LTM2881IV-3PBF

LTM2881 LINER

Document overview

• Manufacturer or author: Provided By www.digicamel.com(FREE DATASHEET DOWNLOAD SITE)
• PDF pages: 20

Technical content

DESCRIPTION

RS485/RS422 µModule Transceiver + Power The L TM®2881 is a complete galvanically isolated full- duplex RS485/RS422 μModule® transceiver. No external components are required. A single supply powers both sides of the interface through an integrated, isolated, low noise, effi cient 5V output DC/DC converter. Coupled inductors and an isolation power transformer provide 2500V RMS of isolation between the line transceiver and the logic interface. This device is ideal for systems where the ground loop is broken allowing for large common mode voltage variation. Uninterrupted communication is guaranteed for common mode transients greater than 30kV/μs. Maximum data rates are 20Mbps or 250kbps in slew limited mode. T ransmit data, DI and receive data, RO, are implemented with event driven low jitter processing. The receiver has a one-eighth unit load supporting up to 256 nodes per bus. A logic supply pin allows easy interfacing with different logic levels from 1.62V to 5.5V, independent of the main supply. Enhanced ESD protection allows this part to withstand up to ±15kV (human body model) on the transceiver interface pins to isolated supplies and ±10kV through the isolation barrier to logic supplies without latch-up or damage. Isolated Half-Duplex RS485 μModule Transceiver

FEATURES

APPLICATIONS

n Isolator μModule Technology n Isolated RS485/RS422 Transceiver: 2500VRMS n Integrated Isolated DC/DC Converter: 1W , 62% Effi ciency n No External Components Required n 20Mbps or Low EMI 250kbps Data Rate n High ESD: ±15kV HBM on Transceiver Interface n High Common Mode Transient Immunity: 30kV/μs n Integrated Selectable 120Ω Termination n 3.3V (L TM2881-3) or 5.0V (L TM2881-5) Operation n 1.62V to 5.5V Logic Supply Pin for Flexible Digital Interface n Common Mode Working Voltage: 560V PEAK n High Input Impedance Failsafe RS485 Receiver n Current Limited Drivers and Thermal Shutdown n Compatible with TIA/EIA-485-A Specifi cation n High Impedance Output During Internal Fault Condition n Low Current Shutdown Mode (< 10μA) n General Purpose CMOS Isolated Channel n Small, Low Profi le (15mm × 11.25mm × 2.8mm) Surface Mount LGA Package n Isolated RS485/RS422 Interface n Industrial Networks n Breaking RS485 Ground Loops

2881 TA01

A RO VL TE RE DE DI GND GND2 VCC 3.3V L TM2881 B Y Z PWR ISOLATION BARRIER L TM2881 Operating Through 35kV/μs CM Transient L, L T , L TC, L TM, Linear Technology, μModule and the Linear logo are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners.

2881 TA01a

PIN CONFIGURATION ABSOLUTE MAXIMUM RATINGS Interface Voltages Signal Voltages ON, RO, DI, DE, Signal Voltages SLO, Operating Temperature Range (Note 1) LGA PACKAGE 32-PIN (15mm s 11.25mm s 2.8mm) TOP VIEW SLODIN RO V L ONREDE DI TE DOUT A B C D E F G H J K L 2345678 V CC2Z GND2 GND BAY VCC TJMAX = 125°C, θJA = 32°C/W WEIGHT = 1g ORDER INFORMATION LEAD FREE FINISH TRAY PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE L TM2881CV-3#PBF L TM2881CV-3#PBF L TM2881V-3 32-Pin (15mm × 11.25mm × 2.8mm) LGA 0°C to 70°C L TM2881IV-3#PBF L TM2881IV-3#PBF L TM2881V-3 32-Pin (15mm × 11.25mm × 2.8mm) LGA –40°C to 85°C L TM2881CV-5#PBF L TM2881CV-5#PBF L TM2881V-5 32-Pin (15mm × 11.25mm × 2.8mm) LGA 0°C to 70°C L TM2881IV-5#PBF L TM2881IV-5#PBF L TM2881V-5 32-Pin (15mm × 11.25mm × 2.8mm) LGA –40°C to 85°C Consult L TC Marketing for parts specifi ed with wider operating temperature ranges. *The temperature grade is identifi ed by a label on the shipping container . For more information on lead free part marking, go to: http://www.linear .com/leadfree/ This product is only offered in trays. For more information go to: http://www.linear .com/packaging/

