LTM2881 Complete Isolated RS485/RS422 µModule Transceiver + Power DESCRIPTION FEATURES n n n n n n n n n n n n n n n n n n Isolator μModule Technology Isolated RS485/RS422 Transceiver: 2500VRMS Integrated Isolated DC/DC Converter: 1W, 62% Efficiency No External Components Required 20Mbps or Low EMI 250kbps Data Rate High ESD: ±15kV HBM on Transceiver Interface High Common Mode Transient Immunity: 30kV/μs Integrated Selectable 120Ω Termination 3.3V (LTM2881-3) or 5.0V (LTM2881-5) Operation 1.62V to 5.5V Logic Supply Pin for Flexible Digital Interface Common Mode Working Voltage: 560VPEAK High Input Impedance Failsafe RS485 Receiver Current Limited Drivers and Thermal Shutdown Compatible with TIA/EIA-485-A Specification High Impedance Output During Internal Fault Condition Low Current Shutdown Mode (< 10μA) General Purpose CMOS Isolated Channel Small, Low Profile (15mm × 11.25mm × 2.8mm) Surface Mount LGA Package n n Coupled inductors and an isolation power transformer provide 2500VRMS 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. Transmit 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. APPLICATIONS n The LTM®2881 is a complete galvanically isolated fullduplex RS485/RS422 μModule® transceiver. No external components are required. A single supply powers both sides of the interface through an integrated, isolated, low noise, efficient 5V output DC/DC converter. Isolated RS485/RS422 Interface Industrial Networks Breaking RS485 Ground Loops L, LT, LTC, LTM, Linear Technology, μModule and the Linear logo are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. TYPICAL APPLICATION Isolated Half-Duplex RS485 μModule Transceiver LTM2881 Operating Through 35kV/μs CM Transient 3.3V MULTIPLE SWEEPS OF COMMON MODE TRANSIENTS VCC LTM2881 PWR VL RO 500V/DIV ISOLATION BARRIER A RE TE DE DI B TWISTED-PAIR CABLE Y Z GND DI RO 1V/DIV 1V/DIV 50ns/DIV 2881 TA01a GND2 2881 TA01 2881f 1 LTM2881 ABSOLUTE MAXIMUM RATINGS PIN CONFIGURATION (Note 1) VCC to GND .................................................. –0.3V to 6V VCC2 to GND2 ............................................... –0.3V to 6V VL to GND .................................................... –0.3V to 6V Interface Voltages (A, B, Y, Z) to GND2 ........................ VCC2 –15V to 15V Signal Voltages ON, RO, DI, DE, RE, TE, DOUT to GND ......................... –0.3V to VL +0.3V Signal Voltages SLO, DIN to GND2 ....................................–0.3V to VCC2 +0.3V Operating Temperature Range LTM2881C................................................ 0°C to 70°C LTM2881I.............................................–40°C to 85°C Storage Temperature Range...................–55°C to 125°C Peak Reflow Temperature (Soldering, 10 sec) ...... 245°C TOP VIEW 1 2 DOUT TE 3 4 5 6 7 8 DI DE RE RO VL ON A B VCC GND C D E F G H J GND2 K L DIN SLO Y Z B A VCC2 LGA PACKAGE 32-PIN (15mm s 11.25mm s 2.8mm) TJMAX = 125°C, θJA = 32°C/W WEIGHT = 1g ORDER INFORMATION LEAD FREE FINISH TRAY PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE LTM2881CV-3#PBF LTM2881CV-3#PBF LTM2881V-3 32-Pin (15mm × 11.25mm × 2.8mm) LGA 0°C to 70°C LTM2881IV-3#PBF LTM2881IV-3#PBF LTM2881V-3 32-Pin (15mm × 11.25mm × 2.8mm) LGA –40°C to 85°C LTM2881CV-5#PBF LTM2881CV-5#PBF LTM2881V-5 32-Pin (15mm × 11.25mm × 2.8mm) LGA 0°C to 70°C LTM2881IV-5#PBF LTM2881IV-5#PBF