Datasheet

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LTC1487

sn1487 1487fs

Ultra-Low Power RS485

with Low EMI, Shutdown

and High Input Impedance

ESCRIPTIO

The LTC

1487 is an ultra-low power differential line trans-

ceiver designed with high impedance inputs allowing up to

256 transceivers to share a single bus. It meets the

requirements of RS485 and RS422. The LTC1487 features

output drivers with controlled slew rate, decreasing the

EMI radiated from the RS485 lines, and improving signal

fidelity with misterminated lines. The CMOS design offers

significant power savings without sacrificing ruggedness

against overload or ESD damage. Typical quiescent cur-

rent is only 80µA while operating and 1µA in shutdown.

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. I/O pins are protected against multiple

ESD strikes of over ±10kV using the Human Body Model.

The LTC1487 is fully specified over the commercial tem-

perature range and is available in 8-pin DIP and SO

packages.

■

High Input Impedance: Up to 256 Transceivers

on the Bus

■

Low Power: I

= 120µA Max with Driver Disabled

■

= 200µA Max with Driver Enabled, No Load

■

1µA Quiescent Current in Shutdown Mode

■

Controlled Slew Rate Driver for Reduced EMI

■

Single 5V Supply

■

ESD Protection to ±10kV On Receiver Inputs and

Driver outputs

■

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

Ground Difference Between Devices on the Data Line

■

Thermal Shutdown Protection

■

Power Up/Down Glitch-Free Driver Outputs Permit

Live Insertion or Removal of Transceiver

■

Driver Maintains High Impedance in Three-State

or with the Power Off

■

Pin Compatible with the LTC485

ITY

■

Battery-Powered RS485/RS422 Applications

■

Low Power RS485/RS422 Transceiver

■

Level Translator

120Ω

LTC1487 • TA01

120Ω

330Ω

LTC1487 LTC1487

EQUIVALENT LOAD OF 256

LTC1487 TRANSCEIVERS

2000 FEET OF TWISTED-PAIR WIRE

4.7nF

RECEIVER INPUT

LTC1487 • TA02

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

Summary of content (8 pages)

PAGE 1
LTC1487 Ultra-Low Power RS485 with Low EMI, Shutdown and High Input Impedance U DESCRIPTIO FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ The LTC®1487 is an ultra-low power differential line transceiver designed with high impedance inputs allowing up to 256 transceivers to share a single bus. It meets the requirements of RS485 and RS422. The LTC1487 features output drivers with controlled slew rate, decreasing the EMI radiated from the RS485 lines, and improving signal fidelity with misterminated lines.
PAGE 2
LTC1487 U U RATI GS W W W W AXI U U ABSOLUTE PACKAGE/ORDER I FOR ATIO (Note 1) Supply Voltage (VCC) .............................................. 12V Control Input Voltage ..................... – 0.5V to VCC + 0.5V Driver Input Voltage ....................... – 0.5V to VCC + 0.5V Driver Output Voltage ........................................... ±14V Receiver Input Voltage .......................................... ±14V Receiver Output Voltage ................ – 0.5V to VCC + 0.
PAGE 3
LTC1487 ELECTRICAL CHARACTERISTICS – 40°C ≤ TA ≤ 85°C, VCC = 5V (Note 4) unless otherwise noted. SYMBOL PARAMETER CONDITIONS VOD1 Differential Driver Output Voltage (Unloaded) IO = 0 ● MIN VOD2 Differential Driver Output Voltage (with Load) R = 50Ω (RS422) R = 27Ω (RS485), Figure 1 ● ● TYP 2.0 1.
PAGE 4
LTC1487 U SWITCHI G CHARACTERISTICS 0°C ≤ TA ≤ 70°C, VCC = 5V (Notes 2, 3) unless otherwise noted.
PAGE 5
LTC1487 U U U PIN FUNCTIONS RO (Pin 1): Receiver Output. If the receiver output is enabled (RE LOW), and A > B by 200mV, RO will be HIGH. If A < B by 200mV, then RO will be LOW. RE (Pin 2): Receiver Output Enable. A LOW enables the receiver output, RO. A HIGH input forces the receiver output into a high impedance state. DE (Pin 3): Driver Outputs Enable. A HIGH on DE enables the driver output. A and B and the chip will function as a line driver.
PAGE 6
LTC1487 U W W SWITCHI G TI E WAVEFOR S 3V f = 1MHz, tr ≤ 10ns, tf ≤ 10ns 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% 10% VDIFF = V(A) – V(B) 10% tr LTC1487 • F05 tf Figure 5. Driver Propagation Delays 3V f = 1MHz, tr ≤ 10ns, tf ≤ 10ns 1.5V DE 1.5V 0V t LZ t ZL(SHDN), t ZL 5V A, B 2.3V OUTPUT NORMALLY LOW 0.5V 2.3V OUTPUT NORMALLY HIGH 0.5V VOL VOH A, B 0V t HZ t ZH(SHDN), t ZH LTC1487 • F06 Figure 6.
PAGE 7
LTC1487 U U W U APPLICATIO S I FOR ATIO High Input Impedance VCC The LTC1487 is designed with a 96kΩ (typ) input impedance to allow up to 256 transceivers to share a single RS485 differential data bus. The RS485 specification requires that a transceiver be able to drive as many as 32 “unit loads.” One unit load (UL) is defined as an impedance that draws a maximum of 1mA with up to 12V across it. Typical RS485 transceivers present between 0.5 and 1 unit load at their inputs.
PAGE 8
LTC1487 U U W U APPLICATIO S I FOR ATIO effects of noise on the line signals. If the line is terminated or the receiver inputs are shorted together, the receiver output will retain the last valid line signal due to the 45mV of hysteresis incorporated in the receiver circuit. If the LTC1487 transceiver inputs are left floating (unterminated), an internal pull-up of 10µA at the A input will force the receiver output to a known high state. In shutdown the LTC1487 typically draws only 1µA of supply current.