ELECTRICAL CHARACTERISTICS

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS Power Supply V CC VCC Supply Voltage L TM2881-3 L TM2881-5 l l 3.0 4.5 3.3 5.0 3.6 5.5 V V VL VL Supply Voltage l 1.62 5.5 V ICCPOFF VCC Supply Current in Off Mode ON = 0V l 01 0 μ A ICCS VCC Supply Current in On Mode L TM2881-3 DE = 0V, RE = VL, No Load L TM2881-5 DE = 0V, RE = VL, No Load l l mA mA VCC2 Regulated VCC2 Output Voltage, Loaded L TM 2881-3 DE = 0V, RE = VL, ILOAD = 100mA L TM2881-5 DE = 0V, RE = VL, ILOAD = 180mA l 4.7 4.7 5.0 5.0 V V VCC2NOLOAD Regulated VCC2 Output Voltage, No Load DE = 0V, RE = VL, No Load 4.8 5.0 5.35 V Effi ciency I CC2 = 100mA (Note 2) 50 % ICC2S VCC2 Short-Circuit Current DE = 0V, RE = VL, VCC2 = 0V l 250 mA Driver OD| Differential Driver Output Voltage R = ∞ (Figure 1) R = 27Ω (RS485) (Figure 1) R = 50Ω (RS422) (Figure 1) l l l 1.5 VCC2 VCC2 VCC2 V V V Δ|V OD| Difference in Magnitude of Driver Differential Output Voltage for Complementary Output States R = 27Ω or R = 50Ω (Figure 1) l 0.2 V VOC Driver Common Mode Output Voltage R = 27Ω or R = 50Ω (Figure 1) l 3V Δ|VOC| Difference in Magnitude of Driver Common Mode Output Voltage for Complementary Output States R = 27Ω or R = 50Ω (Figure 1) l 0.2 V IOZD Driver Three-State (High Impedance) Output Current on Y and Z DE = 0V, (Y or Z) = –7V, +12V l ±10 μA IOSD Maximum Driver Short-Circuit Current – 7V ≤ (Y or Z) ≤ 12V (Figure 2) l – 250 250 mA Receiver R IN Receiver Input Resistance RE = 0V or VL, VIN = –7V, –3V, 3V, 7V, 12V (Figure 3) l 96 125 kΩ RTE Receiver Termination Resistance Enabled TE = V L, VAB = 2V, VB = – 7V, 0V, 10V (Figure 8) l 108 120 156 Ω IIN Receiver Input Current (A, B) ON = 0V V CC2 = 0V or 5V, VIN = 12V (Figure 3) l 125 μA ON = 0V VCC2 = 0V or 5V, VIN = –7V (Figure 3) l –100 μA VTH Receiver Differential Input Threshold Voltage (A-B) –7V ≤ B ≤ 12V l –0.2 0.2 V ΔVTH Receiver Input Failsafe Hysteresis B = 0V 25 mV Receiver Input Failsafe Threshold B = 0V –0.2 –0.05 0 V Logic V IL Logic Input Low Voltage 1.62V ≤ V L ≤ 5.5V l 0.4 V VIH Logic Input High Voltage DIN SLO DI, TE, DE, ON, RE: V L ≥ 2.35V 1.62V ≤ V L < 2.35V l l l l 0.67•VCC2 0.67•VL 0.75•VL V V V V The l denotes the specifi cations which apply over the full operating temperature range, otherwise specifi cations are at TA = 25°C. L TM2881-3 VCC = 3.3V , L TM2881-5 VCC = 5.0V , VL = 3.3V, GND = GND2 = 0V, ON = VL unless otherwise noted.