LTM2881V-5 32-Pin (15mm × 11.25mm × 2.8mm) LGA –40°C to 85°C Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified 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/ 2881f 2 LTM2881 ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. LTM2881-3 VCC = 3.3V, LTM2881-5 VCC = 5.0V, VL = 3.3V, GND = GND2 = 0V, ON = VL unless otherwise noted. SYMBOL PARAMETER CONDITIONS MIN TYP MAX l l 3.0 4.5 3.3 5.0 3.6 5.5 l 1.62 UNITS VCC VCC Supply Voltage LTM2881-3 LTM2881-5 VL VL Supply Voltage 5.5 V ICCPOFF VCC Supply Current in Off Mode ON = 0V l 0 10 μA ICCS VCC Supply Current in On Mode LTM2881-3 DE = 0V, RE = VL , No Load LTM2881-5 DE = 0V, RE = VL , No Load l l 20 15 25 19 mA mA VCC2 Regulated VCC2 Output Voltage, Loaded LTM2881-3 DE = 0V, RE = VL, ILOAD = 100mA LTM2881-5 DE = 0V, RE = VL, ILOAD = 180mA l VCC2NOLOAD Regulated VCC2 Output Voltage, No Load DE = 0V, RE = VL , No Load Efficiency ICC2 = 100mA (Note 2) VCC2 Short-Circuit Current DE = 0V, RE = VL , VCC2 = 0V l Differential Driver Output Voltage R = ∞ (Figure 1) R = 27Ω (RS485) (Figure 1) R = 50Ω (RS422) (Figure 1) l l l R = 27Ω or R = 50Ω (Figure 1) l Power Supply ICC2S 4.7 4.7 5.0 5.0 4.8 5.0 V V V V 5.35 V 250 mA VCC2 VCC2 VCC2 V V V 0.2 V 50 % Driver |VOD| Δ|VOD| Difference in Magnitude of Driver Differential Output Voltage for Complementary Output States 1.5 2 VOC Driver Common Mode Output Voltage R = 27Ω or R = 50Ω (Figure 1) l 3 V 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 RIN Receiver Input Resistance RE = 0V or VL , VIN = –7V, –3V, 3V, 7V, 12V (Figure 3) l 96 125 RTE Receiver Termination Resistance Enabled TE = VL , VAB = 2V, VB = – 7V, 0V, 10V (Figure 8) l 108 120 IIN Receiver Input Current (A, B) ON = 0V VCC2 = 0V or 5V, VIN = 12V (Figure 3) l ON = 0V VCC2 = 0V or 5V, VIN = –7V (Figure 3) l –100 l –0.2 Receiver VTH Receiver Differential Input Threshold Voltage (A-B) –7V ≤ B ≤ 12V ΔVTH Receiver Input Failsafe Hysteresis B = 0V Receiver Input Failsafe Threshold B = 0V kΩ 156 Ω 125 μA μA 0.2 25 –0.2 –0.05 V mV 0 V Logic VIL Logic Input Low Voltage 1.62V ≤ VL ≤ 5.5V l Logic Input High Voltage DIN SLO DI, TE, DE, ON, RE: VL ≥ 2.35V 1.62V ≤ VL < 2.35V l l 0.67•VCC2 2 V V l l 0.67•VL 0.75•VL V V VIH 0.4 V 2881f 3 LTM2881 ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. LTM2881-3 VCC = 3.3V, LTM2881-5 VCC = 5.0V, VL = 3.3V, GND = GND2 = 0V, ON = VL unless otherwise noted. SYMBOL PARAMETER IINL Logic Input Current VHYS Logic Input Hysteresis (Note 2) VOH Output High Voltage Output High, ILOAD = –4mA (Sourcing), 5.5V ≥ VL ≥ 3V Output High, ILOAD = –1mA (Sourcing), 1.62V ≤ VL < 3V l VL –0.4 V l VL –0.4 V Output Low, ILO AD = 4mA (Sinking), 5.5V ≥ VL ≥ 3V Output High, ILOAD = 1mA (Sinking), 1.62V ≤ VL < 3V l 0.4 V l 0.4 V VOL Output Low Voltage CONDITIONS MIN l TYP MAX 0 ±1 150 UNITS μA mV IOZR Three-State (High Impedance) Output Current RE = VL , 0V ≤ RO ≤ VL on RO l ±1 μA IOSR Short-Circuit Current l ±85 mA 0V ≤ (RO or DOUT) ≤ VL SWITCHING CHARACTERISTICS The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. LTM2881-3 VCC = 3.3V, LTM2881-5 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) tPLHD tPHLD Driver Input to Output RDIFF = 54Ω, CL = 100pF (Figure 4) l 20 60 85 Mbps ns ΔtPD Driver Input to Output Difference |tPLHD – tPHLD| RDIFF = 54Ω, CL = 100pF (Figure 4) l 1 8 ns tSKEWD Driver Output