LTC487IN

LTC487 Quad Low Power RS485 Driver 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 SN75174, DS96174, µA96174, and DS96F174 The LTC487® is a low power differential bus/line driver designed for multipoint data transmission standard RS485 applications with extended common-mode range (– 7V to 12V). 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) and over the 4.75V to 5.25V supply voltage range. APPLICATI ■ ■ S Low Power RS485/RS422 Drivers Level Translator and LTC are registered trademarks and LT is a trademark of Linear Technology Corporation. TYPICAL APPLICATI 10k EN 12 4 2 DI 1 DRIVER 3 1/4 LTC487 LTC487 • TA01 EN 12 CABLE LENGTH (FT) 4 2 120Ω 4000 FT BELDEN 9841 120Ω 1 RECEIVER 3 RO 1/4 LTC489 100 U RS485 Cable Length Specification* 1k 10 10k 100k 1M 2.5M 10M DATA RATE (bps) LTC487 • TA09 UO UO * APPLIES FOR 24 GAUGE, POLYETHYLENE DIELECTRIC TWISTED PAIR 1 LTC487 ABSOLUTE (Note 1) AXI U RATI GS PACKAGE/ORDER I FOR ATIO TOP VIEW DI1 DO1A DO1B EN12 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 Commercial ............................................ 0°C to 70°C Industrial ........................................... – 40°C to 85°C Storage Temperature Range ................. – 65°C to 150°C Lead Temperature (Soldering, 10 sec.)................. 300°C 1 2 3 4 5 6 7 8 16 VCC 15 DI4 14 DO4A 13 DO4B 12 EN34 11 DO3B 10 DO3A 9 DI3 ORDER PART NUMBER LTC487CN LTC487CSW LTC487IN LTC487ISW DO2B DO2A DI2 GND 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 SYMBOL VOD1 VOD2 VOD VOC ⏐VOC ⏐ VCC = 5V ± 5%, 0°C ≤ TA ≤ 70°C (Commercial), – 40°C ≤ TA ≤ 85°C (Industrial) (Note 2, 3) PARAMETER Differential Driver Output Voltage (Unloaded) Differential Driver Output Voltage (With Load) Change in Magnitude of Driver Differential Output Voltage for Complementary Output States Driver Common-Mode Output Voltage Change in Magnitude of Driver Common-Mode Output Voltage for Complementary Output States Input High Voltage Input Low Voltage Input Current Supply Current Driver Short-Circuit Current, VOUT = High Driver Short-Circuit Current, VOUT = Low High Impedance State Output Current No Load VO = – 7V VO = 12V VO = – 7V to 12V Output Enabled Output Disabled IOSD1 IOSD2 IOZ 110 110 100 100 ± 10 DI, EN12, EN34 2.0 CONDITIONS IO = 0 R = 50Ω; (RS422) R = 27Ω; (RS485) (Figure 3) R = 27Ω or R = 50Ω (Figure 3) 2 1.5 5 0.2 3 0.2 V 0.8 ±2 200 200 250 250 ± 200 V µA µA µA mA mA µA MIN TYP MAX 5 UNITS V V V V V V VIH VIL IIN1 ICC SWITCHI G CHARACTERISTICS VCC = 5V ± 5%, 0°C ≤ TA ≤ 70°C (Note 2, 3) SYMBOL tPLH tPHL tSKEW tr, tf tZH tZL tLZ tHZ PARAMETER Driver Input to Output Driver Input to Output Driver Output to Output Driver Rise or Fall Time Driver Enable to Output High Driver Enable to Output Low Driver Disable Time from Low Driver Disable Time from High CL = 100pF (Figures 2, 5) S2 Closed CL = 100pF (Figures 2, 5) S1 Closed CL = 15pF (Figures 2, 5) S1 Closed CL = 15pF (Figures 2, 5) S2 Closed 5 CONDITIONS RDIFF = 54Ω, CL1 = CL2 = 100pF (Figures 1, 4) MIN 10 10 TYP 30 30 5 20 35 35 35 35 MAX 50 50 15 25 70 70 70 70 UNITS ns ns ns ns ns ns ns ns 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 pins are negative. All voltages are referenced to device GND unless otherwise specified. Note 3: All typicals are given for VCC = 5V and Temperature = 25°C. 2 U W U U WW W U LTC487 TYPICAL PERFOR A CE CHARACTERISTICS Driver Output High Voltage vs Output Current –96 TA = 25°C 64 OUTPUT CURRENT (mA) OUTPUT CURRENT (mA) –72 48 OUTPUT CURRENT (mA) – 48 –24 0 0 1 2 3 4 LTC487 • TPC01 OUTPUT VOLTAGE (V) TTL Input Threshold vs Temperature 1.63 INPUT THRESHOLD VOLTAGE (V) 5.0 1.61 TIME (ns) 4.0 SUPPLY CURRENT (µA) 1.59 1.57 1.55 –50 0 50 TEMPERATURE (°C ) DIFFERENTIAL VOLTAGE (V) UW 100 Driver Differential Output Voltage vs Output Current TA = 25°C 80 Driver Output Low Voltage vs Output Current TA = 25°C 60 32 40 16 20 0 0 1 2 3 4 LTC487• TPC02 0 0 1 2 3 4 LTC487 • TPC03 OUTPUT VOLTAGE (V) OUTPUT VOLTAGE (V) Driver Skew vs Temperature 130 Supply Current vs Temperature 120 3.0 110 2.0 100 1.0 –50 0 50 100 LTC487 • TPC05 90 –50 0 50 100 LTC487 • TPC06 LTC487 • TPC04 TEMPERATURE (°C ) TEMPERATURE (°C ) Driver Differential Output Voltage vs Temperature 2.3 RO = 54Ω 2.1 1.9 1.7 1.5 –50 0 50 100 LTC487 • TPC07 TEMPERATURE (°C ) 3 LTC487 PI FU CTI UO H H L 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. EN12 (Pin 4): Driver 1 and 2 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. FU CTI INPUT DI H L X SWITCHI G TI E WAVEFOR S 3V DI 0V t PLH B VO A VO –VO 1/2 VO 80% 10% tr t SKEW VDIFF = V(A) – V(B) 1/2 VO 90% 20% tf LTC487 • TA05 1.5V Figure 1. Driver Propagation Delays 1.5V f = 1MHz : t r ≤ 10ns : t f ≤ 10ns 3V EN12 0V t ZL 5V A, B VOL VOH A, B 0V tZH 2.3V 2.3V Figure 2. Driver Enable and Disable Times 4 W W UO U U U U S GND (Pin 8): GND connection. DI3 (Pin 9): Driver 3 input. Refer to DI1. DO3A (Pin 10): Driver 3 output. DO3B (Pin 11): Driver 3 output. EN34 (Pin 12): Driver 3 and 4 outputs enabled. 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; 4.75 < VCC < 5.25. TABLE ENABLES OUTPUTS OUT A H L Z OUT B L H Z H: High Level L: Low Level X: Irrelevant Z: High Impedance (Off) EN12 or EN34 f = 1MHz : t r < 10ns : t f < 10ns 1.5V t PHL t SKEW 1.5V t LZ OUTPUT NORMALLY LOW 0.5V OUTPUT NORMALLY HIGH tHZ 0.5V LTC487 • TA06 LTC487 TEST CIRCUITS A R VOD R B B DI DRIVER 1 EN12 CL1 A RDIFF 500Ω S1 VCC OUTPUT UNDER TEST VOC CL2 LTC487 • TA03 CL S2 LTC487 • TA02 LTC487 • TA04 Figure 3. Driver DC Test Load Figure 4. Driver Timing Test Circuit Figure 5. Driver Timing Test Load #2 APPLICATI S I FOR ATIO Typical Application A typical connection of the LTC487 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 LTC487 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 EN12 4 DX 1 DX 2 1/4 LTC487 SHIELD 3 120Ω EN12 4 DX 1 DX 2 1/4 LTC487 2 3 1 Figure 6. Typical Connection U 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 LTC487 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. EN12 SHIELD 2 120Ω 1 1/4 LTC489 RX 4 3 RX EN12 4 RX 3 RX 1/4 LTC489 LTC487 • TA07 W U UO 5 LTC487 APPLICATI S I FOR ATIO 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, leading to relatively low overall loss (Figure 7). 10 LOSS PER 100 FT (dB) 1.0 0.1 0.1 1.0 10 100 LTC487 • TA08 FREQUENCY (MHz) 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. 10k CABLE LENGTH (FT) 1k 100 10 10k 100k 1M 2.5M 10M DATA RATE (bps) LTC487 • TA09 Figure 8. Cable Length vs Data Rate 6 U Cable Termination The proper termination of the cable is very important. If the cable is not terminated with its characteristic impedance, 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). PROBE HERE Rt DX DRIVER RECEIVER RX Rt = 120Ω Rt = 47Ω Rt = 470Ω LTC487 • TA10 W U UO Figure 9. Termination Effects If the cable is loaded excessively (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/foot). If the cable is lightly loaded (470Ω), 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 feet of cable. LTC487 APPLICATI S I FOR ATIO 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, causing 33mA of DC current to flow in the cable when no data is being sent. This DC current is about 220 times greater than the supply current of the LTC487. One way to eliminate the unwanted current is by AC coupling the termination resistors as shown in Figure 10. 