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1 ltc491 n low power: i cc = 300 m a typical n designed for rs485 or rs422 applications n single +5v supply n C7v 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 in three-state or 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 n pin compatible with the sn75180 differential driver and receiver pair d u escriptio s f ea t u re the ltc491 is a low power differential bus/line transceiver 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. 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. 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 u s a o pp l ic at i u a o pp l ic at i ty p i ca l ltc491 ?ta01 120 w 120 w 120 w 120 w 4000 ft 24 gauge twisted pair 4000 ft 24 gauge twisted pair receiver ltc491 driver receiver ltc491 driver r d r d 5 2 11 12 10 9 4 de 3 reb de reb
ltc491 2 a u g w a w u w a r b s o lu t exi t i s wu u package / o rder i for atio order part number ltc491cn ltc491cs ltc491in ltc491is (note 1) supply voltage (v cc ) ............................................... 12v control input voltages ..................... C0.5v to v cc +0.5v control input currents .......................... C50ma to 50ma driver input voltages ....................... C0.5v to v cc +0.5v driver input currents ............................ C25ma to 25ma driver output voltages .......................................... 14v receiver input voltages ......................................... 14v receiver output voltages ................ C0.5v to v cc +0.5v operating temperature range ltc491c.................................................. 0 c to 70 c ltc491i.............................................. C 40 c to 85 c storage temperature range ................. C 65 c to 150 c lead temperature (soldering, 10 sec.)................. 300 c 14 13 12 11 10 9 8 7 6 5 4 3 2 1 top view s package 14-lead plastic soic nc n package 14-lead plastic dip y nc z b nc v cc d gnd gnd r reb de ltc491 ?poi01 a r d consult factory for military grade parts. e lectr ic al c c hara terist ics c d 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 l 3v d v oc change in magnitude of driver common mode l 0.2 v output voltage for complementary output states v ih input high voltage d, de, reb l 2.0 v v il input low voltage l 0.8 v l in1 input current l 2 m a l in2 input current (a, b) v cc = 0v or 5.25v v in = 12v l 1.0 ma v in = C7v 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 = C0.2v 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, outputs enabled l 300 500 m a or v cc outputs disabled l 300 500 m a r in receiver input resistance C 7v v cm 12v l 12 k w i osd1 driver short circuit current, v out = high v o = C7v 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
3 ltc491 symbol parameter conditions min typ max units t plh driver input to output r diff = 54 w , c l1 = c l2 = 100pf l 10 30 50 ns t phl driver input to output l 10 30 50 ns t skew driver output to output l 5ns t r , t f driver rise or fall time l 51525 ns t zh driver enable to output high c l = 100pf (figures 4, 6) s2 closed l 40 70 ns t zl driver enable to output low c l = 100pf (figures 4, 6) s1 closed l 40 70 ns t lz driver disable time from low c l = 15pf (figures 4, 6) s1 closed l 40 70 ns t hz driver disable time from high c l = 15pf (figures 4, 6) s2 closed l 40 70 ns t plh receiver input to output r diff = 54 w , c l1 = c l2 = 100pf l 40 70 150 ns t phl receiver input to output l 40 70 150 ns t skd t plh C t phl differential receiver skew l 13 ns t zl receiver enable to output low c l = 15pf (figures 3, 8) s1 closed l 20 50 ns t zh receiver enable to output high c l = 15pf (figures 3, 8) s2 closed l 20 50 ns t lz receiver disable from low c l = 15pf (figures 3, 8) s1 closed l 20 50 ns t hz receiver disable from high c l = 15pf (figures 3, 8) s2 closed l 20 50 ns s u gc c hara terist ics wi tch i v cc = 5v 5% (figures 2, 5) (figures 2, 7) 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. the l 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. nc (pin 1): not connected. r (pin 2): receiver output. if the receiver output is enabled (reb low), then if a > b by 200mv, r will be high. if a < b by 200mv, then r will be low. reb (pin 3): receiver output enable. a low enables the receiver output, r. a high input forces the receiver output into a high impedance state. de (pin 4): driver output enable. a high on de enables the driver outputs, a and b. a low input forces the driver outputs into a high impedance state. d (pin 5): driver input. if the driver outputs are enabled (de high), then a low on d forces the driver outputs a low and b high. a high on d will force a high and b low. gnd (pin 6): ground connection. gnd (pin 7): ground connection. nc (pin 8): not connected. y (pin 9): driver output. z (pin 10): driver output. b (pin 11): receiver input. a (pin 12): receiver input. nc (pin 13): not connected. v cc (pin 14): positive supply; 4.75v v cc 5.25v. pi u fu u c u s o ti
ltc491 4 cc hara terist ics uw a t y p i ca lper f o r c e ttl input threshold vs temperature driver skew vs temperature supply current vs temperature driver output high voltage vs driver differential output voltage vs driver output low voltage vs output current t a = 25 c output current t a = 25 c output current t a = 25 c output voltage (v) 0 output current (ma) 0 ?4 4 8 ?2 ?6 1234 ltc491 ?tpc01 output voltage (v) 0 output current (ma) 0 20 40 60 80 1234 ltc491 ?tpc03 temperature ( c ) ?0 input threshold voltage (v) 1.55 1.57 1.59 1.61 1.63 0 50 100 ltc491 ?tpc04 temperature ( c ) ?0 differential voltage (v) 1.5 1.7 1.9 2.1 2.3 0 50 100 ltc491 ?tpc07 driver differential output voltage vs receiver t plh t phl vs receiver output low voltage vs temperature r o = 54 w temperature temperature at i = 8ma temperature ( c ) ?0 output voltage (v) 0 0.2 0.4 0.6 0.8 0 50 100 ltc491 ?tpc09 temperature ( c ) ?0 time (ns) 3.0 4.0 5.0 6.0 7.0 0 50 100 ltc491 ?tpc08 temperature ( c ) ?0 time (ns) 1.0 2.0 3.0 4.0 5.0 0 50 100 ltc491 ?tpc05 temperature ( c ) ?0 supply current ( m a) 310 320 330 340 350 0 50 100 ltc491 ?tpc06 output voltage (v) 0 output current (ma) 0 16 32 48 64 1234 ltc491 ?tpc02
5 ltc491 test circuits figure 2. driver/receiver timing test circuit figure 1. driver dc test load figure 3. receiver timing test load figure 4. driver timing test load ltc491 ?ta04 receiver output c l s1 1k w cc v s2 1k w ltc491 ?ta05 output under test c l s1 500 cc v w s2 ltc491 ?ta02 y z r r v od2 v oc ltc491 ?ta03 driver d y z receiver r diff a b 15pf c l1 c l2 r
ltc491 6 ti w e wavefor s u g witchi w s v ol ltc491 ?ta08 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 ltc491 ?ta07 a, b de 3v 0v f = 1mhz : t r 10ns : t r 10ns v ol v oh 1.5v 1.5v 5v output normally low t zl 2.3v t lz 0.5v a, b 0v t zh 2.3v output normally high t hz 0.5v ? o ltc491 ?ta06 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 figure 5. driver propagation delays figure 6. driver enable and disable times figure 7. receiver propagation delays ltc491 ?ta09 r reb 3v 0v f = 1mhz : t r 10ns : t f 10ns v ol v oh 1.5v 1.5v 5v output normally low t zl 1.5v t lz 0.5v r 0v t zh 1.5v output normally high t hz 0.5v figure 8. receiver enable and disable times
7 ltc491 u s a o pp l ic at i wu u i for atio typical application a typical connection of the ltc491 is shown in figure 9. two twisted pair wires connect up to 32 driver/receiver pairs for full duplex data transmission. there are no restrictions on where the chips are connected to the wires, and it isnt 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 imped- ance, typically 120 w . the input impedance of a receiver is ltc491 ?ta10 120 w 120 w ltc491 driver receiver ltc491 driver dx rx dx rx 2 5 9 10 11 12 receiver 9 10 11 12 5 2 120 w 3 4 3 4 120 w receiver ltc491 driver 9 10 11 12 5 4 3 2 dx rx figure 9. typical connection ltc491 ?ta11 120 w ltc491 driver dx rx 2 5 9 10 11 12 receiver data in data out 3 4 120 w figure 10. line repeater typically 20k w to gnd, or 0.6 unit rs-485 load, so in practice 50 to 60 transceivers can be connected to the same wires. the optional shields around the twisted pair help reduce unwanted noise, and are connected to gnd at one end. the ltc491 can also be used as a line repeater as shown in figure 10. if the cable length is longer than 4000 feet, the ltc491 is inserted in the middle of the cable with the receiver output connected back to the driver input.