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS IINL Logic Input Current l 0± 1 μ A VHYS Logic Input Hysteresis (Note 2) 150 mV VOH Output High Voltage Output High, I LOAD = –4mA (Sourcing), 5.5V ≥ VL ≥ 3V Output High, ILOAD = –1mA (Sourcing), 1.62V ≤ VL < 3V l l VL –0.4 VL –0.4 V V VOL Output Low Voltage Output Low, I LO AD = 4mA (Sinking), 5.5V ≥ VL ≥ 3V Output High, ILOAD = 1mA (Sinking), 1.62V ≤ VL < 3V l l 0.4 0.4 V V IOZR Three-State (High Impedance) Output Current on RO RE = VL, 0V ≤ RO ≤ VL l ±1 μA IOSR Short-Circuit Current 0V ≤ (RO or D OUT) ≤ VL l ±85 mA ELECTRICAL CHARACTERISTICS The l denotes the specifi cations which apply over the full operating temperature range, otherwise specifi cations are at TA = 25°C. L TM2881-3 VCC = 3.3V , L TM2881-5 VCC = 5.0V , VL = 3.3V, GND = GND2 = 0V, ON = VL unless otherwise noted. SWITCHING CHARACTERISTICS The l denotes the specifi cations which apply over the full operating temperature range, otherwise specifi cations are at TA = 25°C. L TM2881-3 VCC = 3.3V , L TM2881-5 VCC = 5.0V , VL = 3.3V, GND = GND2 = 0V, ON = VL unless otherwise noted. SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS Driver SLO = VCC2 fMAX Maximum Data Rate (Note 3) 20 Mbps tPLHD tPHLD Driver Input to Output R DIFF = 54Ω, CL = 100pF (Figure 4) l 60 85 ns ΔtPD Driver Input to Output Difference |tPLHD – tPHLD| RDIFF = 54Ω, CL = 100pF (Figure 4) l 18 n s tSKEWD Driver Output Y to Output Z R DIFF = 54Ω, CL = 100pF (Figure 4) l 1± 8 n s tRD tFD Driver Rise or Fall Time R DIFF = 54Ω, CL = 100pF (Figure 4) l 4 12.5 ns tZLD, tZHD, tLZD, tHZD Driver Output Enable or Disable Time R L = 500Ω, CL = 50pF (Figure 5) l 170 ns Driver SLO = GND2 fMAX Maximum Data Rate (Note 3) 250 kbps tPLHD tPHLD Driver Input to Output R DIFF = 54Ω, CL = 100pF (Figure 4) 1 1.55 μs ΔtPD Driver Input to Output Difference |tPLHD – tPHLD| RDIFF = 54Ω, CL = 100pF (Figure 4) 50 500 ns tSKEWD Driver Output Y to Output Z R DIFF = 54Ω, CL = 100pF (Figure 4) ±200 ±500 ns tRD tFD Driver Rise or Fall Time R DIFF = 54Ω, CL = 100pF (Figure 4) l 0.9 1.5 μs tZLD, tZHD, tLZD, tHZD Driver Output Enable or Disable Time R L = 500Ω, CL = 50pF (Figure 5) l 400 ns

ISOLATION CHARACTERISTICS T A = 25°C, L TM2881-3 VCC = 3.3V , L TM2881-5 VCC = 5.0V , VL = 3.3V unless otherwise noted. SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS VISO Rated Dielectric Insulation Voltage 1 Minute (Derived from 1 Second Test) 2500 V RMS