Y to Output Z RDIFF = 54Ω, CL = 100pF (Figure 4) l 1 ±8 ns tRD tFD Driver Rise or Fall Time RDIFF = 54Ω, CL = 100pF (Figure 4) l 4 12.5 ns tZLD , tZHD , tLZD , tHZD Driver Output Enable or Disable Time RL = 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 RDIFF = 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 RDIFF = 54Ω, CL = 100pF (Figure 4) ±200 ±500 ns tRD tFD Driver Rise or Fall Time RDIFF = 54Ω, CL = 100pF (Figure 4) l 0.9 1.5 μs tZLD , tZHD , tLZD , tHZD Driver Output Enable or Disable Time RL = 500Ω, CL = 50pF (Figure 5) l 400 ns 2881f 4 LTM2881 SWITCHING CHARACTERISTICS The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. LTM2881-3 VCC = 3.3V, LTM2881-5 VCC = 5.0V, VL = 3.3V, GND = GND2 = 0V, ON = VL unless otherwise noted. SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS tPLHR tPHLR Receiver Input to Output CL = 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 1 8 ns tRR tFR Receiver Output Rise or Fall Time CL = 15pF (Figure 6) l 3 12.5 ns tZLR , tZHR , tLZR , tHZR Receiver Output Enable Time RL =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 CL = 15pF, tR and tF < 4ns l 60 100 ns l 325 800 μs Receiver Generic Logic Input tPLHL1 tPHLL1 DIN to DOUT Input to Output Power Supply Generator VCC2 –GND2 Supply Start-Up Time (0V to 4.5V) ON ISOLATION CHARACTERISTICS otherwise noted. SYMBOL PARAMETER VISO Rated Dielectric Insulation Voltage TA = 25°C, LTM2881-3 VCC = 3.3V, LTM2881-5 VCC = 5.0V, VL = 3.3V unless CONDITIONS MIN TYP MAX UNITS 1 Minute (Derived from 1 Second Test) 2500 VRMS 1 Second ±4400 VDC (Note 2) 30 Maximum Working Insulation Voltage (Note 2) 560 Partial Discharge VPR = 1050 VPEAK (Note 2) Common Mode Transient Immunity VIORM VL, No Load kV/μs VPEAK <5 >109 pC Ω Input to Output Resistance (Note 2) 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 specified maximum operating junction temperature may result in device degradation or failure. 2881f 5 LTM2881 TYPICAL PERFORMANCE CHARACTERISTICS TA = 25°C, LTM2881-3 VCC = 3.3V, LTM2881-5 VCC = 5.0V, VL = 3.3V unless otherwise noted. Driver Propagation Delay vs Temperature Driver Skew vs Temperature 2.0 80 1.5 1.5 75 1.0 0.5 0 –0.5 DRIVER PROP DELAY (ns) 2.0 DRIVER SKEW (ns) RECEIVER SKEW (ns) Receiver Skew vs Temperature 1.0 0.5 0 –25 0 25 50 TEMPERATURE (°C) 75 –1.0 –50 100 –25 0 25 50 TEMPERATURE (°C) 75 2881 G01 4.5 126 4.0 124 122 120 118 116 2.0 1.5 0.5 0 100 OUTPUT LOW R = 54Ω 2 0 10 20 30 40 50 OUTPUT CURRENT (mA) 60 0 –50 70 –25 0 25 50 TEMPERATURE (°C) 75 100 2881 G06 Receiver Propagation Delay vs Temperature Supply Current vs Data Rate 120 200 180 3 2 1 115 160 SUPPLY CURRENT (mA) RECEIVER PROP DELAY (ns) SOURCE OUTPUT VOLTAGE (V) R = 100Ω 3 2881 G05 4 110 105 100 R = 54 (–3) 140 120 100 R = 100 (–3) R = 54 (–5) 80 60 R = 100 (–5) 40 95 20 SINK 0 2 3 4 OUTPUT CURRENT (mA) R=∞ 4 1 2881 G04 Receiver Output Voltage vs Output Current (Source and Sink) 100 5 OUTPUT HIGH 2.5 112 1 75 6 3.0 1.0 0 0 25 50 TEMPERATURE (°C) Driver Differential Output Voltage vs Temperature 3.5 114 75 –25 2881 G03 OUTPUT VOLTAGE (V) 128 OUTPUT VOLTAGE (V) RESISTANCE (Ω) 5.0 0 25 50 TEMPERATURE (°C) 50 –50 100 Driver Output Low/High Voltage vs Output Current 130 –25 60 2881 G02 RTERM vs Temperature 110 –50 65 55 –0.5 –1.0 –50 70 5 2881 G07 90 –50 –25 0 25 50 TEMPERATURE (°C) 75 100 2881 G08 0 