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1 ltc490 490fb differential driver and receiver pair n low power: i cc = 300 m a typical n designed for rs485 or rs422 applications n single 5v supply n C 7v to 12v bus common mode range permits 7v ground difference between devices on the bus n thermal shutdown protection n power-up/-down glitch-free driver outputs permit live insertion or removal of package n driver maintains high impedance with the power off n combined impedance of a driver output and receiver allows up to 32 transceivers on the bus n 70mv typical input hysteresis n 28ns typical driver propagation delays with 5ns skew for 2.5mb operation n pin compatible with the sn75179 the ltc ? 490 is a low power differential bus/line trans- ceiver designed for multipoint data transmission standard rs485 applications with extended common mode range (12v to C7v). it also meets the requirements of rs422. the cmos design offers significant power savings over its bipolar counterpart without sacrificing ruggedness against overload or esd damage. 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. both ac and dc specifications are guaranteed from 0 c to 70 c and 4.75v to 5.25v supply voltage range. n low power rs485/rs422 transceiver n level translator features descriptio u applicatio s u typical applicatio u , ltc and lt are registered trademarks of linear technology corporation. ltc490 ?ta01 120 w 120 w 120 w 120 w 4000 ft belden 9841 4000 ft belden 9841 receiver ltc490 driver receiver ltc490 driver r d r d 3 2 7 8 6 5
ltc490 2 490fb (note 1) supply voltage (v cc ) ............................................... 12v driver input currents ........................... C 25ma to 25ma driver input voltages ...................... C0.5v to v cc + 0.5v driver output voltages .......................................... 14v receiver input voltages ......................................... 14v receiver output voltages ............... C0.5v to v cc + 0.5v operating temperature range ltc490c ................................................ 0 c to 70 c ltc490i ............................................. C 40 c to 85 c storage temperature range ................. C 65 c to 150 c lead temperature (soldering, 10 sec).................. 300 c t jmax = 125 c, q ja = 100 c/ w (n8) t jmax = 150 c, q ja = 150 c/ w (s8) n8 package 8-lead pdip s8 package 8-lead plastic so 1 2 3 4 8 7 6 5 top view v cc r d gnd a b z y r d order part number s8 part marking ltc490cn8 ltc490cs8 ltc490in8 ltc490is8 490 490i the l denotes the specificatiions which apply over the full operating temperature range, otherwise specifications are at t a = 25 c. v cc = 5v 5% symbol parameter conditions min typ max units v od1 differential driver output voltage (unloaded) i o = 0 l 5v v od2 differential driver output voltage (with load) r = 50 w (rs422) l 2v r = 27 w (rs485) (figure 1) l 1.5 5 v d v od change in magnitude of driver differential output r = 27 w or r = 50 w (figure 1) l 0.2 v voltage for complementary output states v oc driver common mode output voltage r = 27 w or r = 50 w (figure 1) l 3v d v oc change in magnitude of driver common mode r = 27 w or r = 50 w (figure 1) l 0.2 v output voltage for complementary output states v ih input high voltage (d) l 2.0 v v il input low voltage (d) l 0.8 v l in1 input current (d) l 2 m a l in2 input current (a, b) v cc = 0v or 5.25v v in = 12v l 1ma v in = C 7v l C 0.8 ma v th differential input threshold voltage for receiver C 7v v cm 12v l C 0.2 0.2 v d v th receiver input hysteresis v cm = 0v l 70 mv v oh receiver output high voltage i o = C4ma, v id = 0.2v l 3.5 v v ol receiver output low voltage i o = 4ma, v id = C 0.2v l 0.4 v i ozr three-state output current at receiver v cc = max 0.4v v o 2.4v l 1 m a i cc supply current no load; d = gnd or v cc l 300 500 m a r in receiver input resistance C 7v v o 12v l 12 k w i osd1 driver short-circuit current, v out = high v o = C 7v l 100 250 ma i osd2 driver short-circuit current, v out = low v o = 12v l 100 250 ma i osr receiver short-circuit current 0v v o v cc l 785ma i oz driver three-state output current v o = C 7v to 12v l 2 200 m a absolute axi u rati gs w ww u consult ltc marketing for parts specified with wider operating temperature ranges. package/order i for atio uu w dc electrical characteristics
