Datasheet LTC1687, LTC1686 Datasheet (Linear Technology)

Download

Page 1

FEATURES

■

Precision Propagation Delay Over Temperature:

Receiver/Driver: 18.5ns ±3.5ns

■

High Data Rate:

■

Low t

PLH/tPHL

52Mbps

Skew:

Receiver/Driver: 500ps Typ

■

–7V to 12V RS485 Input Common Mode Range

■

Guaranteed Fail-Safe Operation Over the Entire Common Mode Range

■

High Input Resistance: ≥22k, Even When Unpowered

■

Short-Circuit Protected

■

Thermal Shutdown Protected

■

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

■

Single 5V Supply

■

Pin Compatible with LTC490/LTC491

■

45dB CMRR at 26MHz

APPLICATIONS

■

High Speed RS485/RS422 Full Duplex Transceivers

■

Level Translator

■

Backplane Transceiver

■

STS-1/OC-1 Data Transceiver

■

Signal Repeaters

LTC1686/LTC1687

52Mbps Precision Delay

RS485 Fail-Safe Transceivers

DESCRIPTION

The LTC®1686/LTC1687 are high speed, precision delay, full-duplex RS485 transceivers that can operate at data rates as high as 52Mbps. The devices also meet the requirements of RS422.

A unique architecture provides very stable propagation delays and low skew over a wide common mode and ambient temperature range.

The driver and receiver feature three-state outputs, with disabled driver outputs maintaining high impedance over the entire common mode range. A short-circuit feature detects shorted outputs and substantially reduces driver output current. A similar feature also protects the receiver output from short circuits. Thermal shutdown circuitry protects from excessive power dissipation.

The receiver has a fail-safe feature that guarantees a high output state when the inputs are shorted or are left floating. The LTC1686/LTC1687 RS485 transceivers guarantee receiver fail-safe operation over the range (–7V to 12V). Receiver input resistance remains ≥ 22k when the device is unpowered or disabled.

The LTC1686/LTC1687 operate from a single 5V supply and draw only 7mA of supply current.

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

entire

common mode

TYPICAL APPLICATION

LTC1686 LTC1686

DRIVER

RECEIVER

100Ω 100Ω

400 FT OF CATEGORY 5 UTP

RECEIVER

DRIVER

LTC1686/87 • TA01

10Mbps Data Pulse

400 Feet Category 5 UTP

1686/87 TA02

DRIVER INPUT

RECEIVER INPUT

RECEIVER OUTPUT

2V/DIV

1V/DIV

5V/DIV

CABLE DELAY



100ns/DIV

Page 2

LTC1686/LTC1687

S8 PACKAGE

8-LEAD PLASTIC SO

1 2 3 4

8 7 6 5



TOP VIEW

 R D

GND

A B Z Y

LUTEXI T

WUW

ARB

U G

(Note 1)

Supply Voltage (VDD).............................................. 10V

Control Input Currents .................... –100mA to 100mA

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

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

Driver Output Voltages ................................. +12V/– 7V

Receiver Input Voltages ................................ +12V/–7V

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

Receiver Input Differential ...................................... 10V

PACKAGE

RDER I FOR ATIO

ORDER PART

NUMBER

LTC1686CS8

S8 PART MARKING

= 125°C, θ

JMAX

Consult factory for Industrial and Military grade parts.

= 150°C/ W

1686

Driver Short-Circuit Duration

: –7V to 10V)...................................... Indefinite

OUT

Receiver Short-Circuit Duration

: 0V to VDD) ........................................ Indefinite

OUT

Operating Temperature Range .................... 0°C to 70°C

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

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

GND GND

14-LEAD PLASTIC SO

JMAX

TOP VIEW

S PACKAGE

= 125°C, θ

14 13 12 11 10

9 87

= 90°C/W

NC A B Z

ORDER PART

NUMBER

LTC1687CS

DC ELECTRICAL CHARACTERISTICS

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

OD1

OD2

∆V

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

IN1

IN2

∆V

Differential Driver Output (Unloaded) I Differential Driver Output (With Load) R = 50Ω (RS422) 2.0 V

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

Driver Common Mode Output Voltage R = 27Ω or 50Ω, VDD = 5V, Figure 1 ● 23V

Mode Output Voltage for Complementary Output States

Input High Voltage D, DE, RE ● 2V Input Low Voltage D, DE, RE ● 0.8 V Input Current D, DE, RE ● –1 1 µA Input Current (A, B) VA, VB = 12V, VDD = 0V or 5.25V ● 500 µA

