LTC486 Quad Low Power RS485 Driver U FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ DESCRIPTIO Very Low Power: ICC = 110µA Typ Designed for RS485 or RS422 Applications Single 5V Supply – 7V to 12V Bus Common-Mode Range Permits ± 7V GND Difference Between Devices on the Bus Thermal Shutdown Protection Power-Up/Down Glitch-Free Driver Outputs Permit Live Insertion/Removal of Package Driver Maintains High Impedance in Three-State or with the Power Off 28ns Typical Driver Propagation Delays with 5ns Skew Pin Compatible with the SN75172, DS96172, µA96172, and DS96F172 The LTC486 is a low power differential bus/line driver designed for multipoint data transmission standard RS485 applications with extended common-mode range (12V to –7V). It also meets RS422 requirements. The CMOS design offers significant power savings over its bipolar counterpart without sacrificing ruggedness against overload or ESD damage. The driver features 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. Both AC and DC specifications are guaranteed from 0°C to 70°C (Commercial), – 40°C to 85°C (Industrial), over the 4.75V to 5.25V supply voltage range. UO APPLICATI ■ ■ S Low Power RS485/RS422 Drivers Level Translator UO TYPICAL APPLICATI RS485 Cable Length Specification* EN EN 4 DI 1 DRIVER 12 2 120Ω 120Ω 4000 FT BELDEN 9841 4 RECEIVER 1 3 RO CABLE LENGTH (FT) 10k 1k 100 1/4 LTC488 1/4 LTC486 EN LTC486 • TA01 10 10k 100k 1M 2.5M 10M DATA RATE (bps) LTC486 • TA09 * APPLIES FOR 24 GAUGE, POLYETHYLENE DIELECTRIC TWISTED PAIR 1 LTC486 W U U W W W AXI U U ABSOLUTE PACKAGE/ORDER I FOR ATIO RATI GS (Note 1) ORDER PART NUMBER TOP VIEW Supply Voltage (VCC) ............................................... 12V Control Input Voltages .................... – 0.5V to VCC + 0.5V Driver Input Voltages ...................... – 0.5V to VCC + 0.5V Driver Output Voltages ......................................... ± 14V Control Input Currents ....................................... ± 25mA Driver Input Currents ......................................... ± 25mA Operating Temperature Range LTC486C ................................................ 0°C to 70°C LTC486I ............................................ – 40°C to 85°C Storage Temperature Range ................. – 65°C to 150°C Lead Temperature (Soldering, 10 sec)................. 300°C DI1 1 16 VCC DO1A 2 15 DI4 DO1B 3 14 DO4A EN 4 13 DO4B DO2B 5 12 EN DO2A 6 11 DO3B DI2 7 10 DO3A GND 8 9 LTC486CN LTC486CSW LTC486IN LTC486ISW DI3 N PACKAGE S PACKAGE 16-LEAD PLASTIC DIP 16-LEAD PLASTIC SOL TJMAX = 125°C, θJA = 70°C/W (N) TJMAX = 150°C, θJA = 95°C/W (S) Consult factory for Military grade parts DC ELECTRICAL CHARACTERISTICS VCC = 5V ± 5%, 0°C ≤ Temperature ≤ 70°C (Commercial), – 40°C ≤ Temperature ≤ 85°C (Industrial) (Note 2, 3) SYMBOL PARAMETER CONDITIONS VOD1 Differential Driver Output Voltage (Unloaded) IO = 0 VOD2 Differential Driver Output Voltage (With Load) R = 50Ω; (RS422) VOD Change in Magnitude of Driver Differential Output Voltage for Complementary Output States R = 27Ω or R = 50Ω (Figure 1) TYP MAX VOC Driver Common-Mode Output Voltage ❘VOC❘ Change in Magnitude of Driver Common-Mode Output Voltage for Complementary Output States VIH Input High Voltage VIL Input Low Voltage 0.8 V IIN1 Input Current ±2 µA ICC Supply Current 5 R = 27Ω; (RS485) (Figure 1) DI, EN, EN No Load MIN 2 UNITS V V 1.5 5 V 0.2 V 3 V 0.2 V 2.0 V Output Enabled 110 200 µA Output Disabled 110 200 µA 100 250 mA IOSD1 Driver Short-Circuit Current, VOUT = High VO = – 7V IOSD2 Driver Short-Circuit Current, VOUT = Low VO = 12V 100 250 mA IOZ High Impedance State Output Current VO = – 7V to 12V ± 10 ± 200 µA UNITS U SWITCHI G CHARACTERISTICS VCC = 5V ± 5%, 0°C ≤ Temperature ≤ 70°C (Commercial), – 40°C ≤ Temperature ≤ 85°C (Industrial) (Note 2, 3) SYMBOL PARAMETER CONDITIONS tPLH Driver Input to Output tPHL Driver Input to Output RDIFF = 54Ω, CL1 = CL2 = 100pF (Figures 2, 4) tSKEW Driver Output to Output tr, tf Driver