3 ltc490 490fb 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 ground unless otherwise specified. note 3: all typicals are given for v cc = 5v and temperature = 25 c. symbol parameter conditions min typ max units t plh driver input to output r diff = 54 w , c l1 = c l2 = 100pf (figures 2, 3) l 10 30 50 ns t phl driver input to output r diff = 54 w , c l1 = c l2 = 100pf (figures 2, 3) l 10 30 50 ns t skew driver output to output r diff = 54 w , c l1 = c l2 = 100pf (figures 2, 3) l 5ns t r , t f driver rise or fall time r diff = 54 w , c l1 = c l2 = 100pf (figures 2, 3) l 5525 ns t plh receiver input to output r diff = 54 w , c l1 = c l2 = 100pf (figures 2, 4) l 40 70 150 ns t phl receiver input to output r diff = 54 w , c l1 = c l2 = 100pf (figures 2, 4) l 40 70 150 ns t skd t plh C t phl differential receiver skew r diff = 54 w , c l1 = c l2 = 100pf (figures 2, 4) l 13 ns switchi g characteristics u driver output high voltage driver differential output voltage driver output low voltage vs output current vs output current vs output current typical perfor a ce characteristics uw output voltage (v) 0 output current (ma) 0 ?4 4 8 ?2 ?6 1234 ltc490 ?tpc01 t a = 25 c output voltage (v) 0 output current (ma) 0 16 32 48 64 1234 ltc490 ?tpc02 t a = 25 c output voltage (v) 0 output current (ma) 0 20 40 60 80 1234 ltc490 ?tpc03 t a = 25 c the l denotes the specificatiions which apply over the full operating temperature range, otherwise specifications are at t a = 25 c. v cc = 5v 5%
ltc490 4 490fb ttl input threshold vs temperature driver skew vs temperature supply current vs temperature typical perfor a ce characteristics uw temperature ( c ) ?0 input threshold voltage (v) 1.55 1.57 1.59 1.61 1.63 0 50 100 ltc490 ?tpc04 temperature ( c ) ?0 time (ns) 5 4 3 2 1 0 50 100 ltc490 ?tpc05 temperature ( c ) ?0 supply current ( m a) 310 320 330 340 350 0 50 100 ltc490 ?tpc06 driver differential output voltage receiver ? t plh -t phl ? receiver output low voltage vs temperature vs temperature vs temperature temperature ( c ) ?0 differential voltage (v) 1.5 1.7 1.9 2.1 2.3 0 50 100 ltc490 ?tpc07 r o = 54 w temperature ( c ) ?0 time (ns) 7 6 5 4 3 0 50 100 ltc490 ?tpc08 temperature ( c ) ?0 output voltage (v) 0 0.2 0.4 0.6 0.8 0 50 100 ltc490 ?tpc09 i = 8ma
5 ltc490 490fb ? o ltc490 ?f03 d 3v 0v 1.5v t plh 1.5v v diff = v(y) ?v(z) v o 80% 20% 50% 10% z y t skew t r f = 1mhz : t r 10ns : t f 10ns 90% 50% t phl t f v o t skew 1/2 v o 1/2 v o uu u pi fu ctio s v cc (pin 1): positive supply; 4.75v v cc 5.25v. r (pin 2): receiver output. if a > b by 200mv, r will be high. if a < b by 200mv, then r will be low. d (pin 3): driver input. a low on d forces the driver outputs y low and z high. a high on d will force y high and z low. gnd (pin 4): ground connection. y (pin 5): driver output. z (pin 6): driver output. b (pin 7): receiver input. a (pin 8): receiver input. figure 4. receiver propagation delays figure 3. driver propagation delays figure 1. driver dc test load figure 2. driver/receiver timing test circuit test circuits ltc490 ?f01 y z r r v od2 v oc ltc490 ?f02 driver d y z receiver r diff a b 15pf c l1 c l2 r switchi g ti e wavefor s uw w v ol ltc490 ?f04 a-b v od2 0v t plh 0v output v oh 1.5v f = 1mhz ; t r 10ns : t f 10ns t phl ? od2 1.5v input r