Differential Input Threshold Voltage –7V ≤ VCM ≤ 12V ● –0.3 0.3 V for Receiver

Receiver Input Hysteresis VCM = 0V 25 mV Receiver Output High Voltage I

= 0 ● V

OUT

R = 27Ω (RS485), Figure 1 ● 1.5 V

, VB = –7V, VDD = 0V or 5.25V ● –500 µA

= –4mA, VID = 300mV ● 3.5 4.8 V

OUT

VDD = 5V ± 5% unless otherwise noted (Notes 2, 3).

Page 3

LTC1686/LTC1687

DC ELECTRICAL CHARACTERISTICS

VDD = 5V ±5% unless otherwise noted (Notes 2, 3).

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

V I

OZR

Receiver Output Low Voltage I

= 4mA, VID = –300mV ● 0.4 V

OUT

Three-State (High Impedance) Output 0.4V ≤ V

≤ 2.4V ● –1 1 µA

OUT

Current at Receiver

OZD

Three-State (High Impedance) Output V

= –7V to 12V ● –200 200 µA

OUT

Current at Driver

LOAD

OSD1

OSD2

OSR

Receiver and Driver Output Load Capacitance (Note 4) ● 500 pF Supply Current No Load, Pins D, DE, RE = 0V or V Driver Short-Circuit Current, V Driver Short-Circuit Current, V Receiver Short-Circuit Current V

= HIGH V

OUT

= LOW V

OUT

= –7V or 10V (Note 5) ● 20 mA

OUT

= –7V or 10V (Note 5) ● 20 mA

OUT

= 0V or V

(Note 5) ● 20 mA

● 712 mA

Input Resistance –7V ≤ VCM ≤ 12V ● 22 kΩ Input Capacitance A, B, D, DE, RE Inputs (Note 4) 3 pF Open-Circuit Input Voltage VDD = 5V (Note 4), Figure 5 ● 3.2 3.3 3.4 V

Fail-Safe Time to Detect Fail-Safe Condition 2 µs Time

CMRR Receiver Input Common Mode VCM = 2.5V, f = 26MHz 45 dB

Rejection Ratio

SWITCHING CHARACTERISTICS

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

, t

PLH

SKEW

tr, t

PLH

SQD

PKG-PKG

Driver Input-to-Output Propagation Delay R

PHL

Figures 3, 5

Driver Output A-to-Output B Skew R

Figures 3, 5

Driver Rise/Fall Time R

Figures 3, 5 Driver Enable to Output High CL = 100pF, S2 Closed, Figures 4, 6 ● 25 50 ns Driver Enable to Output Low CL = 100pF, S1 Closed, Figures 4, 6 ● 25 50 ns Driver Disable from Low CL = 15pF, S1 Closed, Figures 4, 6 ● 25 50 ns Driver Disable from High CL = 15pF, S2 Closed, Figures 4, 6 ● 25 50 ns

, t

Receiver Input-to-Output Propagation Delay CL = 15pF, Figures 3, 7 ● 15 18.5 22 ns

PHL

Receiver Skew t

PLH

– t

 C

PHL

Receiver Enable to Output Low CL = 15pF, S1 Closed, Figures 2, 8 ● 25 50 ns Receiver Enable to Output High CL = 15pF, S2 Closed, Figures 2, 8 ● 25 50 ns Receiver Disable from Low CL = 15pF, S1 Closed, Figures 2, 8 ● 25 50 ns Receiver Disable from High CL = 15pF, S2 Closed, Figures 2, 8 ● 25 50 ns Maximum Receiver Input (Note 4) ● 2000 ns

Rise/Fall Times Package-to-Package Skew CL = 15pF, Same Temperature (Note 4) 1.5 ns Minimum Input Pulse Width VDD = 5V ±5% (Note 4) ● 17 19.2 ns Maximum Data Rate VDD = 5V ±5% (Note 4) ● 52 60 Mbps Maximum Input Frequency VDD = 5V ±5% (Note 4) ● 26 30 MHz

VDD = 5V, unless otherwise noted (Notes 2, 3).

