Datasheet

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LTC1685

FEATURES

APPLICATIO S

TYPICAL APPLICATION

DESCRIPTIO

52Mbps, Precision Delay,

RS485 Fail-Safe Transceiver

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Precision Propagation Delay Over Temperature:

Receiver/Driver: 18.5ns

3.5ns

■

High Data Rate:

52Mbps

■

Low t

PLH

PHL

Skew:

Receiver/Driver: 500ps Typ

■

–7V to 12V RS485 Input Common Mode Range

■

Guaranteed Fail-Safe Receiver Operation Over the

Entire Common Mode Range

■

High Receiver Input Resistance: ≥22k, Even When

Unpowered

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Short-Circuit Protected

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Thermal Shutdown Protected

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Driver Maintains High Impedance in Three-State or

with Power Off

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Single 5V Supply

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Pin Compatible with LTC485

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45dB CMRR at 26MHz

The LTC

1685 is a high speed, precision delay RS485

transceiver that can operate at data rates as high as 52Mbps.

The device also meets 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 LTC1685 RS485 transceiver guarantees receiver fail-

safe operation over the

entire

common mode range (–7V

to 12V). Input resistance will remain ≥22k when the device

is unpowered or disabled.

The LTC1685 operates from a single 5V supply and draws

only 7mA of supply current.

10Mbps Data Pulse

400ft Category 5 UTP

1685 TA02

DRIVER INPUT

RECEIVER

INPUT

RECEIVER

OUTPUT

100ns/DIV

1V/DIV

2V/DIV

5V/DIV

CABLE DELAY

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

■

High Speed RS485/RS422 Transceivers

■

Level Translator

■

Backplane Transceiver

■

STS-1/OC-1 Data Transceiver

■

Fast-20, Fast-40 SCSI Transceivers

CC1

GND1

RO1

RE1

DE1

DI1

1685 TA01

CC2

GND2

RO2

RE2

DE2

DI2

Summary of content (12 pages)

PAGE 1
LTC1685 52Mbps, Precision Delay, RS485 Fail-Safe Transceiver U DESCRIPTIO FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ Precision Propagation Delay Over Temperature: Receiver/Driver: 18.5ns ±3.
PAGE 2
LTC1685 U U RATI GS W W W W AXI U U ABSOLUTE PACKAGE/ORDER I FOR ATIO (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 .................................
PAGE 3
LTC1685 DC ELECTRICAL CHARACTERISTICS The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VDD = 5V ±5%, unless otherwise noted.
PAGE 4
LTC1685 U SWITCHING CHARACTERISTICS 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. Note 1: Absolute Maximum Ratings are those values beyond which the life of a device may be impaired.
PAGE 5
LTC1685 U W TYPICAL PERFORMANCE CHARACTERISTICS Receiver Propagation Delay vs Temperature Receiver Maximum Data Rate vs Input Overdrive 25 Driver Propagation Delay vs Temperature 25 70 TA = 25°C 15 10 PROPAGATION DELAY (ns) DATA RATE (Mbps) PROPAGATION DELAY (ns) 60 20 50 40 30 20 5 –25 50 25 0 75 TEMPERATURE (°C) 100 125 0 0.3 1.5 1.0 0.4 0.5 0.6 0.7 RECEIVER INPUT DIFFERENTIAL (V) 1680 G09 0 – 40 – 20 2.
PAGE 6
LTC1685 U U FU CTIO TABLES Receiving Transmitting RE INPUTS DE DI LINE CONDITION B A RE INPUTS DE A–B OUTPUT RO X 1 1 No Fault 0 1 0 0 ≥ 300mV 1 X 1 0 No Fault 1 0 0 0 ≤ – 300mV 0 X 0 X X Hi-Z Hi-Z 0 0 Inputs Open 1 X 1 X Fault 0 0 Inputs Shorted Together A = B = – 7V to 12V 1 1 X X Hi-Z OUTPUTS ±10mA Current Source TEST CIRCUITS A RECEIVER OUTPUT 1k VDD VOD R S1 TEST POINT R CL 15pF VOC 1k S2 1685 F02 B 1685 F01 Figure 2.
PAGE 7
LTC1685 U W W SWITCHI G TI E WAVEFOR S 3V f = 1MHz, t r ≤ 3ns, t f ≤ 3ns 1.5V DI 1.5V 0V t PLH 1/2 VO t PHL B VO A tSKEW 1/2 VO VO 0V –VO t SKEW 90% 90% VDIFF = V(A) – V(B) 10% tr 10% 1586 F05 tf Figure 5. Driver Propagation Delays 3V f = 1MHz, t r ≤ 3ns, t f ≤ 3ns 1.5V DE 1.5V 0V 5V t ZL A, B t LZ 2.5V OUTPUT NORMALLY LOW 0.5V 2.5V OUTPUT NORMALLY HIGH 0.5V VOL VOH A, B 0V t HZ t ZH 1686 F06 Figure 6. Driver Enable and Disable Times VOH 2.
PAGE 8
LTC1685 U U U EQUIVALENT INPUT NETWORKS ≥22k ≥22k A 3.3V A ≥22k ≥22k B B 3.3V DE = 0, RE = 0 OR 1 VDD = 5V VDD = 0V 1685 F09 Figure 9.
PAGE 9
LTC1685 U U W U APPLICATIONS INFORMATION Output Short-Circuit Protection The LTC1685 employs voltage sensing short-circuit 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.
PAGE 10
LTC1685 U W U U APPLICATIONS INFORMATION 2V/DIV DRIVER INPUT 2V/DIV 1V/DIV RECEIVER INPUT 5V/DIV RECEIVER OUTPUT RECEIVER OUTPUT 2V/DIV 1µs/DIV 100ns/DIV 1685 F14a 1685 F12 Figure 12. 1000ft of Category 5 UTP: 4Mbps Figure 14a. 4000ft of Category 5 UTP: 1µs Pulse DRIVER INPUT 2V/DIV 2V/DIV DIFFERENTIAL RECEIVER INPUT 2V/DIV RECEIVER OUTPUT 20ns/DIV 1685 F13 Figure 13.
PAGE 11
LTC1685 U W U U APPLICATIONS INFORMATION 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. High Speed Differential SCSI (Fast-20, Fast-40 HVD) The LTC1685’s high speed, tight propagation delay window and matched driver/receiver propagation delays make it a natural choice as the external transceiver in high speed differential SCSI applications. Note that the ±3.
PAGE 12
LTC1685 U W U U APPLICATIONS INFORMATION parasitic board capacitance. This maintains the good matching characteristics of the low-to-high and high-tolow transitions of the LTC1685. Note that output “A” to output “B” capacitance should also be minimized. If routed adjacent to each other on the same layer, they should be separated by an amount at least as wide as the trace widths.