0.1 R = ∞ (–3) R = ∞ (–5) 1 DATA RATE (Mbps) 10 2881 G09 2881f 6 LTM2881 TYPICAL PERFORMANCE CHARACTERISTICS VCC = 5.0V, VL = 3.3V unless otherwise noted. VCC2 Surplus Current vs Temperature VCC Supply Current vs Temperature at ILOAD = 100mA on VCC2 VCC2 vs Load Current 250 350 6 250 200 LTM2881-5 150 100 200 LTM2881-5 LTM2881-5 (RS485 60mA) 150 LTM2881-5 (RS485 90mA) LTM2881-3 (RS485 60mA) 100 5 VOLTAGE (V) SURPLUS CURRENT (mA) LTM2881-3 300 ICC CURRENT (mA) TA = 25°C, LTM2881-3 VCC = 3.3V, LTM2881-5 LTM2881-3 4 3 50 50 LTM2881-3 (RS485 90mA) 0 –50 –25 0 25 50 TEMPERATURE (°C) 75 100 0 –50 2 –25 0 25 50 TEMPERATURE (°C) 75 100 10 2881 G11 2881 G10 VCC2 Power Efficiency 20 40 60 80 100 120 140 160 180 VCC2 LOAD CURRENT (mA) 2881 G12 VCC2 Load Step (100mA) VCC2 Noise 70 LTM2881-5 EFFICIENCY (%) 60 VCC2 100mV/DIV 50 LTM2881-3 10mV/DIV 40 ILOAD 50mA/DIV 30 20 1000μs/DIV 2881 G14 200μs/DIV 2881 G15 10 0 150 50 100 ICC2 OUTPUT CURRENT (mA) 200 2881 G13 2881f 7 LTM2881 PIN FUNCTIONS LOGIC SIDE (VCC , VL, GND) ISOLATED SIDE (VCC2, GND2) DOUT (Pin A1): General Purpose Logic Output. Logic output connected through isolation path to DIN . Under the condition of an isolation communication failure DOUT is in a high impedance state. DIN (Pin L1): General Purpose Isolated Logic Input. Logic input on the isolated side relative to VCC2 and GND2. A logic high on DIN will generate a logic high on DOUT. A logic low on DIN will generate a logic low on DOUT. TE (Pin A2): Terminator Enable. A logic high enables a termination resistor (typically 120Ω) between pins A and B. 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. 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 enabled (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. 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. VCC2 (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. VL (Pin A7): Logic Supply. Interface supply voltage for pins RO, RE, TE, DI, DE, DOUT, 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. VCC (Pins B6-B8): Supply Voltage. Recommended operating voltage is 3V to 3.6V for LTM2881-3 and 4.5V to 5.5V for LTM2881-5. 2881f 8 LTM2881 BLOCK DIAGRAM VCC 2.2μF VCC2 5V REG ISOLATED DC/DC CONVERTER 2.2μF VL 2.2μF A RO RX B RE ISOLATED COMM INTERFACE DE ISOLATED COMM INTERFACE 120Ω DI Y DX ON Z SLO TE DIN DOUT GND GND2 2881 BD = LOGIC SIDE COMMON = ISOLATED SIDE COMMON TEST CIRCUITS Y GND OR VL DI Y + DRIVER R GND OR VL VOD – Z R + – DI IOSD DRIVER VOC Z 2881 F01 –7V TO 12V 2881 F02 Figure 1. Driver DC Characteristics Figure 2. Driver Output Short-Circuit Current IIN VIN + – + – A OR B RECEIVER B OR A 2881 F03 V RIN = IN IIN Figure 3. Receiver Input Current and Input Resistance 2881f 9 LTM2881 TEST CIRCUITS VL Y DI tPLHD DI tPHLD 0V tSKEWD CL DRIVER RDIFF Y, Z VOD 1/2 VOD CL Z 90% 2881 F04a (Y-Z) 10% 90% 0 0 10% tRD tFD 2881 F04b Figure 4. Driver Timing Measurement RL Y VL OR GND DI CL GND OR VCC2 DRIVER RL DE Z CL VL DE 1/2 VL 0V tZLD tLZD VCC2 Y OR Z 1/2 VCC2 Z OR Y 1/2 VCC2 0.5V VCC2 OR GND 2881 F05a 0.5V 0V 2881 F05b tZHD tHZD Figure 5. Driver Enable and Disable Timing Measurements tR ±VAB/2 VCM ±VAB/2 VAB A-B –VAB A RECEIVER B RO CL 2881 F06a VL RO 0 90% 10% tF 90% 0 10% tPLHR 90% 1/2 VL 10% tRR tPHLR 1/2 VL 90% 10% tFR 2881 F06b Figure 6. Receiver Propagation Delay Measurements 2881f 10 LTM2881 TEST