= 54Ω, CL1 = CL2 = 100pF, ● 15 18.5 22 ns

DIFF

= 54Ω, CL1 = CL2 = 100pF, 500 ps

DIFF

= 54Ω, CL1 = CL2 = 100pF, 3.5 ns

DIFF

= 15pF, Figures 3, 7 500 ps

Page 4

LTC1686/LTC1687

TEMPERATURE (°C)

–25

SUPPLY CURRENT (mA)

100

1686/87 G03

025 75

BOTH DRIVER AND RECEIVER ENABLED AND LOADED 25Mbps DATA RATE

RECEIVER INPUT OVERDRIVE (V)

0.3 0.5

RECEIVER PROPAGATION DELAY (ns)

0.7

1.25

1.5

1686/87 G06

1.0

2.0

2.5

TA = 25°C

ELECTRICAL CHARACTERISTICS

The ● denotes specifications which apply over the full operating temperature range.

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

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

Note 3: All typicals are given for V Note 4: Guaranteed by design, but not tested. Note 5: Short-circuit current does not represent output drive capability.

When the output detects a short-circuit condition, output drive current is significantly reduced (from hundreds of mA to 20mA max) until the short is removed.

TYPICAL PERFORMANCE CHARACTERISTICS

Receiver Input CMRR

46.5

46.0

45.5

45.0

44.5

44.0

43.5

43.0

42.5

COMMON MODE REJECTION RATIO (dB)

TA = 25°C

42.0 10

1k 100k 1M

FREQUENCY (Hz)

1686/87 G01

Supply Current vs Data Rate

BOTH DRIVER AND RECEIVER ENABLED AND LOADED

= 25°C

SUPPLY CURRENT (mA)

10 20 50

DATA RATE (Mbps)

4030

1686/87 G02

= 5V, TA = 25°C.

Supply Current vs Temperature

Receiver Propagation Delay vs Load Capacitance

TA = 25°C

PROPAGATION DELAY (ns)

LOAD CAPACITANCE (pF)

25 35 55

105 205

1686/87 G04

Receiver Propagation Delay vs Common Mode

TA = 25°C

PROPAGATION DELAY (ns)

–4 0

–2

–7

RECEIVER COMMON MODE (V)

Receiver Propagation Delay vs Input Overdrive

412

1686/87 G05

Page 5

TEMPERATURE (°C)

–20

PROPAGATION DELAY (ns)

20 40 60

1686/87 G07

80 100

TYPICAL PERFORMANCE CHARACTERISTICS

LTC1686/LTC1687

Receiver Propagation Delay vs Temperature

PROPAGATION DELAY (ns)

–50 –25

TEMPERATURE (°C)

Driver Propagation Delay vs Driver Input Voltage

PROPAGATION DELAY (ns)

VDD = 5V INPUT THRESHOLD = 1.5V

= 25°C

100

1686/87 G09

125

Receiver Maximum Data Rate vs Input Overdrive

TA = 25°C



DATA RATE (Mbps)

0.3

0.4 0.5 RECEIVER INPUT DIFFERENTIAL (V)

0.6 1.0

Driver Propagation Delay vs Temperature

0.7 1.5 2.5

1686/87 G10

Driver Propagation Delay vs Capacitive Load

19.0 TA = 25°C

18.5

18.0

17.5

17.0

PROPAGATION DELAY (ns)

16.5

PIN FUNCTIONS

LTC1686 V

(Pin 1): Positive Supply, 5V to ± 5%. Bypass with

0.1µF ceramic capacitor. R (Pin 2): Receiver Output. If A ≥ B by 300mV, then R will

be high. If A ≤ B by 300mV, then R will be low. D (Pin 3): Driver Input. Controls the states of the Y and Z

outputs. Do not float.

GND

(Pin 4): Ground. Y (Pin 5): Noninverting Driver Output. Z (Pin 6): Inverting Driver Output.

2.5

3.0 DRIVER INPUT VOLTAGE (V)

3.5

4.0

UUU

4.5

1686/87 G08

5.0

16.0 5

25 50 75

LOAD CAPACITANCE (pF)

100 150

1686/87 G11

B (Pin 7): Inverting Receiver Input. A (Pin 8): Noninverting Receiver Input.

LTC1687 NC (Pins 1, 8, 13): No Connection.