ltc490 6 490fb typical application a typical connection of the ltc490 is shown in figure 5. two twisted-pair wires connect two driver/receiver pairs for full duplex data transmission. note that the driver and receiver outputs are always enabled. if the outputs must be disabled, use the ltc491. there are no restrictions on where the chips are con- nected, and it isnt necessary to have the chips connected at the ends of the wire. however, the wires must be terminated only at the ends with a resistor equal to their characteristic impedance, typically 120 w . because only one driver can be connected on the bus, the cable can be terminated only at the receiving end. the optional shields around the twisted pair help reduce unwanted noise, and are connected to gnd at one end. the ltc490 can also be used as a line repeater as shown in figure 6. if the cable length is longer than 4000 feet, the ltc490 is inserted in the middle of the cable with the receiver output connected back to the driver input. thermal shutdown the ltc490 has a thermal shutdown feature which pro- tects 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 ltc490 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 con- tention faults when two drivers are active on the bus at the same time. cables 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 figure 5. typical connection figure 6. line repeater applicatio s i for atio wu uu ltc490 ?f05 120 w 120 w shield ltc490 driver receiver ltc490 driver dx rx dx rx 2 3 5 6 7 8 receiver 0.01 m f 1 4 5v shield 8 7 6 5 0.01 m f 5v 4 2 3 1 + + ltc490 ?f06 120 w ltc490 driver dx rx 2 3 5 6 7 8 receiver data in data out
7 ltc490 490fb less flexible, more bulky, and more costly than twisted pairs. many cable manufacturers offer a broad range of 120 w cables designed for rs485 applications. 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). 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 applicatio s i for atio wu uu 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, 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 (47 w ), 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 re- flected 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 figure 7. attenuation vs frequency for belden 9841 figure 8. rs485 cable length specification. applies for 24 gauge, polyethylene dielectric twisted pair figure 9. termination effects frequency (mhz) 0.1 0.1 loss per 100 ft (db) 1.0 10 1.0 10 100 ltc490 ?f07 data rate (bps) 10k 10 cable length (ft) 100 1k 10k 100k 1m 10m ltc490 ?f08 2.5m rt driver dx receiver rx rt = 120 w rt = 47 w rt = 470 w ltc490 ?f09 probe here
ltc490 8 490fb cable is lightly loaded (470 w ), 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. ac cable termination cable termination resistors are necessary to prevent un- wanted reflections, but they consume power. the typical differential output voltage of the driver is 2v when the cable is terminated with two 120 w resistors, causing 33ma of dc current to flow in the cable when no data is being sent. this dc current is about 60 times greater than the supply current of the ltc490. one way to eliminate the unwanted current is by ac coupling the termination resis- tors as shown in figure 10. the coupling capacitor must allow high frequency energy to flow to the termination, but block dc and low frequen- cies. 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 120 w cables. applicatio s i for atio wu uu 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/(120 w c). fault protection all of ltcs rs485 products are protected against esd transients up to 2kv using the human body model (100pf, 1.5k w ). however, some applications need more protection. the best protection method is to connect a bidirectional transzorb ? from each line side pin to ground (figure 11). a transzorb is a silicon transient voltage suppressor that has exceptional surge handling capabili- ties, fast response time, and low series resistance. they are available from general instruments, gsi 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, dont forget to check how much the added parasitic capacitance will load down the bus. transzorb is a registered trademark of general instruments, gsi figure 10. ac coupled termination figure 11. esd protection with transzorbs ltc490 ?f10 120 w receiver rx c c = line length (ft) 16.3pf ltc490 ?f11 120 w driver z y