CIRCUITS VL RE 1/2 VL 0V A 0V OR VCC2 RL RO RECEIVER B VCC2 OR 0V VL OR GND CL RE tZLR VL tLZR 1/2 VL RO 0.5V VOL VOH 0.5V 1/2 VL RO 0V 2881 F07a 2881 F07b tZHR tHZR Figure 7. Receiver Enable/Disable Time Measurements IA RTE = VAB IA VL A RO + – RECEIVER TE VAB 1/2 VL 0V IA B + – TE tRTEN tRTZ 90% 10% VB 2881 F08 Figure 8. Termination Resistance and Timing Measurements FUNCTIONAL TABLE LOGIC INPUTS MODE A, B Y, Z RO DC/DC CONVERTER TERMINATOR RIN Hi-Z Enabled On Off ON RE TE DE 1 0 0 0 Receive 1 0 0 1 Transceiver RIN Driven Enabled On Off 1 1 0 1 Transmit RIN Driven Hi-Z On Off 1 0 1 0 Receive + Term On RTE Hi-Z Enabled On On 0 X X X Off RIN Hi-Z Hi-Z Off Off 2881f 11 LTM2881 APPLICATIONS INFORMATION Overview The LTM2881 μModule transceiver provides a galvanicallyisolated 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 LTM2881 is ideal for use in networks where grounds can take on different voltages. Isolation in the LTM2881 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. The DC/DC converter is connected to a low dropout regulator (LDO) to provide a regulated low noise 5V output. The internal power solution is sufficient to support the transceiver interface at its maximum specified load and data rate, and external pins are supplied for extra decoupling (optional) and heat dissipation. The logic supplies, VCC and VL have a 2.2μF decoupling capacitance to GND and the isolated supply VCC2 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, VCC2 Surplus Current vs Temperature. DC/DC Converter The LTM2881 contains a fully integrated isolated DC/DC converter, including the transformer, so that no external components are necessary. The logic side contains a fullbridge 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 rectified by a full-wave voltage doubler. This topology eliminates transformer saturation caused by secondary imbalances. 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. 2881f 12 LTM2881 APPLICATIONS INFORMATION 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). 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 LTM2881. 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 VL) 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. The LTM2881 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 LTM2881 driver outputs in normal and SLO mode operating at 250kbps. SLO mode significantly reduces the high frequency harmonics. Y-Z 10dB/DIV SLO Mode 0 6.25 FREQUENCY 1.25MHz/DIV 12.5 2881 F09a Figure 9a. Frequency Spectrum SLO Mode 125kHz Input 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 reflect 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 logichigh 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 Y-Z 10dB/DIV Receiver and Failsafe 0 6.25 FREQUENCY 1.25MHz/DIV 12.5 2881 F09b Figure 9b. Normal Mode Frequency Spectrum 125kHz Input 2881f 13 LTM2881 APPLICATIONS INFORMATION Receiver Input Resistance The receiver input resistance from A or B to GND2 is greater than 96k permitting up to a total of 256 receivers per system without exceeding the RS485 receiver loading specification. The input resistance of the receiver is unaffected by enabling/disabling the receiver or by powering/unpowering the part. The equivalent input resistance looking into A and B is shown in Figure 10. A 2881 F11 Figure 11. Curve Trace Between A and B with Termination Enabled and Disabled >96k 60Ω 130 128 TE 126 >96k 2881 F10 RESISTANCE (Ω) 60Ω B Figure 10. Equivalent Input Resistance into A and B 124 122 120 118 116 114 112 Switchable Termination Proper cable termination is very important for signal fidelity. If the cable is not terminated with its characteristic impedance, reflections will distort the signal waveforms. 