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. The CMOS design offers significant power savings without sacrificing ruggedness against overload or ESD damage. Typical quiescent current is only 80µA while operating and 1µA in shutdown. High Input Impedance: Up to 256 Transceivers on the Bus Low Power: ICC = 120µA Max with Driver Disabled ICC = 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 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. UO APPLICATI ■ ■ ■ S The LTC1487 is fully specified over the commercial temperature range and is available in 8-pin DIP and SO packages. Battery-Powered RS485/RS422 Applications Low Power RS485/RS422 Transceiver Level Translator , LTC and LT are registered trademarks of Linear Technology Corporation. UO TYPICAL APPLICATI 1 RO 2 RE 3 DE 4 DI LTC1487 R R 2000 FEET OF TWISTED-PAIR WIRE 7 7 120Ω D 120Ω 6 6 D 330Ω 1 RO 2 RE 3 DE 4 DI DI RECEIVER INPUT LTC1487 A B 4.7nF RO EQUIVALENT LOAD OF 256 LTC1487 TRANSCEIVERS LTC1487 • TA01 LTC1487 • TA02 1 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 + 0.5V Operating Temperature Range ............. 0°C ≤ TA ≤ 70°C Lead Temperature (Soldering, 10 sec)................. 300°C ORDER PART NUMBER TOP VIEW RO 1 8 VCC RE 2 7 B DE 3 6 A 5 GND DI 4 R D N8 PACKAGE 8-LEAD PDIP S8 PACKAGE 8-LEAD PLASTIC SO LTC1487CN8 LTC1487CS8 S8 PART MARKING 1487 TJMAX = 125°C, θJA = 130°C/ W (N8) TJMAX = 125°C, θJA = 150°C/ W (S8) Consult factory for Industrial and Military grade parts. ELECTRICAL CHARACTERISTICS 0°C ≤ TA ≤ 70°C, VCC = 5V (Notes 2, 3) unless otherwise noted. SYMBOL PARAMETER CONDITIONS MIN VOD1 Differential Driver Output Voltage (Unloaded) IO = 0 ● VOD2 Differential Driver Output Voltage (with Load) R = 50Ω (RS422) R = 27Ω (RS485), Figure 1 ● ● TYP 2.0 1.5 MAX UNITS 5 V 5 V V ∆VOD Change in Magnitude of Driver Differential Output Voltage for Complementary Output States R = 27Ω or R = 50Ω, Figure 1 ● 0.2 V VOC Driver Common-Mode Output Voltage R = 27Ω or R = 50Ω, Figure 1 ● 3 V ∆VOC Change in Magnitude of Driver Common-Mode Output Voltage for Complementary Output States R = 27Ω or R = 50Ω, Figure 1 ● 0.2 V VIH Input High Voltage DE, DI, RE ● VIL Input Low Voltage DE, DI, RE ● 0.8 IIN1 Input Current DE, DI, RE ● ±2 µA IIN2 Input Current (A, B) DE = 0, VCC = 0V or 5.25V, VIN = 12V DE = 0, VCC = 0V or 5.25V, VIN = – 7V ● ● 0.30 – 0.15 mA mA VTH Differential Input Threshold Voltage for Receiver – 7V ≤ VCM ≤ 12V ● ∆VTH Receiver Input Hysteresis VCM = 0V ● VOH Receiver Output High Voltage IO = – 4mA, VID = 200mV ● VOL Receiver Output Low Voltage IO = 4mA, VID = – 200mV ● 0.4 V IOZR Three-State (High Impedance) Output Current at Receiver VCC = Max, 0.4V ≤ VO ≤ 2.4V ● ±1 µA RIN Receiver Input Resistance – 7V ≤ VCM ≤ 12V ● ICC Supply Current No Load, Output Enabled No Load, Output Disabled ● ● ISHDN Supply Current in Shutdown Mode DE = 0V, RE = VCC IOSD1 Driver Short-Circuit Current, VOUT = HIGH – 7V ≤ VO ≤ 12V ● IOSD2 Driver Short-Circuit Current, VOUT = LOW – 7V ≤ VO ≤ 12V ● IOSR Receiver Short-Circuit Current 0V ≤ VO ≤ VCC ● 2 2 V – 0.2 0.2 45 V mV 3.5 70 V V 96 kΩ 120 80 200 120 µA µA 1 10 µA 35 250 mA 35 250 mA 7 85 mA 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 1.5 VOC Driver Common-Mode Output Voltage R = 27Ω or R = 50Ω, Figure 1 ● VTH Differential Input Threshold Voltage for Receiver – 7V ≤ VCM ≤ 12V ● ∆VTH Receiver Input Hysteresis VCM = 0V ● 45 ICC Supply Current No Load, Output Enabled No Load, Output Disabled ● ● 120 80 ISHDN Supply Current in Shutdown Mode DE = 0V, RE = VCC tPLH Driver Input to Output tPHL Driver Input to Output RDIFF = 54Ω, CL1 = CL2 = 100pF, (Figures 3, 5) tSKEW Driver Output to Output ● t r , tf Driver Rise or Fall Time ● 150 tPLH Receiver Input to Output ● 30 tPHL Receiver Input to Output ● 30 tSKD tPLH – tPHL Differential Receiver Skew ● fMAX Maximum Data Rate ● RDIFF = 54Ω, CL1 = CL2 = 100pF, (Figures 3, 7) U SWITCHI G CHARACTERISTICS – 0.2 MAX UNITS 5 V 5 V V 3 V 0.2 V mV 200 120 µA µA 10 µA ● 150 1200 ns ● 150 1200 ns 1 100 600 ns 2000 ns 140 250 ns 140 250 13 ns ns 250 kbps 0°C ≤ TA ≤ 70°C, VCC = 5V (Notes 2, 3) unless otherwise noted. SYMBOL PARAMETER CONDITIONS MIN tPLH Driver Input to Output tPHL Driver Input to Output RDIFF = 54Ω, CL1 = CL2 = 100pF, (Figures 3, 5) tSKEW Driver Output to Output tr, tf Driver Rise or Fall Time ● tZH Driver Enable to Output High CL = 100pF (Figures 4, 6), S2 Closed ● tZL Driver Enable to Output Low CL = 100pF (Figures 4, 6), S1 Closed tLZ Driver Disable Time from Low tHZ Driver Disable Time from High tPLH Receiver Input to Output tPHL Receiver Input to Output tSKD tPLH – tPHL Differential Receiver Skew tZL Receiver Enable to Output Low tZH tLZ TYP MAX UNITS ● 150 1200 ns ● 150 1200 ns 600 ns 150 1200 ns 100 1500 ns ● 100 1500 ns CL = 15pF (Figures 4, 6), S1 Closed ● 150 1500 ns CL = 15pF (Figures 4, 6), S2 Closed ● 150 1500 ns RDIFF = 54Ω, CL1 = CL2 = 100pF, (Figures 3, 7) ● 30 140 250 ns ● 30 140 250 250 ● ns ● 13 CRL = 15pF (Figures 2, 8), S1 Closed ● 20 50 ns Receiver Enable to Output High CRL = 15pF (Figures 2, 8), S2 Closed ● 20 50 ns Receiver Disable from Low CRL = 15pF (Figures 2, 8), S1 Closed ● 20 50 ns tHZ Receiver Disable from High CRL = 15pF (Figures 2, 8), S2 Closed ● 20 50 fMAX Maximum Data Rate tSHDN Time to Shutdown DE = 0, RE = ● 250 ● 50 ns ns kbps 200 600 ns 3 LTC1487 U SWITCHI G CHARACTERISTICS 0°C ≤ TA ≤ 70°C, VCC = 5V (Notes 2, 3) unless otherwise noted. SYMBOL PARAMETER CONDITIONS MAX UNITS tZH(SHDN) Driver Enable from Shutdown to Output High CL = 100pF (Figures 4, 6), S2 Closed ● 2000 ns tZL(SHDN) Driver Enable from Shutdown to Output Low tZH(SHDN) Receiver Enable from Shutdown to Output High CL = 100pF (Figures 4, 6), S1 Closed ● 2000 ns CL = 15pF (Figures 2, 8), S2 Closed ● 2000 ns tZL(SHDN) Receiver Enable from Shutdown to Output Low CL = 15pF (Figures 2, 8), S1 Closed ● 2000 ns The ● denotes specifications which apply over the full operating temperature range. Note 1: Absolute maximum ratings are those beyond which the safety of the device cannot be guaranteed. Note 2: All currents into device pins are positive; all currents out ot device pins are negative. All voltages are referenced to device ground unless otherwise specified. MIN TYP Note 3: All typicals are given for VCC = 5V and TA = 25°C. Note 4: The LTC1487 is not tested and is not quality-assurance sampled at – 40°C and at 85°C. These specifications are guaranteed by design, correlation, and/or inference from 0°C, 25°C and/or 70°C tests. U W TYPICAL PERFORMANCE CHARACTERISTICS Driver Differential Output Voltage vs Output Current Supply Current vs Temperature TA = 25°C 400 THERMAL SHUTDOWN WITH DRIVER ENABLED AND NOMINAL LOAD 300 250 200 150 DRIVER ENABLED WITH NO LOAD 100 50 60 50 40 30 20 0 0 25 50 75 100 125 150 175 TEMPERATURE (°C) 2.18 2.16 2.14 2.12 2.10 2.08 2.06 2.04 10 DRIVER DISABLED WITH NO LOAD 0 – 50 –25 2.20 DIFFERENTIAL VOLTAGE (V) 350 RL = 54Ω 2.22 70 OUTPUT CURRENT (mA) SUPPLY CURRENT (µA) 2.24 80 450 2.02 0 2.00 –50 –25 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 OUTPUT VOLTAGE (V) 50 25 0 75 TEMPERATURE (°C) LTC1487 • TPC02 LTC1487 • TPC01 Driver Output Low Voltage vs Output Current Driver Skew vs Temperature TA = 25°C –10 100 125 500 0 TA = 25°C 100 LTC1487 • TPC03 Driver Output High Voltage vs Output Current 120 450 –20 80 60 40 400 –30 –40 TIME (ns) OUTPUT CURRENT (mA) OUTPUT CURRENT (mA) Driver Differential Output Voltage vs Temperature –50 –60 –70 300 250 –80 20 350 200 –90 0 0 1 2 3 OUTPUT VOLTAGE (V) 4 LTC1487 • TPC04 4 –100 0 1 3 4 2 OUTPUT VOLTAGE (V) 5 LTC1487 • TPC05 150 –50 –25 50 25 75 0 TEMPERATURE (°C) 100 125 LTC1487 • G06 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. A LOW input will force the driver outputs into a high impedance state and the chip will function as a line receiver. If RE is HIGH and DE is LOW, the part will enter a low power (1µA) shutdown state. DI (Pin 4): Driver Input. If the driver outputs are enabled (DE HIGH) then a LOW on DI forces the outputs A LOW and B HIGH. A HIGH on DI with the driver outputs enabled will force A HIGH and B LOW. GND (Pin 5): Ground. A (Pin 6): Driver Output/Receiver Input. B (Pin 7): Driver Output/Receiver Input. VCC (Pin 8): Positive Supply. 