Datasheet LTC491 Datasheet (Linear Technology)

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Page 1

LTC491

Differential Driver and

Receiver Pair

EATU

■

Low Power: ICC = 300µA Typical

■

Designed for RS485 or RS422 Applications

■

Single +5V Supply

■

–7V to +12V Bus Common Mode Range

Permits ±7V Ground Difference Between Devices on the Bus

■

Thermal Shutdown Protection

■

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

■

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

■

Combined Impedance of a Driver Output and Receiver Allows up to 32 Transceivers on the Bus

■

70mV Typical Input Hysteresis

■

28ns Typical Driver Propagation Delays with 5ns Skew

■

Pin Compatible with the SN75180

PPLICATI

DUESCRIPTIO

The LTC491 is a low power differential bus/line transceiver designed for multipoint data transmission standard RS485 applications with extended common mode range (+12V to –7V). It also meets the requirements of RS422.

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

The driver and receiver feature 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.

The receiver has a fail safe feature which guarantees a high output state when the inputs are left open.

Both AC and DC specifications are guaranteed from 0°C to 70°C and 4.75V to 5.25V supply voltage range.

■

Low Power RS485/RS422 Transceiver

■

Level Translator

PPLICATITYPICAL

DRIVER

LTC491

RECEIVER

REB

120Ω 120Ω

4000 FT 24 GAUGE TWISTED PAIR

120Ω 120Ω

4000 FT 24 GAUGE TWISTED PAIR

RECEIVER

LTC491

DRIVER

REB

LTC491 • TA01

Page 2

LTC491

PACKAGE

RDER I FOR ATIO

(Note 1)

LUTEXI T

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

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

Control Input Currents .......................... –50mA to 50mA

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

Driver Input Currents ............................ –25mA to 25mA

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

Receiver Input Voltages ......................................... ±14V

Receiver Output Voltages ................ –0.5V to VCC +0.5V

Operating Temperature Range

LTC491C.................................................. 0°C to 70°C

LTC491I.............................................. – 40°C to 85°C

WUW

ARB

U G

TOP VIEW

REB

GND GND

N PACKAGE

14-LEAD PLASTIC DIP

Consult factory for Military grade parts.

14-LEAD PLASTIC SOIC

Z Y

9 87

S PACKAGE

LTC491 • POI01

ORDER PART

NUMBER

LTC491CN LTC491CS LTC491IN LTC491IS

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

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

LECTRICAL C CHARA TERIST

VCC = 5V ±5%

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

OD1

OD2

∆V

∆VOC Change in Magnitude of Driver Common Mode ● 0.2 V

IN1

IN2

∆V

OZR

OSD1

OSD2

OSR

IOZDriver Three-State Output Current VO = –7V to 12V ● ±2 ±200 µA

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

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 ● 3V

Output Voltage for Complementary Output States Input High Voltage D, DE, REB ● 2.0 V Input Low Voltage ● 0.8 V Input Current ● ±2 µA Input Current (A, B) VCC = 0V or 5.25V VIN = 12V ● 1.0 mA

Differential Input Threshold Voltage for Receiver –7V ≤ VCM ≤ 12V ● –0.2 0.2 V Receiver Input Hysteresis VCM = 0V ● 70 mV Receiver Output High Voltage IO = –4mA, V Receiver Output Low Voltage IO = 4mA, V Three-State Output Current at Receiver VCC = Max 0.4V ≤ VO ≤ 2.4V ● ±1 µA Supply Current No Load; D = GND, Outputs Enabled ● 300 500 µA

Receiver Input Resistance –7V ≤ VCM ≤ 12V ● 12 kΩ Driver Short Circuit Current, V Driver Short Circuit Current, V Receiver Short Circuit Current 0V ≤ VO ≤ V

