Datasheet

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LTC2854/LTC2855

285455fb

FEATURES DESCRIPTION

APPLICATIONS

TYPICAL APPLICATION

PRODUCT SELECTION GUIDE

3.3V 20Mbps RS485/RS422

Transceivers with Integrated

Switchable Termination

The LTC

2854 and LTC2855 are low power, 20Mbps

RS485/RS422 transceivers operating on 3.3V supplies.

The receiver includes a logic-selectable 120Ω termina-

tion, one-eighth unit load supporting up to 256 nodes per

bus (C, I-Grade), and a failsafe feature that guarantees a

high output state under conditions of ﬂ oating or shorted

inputs.

The driver maintains a high output impedance over the

entire common mode range when disabled or when the

supply is removed. Excessive power dissipation caused by

bus contention or a fault is prevented by current limiting

all outputs and by a thermal shutdown.

Enhanced ESD protection allows the LTC2854 to withstand

±25kV (human body model) and the LTC2855 to withstand

±15kV on the transceiver interface pins without latchup

or damage.

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Low Power RS485/RS422 Transceiver

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Level Translator

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Backplane Transceiver

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Integrated, Logic-Selectable 120Ω Termination

Resistor

■

3.3V Supply Voltage

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20Mbps Maximum Data Rate

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No Damage or Latchup Up to ±25kV HBM

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High Input Impedance Supports 256 Nodes

(C, I-Grade)

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Operation Up to 125°C (H-Grade)

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Guaranteed Failsafe Receiver Operation Over the

Entire Common Mode Range

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Current Limited Drivers and Thermal Shutdown

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Delayed Micropower Shutdown: 5μA Maximum

(C, I-Grade)

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Power Up/Down Glitch-Free Driver Outputs

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Low Operating Current: 370μA Typical in

Receive Mode

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Compatible with TIA/EIA-485-A Speciﬁ cations

■

Available in 10-Pin 3mm × 3mm DFN, 12-Pin

4mm × 3mm DFN and 16-Pin SSOP Packages

LTC2854 at 20Mbps into 54Ω

PART NUMBER DUPLEX PACKAGE

LTC2854 HALF DFN-10

LTC2855 FULL SSOP-16, DFN-12

285455 TA01

LTC2854

120Ω

LTC2854

120Ω

RERO TE DE DI

LTC2854

120Ω

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

All other trademarks are the property of their respective owners.

20ns/DIV

2V/DIV

285455 TA01b

A-B

Summary of content (16 pages)