120Ω C RECEIVER RX C = LINE LENGTH (FT) x 16.3pF LTC487 • TA11 Figure 10. AC Coupled Termination The coupling capacitor must allow high-frequency energy to flow to the termination, but block 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 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, and 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 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. Because the receiver 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. U has about 70mV of hysteresis, the receiver output will maintain the last data bit received. If the receiver output must be forced to a known state, the circuits of Figure 11 can be used. 5V 110Ω 130Ω 130Ω 110Ω RECEIVER RX 5V 1.5k 140Ω RECEIVER RX 1.5k 100k 5V C 120Ω RECEIVER RX LTC487 • TA12 W U UO Figure 11. Forcing ‘0’ When All Drivers Are Off 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 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 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 more protection. The best protection method is to connect a bidirectional TransZorb® from each line side pin to ground (Figure 12). TransZorb is a registered trademark of General Instruments, GSI 7 LTC487 APPLICATI S I FOR ATIO 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 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. TYPICAL APPLICATI RS232 to RS485 Level Translator with Hysteresis R = 220k Y 10k RS232 IN 5.6k DRIVER 1/4 LTC487 120Ω Z ⎜VY - VZ ⎜ 19k HYSTERESIS = 10kΩ • ———— ≈ ———— R R LTC487 • TA14 PACKAGE DESCRIPTIO 0.300 – 0.325 (7.620 – 8.255) Dimensions in inches (millimeters) unless otherwise noted. N Package, 16-Lead Plastic DIP 0.770* (19.558) MAX 16 15 14 13 12 11 10 9 0.130 ± 0.005 (3.302 ± 0.127) 0.009 – 0.015 (0.229 – 0.381) 0.015 (0.381) MIN ( +0.025 0.325 –0.015 8.255 +0.635 –0.381 ) 0.125 (3.175) MIN 0.045 ± 0.015 (1.143 ± 0.381) 0.100 ± 0.010 (2.540 ± 0.254) *THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS. MOLD FLASH OR PROTURSIONS SHALL NOT EXCEED 0.010 INCH (0.254mm). S Package, 16-Lead Plastic SOL 0.291 – 0.299 (7.391 – 7.595) (NOTE 2) 0.010 – 0.029 × 45° (0.254 – 0.737) 16 0.093 – 0.104 (2.362 – 2.642) 0.037 – 0.045 (0.940 – 1.143) 15 0.005 (0.127) RAD MIN 0° – 8° TYP 0.050 (1.270) TYP 0.009 – 0.013 (0.229 – 0.330) NOTE 1 0.016 – 0.050 (0.406 – 1.270) 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 Linear Technology Corporation 1630 McCarthy Blvd., Milpitas, CA 95035-7487 (408) 432-1900 ● FAX: (408) 434-0507 ● TELEX: 499-3977 U Y DRIVER 120Ω Z LTC487 • TA13 U W UO U UO Figure 12. ESD Protection with TransZorbs 0.045 – 0.065 (1.143 – 1.651) 0.260 ± 0.010* 0.065 (6.604 ± 0.254) (1.651) TYP 1 0.018 ± 0.003 (0.457 ± 0.076) 2 3 4 5 6 7 8 N16 0594 0.398 – 0.413 (10.109 – 10.490) (NOTE 2) 14 13 12 11 10 9 NOTE 1 0.394 – 0.419 (10.007 – 10.643) 0.004 – 0.012 (0.102 – 0.305) 0.014 – 0.019 (0.356 – 0.482) TYP 1 2 3 4 5 6 7 8 SOL16 0494 sn487 487fas LT/GP 0894 0K REV A • PRINTED IN USA © LINEAR TECHNOLOGY CORPORATION 1994