= High VO = –7V ● 100 250 mA

OUT

= Low VO = 12V ● 100 250 mA

OUT

ICSCD

R = 27Ω; (RS485) (Figure 1) ● 1.5 5 V

VIN = –7V ● –0.8 mA

= 0.2V ● 3.5 V

= –0.2V ● 0.4 V

or V

Outputs Disabled ● 300 500 µA

● 785mA

Page 3

LTC491

VCC = 5V ±5%

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

PLH

PHL

SKEW

, t

tZHDriver Enable to Output High CL = 100pF (Figures 4, 6) S2 Closed ● 40 70 ns t

PLH

PHL

SKD

TCH

GC CHARA TERIST

Driver Input to Output R Driver Input to Output ● 10 30 50 ns

Driver Output to Output ● 5ns Driver Rise or Fall Time ● 51525 ns

Driver Enable to Output Low CL = 100pF (Figures 4, 6) S1 Closed ● 40 70 ns Driver Disable Time From Low CL = 15pF (Figures 4, 6) S1 Closed ● 40 70 ns Driver Disable Time From High CL = 15pF (Figures 4, 6) S2 Closed ● 40 70 ns Receiver Input to Output R Receiver Input to Output ● 40 70 150 ns  t

– t

PLH

Receiver Enable to Output Low CL = 15pF (Figures 3, 8) S1 Closed ● 20 50 ns Receiver Enable to Output High CL = 15pF (Figures 3, 8) S2 Closed ● 20 50 ns Receiver Disable From Low CL = 15pF (Figures 3, 8) S1 Closed ● 20 50 ns Receiver Disable From High CL = 15pF (Figures 3, 8) S2 Closed ● 20 50 ns

 Differential Receiver Skew ● 13 ns

PHL

ICS

= 54Ω, CL1 = CL2 = 100pF ● 10 30 50 ns

DIFF

(Figures 2, 5)

= 54Ω, CL1 = CL2 = 100pF ● 40 70 150 ns

DIFF

(Figures 2, 7)

● denotes specifications which apply over the full operating

The temperature range.

Note 1: “Absolute Maximum Ratings” are those beyond which the safety of the device cannot be guaranteed.

FUUC

U S

NC (Pin 1): Not Connected. R (Pin 2): Receiver output. If the receiver output is enabled

(REB low), then if A > B by 200mV, R will be high. If A < B by 200mV, then R will be low.

REB (Pin 3): Receiver output enable. A low enables the receiver output, R. A high input forces the receiver output into a high impedance state.

DE (Pin 4): Driver output enable. A high on DE enables the driver outputs, A and B. A low input forces the driver outputs into a high impedance state.

D (Pin 5): Driver input. If the driver outputs are enabled (DE high), then A low on D forces the driver outputs A low and B high. A high on D will force A high and B low.

Note 2: All currents into device pins are positive; all currents out of device pins are negative. All voltages are referenced to device ground unless otherwise specified.

Note 3: All typicals are given for V

= 5V and Temperature = 25°C.

GND (Pin 6): Ground Connection. GND (Pin 7): Ground Connection. NC (Pin 8): Not Connected. Y (Pin 9): Driver output. Z (Pin 10): Driver output. B (Pin 11): Receiver input. A (Pin 12): Receiver input. NC (Pin 13): Not Connected. V

(Pin 14): Positive supply; 4.75V ≤ VCC ≤ 5.25V.

Page 4

LTC491

OUTPUT VOLTAGE (V)

OUTPUT CURRENT (mA)

1234

LTC491 • TPC03

TEMPERATURE (°C )

–50

OUTPUT VOLTAGE (V)

0.2

0.4

0.6

0.8

0 50 100

LTC491 • TPC09

PICA

LPER

CCHARA TERIST

ICS

Driver Output High Voltage vs Driver Differential Output Voltage vs Driver Output Low Voltage vs Output Current TA = 25°C Output Current TA = 25°C Output Current TA = 25°C

–96

–72

– 4 8

OUTPUT CURRENT (mA)

–24

1234

OUTPUT VOLTAGE (V)

