

t

PHL

t

SKEW

90%

10%

t

f

O

1785/91 F05

Figure 5. Driver Propagation Delays

5V

0V

5V

V

OL

V

OH

0V

1.5V

f = 125kHz, tr ≤ 10ns, tf ≤ 10ns

t

ZL(SHDN),tZL

2.3V

2.3V

OUTPUT NORMALLY LOW

OUTPUT NORMALLY HIGH

t

ZH(SHDN),tZH

1.5V

t

LZ

0.5V

0.5V

t

HZ

178591 F06

RO

A – B

RE

RO

RO

Figure 6. Driver Enable and Disable Times

V

OH

V

OL

V

OD2

–V

OD2

t

PHL

1.5V

f = 125kHz, tr ≤ 10ns, tf ≤ 10ns

0V 0V

OUTPUT

INPUT

t

PLH

1.5V

178591 F07

Figure 7. Receiver Propagation Delays

5V

0V

5V

0V

1.5V
f = 125kHz, tr ≤ 10ns, tf ≤ 10ns

t

, t

ZL(SHDN)

ZL

1.5V

1.5V

OUTPUT NORMALLY LOW

OUTPUT NORMALLY HIGH

t

, t

ZH(SHDN)

ZH

1.5V

t

LZ

0.5V

0.5V

t

HZ

178591 F08

Figure 8. Receiver Enable and Disable Times

178591fc

9

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 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 toler­ance 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 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 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.

10

178591fc

APPLICATIONS INFORMATION

LT1785/LT1785A/

LT1791/LT1791A

Slew Limiting for EMI Emissions Control

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.

12

A

RX

TX

120Ω

11

B

10

Z

9

Y

RO

RE

DE

2

3

4

5

DI

Data Network Cable Selection and Termination

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 72.

9

120Ω

Y

10

Z

LT1791LT1791

B

11

12

RX

A

5

4

3

2

DI

DE

RE

RO

178591 F09

TX

Figure 9. Full-Duplex RS422

178591fc

11

LT1785/LT1785A/
LT1791/LT1791A

PACKAGE DESCRIPTION

N8 Package

8-Lead PDIP (Narrow 0.300)

(LTC DWG # 05-08-1510)

87 6

.255 ± .015*

(6.477 ± 0.381)

.400*

(10.160)

MAX

5

12

.300 – .325

(7.620 – 8.255)

.065

(1.651)

.008 – .015

(0.203 – 0.381)

+.035

.325

–.015
+0.889

8.255

()

–0.381

NOTE:

1. DIMENSIONS ARE

*THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS.

MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED .010 INCH (0.254mm)

INCHES

MILLIMETERS

TYP

.045 – .065

(1.143 – 1.651)

.100

(2.54)

BSC

3

4

.130 ± .005

(3.302 ± 0.127)

.120

(3.048)

MIN

.018 ± .003

(0.457 ± 0.076)

.020

(0.508)

MIN

N8 1002

12

178591fc

PACKAGE DESCRIPTION

.050 BSC

S8 Package

8-Lead Plastic Small Outline (Narrow 0.150)

(LTC DWG # 05-08-1610)

.189 – .197

.045 ±.005

(4.801 – 5.004)

8

NOTE 3

7

LT1785/LT1785A/

LT1791/LT1791A

5

6

.245
MIN

.030 ±.005

TYP

RECOMMENDED SOLDER PAD LAYOUT

.010 – .020

(0.254 – 0.508)

.008 – .010

(0.203 – 0.254)

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)

×

°

45

.016 – .050

(0.406 – 1.270)

INCHES

(MILLIMETERS)

.160

±.005

.228 – .244

(5.791 – 6.197)

0°– 8° TYP

.053 – .069

(1.346 – 1.752)

.014 – .019

(0.355 – 0.483)

TYP

.150 – .157

(3.810 – 3.988)

NOTE 3

1

3

2

4

.004 – .010

(0.101 – 0.254)

.050

(1.270)