1 Second ±4400 V DC

Common Mode T ransient Immunity (Note 2) 30 kV/μs VIORM Maximum Working Insulation Voltage (Note 2) 560 V PEAK Partial Discharge V PR = 1050 VPEAK (Note 2) <5 pC Input to Output Resistance (Note 2) >10 9 Ω Input to Output Capacitance (Note 2) 6 pF Creepage Distance (Note 2) 9.48 mm Note 1: Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. Exposure to any Absolute Maximum Rating condition for extended periods may affect device reliability and lifetime. Note 2: Guaranteed by design and not subject to production test. Note 3: Maximum Data rate is guaranteed by other measured parameters and is not tested directly. Note 4: This μModule transceiver includes over temperature protection that is intended to protect the device during momentary overload conditions. Junction temperature will exceed 125°C when over temperature protection is active. Continuous operation above specifi ed maximum operating junction temperature may result in device degradation or failure. SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS Receiver t PLHR tPHLR Receiver Input to Output C L = 15pF, VCM = 2.5V, |VAB| = 1.4V, tR and tF < 4ns, (Figure 6) l 100 140 ns tSKEWR Differential Receiver Skew |tPLHR - tPHLR| CL = 15pF (Figure 6) l 18 n s tRR tFR Receiver Output Rise or Fall Time C L = 15pF (Figure 6) l 3 12.5 ns tZLR, tZHR, tLZR, tHZR Receiver Output Enable Time R L =1kΩ, CL = 15pF (Figure 7) l 50 ns tRTEN, tRTZ Termination Enable or Disable Time RE = 0V, DE = 0V, VAB = 2V, VB = 0V (Figure 8) l 100 μs Generic Logic Input tPLHL1 tPHLL1 DIN to DOUT Input to Output C L = 15pF, tR and tF < 4ns l 60 100 ns Power Supply Generator VCC2–GND2 Supply Start-Up Time (0V to 4.5V) ON VL, No Load l 325 800 μs SWITCHING CHARACTERISTICS The l denotes the specifi cations which apply over the full operating temperature range, otherwise specifi cations are at TA = 25°C. L TM2881-3 VCC = 3.3V , L TM2881-5 VCC = 5.0V , VL = 3.3V, GND = GND2 = 0V, ON = VL unless otherwise noted.

TEMPERATURE (°C) RECEIVER SKEW (ns) 2.0 0.5 1.5 1.0 –0.5 –1.0

2881 G01

1007550250–25–50 Receiver Skew vs Temperature Driver Skew vs Temperature Driver Propagation Delay vs Temperature TEMPERATURE (°C) DRIVER SKEW (ns) 2.0 0.5 1.5 1.0 –0.5 –1.0

2881 G02

1007550250–25–50 TEMPERATURE (°C) DRIVER PROP DELAY (ns)

2881 G03

1007550250–25–50 TYPICAL PERFORMANCE CHARACTERISTICS Receiver Output Voltage vs Output Current (Source and Sink) Receiver Propagation Delay vs Temperature Supply Current vs Data Rate RTERM vs Temperature Driver Output Low/High Voltage vs Output Current Driver Differential Output Voltage vs Temperature TEMPERATURE (°C) RESISTANCE (Ω) 130 110 112 114 116 118 120 122 124 126 128

2881 G04

1007550250–25–50 OUTPUT CURRENT (mA) OUTPUT VOL TAGE (V) 5.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5

2881 G05

TEMPERATURE (°C) OUTPUT VOL TAGE (V)

2881 G06

1007550250–25–50 R = ∞ R = 100Ω R = 54Ω TA = 25°C, L TM2881-3 VCC = 3.3V , L TM2881-5 VCC = 5.0V , VL = 3.3V unless otherwise noted. OUTPUT CURRENT (mA) OUTPUT VOL TAGE (V)