LTC491 • TPC01

OUTPUT CURRENT (mA)

1234

OUTPUT VOLTAGE (V)

LTC491 • 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)

350

340

330

1.57

INPUT THRESHOLD VOLTAGE (V)

1.55 –50

Driver Differential Output Voltage vs Receiver  t Temperature RO = 54Ω Temperature Temperature at I = 8mA

2.3

2.1

1.9

1.7

DIFFERENTIAL VOLTAGE (V)

1.5 –50

0 50 100

TEMPERATURE (°C )

0 50 100

TEMPERATURE (°C )

LTC491 • TPC04

LTC491 • TPC07

2.0

1.0 –50

7.0

6.0

5.0

TIME (ns)

4.0

3.0 –50

0 50 100 TEMPERATURE (°C )

PLH tPHL

0 50 100 TEMPERATURE (°C )

LTC491 • TPC05

 vs Receiver Output Low Voltage vs

LTC491 • TPC08

SUPPLY CURRENT (µA)

320

310

–50

0 50 100 TEMPERATURE (°C )

LTC491 • TPC06

Page 5

TEST CIRCUITS

OD2

LTC491 • TA02

Figure 1. Driver DC Test Load

LTC491

RECEIVER

OUTPUT

DRIVER

DIFF

RECEIVER

B 15pF

LTC491 • TA03

Figure 2. Driver/Receiver Timing Test Circuit

1kΩ

LTC491 • TA04

OUTPUT

UNDER TEST

500

Ω

LTC491 • TA05

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

Page 6

LTC491

WITCHI

–V

A, B

1/2 V

10%

50%

1.5V

PLH

f = 1MHz : t

DIFF

≤ 10ns : t

= V(Y) – V(Z)

≤ 10ns

1/2 V

1.5V

PHL

90%

50%

20%

SKEW

LTC491 • TA06

WAVEFORS

80%

SKEW

Figure 5. Driver Propagation Delays

1.5V 1.5V

2.3V

f = 1MHz : t

≤ 10ns : tr ≤ 10ns

OUTPUT NORMALLY LOW

OUTPUT NORMALLY HIGH

0.5V

LTC491 • TA07

Figure 6. Driver Enable and Disable Times

A-B

REB

OD2

–V

OD2

PLH

1.5V

f = 1MHz ; t

INPUT

≤ 10ns : t

OUTPUT

≤ 10ns

PHL

1.5V

LTC491 • TA08

Figure 7. Receiver Propagation Delays

1.5V 1.5V

1.5V

f = 1MHz : t

≤ 10ns : tf ≤ 10ns

OUTPUT NORMALLY LOW

OUTPUT NORMALLY HIGH

0.5V

LTC491 • TA09

Figure 8. Receiver Enable and Disable Times

Page 7

LTC491

PPLICATI

I FOR ATIO

Typical Application

A typical connection of the LTC491 is shown in Figure 9. Two twisted pair wires connect up to 32 driver/receiver pairs for full 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 input impedance of a receiver is

RECEIVER

DRIVER

120Ω

typically 20kΩ to GND, or 0.6 unit RS-485 load, so in practice 50 to 60 transceivers can be connected to the same wires. The optional shields around the twisted pair help reduce unwanted noise, and are connected to GND at one end.

The LTC491 can also be used as a line repeater as shown in Figure 10. If the cable length is longer than 4000 feet, the LTC491 is inserted in the middle of the cable with the receiver output connected back to the driver input.

120Ω

RECEIVER

DRIVER

LTC491

2 3

9 10 11 12

RECEIVER

DRIVER

DX RX

Figure 9. Typical Connection

RECEIVER DATA IN

DRIVERDX

LTC491

120Ω

DATA OUT

LTC491 • TA11

LTC491

LTC491 • TA10

Figure 10. Line Repeater

Page 8

LTC491

PPLICATI

I FOR ATIO

Thermal Shutdown

The LTC491 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 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 LTC491 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.