110 –10 15 2881 G11 Figure 12. Termination Resistance vs Common Mode Voltage The integrated switchable termination resistor provides logic control of the line termination for optimal performance when configuring transceiver networks. 150 10 MAGNITUDE (Ω) PHASE 140 0 130 –10 120 MAGNITUDE –20 110 100 0.1 PHASE (DEGREES) When the TE pin is high, the termination resistor is enabled and the differential resistance from A to B is 120Ω. Figure 11 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 12. The integrated termination resistor has a high frequency response which does not limit performance at the maximum specified data rate. Figure 13 shows the magnitude and phase of the termination impedance versus frequency. The termination resistor cannot be enabled by TE if the device is unpowered, ON is low or the LTM2881 is in thermal shutdown. –5 0 5 10 COMMON MODE VOLTAGE (V) –30 –40 1 FREQUENCY (MHz) 10 2881 F13 Figure 13. Termination Magnitude and Phase vs Frequency 2881f 14 LTM2881 APPLICATIONS INFORMATION 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 configuration of Figure 4. 250 • 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. 230 190 170 LTM2881-3 R=54 CL=1000p R=54 CL=100p R=54 CL=0 150 130 110 90 LTM2881-5 R=54 CL=1000p R=54 CL=100p R=54 CL=0 70 VCC 50 0.1 1 DATA RATE (Mbps) 10 2881 F14 Figure 14. Supply Current vs Data Rate GND2 VCC2 ON VL RO A RE B SLOT SUPPLY CURRENT (mA) 210 • If the LTM2881 voltage supply is hot plugged without additional protection, damage may occur. Refer to Linear Technology Application Note 88, entitled “Ceramic Capacitors Can Cause Overvoltage Transients” for a detailed discussion of this problem. To protect against hot plug transients use tantalum for aforementioned additional capacitor. DE PCB Layout Isolation Considerations The high integration of the LTM2881 makes the PCB board layout very simple. However, to optimize its electrical isolation characteristics and thermal performance, some layout considerations are still necessary. Figure 15 is a suggested layout for good thermal performance and to optimize isolation characteristics. Z DI Y TE SLO DO1 9.48mm DL1 GND 2881 F15 Figure 15. PCB Recommended Layout • Use large PCB copper areas for high current paths, including VCC , GND, VCC2 , and GND2. It helps to minimize the PCB conduction loss and thermal stress. • The LTM2881 includes 2.2μF ceramic decoupling capacitors on VCC to GND, VL to GND, and VCC2 to GND2 supply pins. Further decoupling capacitance (10μF) can be added within one-quarter inch away from the VCC, VL , and/or VCC2 pin. 2881f 15 LTM2881 APPLICATIONS INFORMATION Cable Length versus Data Rate CABLE LENGTH (FT) For a given data rate, the maximum transmission distance is bounded by the cable properties. A typical curve of cable length versus data rate compliant with the RS485 standard is shown in Figure 16. Three regions of this curve reflect different performance limiting factors in data transmission. In