4.75V < VCC < 5.25V. U U FU CTIO TABLES LTC1487 Transmitting LTC1487 Receiving INPUTS OUTPUTS INPUTS OUTPUTS RE DE DI B A RE DE A–B RO X 1 1 0 1 0 0 ≥ 0.2V 1 X 1 0 1 0 0 0 ≤ – 0.2V 0 0 0 X Z Z 0 0 Inputs Open 1 1 0 X Z* Z* 1 0 X Z* *Shutdown mode *Shutdown mode TEST CIRCUITS A R VOD 1k VCC 1k CRL VOC R S1 TEST POINT RECEIVER OUTPUT S2 B LTC1487 • F01 LTC1487 • F02 Figure 1. Driver DC Test Load Figure 2. Receiver Timing Test Load 3V DE A DI CL1 S1 RO RDIFF B A B CL2 RE 15pF OUTPUT UNDER TEST VCC 500Ω S2 CL LTC1487 • F03 LTC1487 • F04 Figure 3. Driver/Receiver Timing Test Circuit Figure 4. Driver Timing Test Load 5 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. Driver Enable and Disable Times VOH 1.5V RO VOL f = 1MHz, tr ≤ 10ns, tf ≤ 10ns t PHL VOD2 A–B –VOD2 1.5V OUTPUT 0V t PLH 0V INPUT LTC1487 • F07 Figure 7. Receiver Propagation Delays 3V 1.5V RE 5V RO RO 1.5V f = 1MHz, tr ≤ 10ns, tf ≤ 10ns 0V t ZL(SHDN), tZL t LZ 1.5V OUTPUT NORMALLY LOW 0.5V 1.5V OUTPUT NORMALLY HIGH 0.5V 0V t ZH(SHDN), tZH t HZ Figure 8. Receiver Enable and Disable Times 6 LTC1487 • F08 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. The 96kΩ input impedance of the LTC1487 will draw only 125µA under the same 12V condition, presenting only 0.125UL to the bus. As a result, 256 LTC1487 transceivers (32UL/0.125UL = 256) can be connected to a single RS485 data bus without exceeding the RS485 driver load specification. The LTC1487 meets all other RS485 specifications, allowing it to operate equally well with standard RS485 transceiver devices or high impedance transceivers. CMOS Output Driver The RS485 specification requires that a transceiver withstand common-mode voltages of up to 12V or –7V at the RS485 line connections. Additionally, the transceiver must be immune to both ESD and latch-up. This rules out traditional CMOS drivers, which include parasitic diodes from their driver outputs to each supply rail (Figure 9). The LTC1487 uses a proprietary process enhancement which adds a pair of Schottky diodes to the output stage (Figure 10), preventing current from flowing when the commonmode voltage exceeds the supply rails. Latch-up at the output drivers is virtually eliminated and the driver is prevented from loading the line under RS485 specified fault conditions. A proprietary output protection structure protects the transceiver line terminals against ESD strikes (Human Body Model) of up to ±10kV. P1 D1 OUTPUT N1 P1 D1 OUTPUT LOGIC SD4 N1 D2 LTC1487 • F10 Figure 10. LTC1487 Output Stage When two or more drivers are connected to the same transmission line, a potential condition exists whereby more than two drivers are simultaneously active. If one or more drivers is sourcing current while another driver is sinking current, excessive power dissipation may occur within either the sourcing or sinking element. This condition is defined as driver contention, since multiple drivers are competing for one transmission line. The LTC1487 provides a current limiting scheme to prevent driver contention failure. When driver contention occurs, the current drawn is limited to about 70mA, preventing excessive power dissipation within the drivers. The LTC1487 has a thermal shutdown feature which protects the part from excessive power dissipation. Under extreme fault conditions, up to 250mA can flow through the part, causing rapid internal temperature