9 ltc490 490fb typical applicatio s u rs232 receiver rs232 to rs485 level transistor with hysteresis ltc490 ?ta02 5.6k receiver rs232 in 1/2 ltc490 rx 120 w driver y z r = 220k 10k rs232 in 5.6k ltc490 ?ta03 hysteresis = 10k ? ? 1/2 ltc490 ? vy ?vz ? r 19k r
ltc490 10 490fb n8 package 8-lead pdip (narrow .300 inch) (reference ltc dwg # 05-08-1510) u package descriptio n8 1002 .065 (1.651) typ .045 ?.065 (1.143 ?1.651) .130 .005 (3.302 0.127) .020 (0.508) min .018 .003 (0.457 0.076) .120 (3.048) min 12 3 4 87 6 5 .255 .015* (6.477 0.381) .400* (10.160) max .008 ?.015 (0.203 ?0.381) .300 ?.325 (7.620 ?8.255) .325 +.035 ?015 +0.889 0.381 8.255 () note: 1. dimensions are inches millimeters *these dimensions do not include mold flash or protrusions. mold flash or protrusions shall not exceed .010 inch (0.254mm) .100 (2.54) bsc
11 ltc490 490fb u package descriptio s8 package 8-lead plastic small outline (narrow .150 inch) (reference ltc dwg # 05-08-1610) .016 ?.050 (0.406 ?1.270) .010 ?.020 (0.254 ?0.508) 45 0 ?8 typ .008 ?.010 (0.203 ?0.254) so8 0303 .053 ?.069 (1.346 ?1.752) .014 ?.019 (0.355 ?0.483) typ .004 ?.010 (0.101 ?0.254) .050 (1.270) bsc 1 2 3 4 .150 ?.157 (3.810 ?3.988) note 3 8 7 6 5 .189 ?.197 (4.801 ?5.004) note 3 .228 ?.244 (5.791 ?6.197) .245 min .160 .005 recommended solder pad layout .045 .005 .050 bsc .030 .005 typ inches (millimeters) note: 1. dimensions in 2. drawing not to scale 3. these dimensions do not include mold flash or protrusions. mold flash or protrusions shall not exceed .006" (0.15mm) 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 represen- tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
ltc490 12 490fb ? linear technology corporation 1993 lt/tp 0104 1k rev b ? printed in usa linear technology corporation 1630 mccarthy blvd., milpitas, ca 95035-7417 (408) 432-1900 l fax: (408) 434-0507 l www.linear.com related parts part number description comments ltc486/ltc487 low power quad rs485 drivers 110 m a supply current ltc488/ltc489 low power quad rs485 receivers 7ma supply current ltc1480 3.3v supply rs485 transceiver lower supply voltage ltc1481 low power rs485 transceiver with shutdown lowest power ltc1482 rs485 transceiver with carrier detect 15kv esd, fail-safe ltc1483 low power, low emi rs485 transceiver slew rate limited driver outputs, lowest power ltc1484 rs485 transceiver with fail-safe 15kv esd, msop package ltc1485 10mbps rs485 transceiver high speed ltc1518/ltc1519 52mbps quad rs485 receivers higher speed, ltc488/ltc489 pin-compatible ltc1520 lvds-compatible quad receiver 100mv threshold, low channel-to-channel skew ltc1535 2500v isolated rs485 transceiver full-duplex, self-powered using external transformer ltc1685 52mbps rs485 transceiver industry-standard pinout, 500ps propagation delay skew ltc1686/ltc1687 52mbps full-duplex rs485 transceiver ltc490/ltc491 pin compatible ltc1688/ltc1689 100mbps quad rs485 drivers highest speed, ltc486/ltc487 pin compatible ltc1690 full-duplex rs485 transceiver with fail-safe 15kv esd, ltc490 pin compatible lt1785/ltc1785a 60v protected rs485 transceivers 15kv esd, fail-safe (lt1785a) lt1791/ltc1791a 60v protected full-duplex rs485 transceivers 15kv esd, fail-safe (lt1791a), ltc491 pin compatible

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