the flat region of the curve, maximum distance is determined by resistive loss in the cable. The downward sloping region represents limits in distance and rate due to the AC losses in the cable. The solid vertical line represents the specified maximum data rate in the RS485 standard. The dashed line at 250kbps shows the maximum data rate when SLO is low. The dashed line at 20Mbps shows the maximum data rate when SLO is high. 10k LOW-EMI MODE MAX DATA RATE 1k NORMAL MODE MAX DATA RATE 100 RS485 MAX DATA RATE 10 10k 100k 1M 10M DATA RATE (bps) 100M 2881 F16 Figure 16. Cable Length vs Data Rate 2881f 16 LTM2881 TYPICAL APPLICATIONS Full-Duplex RS485 Connection VCC VCC LTM2881 PWR VL A ISOLATION BARRIER RO RE TE DE DI B Y Z GND GND2 2881 TA02 Isolated System Fault Detection VCC VCC LTM2881 VL A ISOLATION BARRIER RO RE TE DE DI 330k B Y Z DIN DOUT FAULT GND GND2 2881 TA03 Switched 5V Power with Isolated CMOS Logic Connection with Low Voltage Interface VCC 1.8V VCC VL RO ISOLATION BARRIER TE DE DI GND IRLML6402 B LTM2881 330k Z DIN DOUT CMOS OUTPUT REGULATED 5V SWITCHED 5V A RE OFF ON PWR VCC2 GND2 CMOS INPUT 2881 TA04 2881f 17 LTM2881 TYPICAL APPLICATIONS 4-Wire Full Duplex Self Biasing for Unshielded CAT5 Connection VCC VCCB ISOLATION BARRIER RO RE DE DI LTM2881 LTM2881 PWR A Y 51Ω B Z 10nF Y 51Ω A 51Ω Z GND GND2 VCC PWR ISOLATION BARRIER VCC VL VL DE DI RE RO B 51Ω 10nF GND2 GND 2881 TA04a BUS INHERITED B 2881f 18 LTM2881 PACKAGE DESCRIPTION LGA Package 32-Lead (15mm × 11.25mm × 2.8mm) (Reference LTC DWG # 05-08-1773 Rev θ) DETAIL A 2.69 – 2.95 8 aaa Z 7 6 5 4 3 2 1 PAD 1 A PAD “A1” CORNER B 4 C D E 15.00 BSC 12.70 BSC MOLD CAP F G SUBSTRATE H 0.290 – 0.350 2.400 – 2.600 J Z bbb Z DETAIL B K L X aaa Z 11.25 BSC Y 8.89 BSC DETAIL B PACKAGE TOP VIEW 4.445 3.175 1.905 0.635 0.635 1.905 3 0.630 ±0.025 Ø 32x eee S X Y 3.175 PADS SEE NOTES PACKAGE BOTTOM VIEW 0.630 ±0.025 Ø 32x 4.445 1.27 BSC eee S X Y DETAIL c DETAIL A 6.350 5.080 DETAIL C NOTES: 1. DIMENSIONING AND TOLERANCING PER ASME Y14.5M-1994 2. ALL DIMENSIONS ARE IN MILLIMETERS 0.000 3 LAND DESIGNATION PER JESD MO-222 4 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 5. PRIMARY DATUM -Z- IS SEATING PLANE 6. THE TOTAL NUMBER OF PADS: 32 5.080 6.350 COMPONENT PIN “A1” LTMXXXXXX μModule TRAY PIN 1 BEVEL SYMBOL TOLERANCE aaa 0.10 bbb 0.10 eee 0.05 PACKAGE IN TRAY LOADING ORIENTATION LGA 32 0308 REV Ø SUGGESTED PCB LAYOUT TOP VIEW 2881f 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. 19 LTM2881 TYPICAL APPLICATION Multi-Node Network with End Termination and Single Ground Connection on Isolation Bus VCCC VCCA VCC LTM2881 PWR VL ISOLATION BARRIER CABLE SHIELD OR GROUND RETURN Y Z Z GND2 GND2 A A RE TE VCC2 DE DI GND C Z GND B Y VL RO PWR GND2 DI B Y DE A B VCC1 RE TE A PWR ISOLATION BARRIER RO VCC LTM2881 ISOLATION BARRIER GND DI DE TE RE RO VCC VL VCCB LTM2881 B 2881 TA05 B RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LTC1535 Isolated RS485 Transceiver 2500VRMS Isolation in Surface Mount Package LT1785 ±60V Fault-Protected Transceiver Half Duplex LT1791 ±60V Fault-Protected Transceiver Full Duplex LTC2861 20Mbps RS485 Transceivers with Integrated Switchable Termination Full Duplex 15kV ESD 2881f 20 Linear Technology Corporation LT 1109 • PRINTED IN USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2009