rise. The thermal shutdown circuit will disable the driver outputs when the internal temperature reaches 150°C and turns them back on when the temperature cools to 130°C. This cycle will repeat as necessary until the fault condition is removed. Receiver Inputs VCC LOGIC SD3 D2 LTC1487 • F09 Figure 9. Conventional CMOS Output Stage The LTC1487 receiver features an input common-mode range covering the entire RS485 specified range of –7V to 12V. Internal 96k input resistors from each line terminal to ground provide the 0.125UL load to the RS485 bus. Differential signals of greater than ±200mV within the specified input common-mode range will be converted to a TTL-compatible signal at the receiver output. A small amount of input hysteresis is included to minimize the 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 circuits as described herein will not infringe on existing patent rights. 7 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. In order to guarantee that the part goes into shutdown, RE must be HIGH and DE must be LOW for at least 600ns simultaneously. If this time duration is less than 50ns the part will not enter shutdown mode. Toggling either RE or DE will wake the LTC1487 back up within 3.5µs. Low Power Operation If the driver is active immediately prior to shutdown, the supply current will not drop to 1µA until the driver outputs have reached a steady state; this can take as long as 2.6µs under worst case conditions. If the driver is disabled prior to shutdown the supply current will drop to 1µA immediately. The LTC1487 draws very little supply current whenever the driver outputs are disabled. In shutdown mode, the quiescent current is typically less than 1µA. With the receiver active and the driver outputs disabled, the LTC1487 will typically draw 80µA quiescent current. With the driver outputs enabled but unterminated, quiescent current will rise slightly as one of the two outputs sources current into the internal receiver input resistance. With the minimum receiver input resistance of 70k and the maximum output swing of 5V, the quiescent current will rise by a maximum of 72µA. Typical quiescent current rise with the driver enabled is about 40µA. The quiescent current rises significantly if the driver is enabled when it is externally terminated. With 1/2 termination load (120Ω between the driver outputs), the quiescent current will jump to at least 13mA as the drivers force a minimum of 1.5V across the termination resistance. With a fully terminated 60Ω line attached, the current will rise to greater than 25mA with the driver enabled, completely overshadowing the extra 40µA drawn by the internal receiver inputs. Shutdown Mode Both the receiver output (RO) and the driver outputs (A, B) can be placed in three-state mode by bringing RE HIGH and DE LOW respectively. In addition, the LTC1487 will enter shutdown mode when RE is HIGH and DE is LOW. Slew Rate and Propagation Delay Many digital encoding schemes are dependent upon the difference in the propagation delay times of the driver and receiver. Figure 11 shows the test circuit for the LTC1487 propagation delay. 100pF TTL IN t r, t f < 6ns D BR R R 100Ω RECEIVER OUT LTC1487 • F11 100pF Figure 11. Receiver Propagation Delay Test Circuit The receiver delay times are: tPLH – tPHL = 13ns Typ, VCC = 5V The LTC1487 drivers feature controlled slew rate to reduce system EMI and improve signal fidelity by reducing reflections due to misterminated cables. The driver’s skew times are: Skew = 250ns Typ, VCC = 5V 600ns Max, VCC = 5V, TA = – 40°C to 85°C U PACKAGE DESCRIPTION For package descriptions consult the 1994 Linear Databook Volume III. 8 Linear Technology Corporation LT/GP 0395 10K • PRINTED IN THE USA 1630 McCarthy Blvd., Milpitas, CA 95035-7487 (408) 432-1900 ● FAX: (408) 434-0507 ● TELEX: 499-3977 LINEAR TECHNOLOGY CORPORATION 1995