BSC

SO8 0303

178591fc

13

LT1785/LT1785A/
LT1791/LT1791A

PACKAGE DESCRIPTION

N Package

14-Lead PDIP (Narrow 0.300)

(LTC DWG # 05-08-1510)

14

.255 ± .015*

(6.477 ± 0.381)

1213

.770*

(19.558)

MAX

11

8910

.300 – .325

(7.620 – 8.255)

(0.508)

.008 – .015

(0.203 – 0.381)

+.035

.325

–.015

+0.889

8.255

()

–0.381

NOTE:

1. DIMENSIONS ARE

*THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS.
MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED .010 INCH (0.254mm)

INCHES

MILLIMETERS

.020

MIN

.130 ± .005

(3.302 ± 0.127)

.120

(3.048)

MIN

.005

(0.127)

MIN

2

31

.045 – .065

(1.143 – 1.651)

.100

(2.54)

BSC

6

7

.065

(1.651)

TYP

.018 ± .003

(0.457 ± 0.076)

N14 1103

5

4

14

178591fc

PACKAGE DESCRIPTION

LT1785/LT1785A/

LT1791/LT1791A

S Package

14-Lead Plastic Small Outline (Narrow 0.150)

(LTC DWG # 05-08-1610)

.050 BSC

N

.245
MIN

1 2 3 N/2

.030 ±.005

TYP

RECOMMENDED SOLDER PAD LAYOUT

.010 – .020

(0.254 – 0.508)

.008 – .010

(0.203 – 0.254)

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)

×

°

45

.016 – .050

(0.406 – 1.270)

INCHES

(MILLIMETERS)

.045 ±.005

.160 ±.005

.228 – .244

(5.791 – 6.197)

0° – 8° TYP

.053 – .069

(1.346 – 1.752)

.014 – .019

(0.355 – 0.483)

TYP

.337 – .344

(8.560 – 8.738)

NOTE 3

13

12

11

4

10

5

.050

(1.270)

BSC

14

N

1

3

2

8

9

.150 – .157

(3.810 – 3.988)

N/2

7

6

NOTE 3

.004 – .010

(0.101 – 0.254)

S14 0502

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.

178591fc

15

LT1785/LT1785A/
LT1791/LT1791A

TYPICAL APPLICATION

RO

1

RX

2

RE

DE

DI

LT1785

3

4

TX

R

T

120Ω

7

B

6

A

6

7

A

B

LT1785

4

DI

DE

1

3

2

RE

RO

6

7

A

B

LT1785

4

3

DI

DE

1

2

RE

RO

R

120Ω

T

7

6

RX

B

A

LT1785

1

RO

2

RE

3

DE

4

TX

DI

178591 F10

Figure 10. Half-Duplex RS485 Network Operation

RAYCHEM

POLYSWITCH

TR600-150

× 2

COMPOSITE

47Ω

47Ω

CARBON

5W

0.1µF
300V

RT,120Ω

178591 F11

RO

RE

DE

8

V

1

RX

2

LT1785

3

4

DI

TX

CC

7

B

6

A

5

1.5KE36CA

Figure 11. RS485 Network with 120V AC Line Fault Protection

RELATED PARTS

PART NUMBER DESCRIPTION COMMENTS

LTC485 Low Power RS485 Interface Transceiver I

LTC491 Differential Driver and Receiver Pair I

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 2500V

LTC1685 52Mbps RS485 Half-Duplex Transceiver Propagation Delay Skew 500ps (Typ)

LTC1687 52Mbps RS485 Full-Duplex Transceiver Propagation Delay Skew 500ps (Typ)

Linear Technology Corporation

16

1630 McCarthy Blvd., Milpitas, CA 95035-7417

[phone] ● FAX: [phone]

●

www.linear.com

= 300µA (Typ)

CC

= 300µA

CC

Isolation in Surface Mount Package

RMS

LT 0409 REV C • PRINTED IN USA

© LINEAR TECHNOLOGY CORPORATION 1998

178591fc