2881 G07

TEMPERATURE (°C) RECEIVER PROP DELAY (ns) 120 115 110 105 100

2881 G08

1007550250–25–50 DATA RATE (Mbps) SUPPL Y CURRENT (mA) 200 180 140 160 100 120

2881 G09

1010.1 R = 54 (–3) R = 100 (–3) R = 54 (–5) R = 100 (–5) R = ∞ (–3) R = ∞ (–5)

TYPICAL PERFORMANCE CHARACTERISTICS TA = 25°C, L TM2881-3 VCC = 3.3V , L TM2881-5 VCC = 5.0V , VL = 3.3V unless otherwise noted. VCC2 vs Load Current VCC Supply Current vs Temperature at ILOAD = 100mA on VCC2 VCC2 Surplus Current vs Temperature TEMPERATURE (°C) ICC CURRENT (mA) 350 250 300 200 150 100

2881 G10

100755025–25 0–50 L TM2881-3 L TM2881-5 TEMPERATURE (°C) SURPLUS CURRENT (mA) 250 200 150 100

2881 G11

100755025–25 0–50 L TM2881-5 (RS485 60mA) L TM2881-3 (RS485 60mA) L TM2881-3 (RS485 90mA) L TM2881-5 (RS485 90mA) VCC2 LOAD CURRENT (mA) VOL TAGE (V)

2881 G12

18016010 20 40 60 80 100 120 140 L TM2881-5 L TM2881-3 ICC2 OUTPUT CURRENT (mA) EFFICIENCY (%) 20050 100

2881 G13

1000μs/DIV 2881 G14 VCC2 100mV/DIV ILOAD 50mA/DIV 200μs/DIV 2881 G15 10mV/DIV VCC2 Power Effi ciency V CC2 Load Step (100mA) V CC2 Noise

LOGIC SIDE (VCC, VL, GND) DOUT (Pin A1): General Purpose Logic Output. Logic output connected through isolation path to D IN. Under the condition of an isolation communication failure DOUT is in a high impedance state. TE (Pin A2): Terminator Enable. A logic high enables a termination resistor (typically 120Ω) between pins A and B. DI (Pin A3): Driver Input. If the driver outputs are enabled (DE high), then a low on DI forces the driver noninverting output (Y) low and the inverting output (Z) high. A high on DI, with the driver outputs enabled, forces the driver noninverting output (Y) high and inverting output (Z) low. DE (Pin A4): Driver Enable. A logic low disables the driver leaving the outputs Y and Z in a high impedance state. A logic high enables the driver. RE (Pin A5): Receiver Enable. A logic low enables the receiver output. A logic high disables RO to a high impedance state. RO (Pin A6): Receiver Output. If the receiver output is ena bled (RE low) and if A – B is > 200mV, RO is a logic high, if A – B is < 200mV RO is a logic low. If the receiver inputs are open, shorted, or terminated without a valid signal, RO will be high. Under the condition of an isolation communication failure RO is in a high impedance state. V L (Pin A7): Logic Supply. Interface supply voltage for pins RO, RE, TE, DI, DE, D OUT, and ON. Recommended operating voltage is 1.62V to 5.5V. ON (Pin A8): Enable. Enables power and data communication through the isolation barrier. If ON is high the part is enabled and power and communications are functional to the isolated side. If ON is low the logic side is held in reset and the isolated side is unpowered. GND (Pins B1-B5): Circuit Ground. V CC (Pins B6-B8): Supply Voltage. Recommended operating voltage is 3V to 3.6V for L TM2881-3 and 4.5V to 5.5V for L TM2881-5. PIN FUNCTIONS ISOLATED SIDE (VCC2, GND2) DIN (Pin L1): General Purpose Isolated Logic Input. Logic input on the isolated side relative to V CC2 and GND2. A logic high on D IN will generate a logic high on D OUT. A logic low on DIN will generate a logic low on DOUT. SLO (Pin L2): Driver Slew Rate Control. A low input, relative to GND2, will force the driver into a reduced slew rate mode for reduced EMI. A high input, relative to GND2, puts the driver into full speed mode to support maximum data rates. Y (Pin L3): Non Inverting Driver Output. High impedance when the driver is disabled. Z (Pin L4): Inverting Driver Output. High impedance when the driver is disabled. B (Pin L5): Inverting Receiver Input. Impedance is > 96kΩ in receive mode with TE low or unpowered. A (Pin L6): Non Inverting Receiver Input. Impedance is > 96kΩ in receive mode with TE low or unpowered. V CC2 (Pins L7-L8): Isolated Supply Voltage. Internally generated from VCC by an isolated DC/DC converter and regulated to 5V. GND2 (Pins K1-K8): Isolated Side Circuit Ground. The pads should be connected to the isolated ground and/or cable shield.