Rise or Fall Time tZH Driver Enable to Output High 2 MIN TYP MAX 10 30 50 ns 10 30 50 ns 5 15 ns 15 25 ns 35 70 ns 5 CL = 100pF (Figures 3, 5) S2 Closed LTC486 U SWITCHI G CHARACTERISTICS VCC = 5V ± 5%, 0°C ≤ Temperature ≤ 70°C (Commercial), – 40°C ≤ Temperature ≤ 85°C (Industrial) (Note 2, 3) SYMBOL PARAMETER CONDITIONS TYP MAX tZL Driver Enable to Output Low CL = 100pF (Figures 3, 5) S1 Closed MIN 35 70 UNITS ns tLZ Driver Disable Time from Low CL = 15pF (Figures 3, 5) S1 Closed 35 70 ns tHZ Driver Disable Time from High CL = 15pF (Figures 3, 5) S2 Closed 35 70 ns pins are negative. All voltages are referenced to device ground unless otherwise specified. Note 3: All typicals are given for VCC = 5V and temperature = 25°C. 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 of device W U W SWITCHI G TI E WAVEFOR S 3V DI f = 1MHz : t r < 10ns : t f < 10ns 1.5V 1.5V 0V t PLH t PHL B VO A t SKEW 1/2 VO VO 80% –VO 90% VDIFF = V(A) – V(B) 10% tr 20% tf Figure 4. Driver Propagation Delays 3V EN t SKEW 1/2 VO LTC486 • TA05 f = 1MHz : t r ≤ 10ns : t f ≤ 10ns 1.5V 1.5V 0V t ZL 5V A, B VOL VOH A, B t LZ 2.3V OUTPUT NORMALLY LOW 2.3V OUTPUT NORMALLY HIGH 0.5V 0.5V 0V tHZ tZH LTC486 • TA06 Figure 5. Driver Enable and Disable Times 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 64 –72 – 48 –24 80 OUTPUT CURRENT (mA) OUTPUT CURRENT (mA) –96 OUTPUT CURRENT (mA) Driver Output Low Voltage vs Output Current TA = 25°C 48 32 16 0 0 0 1 2 3 OUTPUT VOLTAGE (V) 4 LTC486 • TPC01 60 40 20 0 0 1 2 3 OUTPUT VOLTAGE (V) 4 LTC486• TPC02 0 1 2 3 OUTPUT VOLTAGE (V) 4 LTC486 • TPC03 3 LTC486 U W TYPICAL PERFOR A CE CHARACTERISTICS Driver Skew vs Temperature 1.63 5 1.61 4 TIME (ns) INPUT THRESHOLD VOLTAGE (V) TTL Input Threshold vs Temperature 1.59 1.57 3 2 1.55 –50 0 50 1 –50 100 TEMPERATURE (°C ) 50 130 DIFFERENTIAL VOLTAGE (V) 120 110 100 0 50 2.1 1.9 1.7 1.5 –50 100 TEMPERATURE (°C ) 0 50 TABLE UO ENABLES OUTPUTS U DI EN EN OUTA OUTB H L H L X H H X X L X X L L H H L H L Z L H L H Z 4 100 LTC486 • TPC06 TEMPERATURE (°C ) INPUT LTC486 • TPC05 2.3 90 –50 FU CTI 100 Driver Differential Output Voltage vs Temperature RO = 54Ω Supply Current vs Temperature SUPPLY CURRENT (µA) 0 TEMPERATURE (°C ) LTC486 • TPC04 H: High Level L: Low Level X: Irrelevant Z: High Impedance (Off) LTC486 • TPC07 LTC486 UO U U PI FU CTI S DI1 (Pin 1): Driver 1 Input. If Driver 1 is enabled, then a low on DI1 forces the driver outputs DO1A low and DO1B high. A high on DI1 with the driver outputs enabled will force DO1A high and DO1B low. DO1A (Pin 2): Driver 1 Output. DO1B (Pin 3): Driver 1 Output. EN (Pin 4): Driver Outputs Enabled. See Function Table for details. DO2B (Pin 5): Driver 2 Output. DO2A (Pin 6): Driver 2 Output. DI2 (Pin 7): Driver 2 Input. Refer to DI1. TEST CIRCUITS GND (Pin 8): Ground Connection. DI3 (Pin 9): Driver 3 Input. Refer to DI1. DO3A (Pin 10): Driver 3 Output. DO3B (Pin 11): Driver 3 Output. EN (Pin 12): Driver Outputs Disabled. See Function Table for details. DO4B (Pin 13): Driver 4 Output. DO4A (Pin 14): Driver 4 Output. DI4 (Pin 15): Driver 4 Input. Refer to DI1. VCC (Pin 16): Positive Supply; 4.75V < VCC < 5.25V . S1 EN A VCC CI1 R OUTPUT UNDER TEST A VOD DI R DRIVER RDIFF CL VOC B 500Ω S2 B CI2 LTC486 • TA04 LTC486 • TA03 EN LTC486 • TA02 W U UO APPLICATI Figure 2. Driver Timing Test Circuit Figure 3. Driver Timing Test Load #2 U Figure 1. Driver DC Test Load S I FOR ATIO Typical Application A typical connection of the LTC486 is shown in Figure 6. A twisted pair of wires connect up to 32 drivers and receivers for half 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. The optional shields around the twisted pair help reduce unwanted noise, and are connected to GND at one end. Thermal Shutdown The LTC486 has a thermal shutdown feature which protects the part from excessive power dissipation. If the outputs of the driver are accidently shorted to a power supply or low impedance source, up to 250mA can flow through the part. The thermal shutdown circuit disables the driver outputs when the internal temperature reaches 150°C and turns them back on when the temperature cools to 130°C. If the outputs of two or more LTC486 drivers are shorted directly, the driver outputs can not supply enough current to activate the thermal shutdown. Thus, the thermal shutdown circuit will not prevent contention faults when two drivers are active on the bus at the same time. Cable 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. 