R (Pin 2): Receiver Output. If A ≥ B by 300mV, then R will

be high. If A ≤ B by 300mV, then R will be low. RE (Pin 3): Receiver Enable. RE = low enables the receiver.

RE = high forces receiver output into high impedance state. Do not float.

Page 6

LTC1686/LTC1687

UUU

PIN FUNCTIONS

DE (Pin 4): Driver Enable. DE = high enables the driver. DE = low will force the driver output into a high impedance state. Do not float.

D (Pin 5): Driver Input. Controls the states of the Y and Z outputs when DE = high. Do not float.

GND (Pins 6, 7): Ground. Y (Pin 9): Noninverting Driver Output.

U U

FU CTIO TABLES

Transmitting

INPUTS LINE OUTPUTS

RE DE D CONDITION Z Y

X 1 1 No Fault 0 1 X 1 0 No Fault 1 0 X 0 X X Hi- Z Hi- Z X 1 X Fault ±10mA Current

(LTC1687)

Source

Z (Pin 10): Inverting Driver Output. B (Pin 11): Inverting Receiver Input. A (Pin 12): Noninverting Receiver Input. V

(Pin 14): Positive Supply, 5V to ± 5%. Bypass with

0.1µF ceramic capacitor.

Receiving

INPUTS OUTPUT

RE DE A – B R

0X ≥ 300mV 1 0X ≤ –300mV 0 0 X Inputs Open 1 0 X Inputs Shorted Together 1

A = B = –7V to 12V

1 X X Hi- Z



TEST CIRCUITS

Figure 1. Driver DC Test Load

DIFF

Figure 3. Driver/Receiver Timing Test Circuit Figure 4. Driver Timing Test Load #2

1686/87 • F01

RECEIVER

OUTPUT

TEST POINT

15pF



1686/87 F02

Figure 2. Driver DC Test Load

15pF

1686/87 F03

OUTPUT

UNDER TEST

500Ω

1686/87 F04

Page 7

UW W

SWITCHI G TI E WAVEFOR S

LTC1686/LTC1687

Y, Z

–V

1/2 V

1.5V

10%

f = 1MHz, tr ≤ 3ns, tf ≤ 3ns

PLH

SKEW

90%

= V(Y) – V(Z)

DIFF

1.5V

1/2 V

PHL

SKEW

90%

10%

1686/87 F05

Figure 5. Driver Propagation Delays

1.5V

2.5V

f = 1MHz, tr ≤ 3ns, tf ≤ 3ns

OUTPUT NORMALLY LOW

OUTPUT NORMALLY HIGH

1.5V

0.5V

1686/87 F06

A – B

Figure 6. Driver Enable and Disable Times

–V

OD2

PHL

2.5V

f = 1MHz, tr ≤ 3ns, tf ≤ 3ns

OUTPUT

INPUT

PLH

2.5V

1686/87 F07

Figure 7. Receiver Propagation Delays

1.5V

f = 1MHz, tr ≤ 3ns, tf ≤ 3ns

2.5V

OUTPUT NORMALLY LOW

OUTPUT NORMALLY HIGH

1.5V

0.5V

1686/87 F08

Figure 8. Receiver Enable and Disable Times

Page 8

LTC1686/LTC1687

U

EQUIVALE T I PUT NETWORKS

≥22k

RE = 0 OR 1, V

WUU

3.3V

= 5V VDD = 0V

Figure 9. Input Thevenin Equivalent

APPLICATIONS INFORMATION

THEORY OF OPERATION

Unlike typical CMOS transceivers whose propagation delay can vary by as much as 500% from package to package and show significant temperature drift, the LTC1686/LTC1687 employ a novel architecture that produces a tightly controlled and temperature compensated propagation delay. The differential timing skew is also minimized between rising and falling output edges of the receiver output and the complementary driver outputs.

The precision timing features of the LTC1686/LTC1687 reduce overall system timing constraints by providing a narrow ± 3.5ns window during which valid data appears at the receiver/driver output. The driver and receiver will have propagation delays that typically match to within 1ns.

In clocked data systems, the low skew minimizes duty cycle distortion of the clock signal. The LTC1686/LTC1687 can be used at data rates of 52Mbps with less than 5% duty cycle distortion (depending on cable length). When a clock signal is used to retime parallel data, the maximum recommended data transmission rate is 26Mbps to avoid timing errors due to clock distortion.