2881 F01

2881 F02

2881 F03

Figure 1. Driver DC Characteristics Figure 2. Driver Output Short-Circuit Current Figure 3. Receiver Input Current and Input Resistance

2881 F06a

2881 F06b

Figure 6. Receiver Propagation Delay Measurements

2881 F05a

2881 F05b

Figure 5. Driver Enable and Disable Timing Measurements

2881 F04a

2881 F04b

Figure 4. Driver Timing Measurement

2881 F07a

2881 F07b

Figure 7. Receiver Enable/Disable Time Measurements

2881 F08

Figure 8. Termination Resistance and Timing Measurements

The L TM2881 μModule transceiver provides a galvanically- isolated robust RS485/RS422 interface, powered by an integrated, regulated DC/DC converter, complete with decoupling capacitors. A switchable termination resistor is integrated at the receiver input to provide proper termination to the RS485 bus. The L TM2881 is ideal for use in networks where grounds can take on different voltages. Isolation in the L TM2881 blocks high voltage differences and eliminates ground loops and is extremely tolerant of common mode transients between ground potentials. Error free operation is maintained through common mode events greater than 30kV/μs providing excellent noise isolation. DC/DC Converter The L TM2881 contains a fully integrated isolated DC/DC converter, including the transformer, so that no external components are necessary. The logic side contains a full- bridge driver, running about 2MHz, and is AC-coupled to a single transformer primary. A series DC blocking capacitor prevents transformer saturation due to driver duty cycle imbalance. The transformer scales the primary voltage, and is rectifi ed by a full-wave voltage doubler. This topology eliminates transformer saturation caused by secondary imbalances. APPLICATIONS INFORMATION The DC/DC converter is connected to a low dropout reg- ulator (LDO) to provide a regulated low noise 5V output. The internal power solution is suffi cient to support the transceiver interface at its maximum specifi ed load and data rate, and external pins are supplied for extra decoupling (optional) and heat dissipation. The logic supplies, V CC and V L have a 2.2μF decoupling capacitance to GND and the isolated supply V CC2 has a 2.2μF decoupling capacitance to GND2 within the μModule package. Surplus current is available to external applications. The amount of surplus current is dependent upon the implementation and current delivered to the RS485 driver and line load. An example of available surplus current is shown in the Typical Performance Characteristics graph, V CC2 Surplus Current vs Temperature. Driver The driver provides full RS485 and RS422 compatibility. When enabled, if DI is high, Y–Z is positive. When the driver is disabled, both outputs are high impedance with less than 10μA of leakage current over the entire common mode range of –7V to 12V, with respect to GND2.

