Datasheet LTC487 Datasheet (Linear Technology)

LTC487

Quad Low Power

RS485 Driver

EATU

F

■

Very Low Power: ICC = 110µA Typ

■

Designed for RS485 or RS422 Applications

■

Single 5V Supply

■

–7V to 12V Bus Common-Mode Range Permits

RE

S

±7V GND Difference Between Devices on the Bus

■

Thermal Shutdown Protection

■

Power-Up/Down Glitch-Free Driver Outputs Permit
Live Insertion/Removal of Package

■

Driver Maintains High Impedance in Three-State or
with the Power Off

■

28ns Typical Driver Propagation Delays with
5ns Skew

■

Pin Compatible with the SN75174, DS96174,
µA96174, and DS96F174

U

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PPLICATI

A

■

Low Power RS485/RS422 Drivers

■

Level Translator

S

DUESCRIPTIO

The LTC487® is a low power differential bus/line driver
designed for multipoint data transmission standard RS485
applications with extended common-mode range (– 7V to
12V). It also meets RS422 requirements.

The CMOS design offers significant power savings over its
bipolar counterpart without sacrificing ruggedness against
overload or ESD damage.

The driver features three-state outputs, with the driver
outputs maintaining high impedance over the entire com­mon-mode range. Excessive power dissipation caused by
bus contention or faults is prevented by a thermal shut­down circuit which forces the driver outputs into a high
impedance state.

Both AC and DC specifications are guaranteed from 0°C to
70°C (Commercial), –40°C to 85°C (Industrial) and over
the 4.75V to 5.25V supply voltage range.

DI

EN 12

1

DRIVER

1/4 LTC487

and LTC are registered trademarks and LT is a trademark of Linear Technology Corporation.

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PPLICATITYPICAL

RS485 Cable Length Specification*

10k

EN 12

4

2

120Ω 120Ω

3

4000 FT BELDEN 9841

2

1

4

RECEIVER

1/4 LTC489

3

LTC487 • TA01

RO

1k

100

CABLE LENGTH (FT)

10

10k

* APPLIES FOR 24 GAUGE, POLYETHYLENE

DIELECTRIC TWISTED PAIR

100k 1M 10M

DATA RATE (bps)

2.5M

LTC487 • TA09

1

LTC487

WU

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PACKAGE

/

O

RDER I FOR ATIO

W

O

A

(Note 1)

LUTEXI T

S

Supply Voltage (VCC) ............................................... 12V

Control Input Voltages .................... –0.5V to VCC + 0.5V

Driver Input Voltages ...................... –0.5V to VCC + 0.5V

Driver Output Voltages .......................................... ±14V

Control Input Currents ........................................ ±25mA

Driver Input Currents .......................................... ±25mA

Operating Temperature Range

Commercial ............................................ 0°C to 70°C

Industrial ........................................... – 40°C to 85°C

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

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

LECTRICAL C CHARA TERIST

E

VCC = 5V ±5%, 0°C ≤ TA ≤ 70°C (Commercial), –40°C ≤ TA ≤ 85°C (Industrial) (Note 2, 3)

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

V

OD1

V

OD2

V

OD

V

OC

V

 Change in Magnitude of Driver Common-Mode 0.2 V

OC

V

IH

V

IL

I

IN1

I

CC

I

OSD1

I

OSD2

I

OZ

Differential Driver Output Voltage (Unloaded) IO = 0 5 V
Differential Driver Output Voltage (With Load) R = 50Ω; (RS422) 2 V

Change in Magnitude of Driver Differential R = 27Ω or R = 50Ω 0.2 V
Output Voltage for Complementary Output States (Figure 3)

Driver Common-Mode Output Voltage 3V

Output Voltage for Complementary Output States
Input High Voltage DI, EN12, EN34 2.0 V
Input Low Voltage 0.8 V
Input Current ±2 µA
Supply Current No Load Output Enabled 110 200 µA

Driver Short-Circuit Current, V
Driver Short-Circuit Current, V
High Impedance State Output Current VO = –7V to 12V ±10 ±200 µA

A

WUW

OUT
OUT

U

ARB

G

I

S

TOP VIEW

1

DI1

2

DO1A

3

DO1B

4

EN12

5

DO2B

6

DO2A

DI2

7

GND

8

N PACKAGE

16-LEAD PLASTIC DIP



T

= 125°C, θJA = 70°C/W (N)

JMAX

= 150°C, θJA = 95°C/W (S)

T

JMAX

Consult factory for Military grade parts.