5 LTC486 W U U UO S I FOR ATIO APPLICATI EN EN 4 DX 1 SHIELD SHIELD 3 2 120Ω DX 120Ω 2 4 3 RX RX 1 12 12 EN EN 1/4 LTC486 EN 4 DX 1 3 EN 1/4 LTC488 4 1 DX 3 RX 2 2 12 EN 1/4 LTC486 RX 12 EN 1/4 LTC488 LTC486 • TA07 Figure 6. Typical Connection LOSS PER 100 FT (dB) 10 1k 100 10 10k 100k 1M 2.5M 10M DATA RATE (bps) 1 LTC486 • TA09 Figure 8. Cable Length vs Data Rate 0.1 0.1 1 10 100 FREQUENCY (MHz) LTC486 • TA08 Figure 7. Attenuation vs Frequency for Belden 9841 When using low loss cables, Figure 8 can be used as a guideline for choosing the maximum line length for a given data rate. With lower quality PVC cables, the dielectric loss factor can be 1000 times worse. PVC twisted pairs have terrible losses at high data rates (>100kbs) and greatly reduce the maximum cable length. At low data rates however, they are acceptable and much more economical. Cable Termination The proper termination of the cable is very important. If the cable is not terminated with its characteristic impedance, 6 10k CABLE LENGTH (FT) Losses in a transmission line are a complex combination of DC conductor loss, AC losses (skin effect), leakage, and AC losses in the dielectric. In good polyethylene cables such as the Belden 9841, the conductor losses and dielectric losses are of the same order of magnitude, with relatively low overall loss (Figure 7). distorted waveforms will result. In severe cases, distorted (false) data and nulls will occur. A quick look at the output of the driver will tell how well the cable is terminated. It is best to look at a driver connected to the end of the cable, since this eliminates the possibility of getting reflections from two directions. Simply look at the driver output while transmitting square wave data. If the cable is terminated properly, the waveform will look like a square wave (Figure 9). If the cable is loaded excessively (e.g., 47Ω), the signal initially sees the surge impedance of the cable and jumps to an initial amplitude. The signal travels down the cable and is reflected back out of phase because of the mistermination. 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/ft). If the cable is lightly loaded (e.g., 470Ω), LTC486 W U U UO S I FOR ATIO APPLICATI PROBE HERE Rt DX DRIVER RECEIVER RX Rt = 120Ω Rt = 47Ω represents an electrical one-tenth wavelength. The value of the coupling capacitor should therefore be set at 16.3pF per foot of cable length for 120Ω cables. With the coupling capacitors in place, power is consumed only on the signal edges, not when the driver output is idling at a 1 or 0 state. A 100nF capacitor is adequate for lines up to 4000 feet in length. Be aware that the power savings start to decrease once the data rate surpasses 1/(120Ω × C). Receiver Open-Circuit Fail-Safe Rt = 470Ω LTC486 • TA10 Figure 9. Termination Effects 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 ft. of cable. AC Cable Termination Cable termination resistors are necessary to prevent unwanted reflections, but they consume power. The typical differential output voltage of the driver is 2V when the cable is terminated with two 120Ω resistors. When no data is being sent 33mA of DC current flows in the cable . This DC current is about 220 times greater than the supply current of the LTC486. One way to eliminate the unwanted current is by AC coupling the termination resistors as shown in Figure 10. Some data encoding schemes require that the output of the receiver maintains