Driver Overvoltage and Overcurrent Protection The driver outputs are protected from short circuits to any voltage within the absolute maximum range of (VCC2 –15V) to (GND2 +15V) levels. The maximum VCC2 current in this condition is 250mA. If the pin voltage exceeds about ±10V, current limit folds back to about half of the peak value to reduce overall power dissipation and avoid damaging the part. The device also features thermal shutdown protection that disables the driver and receiver output in case of excessive power dissipation (See Note 4 in the Electrical Characteristics section). SLO Mode The L TM2881 features a logic-selectable reduced slew rate mode (SLO mode) that softens the driver output edges to reduce EMI emissions from equipment and data cables. The reduced slew rate mode is entered by taking the SLO pin low to GND2, where the data rate is limited to about 250kbps. Slew limiting also mitigates the adverse effects of imperfect transmission line termination caused by stubs or mismatched cables. Figures 9a and 9b show the frequency spectrums of the L TM2881 driver outputs in normal and SLO mode operating at 250kbps. SLO mode signifi cantly reduces the high frequency harmonics. Receiver and Failsafe With the receiver enabled, when the absolute value of the differential voltage between the A and B pins is greater than 200mV, the state of RO will refl ect the polarity of (A-B). During data communication the receiver detects the state of the input with symmetric thresholds around 0V . The symmetric thresholds preserve duty cycle for attenuated signals with slow transition rates on high capacitive busses, or long cable lengths. The receiver incorporates a failsafe feature that guarantees the receiver output to be a logic- high during an idle bus, when the inputs are shorted, left open or terminated, but not driven. The failsafe feature eliminates the need for system level integration of network pre-biasing by guaranteeing a logic-high on RO under APPLICATIONS INFORMATION Figure 9a. Frequency Spectrum SLO Mode 125kHz Input Figure 9b. Normal Mode Frequency Spectrum 125kHz Input FREQUENCY 1.25MHz/DIV Y-Z 10dB/DIV

2881 F09a

12.56.250 FREQUENCY 1.25MHz/DIV Y-Z 10dB/DIV

2881 F09b

12.56.250 the conditions of an idle bus. Further network biasing constructed to condition transient noise during an idle state is unnecessary due to the common mode transient rejection of the L TM2881. The failsafe detector monitors A and B in parallel with the receiver and detects the state of the bus when A-B is above the input failsafe threshold for longer than about 3μs with a hysteresis of 25mV. This failsafe feature is guaranteed to work for inputs spanning the entire common mode range of –7V to 12V. The receiver output is internally driven high (to V L) or low (to GND) with no external pull-up needed. When the receiver is disabled the RO pin becomes Hi-Z with leakage of less than ±1μA for voltages within the supply range.

Figure 10. Equivalent Input Resistance into A and B without exceeding the RS485 receiver loading specifi cation. Figure 13. Termination Magnitude and Phase vs Frequency Figure 12. Termination Resistance vs Common Mode Voltage Figure 11. Curve T race Between A and B with Termination mance when confi guring transceiver networks. Figure 12. The integrated termination resistor has a high

2881 F11

2881 F13

optimize isolation characteristics.

• Use large PCB copper areas for high current paths, including V CC, GND, V CC2, and GND2. It helps to minimize the PCB conduction loss and thermal stress.
• The L TM2881 includes 2.2μF ceramic decoupling capacitors on V CC to GND, V L to GND, and V CC2 to GND2 supply pins. Further decoupling capacitance (10μF) can be added within one-quarter inch away from the V CC, VL, and/or VCC2 pin.

Figure 15. PCB Recommended Layout

2881 F15

• If the L TM2881 voltage supply is hot plugged without additional protection, damage may occur. Refer to Linear Technology Application Note 88, entitled “Ceramic Capacitors Can Cause Overvoltage T ransients” for a detailed discussion of this problem. To protect against hot plug transients use tantalum for aforementioned additional capacitor.
• Do not place copper on the PCB between the inner rows of pads. This area must remain open to withstand the rated isolation voltage. The PCB may also be slotted in this area to insure contamination does not compromise the isolation voltage. Supply Current The static supply current is dominated by power delivered to the termination resistance. Power supply current increases with data rate due to capacitive loading. Figure 14 shows supply current versus data rate for three different loads for the circuit confi guration of Figure 4.

Figure 14. Supply Current vs Data Rate

2881 F14

Figure 16. Cable Length vs Data Rate

2881 F16DATA RATE (bps)

20Mbps shows the maximum data rate when SLO is high.