16

V

CC

15

DI4

14

DO4A

13

DO4B

12

EN34

11

DO3B
DO3A

10

DI3

9

S PACKAGE

16-LEAD PLASTIC SOL

ORDER PART

NUMBER

LTC487CN
LTC487CS
LTC487IN
LTC487IS

ICSCD

R = 27Ω; (RS485) (Figure 3) 1.5 5 V

Output Disabled 110 200 µA

= High VO = –7V 100 250 mA
= Low VO = 12V 100 250 mA

U

S

WI

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

t

PLH

t

PHL

t

SKEW

t

r, tf

t

ZH

t

ZL

t

LZ

t

HZ

Note 1: Absolute maximum ratings are those beyond which the safety of
the device cannot be guaranteed.

Note 2: All currents into device pins are positive; all currents out of device

I

TCH

GC CHARA TERIST

Driver Input to Output R
Driver Input to Output
Driver Output to Output 515ns
Driver Rise or Fall Time 5 20 25 ns
Driver Enable to Output High CL = 100pF (Figures 2, 5) S2 Closed 35 70 ns
Driver Enable to Output Low CL = 100pF (Figures 2, 5) S1 Closed 35 70 ns
Driver Disable Time from Low CL = 15pF (Figures 2, 5) S1 Closed 35 70 ns
Driver Disable Time from High CL = 15pF (Figures 2, 5) S2 Closed 35 70 ns

VCC = 5V ±5%, 0°C ≤ TA ≤ 70°C (Note 2, 3)

ICS

= 54Ω, CL1 = CL2 = 100pF 10 30 50 ns

DIFF

(Figures 1, 4)

pins are negative. All voltages are referenced to device GND unless
otherwise specified.

Note 3: All typicals are given for V

10 30 50 ns

= 5V and Temperature = 25°C.

CC

2

UW

TEMPERATURE (°C )

–50

SUPPLY CURRENT (µA)

90

100

110

120

130

0 50 100

LTC487 • TPC06

OUTPUT VOLTAGE (V)

0

OUTPUT CURRENT (mA)

0

20

40

60

80

1234

LTC487 • TPC03

TA = 25°C

Y

PICA

LPER

F

O

R

AT

CCHARA TERIST

E

C

ICS

Driver Output High Voltage Driver Differential Output Voltage Driver Output Low Voltage
vs Output Current vs Output Current vs Output Current

LTC487

–96

–72

– 4 8

OUTPUT CURRENT (mA)

–24

0

0

1234

OUTPUT VOLTAGE (V)

TA = 25°C

LTC487 • TPC01

64

48

32

OUTPUT CURRENT (mA)

16

0

0

1234

OUTPUT VOLTAGE (V)

TA = 25°C

LTC487• TPC02

TTL Input Threshold vs Temperature Driver Skew vs Temperature Supply Current vs Temperature

1.63

1.61

1.59

5.0

4.0

3.0

TIME (ns)

1.57

INPUT THRESHOLD VOLTAGE (V)

1.55
–50

0 50 100
TEMPERATURE (°C )

LTC487 • TPC04

2.0

1.0
–50

0 50 100
TEMPERATURE (°C )

Driver Differential Output
Voltage vs Temperature

2.3

2.1

1.9

1.7

DIFFERENTIAL VOLTAGE (V)

1.5
–50

0 50 100
TEMPERATURE (°C )

LTC487 • TPC05

RO = 54Ω

LTC487 • TPC07

3

LTC487

PI

U

FUUC

TI

O

U
S

DI1 (Pin 1): Driver 1 input. If Driver 1 is enabled, then a low
on DI1 forces the driver outputs DO1A low and DO1B high.
A high on DI1 with the driver outputs enabled will force
DO1A high and DO1B low.

DO1A (Pin 2): Driver 1 output.
DO1B (Pin 3): Driver 1 output.
EN12 (Pin 4): Driver 1 and 2 outputs enabled. See Func-

tion Table for details.