a known state (usually a logic 1) when the data is finished transmitting and all drivers on the line are forced into three-state. All LTC RS485 receivers have a fail-safe feature which guarantees the output to be in a logic 1 state when the receiver inputs are left floating (open-circuit). However, when the cable is terminated with 120Ω, the differential inputs to the receiver are shorted together, not left floating. If the receiver output must be forced to a known state, the circuits of Figure 11 can be used. The termination resistors are used to generate a DC bias which forces the receiver output to a known state, in this case a logic 0. The first method consumes about 208mW and the second about 8mW. The lowest power 5V 110Ω 130Ω 130Ω 110Ω RECEIVER RX RECEIVER RX RECEIVER RX 5V 120Ω 1.5k C RECEIVER RX 140Ω C = LINE LENGTH (FT) × 16.3pF 1.5k LTC486 • TA11 Figure 10. AC Coupled Termination The coupling capacitor allows high frequency energy to flow to the termination, but blocks DC and low frequencies. The dividing line between high and low frequency depends on the length of the cable. The coupling capacitor must pass frequencies above the point where the line 100k C 5V 120Ω LTC486 • TA12 Figure 11. Forcing “0” When All Dirvers Are Off 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. 7 LTC486 W U U UO APPLICATI S I FOR ATIO solution is to use an AC termination with a pull-up resistor. Simply swap the receiver inputs for data protocols ending in logic 1. Fault Protection breakdown voltages and prices. Be sure to pick a breakdown voltage higher than the common-mode voltage required for your application (typically 12V). Also, don’t forget to check how much the added parasitic capacitance will load down the bus. 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 greater protection. The best protection method is to connect a bidirectional TransZorb® from each line side pin to ground (Figure 12). A TransZorb® is a silicon transient voltage suppressor that has exceptional surge handling capabilities, fast response time, and low series resistance. They are available from General Semiconductor Industries and come in a variety of Y 120Ω DRIVER Z LTC486 • TA13 Figure 12. ESD Protection TransZorb® is a registrated trademark of General Instruments, GSI UO TYPICAL APPLICATI RS232 to RS485 Level Translator with Hysteresis R = 220k Y 10k RS232 IN 120Ω DRIVER 5.6k Z 1/4 LTC486 ⎜VY - VZ ⎜ 19k HYSTERESIS = 10k × ———— ≈ —— R R LTC486 • TA14 U PACKAGE DESCRIPTIO Dimensions in inches (millimeters) unless otherwise noted. 0.300 – 0.325 (7.620 – 8.255) N Package 16-Lead Plastic DIP 0.009 – 0.015 (0.229 – 0.381) ( 0.130 ± 0.005 (3.302 ± 0.127) 0.015 (0.381) MIN +0.025 0.325 –0.015 8.255 0.005 (0.127) RAD MIN +0.635 –0.381 ) 0.065 (1.651) TYP 0.125 (3.175) MIN 0.045 ± 0.015 (1.143 ± 0.381) 15 14 13 12 11 10 9 1 2 3 4 5 6 7 8 10 9 0.260 ± 0.010 (6.604 ± 0.254) 0.018 ± 0.003 (0.457 ± 0.076) 0.100 ± 0.010 (2.540 ± 0.254) 0.093 – 0.104 (2.362 – 2.642) 0.037 – 0.045 (0.940 – 1.143) 16 15 14 13 12 11 0° – 8° TYP 0.009 – 0.013 (0.229 – 0.330) NOTE 1 0.050 (1.270) TYP 0.014 – 0.019 0.016 – 0.050 (0.356 – 0.482) (0.406 – 1.270) TYP NOTE: 1. PIN 1 IDENT, NOTCH ON TOP AND CAVITIES ON THE BOTTOM OF PACKAGES ARE THE MANUFACTURING OPTIONS. THE PART MAY BE SUPPLIED WITH OR WITHOUT ANY OF THE OPTIONS. 2. THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS. MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.006 INCH (0.15mm). 8 16 0.398 – 0.413 (10.109 – 10.490) (NOTE 2) 0.291 – 0.299 (7.391 – 7.595) (NOTE 2) 0.010 – 0.029 × 45° (0.254 – 0.737) S Package 16-Lead Plastic SOL 0.770 (19.558) MAX 0.045 – 0.065 (1.143 – 1.651) Linear Technology Corporation 0.004 – 0.012 (0.102 – 0.305) 0.394 – 0.419 (10.007 – 10.643) NOTE 1 1 2 3 4 5 6 7 8 sn486 486fas LT/GP 0294 5K REV A • PRINTED IN USA 1630 McCarthy Blvd., Milpitas, CA 95035-7487 (408) 432-1900 ● FAX: (408) 434-0507 ● TELEX: 499-3977 © LINEAR TECHNOLOGY CORPORATION 1994