2881 TA02

B Y Z PWR GND2 L TM2881 ISOLATION BARRIER AVL RE DE TE RO DI DOUT DIN GNDFAUL T VCC VCC B Y Z GND2 330k

2881 TA03

2881 TA04

A VCC2 RO VL TE RE DE DI DOUT GND GND2 VCC VCC CMOS INPUTCMOS OUTPUT B Z 1.8V DIN PWR ONOFF REGULATED 5V SWITCHED 5V IRLML6402 330k L TM2881 ISOLATION BARRIER Full-Duplex RS485 Connection Isolated System Fault Detection Switched 5V Power with Isolated CMOS Logic Connection with Low Voltage Interface TYPICAL APPLICATIONS

B Y Z PWR GND2 L TM2881 ISOLATION BARRIER Y VL RE RO DE DI GND VCC VCCB

2881 TA04a

Z A B PWR GND2 BUS INHERITED L TM2881 10nF 51Ω 51Ω 51Ω 51Ω 10nF ISOLATION BARRIER B 4-Wire Full Duplex Self Biasing for Unshielded CAT5 Connection

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. PACKAGE DESCRIPTION NOTES: 1. DIMENSIONING AND TOLERANCING PER ASME Y14.5M-1994 2. ALL DIMENSIONS ARE IN MILLIMETERS LAND DESIGNATION PER JESD MO-222 5. PRIMARY DATUM -Z- IS SEATING PLANE 6. THE TOTAL NUMBER OF PADS: 32 DETAILS OF PAD #1 IDENTIFIER ARE OPTIONAL, BUT MUST BE LOCATED WITHIN THE ZONE INDICATED. THE PAD #1 IDENTIFIER MAY BE EITHER A MOLD OR MARKED FEATURE DETAIL B DETAIL B SUBSTRATE MOLD CAP 0.290 – 0.350 2.400 – 2.600 bbb Z Z PACKAGE TOP VIEW 11.25 BSC 15.00 BSC PAD “A1” CORNER X Y aaa Z aaa Z PACKAGE BOTTOM VIEW PADS SEE NOTES SUGGESTED PCB LAYOUT TOP VIEW LGA 32 0308 REV Ø L TMXXXXXX μModule TRAY PIN 1 BEVEL PACKAGE IN TRAY LOADING ORIENTATION COMPONENT PIN “A1” 8.89 BSC 1.27 BSC PAD 1 0.635 0.635 1.905 1.905 3.175 3.175 4.445 4.445 6.350 6.350 5.080 5.080 0.000 SYMBOL aaa bbb eee TOLERANCE 0.10 0.10 0.05 DETAIL A 0.630 ±0.025 Ø 32x S YXeee DETAIL C 0.630 ±0.025 Ø 32x S YXeee F G H L J K E A B C D 2143567 2.69 – 2.95 DETAIL A 12.70 BSC DETAIL c 32-Lead (15mm × 11.25mm × 2.8mm) (Reference L TC DWG # 05-08-1773 Rev θ)

Linear Technology Corporation 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear .com © LINEAR TECHNOLOGY CORPORATION 2009 LT 1109 • PRINTED IN USA RELATED PARTS TYPICAL APPLICATION PART NUMBER DESCRIPTION COMMENTS L TC1535 Isolated RS485 Transceiver 2500V RMS Isolation in Surface Mount Package L T1785 ±60V Fault-Protected Transceiver Half Duplex L T1791 ±60V Fault-Protected Transceiver Full Duplex L TC2861 20Mbps RS485 Transceivers with Integrated Switchable Termination Full Duplex 15kV ESD AVL VCC1 RE DE TE RO DI GND VCC VCCA

2881 TA05

B Y Z A PWR GND2 A VL RE DE TE RO DI GND VCC VCCC B Y Z PWR GND2 AVL RE DE TE RO DI GND VCC VCCB B Y Z PWR GND2 L TM2881 L TM2881L TM2881 VCC2 C B CABLE SHIELD OR GROUND RETURN ISOLATION BARRIER ISOLATION BARRIER ISOLATION BARRIER B Multi-Node Network with End Termination and Single Ground Connection on Isolation Bus