DO2B (Pin 5): Driver 2 output.
DO2A (Pin 6): Driver 2 output.
DI2 (Pin 7): Driver 2 input. Refer to DI1.

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FUUC

INPUT ENABLES OUTPUTS

TI

DI EN12 or EN34 OUT A OUT B

HH HL
LH LH
XL ZZ

TABLE

GND (Pin 8): GND connection.
DI3 (Pin 9): Driver 3 input. Refer to DI1.
DO3A (Pin 10): Driver 3 output.
DO3B (Pin 11): Driver 3 output.
EN34 (Pin 12): Driver 3 and 4 outputs enabled. See

Function Table for details.

DO4B (Pin 13): Driver 4 output.
DO4A (Pin 14): Driver 4 output.
DI4 (Pin 15): Driver 4 input. Refer to DI1.
VCC (Pin 16): Positive supply; 4.75 < VCC < 5.25.

H: High Level
L: Low Level
X: Irrelevant
Z: High Impedance (Off)

WITCHI

3V

DI

0V

B

A

V

O

–V

O

3V

EN12

0V

5V

A, B

V

OL

V

OH

A, B

0V

U

G

V

O

TI

E

1/2 V

W

O

WAVEFORS

1.5V

t

PLH

t

SKEW

80%
10%
t

r

Figure 1. Driver Propagation Delays

1.5V 1.5V

t

ZL

2.3V

2.3V

t

ZH

W

S

f = 1MHz : t 10ns : t 10ns

f = 1MHz : t 10ns : t 10ns

<<

rf

V = V(A) – V(B)

DIFF

≤≤

rf

OUTPUT NORMALLY LOW

OUTPUT NORMALLY HIGH

1/2 V

1.5V

t

PHL

t

O

t

LZ

t

HZ

90%

t

f

SKEW

20%

LTC487 • TA05

0.5V

0.5V

LTC487 • TA06

Figure 2. Driver Enable and Disable Times

4

LTC487

TEST CIRCUITS

A

R

V

OD

R

V

B

Figure 3. Driver DC Test Load

PPLICATI

A

OC

LTC487 • TA02

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I FOR ATIO

EN12

A

DI

DRIVER 1

R

B

Figure 4. Driver Timing Test Circuit

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Typical Application

A typical connection of the LTC487 is shown in Figure 6.
A twisted pair of wires connect up to 32 drivers and
receivers for half duplex data transmission. There are no
restrictions on where the chips are connected to the wires,
and it isn’t necessary to have the chips connected at the
ends. However, the wires must be terminated only at
the ends with a resistor equal to their characteristic
impedance, typically 120Ω. The optional shields around
the twisted pair help reduce unwanted noise, and are
connected to GND at one end.

Thermal Shutdown

The LTC487 has a thermal shutdown feature which pro­tects the part from excessive power dissipation. If the
outputs of the driver are accidently shorted to a power
supply or low impedance source, up to 250mA can flow

C

DIFF

L1

C

L2

LTC487 • TA03

OUTPUT

UNDER TEST

500

C

Ω

L

S1

S2

LTC487 • TA04

V

CC

Figure 5. Driver Timing Test Load #2

through the part. The thermal shutdown circuit disables
the driver outputs when the internal temperature reaches
150°C and turns them back on when the temperature
cools to 130°C. If the outputs of two or more LTC487
drivers are shorted directly, the driver outputs can not
supply enough current to activate the thermal shutdown.
Thus, the thermal shutdown circuit will not prevent con­tention faults when two drivers are active on the bus at the
same time.

Cable and Data Rate

The transmission line of choice for RS485 applications is
a twisted pair. There are coaxial cables (twinaxial) made
for this purpose that contain straight pairs, but these are
less flexible, more bulky, and more costly than twisted
pairs. Many cable manufacturers offer a broad range of
120Ω cables designed for RS485 applications.

DX

EN12

SHIELD

1

2

120Ω

EN12

4

RX

1/4 LTC4891/4 LTC487

3

EN12

DX

4

SHIELD

3

2

4

3

1

DX

2

DX

120Ω

1

RX

EN12

4

2

RX

1

1/4 LTC4891/4 LTC487

3

LTC487 • TA07

RX

Figure 6. Typical Connection

5

LTC487

PPLICATI

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Losses in a transmission line are a complex combination
of DC conductor loss, AC losses (skin effect), leakage, and
AC losses in the dielectric. In good polyethylene cables
such as the Belden 9841, the conductor losses and dielec­tric losses are of the same order of magnitude, leading to
relatively low overall loss (Figure 7).