ltc491 8 u s a o pp l ic at i wu u i for atio thermal shutdown the ltc491 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 ltc491 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 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 over all loss (figure 11). when using low loss cables, figure 12 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. data rate (bps) 10k 10 cable length (ft) 100 1k 10k 100k 1m 10m ltc491 ?ta13 2.5m figure 12. cable length vs data rate figure 11. attenuation vs frequency for belden 9481 frequency (mh z ) 0.1 0.1 loss per 100 ft (db) 1.0 10 1.0 10 100 ltc491 ?ta12
9 ltc491 u s a o pp l ic at i wu u i for atio 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 13). rt driver dx receiver rx rt = 120 w rt = 47 w rt = 470 w ltc491 ?ta14 probe here figure 13. termination effects 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 reflected signal returns to the driver, the ampli- tude 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 (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 ltc491. one way to eliminate the unwanted current is by ac coupling the termination resis- tors as shown in figure 14. ltc491 ?ta15 120 w receiver rx c c = line length (ft) x 16.3pf figure 14. ac coupled termination 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. 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).
ltc491 10 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 16). ltc491 ?ta17 120 w driver z y figure 16. esd protection with transzorbs 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 break- down 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. 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. the receiver of the ltc491 has 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 120 w , the differential inputs to the receiver are shorted together, not left floating. because the receiver has about 70mv of hysteresis, the receiver output will maintain the last data bit received. 140 w receiver rx +5v 1.5k w receiver rx +5v 110 w 130 w 110 w 130 w ltc491 ?ta16 120 w receiver rx c +5v 100k w 1.5k w figure 15. forcing o 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. u s a o pp l ic at i wu u i for atio
11 ltc491 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. u s a o pp l ic at i ty p i ca l rs232 receiver ltc491 ?ta18 5.6k w receiver rs232 in 1/2 ltc491 rx rs232 to rs485 level transistor with hysteresis 120 w driver y z r = 220k w 10k w rs232 in 5.6k w ltc491 ?ta19 hysteresis = 10k w ? ? 1/2 ltc491 ? vy - vz ? r 19k r
ltc491 12 linear technology corporation 1630 mccarthy blvd., milpitas, ca 95035-7487 (408) 432-1900 l fax : (408) 434-0507 l telex : 499-3977 ? linear technology corporation 1992 ba/gp 0492 10k rev 0 u package d e sc r i pti o n package 14-lead plastic dip t j max q ja 100 c90 c/w s package 14-lead plastic soic t j max q ja 100 c 110 c/w dimensions in inches (millimeters) unless otherwise noted. n14 0392 0.260 ?0.010 (6.604 ?0.254) 0.770 (19.558) max 3 1 2 4 5 6 7 8 9 10 11 12 13 14 0.009 ?0.015 (0.229 ?0.381) 0.300 ?0.325 (7.620 ?8.255) 0.325 +0.025 0.015 +0.635 0.381 8.255 () 0.015 (0.380) min 0.125 (3.175) min 0.130 ?0.005 (3.302 ?0.127) 0.045 ?0.065 (1.143 ?1.651) 0.065 (1.651) typ 0.018 ?0.003 (0.457 ?0.076) 0.100 ?0.010 (2.540 ?0.254) 0.075 ?0.015 (1.905 ?0.381) 1 2 3 4 0.150 ?0.157 (3.810 ?3.988) 14 13 0.337 ?0.344 (8.560 ?8.738) 0.228 ?0.244 (5.791 ?6.197) 12 11 10 9 5 6 7 8 0.010 ?0.020 (0.254 ?0.508) 0.016 ?0.050 0.406 ?1.270 45 0??8?typ 0.008 ?0.010 (0.203 ?0.254) so14 0392 0.053 ?0.069 (1.346 ?1.752) 0.014 ?0.019 (0.355 ?0.483) 0.004 ?0.010 (0.101 ?0.254) 0.050 (1.270) typ

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