PAGE 1
LTC2854/LTC2855 3.3V 20Mbps RS485/RS422 Transceivers with Integrated Switchable Termination DESCRIPTION FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ The LTC®2854 and LTC2855 are low power, 20Mbps RS485/RS422 transceivers operating on 3.3V supplies. The receiver includes a logic-selectable 120Ω termination, one-eighth unit load supporting up to 256 nodes per bus (C, I-Grade), and a failsafe feature that guarantees a high output state under conditions of ﬂoating or shorted inputs.
PAGE 2
LTC2854/LTC2855 ABSOLUTE MAXIMUM RATINGS (Note 1) Supply Voltage (VCC) ................................... –0.3V to 7V Logic Input Voltages (RE, DE, DI, TE)............ –0.3V to 7V Interface I/O: A, B, Y, Z ...................................... (VCC –15V) to +15V (A-B) or (B-A) with Terminator Enabled ..................6V Receiver Output Voltage (RO) ........ –0.3V to (VCC +0.3V) Operating Temperature (Note 4) LTC2854C, LTC2855C .............................. 0°C to 70°C LTC2854I, LTC2855I .............
PAGE 3
LTC2854/LTC2855 ELECTRICAL CHARACTERISTICS The ● denotes the speciﬁcations which apply over the full operating temperature range, otherwise speciﬁcations are at TA = 25°C, VCC = 3.3V unless otherwise noted (Note 2). SYMBOL PARAMETER CONDITIONS MIN Differential Driver Output Voltage R = ∞, VCC = 3V (Figure 1) R = 27Ω, VCC = 3V (Figure 1) R = 50Ω, VCC = 3.13V (Figure 1) ● ● ● R = 27Ω or R = 50Ω (Figure 1) ● TYP MAX UNITS VCC VCC VCC V V V 0.
PAGE 4
LTC2854/LTC2855 ELECTRICAL CHARACTERISTICS The ● denotes the speciﬁcations which apply over the full operating temperature range, otherwise speciﬁcations are at TA = 25°C, VCC = 3.3V, TE = 0V unless otherwise noted (Note 2).
PAGE 5
LTC2854/LTC2855 TEST CIRCUITS Y GND DI OR VCC DRIVER Z Y R + VOD – R IOSD GND OR DI VCC + VOC – DRIVER Z + – –7V to +12V 285455 F01-2 Figure 1. Driver DC Characteristics Figure 2. Driver Output Short-Circuit Current IIN VIN + – A OR B RECEIVER B OR A 285455 F03 V RIN = IN IIN Figure 3. Receiver Input Current and Input Resistance VCC DI tPLHD DI Y tPHLD 0V CL DRIVER RDIFF tSKEWD VO Y, Z 1/2 VO CL Z 90% 285455 F04a Y-Z 10% 90% 0 0 10% tRD tFD 285455 F04b Figure 4.
PAGE 6
LTC2854/LTC2855 TEST CIRCUITS tR VAB A ±VAB/2 VCM A-B –VAB RO RECEIVER B ±VAB/2 CL 90% 10% tPLHR VCC 90% 1/2 VCC 10% VO RO 285455 F06a tF 90% 0 10% 0 tPHLR 1/2 VCC 90% 10% tRR tFR tSKEWR = tPLHR – tPHLR 285455 F06b Figure 6. Receiver Propagation Delay Measurements RE VCC 1/2 VCC 0V 0V OR VCC RECEIVER VCC OR 0V tZLR, tZLSR A RL RO B CL RE VCC VCC OR GND tLZR VO RO 1/2 VCC 0.5V VOL VOH DI = 0V OR VCC 0.
PAGE 7
LTC2854/LTC2855 TYPICAL PERFORMANCE CHARACTERISTICS Receiver Skew vs Temperature TA = 25°C, VCC = 3.3V, unless otherwise noted. Driver Propagation Delay vs Temperature Driver Skew vs Temperature 2 0 16 PROP DELAY (ns) 1 18 RDIFF = 54Ω CL = 100pF 1 DRIVER SKEW (ns) RECEIVER SKEW (ns) VAB = 1.
PAGE 8
LTC2854/LTC2855 PIN FUNCTIONS (DD/DE/GN) RO (Pin 1): Receiver Output. If the receiver output is enabled (RE low) and A > B by 200mV, then RO will be high. If A < B by 200mV, then RO will be low. If the receiver inputs are open, shorted, or terminated without a signal, RO will be high. RE (Pin 2): Receiver Enable. A low enables the receiver. A high input forces the receiver output into a high impedance state. DE (Pin 3): Driver Enable. A high on DE enables the driver.
PAGE 9