10

1.0

LOSS PER 100 FT (dB)

0.1

0.1

Figure 7. Attenuation vs Frequency for Belden 9841

1.0 10 100

FREQUENCY (MHz)

LTC487 • TA08

Cable Termination

The proper termination of the cable is very important. If the
cable is not terminated with its characteristic impedance,
distorted waveforms will result. In severe cases, distorted
(false) data and nulls will occur. A quick look at the output
of the driver will tell how well the cable is terminated. It is
best to look at a driver connected to the end of the cable,
since this eliminates the possibility of getting reflections
from two directions. Simply look at the driver output while
transmitting square wave data. If the cable is terminated
properly, the waveform will look like a square wave
(Figure 9).

PROBE HERE

DRIVERDX RECEIVER RX

Rt = 120Ω

Rt

When using low loss cables, Figure 8 can be used as a
guideline for choosing the maximum line length for a given
data rate. With lower quality PVC cables, the dielectric loss
factor can be 1000 times worse. PVC twisted pairs have
terrible losses at high data rates (> 100kbs) and greatly
reduce the maximum cable length. At low data rates
however, they are acceptable and much more economical.

10k

1k

100

CABLE LENGTH (FT)

10

10k

Figure 8. Cable Length vs Data Rate

100k 1M 10M

DATA RATE (bps)

2.5M

LTC487 • TA09

Rt = 47Ω

Rt = 470Ω

LTC487 • TA10

Figure 9. Termination Effects

If the cable is loaded excessively (47Ω), the signal initially
sees the surge impedance of the cable and jumps to an
initial amplitude. The signal travels down the cable and is
reflected back out of phase because of the mistermination.
When the reflected signal returns to the driver, the ampli­tude will be lowered. The width of the pedestal is equal to
twice the electrical length of the cable (about 1.5ns/foot).
If the cable is lightly loaded (470Ω), the signal reflects in
phase and increases the amplitude at the driver output. An
input frequency of 30kHz is adequate for tests out to
4000 feet of cable.

6

LTC487

PPLICATI

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AC Cable Termination

Cable termination resistors are necessary to prevent un­wanted reflections, but they consume power. The typical
differential output voltage of the driver is 2V when the
cable is terminated with two 120Ω resistors, causing
33mA of DC current to flow in the cable when no data is
being sent. This DC current is about 220 times greater than
the supply current of the LTC487. One way to eliminate the
unwanted current is by AC coupling the termination resis­tors as shown in Figure 10.

120Ω

C

C = LINE LENGTH (FT) x 16.3pF

Figure 10. AC Coupled Termination

RECEIVER

RX

LTC487 • TA11

The coupling capacitor must allow high-frequency energy
to flow to the termination, but block DC and low frequen­cies. The dividing line between high and low frequency
depends on the length of the cable. The coupling capacitor
must pass frequencies above the point where the line
represents an electrical one-tenth wavelength. The value
of the coupling capacitor should therefore be set at

16.3pF per foot of cable length for 120Ω cables. With the
coupling capacitors in place, power is consumed only on
the signal edges, and not when the driver output is idling
at a 1 or 0 state. A 100nF capacitor is adequate for lines up
to 4000 feet in length. Be aware that the power savings start
to decrease once the data rate surpasses 1/(120Ω × C).

Receiver Open-Circuit Fail-Safe

has about 70mV of hysteresis, the receiver output will
maintain the last data bit received.

If the receiver output must be forced to a known state, the
circuits of Figure 11 can be used.

5V

130Ω110Ω 130Ω

5V

1.5k

140Ω

100k

5V

C

Figure 11. Forcing ‘0’ When All Drivers Are Off

110Ω

1.5k

120Ω

RECEIVER

RECEIVER

RECEIVER

RX

RX

RX

LTC487 • TA12

The termination resistors are used to generate a DC bias
which forces the receiver output to a known state, in this
case a logic 0. The first method consumes about
208mW and the second about 8mW. The lowest power
solution is to use an AC termination with a pull-up resistor.
Simply swap the receiver inputs for data protocols ending
in logic 1.