FAIL-SAFE FEATURES

The LTC1686/LTC1687 have a fail-safe feature that guarantees the receiver output to be in a logic HIGH state when the inputs are either shorted or left open (note that when inputs are left open, large external leakage currents might override the fail-safe circuitry). In order to maintain good

≥22k

1686/87 F09

high frequency performance, it is necessary to slow down the transient response of the fail-safe feature. When a line fault is detected, the output will go HIGH typically in 2µs.

Note that the LTC1686/LTC1687 guarantee receiver failsafe performance over the

entire

(–7V to 12V) common

mode range!

When the inputs are accidentally shorted (by cutting through a cable, for example), the short circuit fail-safe feature will guarantee a high output logic level. Note also that if the line driver is removed and the ground terminated resistors are left in place, the receiver will see this as a “short” and output a logic HIGH. Both of these fail-safe features will keep the receiver from outputting false data pulses under line fault conditions.

Thermal shutdown and short-circuit protection prevent latchup damage to the LTC1686/LTC1687 during fault conditions.

OUTPUT SHORT-CIRCUIT PROTECTION

The LTC1686/LTC1687 employ voltage sensing shortcircuit protection at the output terminals of both the driver and receiver. For a given input polarity, this circuitry determines what the correct output level should be. If the output level is different from the expected, it shuts off the big output devices. For example, if the driver input is >2V, it expects the “A” output to be >3.25V and the “B” output to be <1.75V. If the “A” output is subsequently shorted to a voltage below VDD/2, this circuitry shuts off the big output devices and turns on a smaller device in its place

Page 9

LTC1686/LTC1687

WUU

APPLICATIONS INFORMATION

(the converse applies for the “B” output). The outputs then appear as ±10mA current sources. Note that under normal operation, the output drivers can sink/source >50mA. A time-out period of about 50ns is used in order to maintain normal high frequency operation, even under heavy capacitive loads.

If the cable is shorted at a large distance from the device outputs, it is possible for the short to go unnoticed at the driver outputs due to parasitic cable resistance. Additionally, when the cable is shorted, it no longer appears as a simple transmission line impedance, and the parasitic L’s and C’s might give rise to ringing and even oscillation. All these conditions disappear once the device comes out of short-circuit mode.

For cables with the typical RS485 termination (no DC bias on the cable, such as Figure 10), the LTC1686/LTC1687 will automatically come out of short-circuit mode once the physical short has been removed.

Cable Termination

The recommended cable termination for the LTC1686/ LTC1687 is a single resistor across the two wires at each end of the twisted-pair line (see Figure 10). The LTC1687 can also be used with cable terminations with a DC bias (such as Fast-20 and Fast-40 differential SCSI terminators). When using a biased termination with the LTC1687, however, the DE pin must be held low for at least 200ns after the part has been powered up. This ensures proper start-up into the DC load of the biased termination. Furthermore, when the LTC1687 output is shorted, the DE pin

should be pulsed low for at least 200ns after the short has been removed. Since the LTC1686 driver is always enabled, the LTC1686 should only be used with single resistor termination, as shown in Figure 10.

HIGH SPEED TWISTED-PAIR TRANSMISSION

Data rates up to 52Mbps can be transmitted over 100 feet of category 5 twisted pair. Figure 10 shows the LTC1687 receiving differential data from another LTC1687 transceiver. Figure 11a shows a 26MHz (52Mbps) square wave propagated over 100 feet of category 5 UTP. Figure 11b shows a more stringent case of propagating a 20ns pulse over 100 feet of category 5 UTP. Figure 12 shows a 2MHz (4Mbps) square wave propagated over 1000 feet of category 5 unshielded twisted pair. Note that the LTC1686/ LTC1687 can still perform reliably at this distance and speed. Very inexpensive unshielded telephone grade twisted pair is shown in Figure 13. Despite the noticeable loss at the receiver input, the LTC1686/LTC1687 can still transfer at 30Mbps over 100 feet of telephone grade UTP. Note that under all these conditions, the LTC1686/LTC1687 can pass through a single data pulse equal to the inverse of the data rate (e.g., 20ns for 50Mbps data rate).