LTC2854/LTC2855 FUNCTION TABLES LTC2854 LOGIC INPUTS DE RE TE MODE A, B RO TERMINATOR 0 0 0 RECEIVE RIN DRIVEN OFF 0 0 1 RECEIVE WITH TERM RTERM DRIVEN ON 0 1 0 SHUTDOWN RIN HIGH-Z OFF 0 1 1 TERM ONLY RTERM HIGH-Z ON 1 0 0 TRANSMIT WITH RECEIVE DRIVEN DRIVEN OFF 1 0 1 TRANSMIT WITH RECEIVE AND TERM DRIVEN DRIVEN ON 1 1 0 TRANSMIT DRIVEN HIGH-Z OFF 1 1 1 TRANSMIT WITH TERM DRIVEN HIGH-Z ON LTC2855 LOGIC INPUTS DE RE TE MODE A, B Y, Z RO TE
PAGE 10
LTC2854/LTC2855 APPLICATIONS INFORMATION Driver The driver provides full RS485/RS422 compatibility. When enabled, if DI is high, Y-Z is positive for the full-duplex device (LTC2855) and A-B is positive for the half-duplex device (LTC2854). When the driver is disabled, both outputs are highimpedance. For the full-duplex LTC2855, the leakage on the driver output pins is guaranteed to be less than 10μA over the entire common mode range of –7V to +12V.
PAGE 11
LTC2854/LTC2855 APPLICATIONS INFORMATION When the TE pin is high, the termination resistor is enabled and the differential resistance from A to B is 120Ω. Figure 10 shows the I/V characteristics between pins A and B with the termination resistor enabled and disabled. The resistance is maintained over the entire RS485 common mode range of –7V to +12V as shown in Figure 11. Switchable Termination Proper cable termination is very important for good signal ﬁdelity.
PAGE 12
LTC2854/LTC2855 APPLICATIONS INFORMATION Supply Current The unloaded static supply currents in the LTC2854/ LTC2855 are very low —typically under 500μA for all modes of operation. In applications with resistively terminated cables, the supply current is dominated by the driver load. For example, when using two 120Ω terminators with a differential driver output voltage of 2V, the DC current is 33mA, which is sourced by the positive voltage supply.
PAGE 13
LTC2854/LTC2855 TYPICAL APPLICATION Failsafe “0” Application (Idle State = Logic “0”) VCC 100kΩ RO LTC2854 R I1 B A DI D I2 "A" "B" 285455 TA03 PACKAGE DESCRIPTION DD Package 10-Lead Plastic DFN (3mm × 3mm) (Reference LTC DWG # 05-08-1699) R = 0.115 TYP 6 0.38 ± 0.10 10 0.675 ±0.05 3.50 ±0.05 1.65 ±0.05 2.15 ±0.05 (2 SIDES) 3.00 ±0.10 (4 SIDES) PACKAGE OUTLINE 1.65 ± 0.10 (2 SIDES) PIN 1 TOP MARK (SEE NOTE 6) (DD) DFN 1103 5 0.200 REF 0.25 ± 0.05 0.50 BSC 2.38 ±0.05 (2 SIDES) 1 0.25 ± 0.
PAGE 14
LTC2854/LTC2855 PACKAGE DESCRIPTION DE/UE Package 12-Lead Plastic DFN (4mm × 3mm) (Reference LTC DWG # 05-08-1695 Rev D) 0.70 ±0.05 3.60 ±0.05 2.20 ±0.05 3.30 ±0.05 1.70 ± 0.05 PACKAGE OUTLINE 0.25 ± 0.05 0.50 BSC 2.50 REF RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED 4.00 ±0.10 (2 SIDES) 7 R = 0.115 TYP 0.40 ± 0.10 12 R = 0.05 TYP PIN 1 TOP MARK (NOTE 6) 0.200 REF 3.00 ±0.10 (2 SIDES) 0.75 ±0.05 3.30 ±0.10 1.70 ± 0.10 6 0.25 ± 0.
PAGE 15
LTC2854/LTC2855 PACKAGE DESCRIPTION GN Package 16-Lead Plastic SSOP (Narrow .150 Inch) (Reference LTC DWG # 05-08-1641) .189 – .196* (4.801 – 4.978) .045 ±.005 .009 (0.229) REF 16 15 14 13 12 11 10 9 .254 MIN .150 – .165 .229 – .244 (5.817 – 6.198) .0165 ± .0015 .150 – .157** (3.810 – 3.988) .0250 BSC RECOMMENDED SOLDER PAD LAYOUT 1 .015 ± .004 × 45° (0.38 ± 0.10) .007 – .0098 (0.178 – 0.249) 2 3 4 5 6 7 .0532 – .0688 (1.35 – 1.75) 8 .004 – .0098 (0.102 – 0.249) 0° – 8° TYP .016 – .050 (0.
PAGE 16
LTC2854/LTC2855 TYPICAL APPLICATION Multi-Node Network with End Termination Using the LTC2854 TE = 0V TE = 0V D R D R LTC2854 LTC2854 LTC2854 LTC2854 R R TE = 3.3V TE = 3.3V D D 285455 TA04 RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LTC485 Low Power RS485 Interface Transceiver ICC = 300μA (Typ) LTC491 Differential Driver and Receiver Pair ICC = 300μA LTC1480 3.3V Ultralow Power RS485 Transceiver 3.