Cables 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.

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 over all loss (Figure 11).

When using low loss cables, Figure 12 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.

1.0

LOSS PER 100 ft (dB)

0.1

1.0 10 100

FREQUENCY (MHZ)

LTC491 • TA12

10k

100

CABLE LENGTH (ft)

10k

Figure 12. Cable Length vs Data RateFigure 11. Attenuation vs Frequency for Belden 9481

100k 1M 10M

DATA RATE (bps)

2.5M

LTC491 • TA13

Page 9

LTC491

PPLICATI

I FOR ATIO

Cable Termination

The proper termination of the cable is very important. If the cable is not terminated with it’s 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 13).

PROBE HERE

DRIVERDX RECEIVER RX

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.

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 60 times greater than the supply current of the LTC491. One way to eliminate the unwanted current is by AC coupling the termination resistors as shown in Figure 14.

120Ω

Rt = 120Ω

Rt = 47Ω

Rt = 470Ω

LTC491 • TA14

Figure 13. 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).

C = LINE LENGTH (ft) x 16.3pF

Figure 14. AC Coupled Termination

RECEIVER

LTC491 • TA15

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).

Page 10

LTC491

LTC491 • TA17

120Ω

DRIVER

PPLICATI

I FOR ATIO

Receiver Open-Circuit Fail-Safe

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. The receiver of the LTC491 has 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 has about 70mV of hysteresis, the receiver output will maintain the last data bit received.

+5V

130Ω110Ω 130Ω

+5V

1.5kΩ

140Ω

100kΩ

+5V

110Ω

1.5kΩ

120Ω

RECEIVER

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 16).

Figure 16. ESD Protection with TransZorbs

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.

RECEIVER

Figure 15. Forcing “O” When All Drivers are Off

LTC491 • TA16

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.

Page 11

LTC491

PPLICATITYPICAL

RS232 Receiver

RS232 IN

5.6kΩ

RECEIVER

1/2 LTC491

RS232 to RS485 Level Transistor with Hysteresis

R = 220kΩ

LTC491 • TA18

RS232 IN

5.6kΩ

10kΩ

DRIVER

1/2 LTC491

HYSTERESIS = 10kΩ • ≈

120Ω

VY - VZ  ————

19k

————

LTC491 • TA19

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.

Page 12

LTC491

PACKAGEDESCRIPTI

Dimensions in inches (millimeters) unless otherwise noted.

N Package

14-Lead Plastic DIP

0.770

(19.558)

MAX

1213

8910

0.260 ± 0.010

(6.604 ± 0.254)

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

0.015

(0.380)

MIN

0.075 ± 0.015

(1.905 ± 0.381)

0.130 ± 0.005

(3.302 ± 0.127)

S Package

14-Lead Plastic SOIC

0.337 – 0.344

(8.560 – 8.738)

0.100 ± 0.010

(2.540 ± 0.254)

11 10

0.045 – 0.065

(1.143 – 1.651)

0.018 ± 0.003

(0.457 ± 0.076)

J MAX

100°C90°C/W

0.065

(1.651)

TYP

0.125

(3.175)

MIN

N14 0392

J MAX

100°C 110°C/W

0.228 – 0.244

(5.791 – 6.197)

0.010 – 0.020

(0.254 – 0.508)

0° – 8° TYP

0.016 – 0.050

0.406 – 1.270

× 45°

0.008 – 0.010

(0.203 – 0.254)

Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7487

(408) 432-1900

●

FAX

: (408) 434-0507

●

TELEX

: 499-3977

0.053 – 0.069

(1.346 – 1.752)

0.014 – 0.019

(0.355 – 0.483)

0.150 – 0.157

(3.810 – 3.988)

0.050

(1.270)

TYP

0.004 – 0.010

(0.101 – 0.254)

SO14 0392

BA/GP 0492 10K REV 0

 LINEAR TECHNOLOGY CORPORATION 1992