TRANSMISSION OVER LONG DISTANCES

1Mbps Over 4000 Feet Category 5 UTP

The LTC1685/LTC1686/LTC1687 family of high speed transceivers is capable of 1Mbps transmission over 4000 feet of category 5 UTP. High quality cable provides lower

DRIVER

LTC1687

RECEIVER

100Ω 100Ω

CATEGORY 5 UTP

Figure 10

RECEIVER

LTC1687

DRIVER

LTC1686/87 • F10

Page 10

LTC1686/LTC1687

WUU

APPLICATIONS INFORMATION

1686/87 F11a

DRIVER INPUT

RECEIVER OUTPUT

DRIVER INPUT

RECEIVER INPUT

2V/DIV

10ns/DIV

Figure 11a. 100 Feet of Category 5 UTP: 50Mbps

2V/DIV

CABLE DELAY

1686/87 F13

DRIVER INPUT

DIFFERENTIAL RECEIVER INPUT

RECEIVER OUTPUT

2V/DIV

20ns/DIV

Figure 13. 100 Feet of Telephone Grade UTP: 30Mbps

2V/DIV

1V/DIV

CABLE DELAY

DRIVER INPUT

RECEIVER INPUT

1685 F11b

RECEIVER OUTPUT

5V/DIV

20ns/DIV

Figure 11b. 100 Feet of Category 5 UTP: 20ns Pulse

1686/87 F12

DRIVER INPUT

RECEIVER OUTPUT

2V/DIV

100ns/DIV

Figure 12. 1000 Feet of Category 5 UTP: 4Mbps

DC and AC attenuation over long distances. Figure 14a shows a 1µ s pulse propagated down 4000 feet of category 5 UTP. Notice the significant attenuation at the receiver input and the clean pulse at the receiver output. The DC attenuation is due to the parasitic resistance of the cable. Figure 14b shows a 1Mbps square wave over the same 4000 feet of cable.

1685 F14a

RECEIVER OUTPUT

5V/DIV

1µs/DIV

Figure 14a. 4000 Feet of Category 5 UTP 1µs Pulse

1685 F14b

DRIVER INPUT

RECEIVER OUTPUT

2V/DIV

5V/DIV

1µs/DIV

Figure 14b. 4000 Feet of Category 5 UTP 1Mbps Square Wave

1.6Mbps Over 8000 Feet (1.5 Miles) Category 5 UTP Using Repeaters

The LTC1686/LTC1687 can be used as repeaters to extend the effective length of a high speed twisted-pair line. Figure 15a shows a three repeater configuration using 2000 feet segments of category 5 UTP. Figure 15b shows the

Page 11

LTC1686/LTC1687

WUU

APPLICATIONS INFORMATION

LTC1687

2000 FT 2000 FT 2000 FT 2000 FT

2V/DIV

5V/DIV

2V/DIV

5V/DIV

DELAY OF 8000 FT

OF CABLE

Figure 15b. 1.6Mbps Pulse and Square Wave Signals Over 8000 Feet Category 5 UTP Using Three Repeaters

LTC1687

REPEATER

Figure 15a. 1.6Mbps, 8000 Feet (1.5 Miles) Using Three Repeaters

DRIVER 1 INPUT

RECEIVER 5 OUTPUT

DRIVER 1 INPUT

RECEIVER 5 OUTPUT

2µs/DIV

1686/87 F15b

LTC1687

REPEATER

LTC1687

D R5R4DR3DR2D1

REPEATER

1686/87 F15a

1686/87 F16

DRIVER 1 INPUT

RECEIVER 2 INPUT RECEIVER 3 INPUT RECEIVER 4 INPUT

RECEIVER 5 OUTPUT

2V/DIV

1V/DIV

5V/DIV

2µs/DIV

Figure 16. Intermediate Signals of a 1µs Pulse

propagation of a 600ns pulse through the network of Figure 15A. The bottom two traces show a 1.6Mbps square wave. Notice that the duty cycle does not noticeably degrade. For the case of the single pulse, however, there is a slight degradation of the pulse width.

By slowing down the data rate slightly to 1Mbps, one can obtain minimal pulse width degradation as the signal traverses through the repeater network. Figure 16 shows that the output pulse (bottom trace) is nearly the same width to the input pulse (top trace). The middle three traces of Figure 16 show the signal at the end of each of the first three 2000 feet sections of category 5 UTP. Notice how the LTC1687 repeaters are able to regenerate the signal with little loss. This implies that we can cascade more repeater networks and potentially achieve 1Mbps operation at total distances of over 10,000 feet! A higher data rate can be achieved if the repeaters are spaced closer together.