Some data encoding schemes require that the output of
the receiver maintains a known state (usually a logic 1)
when the data is finished transmitting and all drivers on the
line are forced into three-state. All LTC RS485 receivers
have a fail-safe feature which guarantees the output to be
in a logic 1 state when the receiver inputs are left floating
(open-circuit). However, when the cable is terminated
with 120Ω, the differential inputs to the receiver are
shorted together, not left floating. Because the receiver

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.

Fault Protection

All of LTC’s RS485 products are protected against ESD
transients up to 2kV using the human body model
(100pF, 1.5kΩ). However, some applications need more
protection. The best protection method is to connect a
bidirectional TransZorb® from each line side pin to ground
(Figure 12).

TransZorb is a registered trademark of General Instruments, GSI

7

LTC487

PPLICATI

A

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S

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A TransZorb is a silicon transient voltage suppressor that
has exceptional surge handling capabilities, fast response
time, and low series resistance. They are available from
General Semiconductor Industries and come in a variety of
breakdown voltages and prices. Be sure to pick a break­down voltage higher than the common-mode voltage
required for your application (typically 12V). Also, don’t
forget to check how much the added parasitic capacitance
will load down the bus.

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PPLICATITYPICAL

RS232 to RS485 Level Translator with Hysteresis

R = 220k

Y

RS232 IN

5.6k

10k

DRIVER

1/4 LTC487

120Ω

U

PACKAGEDESCRIPTI

O

Dimensions in inches (millimeters) unless otherwise noted.

Y

DRIVER

120Ω

Z

LTC487 • TA13

Figure 12. ESD Protection with TransZorbs

VY - VZ 

Z

HYSTERESIS = 10kΩ • ≈

————

R

19k

————

R

LTC487 • TA14

N Package, 16-Lead Plastic DIP

0.300 – 0.325

(7.620 – 8.255)

0.009 – 0.015

(0.229 – 0.381)

+0.025

0.325
–0.015

+0.635

8.255

()

–0.381

*THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS.
MOLD FLASH OR PROTURSIONS SHALL NOT EXCEED 0.010 INCH (0.254mm).

0.015

(0.381)

MIN

0.130 ± 0.005

(3.302 ± 0.127)

0.125

(3.175)

MIN

0.045 ± 0.015

(1.143 ± 0.381)

0.100 ± 0.010

(2.540 ± 0.254)

0.045 – 0.065

(1.143 – 1.651)

S Package, 16-Lead Plastic SOL

0.291 – 0.299

(7.391 – 7.595)

0.005

(0.127)

RAD MIN

0.009 – 0.013

(0.229 – 0.330)

NOTE:

1. PIN 1 IDENT, NOTCH ON TOP AND CAVITIES ON THE BOTTOM OF PACKAGES ARE THE MANUFACTURING OPTIONS.
THE PART MAY BE SUPPLIED WITH OR WITHOUT ANY OF THE OPTIONS.

2. THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS.
MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.006 INCH (0.15mm).

(NOTE 2)

0.010 – 0.029

(0.254 – 0.737)

NOTE 1

0.016 – 0.050

(0.406 – 1.270)

× 45°

0° – 8° TYP

0.093 – 0.104

(2.362 – 2.642)

0.050

(1.270)

TYP

0.014 – 0.019

(0.356 – 0.482)

TYP

0.065

(1.651)

TYP

0.018 ± 0.003

(0.457 ± 0.076)

0.037 – 0.045

(0.940 – 1.143)

0.004 – 0.012

(0.102 – 0.305)

0.260 ± 0.010*
(6.604 ± 0.254)

NOTE 1

14

15

16

2

1

(10.109 – 10.490)

15 1413121110 9

16

2345678

1

13

3

0.398 – 0.413

(NOTE 2)

0.770*

(19.558)

MAX

4

12

5

11

6

7

0.394 – 0.419

(10.007 – 10.643)

910

8

N16 0594

SOL16 0494

8

Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7487

[phone]

●

FAX

: [phone]

●

TELEX

: 499-3977

LT/GP 0894 0K REV A • PRINTED IN USA

 LINEAR TECHNOLOGY CORPORATION 1994