Datasheet

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LTC491

491fa

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Low Power: I

= 300µA Typical

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Designed for RS485 or RS422 Applications

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Single 5V Supply

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–7V to 12V Bus Common Mode Range

Permits ±7V Ground Difference Between Devices

on the Bus

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Thermal Shutdown Protection

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Power-Up/-Down Glitch-Free Driver Outputs Permit

Live Insertion or Removal of Package

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Driver Maintains High Impedance in Three-State or

with the Power Off

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Combined Impedance of a Driver Output and

Receiver Allows up to 32 Transceivers on the Bus

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70mV Typical Input Hysteresis

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28ns Typical Driver Propagation Delays with 5ns

Skew for 2.5MB Operation

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Pin Compatible with the SN75180

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Available in 14-Lead PDIP and SO Packages

Differential Driver and

Receiver Pair

The LTC

491 is a low power differential bus/line trans-

ceiver 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.

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Low Power RS485/RS422 Transceiver

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Level Translator

LTC491 • TA01

120Ω 120Ω

4000 FT 24 GAUGE TWISTED PAIR

RECEIVER

LTC491

DRIVER

RECEIVER

LTC491

DRIVER

REB

FEATURES

DESCRIPTIO

APPLICATIO S

TYPICAL APPLICATIO

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

Summary of content (12 pages)

PAGE 1
LTC491 Differential Driver and Receiver Pair U FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ DESCRIPTIO 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
PAGE 2
LTC491 U RATI GS W W W 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 .........................................
PAGE 3
LTC491 U SWITCHI G CHARACTERISTICS The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C.
PAGE 4
LTC491 U W TYPICAL PERFOR A CE CHARACTERISTICS Driver Output High Voltage vs Output Current, TA = 25°C Driver Differential Output Voltage vs Output Current, TA = 25°C OUTPUT CURRENT (mA) OUTPUT CURRENT (mA) –72 – 48 –24 80 OUTPUT CURRENT (mA) 64 –96 48 32 16 0 0 0 1 2 3 1 2 3 4.0 1.59 1.57 3.0 1.0 –50 100 TEMPERATURE (°C ) 0 50 330 320 310 –50 100 6.0 TEMPERATURE (°C ) 5.0 3.0 –50 100 LTC491 • TPC06 0.8 0.6 0.4 0.
PAGE 5
LTC491 TEST CIRCUITS Y R VOD2 R VOC Z LTC491 • F01 Figure 1. Driver DC Test Load CL1 Y D A RDIFF DRIVER RECEIVER Z CL2 B R 15pF LTC491 • F02 Figure 2. Driver/Receiver Timing Test Circuit S1 RECEIVER OUTPUT 1k VCC CL 1k S2 LTC491 • F03 Figure 3. Receiver Timing Test Load S1 VCC OUTPUT UNDER TEST 500Ω CL S2 LTC491 • F04 Figure 4.
PAGE 6
LTC491 U W W SWITCHI G TI E WAVEFOR S 3V D f = 1MHz : tr ≤ 10ns : tf ≤ 10ns 1.5V 1.5V 0V tPHL tPLH VO 80% 50% 10% –VO 90% VDIFF = V(Y) – V(Z) tr 50% 20% tf Z VO Y tSKEW 1/2 VO tSKEW 1/2 VO LTC491 • F05 Figure 5. Driver Propagation Delays 3V DE f = 1MHz : tr ≤ 10ns : tr ≤ 10ns 1.5V 1.5V 0V tZL tLZ 5V A, B VOL 2.3V OUTPUT NORMALLY LOW 2.3V OUTPUT NORMALLY HIGH 0.5V VOH A, B 0.5V 0V tZH LTC491 • F06 tHZ Figure 6.
PAGE 7
LTC491 U W U U APPLICATIO S 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.
PAGE 8
LTC491 W U U UO APPLICATI S I FOR ATIO 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.
PAGE 9
LTC491 W U U UO APPLICATI S I FOR ATIO 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.
PAGE 10
LTC491 W U U UO APPLICATI S I FOR ATIO Fault Protection Y 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). A TransZorb is a silicon transient voltage suppressor that has exceptional surge handling capabilities, fast response time, and low series resistance.
PAGE 11
LTC491 U PACKAGE DESCRIPTIO S Package 14-Lead Plastic Small Outline (Narrow .150 Inch) (Reference LTC DWG # 05-08-1610) .337 – .344 (8.560 – 8.738) NOTE 3 .045 ±.005 .050 BSC 14 N 12 11 10 9 8 N .245 MIN .160 ±.005 .150 – .157 (3.810 – 3.988) NOTE 3 .228 – .244 (5.791 – 6.197) 1 .030 ±.005 TYP 13 2 3 N/2 N/2 RECOMMENDED SOLDER PAD LAYOUT 1 .010 – .020 × 45° (0.254 – 0.508) .008 – .010 (0.203 – 0.254) 2 3 4 5 6 .053 – .069 (1.346 – 1.752) .004 – .010 (0.101 – 0.
PAGE 12
LTC491 U TYPICAL APPLICATIO S RS232 Receiver RS232 to RS485 Level Transistor with Hysteresis R = 220k RS232 IN 5.6k Y RX RECEIVER 10k 1/2 LTC491 LTC491 • TA02 RS232 IN 120Ω DRIVER 5.6k 1/2 LTC491 Z VY - VZ  19k HYSTERESIS = 10kΩ • ———— ≈ ———— R R LTC491 • TA03 RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LTC486/LTC487 Low Power Quad RS485 Drivers 110µA Supply Current LTC488/LTC489 Low Power Quad RS485 Receivers 7mA Supply Current LTC1480 3.