HIGH SPEED BACKPLANE TRANSMISSION

The LTC1686/LTC1687 can also be used in backplane point-to-point transceiver applications, where the user wants to assure operation even when the common mode

goes above or below the rails. It is advisable to terminate the PC traces when approaching maximum speeds. Since the LTC1686/LTC1687 are not intended to drive parallel terminated cables with characteristic impedances much less than that of twisted pair, both ends of the PC trace must be

series terminated

with the characteristic impedance of the trace. For best results, the signal should be routed differentially. The true and complement outputs of the LTC1686/LTC1687 should be routed on adjacent layers of the PC board. The two traces should be routed very symmetrically, minimizing and equalizing parasitics to nearby signal and power/ground layers. For single-ended transmission, route the series terminated single-ended trace over an adjacent ground plane. Then set the (bypassed) negative input of the receiver to roughly 2.5V. Note that single-ended operation might not reach maximum speeds.

LAYOUT CONSIDERATIONS

A ground plane is recommended when using high frequency devices like the LTC1686/LTC1687. A 0.1µF ce- ramic bypass capacitor less than 0.25 inch away from the VDD pin is also recommended.

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

LTC1686/LTC1687

WUU

APPLICATIONS INFORMATION

Long traces bounded by a VDD and/or GND planes can add substantial parasitic capacitance. Parasitic capacitances on the receiver/driver outputs can also unduly slow down both the propagation delay and the rise/fall times.

PACKAGE DESCRIPTION

0.010 – 0.020

(0.254 – 0.508)

0.008 – 0.010

(0.203 – 0.254)

*

DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH  

SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE

DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD  FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE  

× 45°

0.016 – 0.050

0.406 – 1.270

Dimensions in inches (millimeters) unless otherwise noted.

S8 Package

8-Lead Plastic Small Outline (Narrow 0.150)

(LTC DWG # 05-08-1610)

0.053 – 0.069

(1.346 – 1.752)

0°– 8° TYP

0.014 – 0.019

(0.355 – 0.483)

S Package

14-Lead Plastic Small Outline (Narrow 0.150)

(LTC DWG # 05-08-1610)

The receiver inputs are high bandwidth and high impedance. If they are left floating, any capacitive coupling from any other signal can cause a glitch at the receiver output. Thus, if the receiver is not being used, it is advisable to always ground at least one of the two receiver input pins.

0.189 – 0.197* (4.801 – 5.004)

0.004 – 0.010

(0.101 – 0.254)

0.050

(1.270)

TYP

0.228 – 0.244

(5.791 – 6.197)

0.150 – 0.157** (3.810 – 3.988)

SO8 0996

0.337 – 0.344*

0.010 – 0.020

(0.254 – 0.508)

0.008 – 0.010

(0.203 – 0.254)

DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE

*

DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE

 

× 45°

0.016 – 0.050

0.406 – 1.270

0° – 8° TYP

0.053 – 0.069

(1.346 – 1.752)

0.014 – 0.019

(0.355 – 0.483)

0.050

(1.270)

TYP

0.004 – 0.010

(0.101 – 0.254)

0.228 – 0.244

(5.791 – 6.197)

(8.560 – 8.738)

11 10

0.150 – 0.157**

RELATED PARTS

PART NUMBER DESCRIPTION COMMENTS

LTC490 Low Power RS485 Full-Duplex Transceiver ICC = 300µA (Typ), SO-8 Package LTC491 Low Power RS485 Full-Duplex Transceiver ICC = 300µA (Typ), 14-Lead SO Package LTC1518 High Speed Quad RS485 Receiver 52Mbps, Pin Compatible with LTC488 LTC1519 High Speed Quad RS485 Receiver 52Mbps, Pin Compatible with LTC489 LTC1520 High Speed Quad Differential Receiver 52Mbps, ±100mV Threshold, Rail-to-Rail Common Mode LTC1685 High Speed RS485 Transceiver 52Mbps, Pin Compatible with LTC485

16867f LT/TP 1197 4K • PRINTED IN THE USA

 LINEAR TECHNOLOGY CORPORATION 1997

Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417 ● (408) 432-1900 FAX: (408) 434-0507

●

TELEX: 499-3977 ● www.linear-tech.com

(3.810 – 3.988)

S14 0695