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ltc487 487fc typical a pplica t ion descrip t ion quad low power rs485 driver the ltc ? 487 is a low power differential bus/line driver designed for multipoint data transmission standard rs485 applications with extended common mode range (C 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 com- mon 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 0c to 70c (commercial), C40c to 85c (industrial) and over the 4.75v to 5.25v supply voltage range. l , lt, ltc, ltm, linear technology and the linear logo are registered trademarks of linear technology corporation. all other trademarks are the property of their respective owners. fea t ures a pplica t ions n very low power: i cc = 110a typ n designed for rs485 or rs422 applications n single 5v supply n C 7v to 12v bus common mode range permits 7v gnd difference between devices on the bus n thermal shutdown protection n power-up/down glitch-free driver outputs permit live insertion/removal of package n driver maintains high impedance in three-state or with the power off n 28ns typical driver propagation delays with 5ns skew n pin compatible with the sn75174, ds96174, a96174, and ds96f174 n low power rs485/rs422 drivers n level translator 2 driver ltc487 ta01 receiver di en 12 4 3 1 1/4 ltc487 120 120 ro 3 en 12 4 2 1 1/4 ltc489 4000 ft belden 9841 data rate (bps) 10k 10 cable length (ft) 100 1k 10k 100k 1m 10m ltc487 ta09 2.5m rs485 cable length specification* * applies for 24 gauge, polyethylene dielectric twisted pair
ltc487 487fc p in c on f igura t ion a bsolu t e maxi m u m r a t ings supply voltage (v cc ) ................................................ 12v control input voltages .................... C0.5v to v cc + 0.5v driver input voltages ...................... C0.5v to v cc + 0.5v driver output voltages ............................................ 14v control input currents .........................................25ma driver input currents ...........................................25ma operating temperature range commercial ............................................. 0c to 70c industrial .............................................C40c to 85c storage temperature range .................. C65c to 150c lead temperature (soldering, 10 sec.).................. 300c (note 1) 1 2 3 4 5 6 7 8 16 15 14 13 12 11 10 9 di1 do1a do1b en12 do2b do2a di2 gnd di3 do3a do3b en34 do4b do4a di4 v n package 16-lead plastic dip top view cc sw package 16-lead plastic so t jmax = 125c, ja = 70c/w (n) t jmax = 150c, ja = 95c/w (s) consult factory for military grade parts. o r d er i n f or m a t ion lead free finish tape and reel part marking package description temperature range ltc487cn#pbf ltc487cn#trpbf ltc487cn 16-lead plastic dip 0c to 70c ltc487csw#pbf ltc487csw#trpbf ltc487csw 16-lead plastic so 0c to 70c ltc487in#pbf ltc487in#trpbf ltc487in 16-lead plastic dip C40c to 85c ltc487isw#pbf ltc487is#trpbf ltc487isw 16-lead plastic so C40c to 85c consult ltc marketing for parts specified with wider operating temperature ranges. consult ltc marketing for information on non-standard lead based finish parts. for more information on lead free part marking, go to: http://www.linear.com/leadfree/ for more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/
ltc487 487fc d c e lec t rical c harac t eris t ics v cc = 5v 5%, 0c t a 70c (commercial), C40c t a 85c (industrial) (notes 2, 3) note 1: stresses beyond those listed under absolute maximum ratings may cause permanent damage to the device. exposure to any absolute maximum rating condition for extended periods may affect device reliability and lifetime. symbol parameter conditions min typ max units v od1 differential driver output voltage (unloaded) i o = 0 5 v v od2 differential driver output voltage (with load) r = 50 ?; (rs422) 2 v r = 27?; (rs485) (figure 3) 1.5 5 v v od change in magnitude of driver differential output voltage for complementary output states r = 27? or r = 50? (figure 3) 0.2 v v oc driver common mode output voltage 3 v ? v oc ? change in magnitude of driver common mode output voltage for complementary output states 0.2 v v ih input high voltage di, en12, en34 2.0 v v il input low voltage 0.8 v i in1 input current 2 a i cc supply current no load output enabled 110 200 a output enabled 110 200 a i osd1 driver short-circuit current, v out = high v o = C 7v 100 250 ma i osd2 driver short-circuit current, v out = low v o = 12v 100 250 ma i oz high impedance state output current v o = C7v to 12v 10 200 a s wi t ching c harac t eris t ics v cc = 5v 5%, 0c t a 70c (notes 2, 3) symbol parameter conditions min typ max units t plh driver input to output r diff = 54?, c l1 = c l2 = 100pf 10 30 50 ns t phl driver input to output (figures 1, 4) 10 30 50 ns t skew driver output to output 5 15 ns t r, t f driver rise or fall time 5 20 25 ns t zh driver enable to output high c l = 100pf (figures 2, 5) s2 closed 35 70 ns t zl driver enable to output low c l = 100pf (figures 2, 5) s1 closed 35 70 ns t lz driver disable time from low c l = 15pf (figures 2, 5) s1 closed 35 70 ns t hz driver disable time from high c l = 15pf (figures 2, 5) s2 closed 35 70 ns 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 v cc = 5v and temperature = 25c.
ltc487 487fc typical p er f or m ance c harac t eris t ics ttl input threshold vs temperature driver skew vs temperature supply current vs temperature driver differential output voltage vs temperature driver output high voltage vs output current driver differential output voltage vs output current driver output low voltage vs output current output voltage (v) 0 output current (ma) 0 ?24 ? 48 ?72 ?96 1 2 3 4 487 g01 t a = 25c output voltage (v) 0 output current (ma) 0 16 32 48 64 1 2 3 4 487 g02 t a = 25c output voltage (v) 0 output current (ma) 0 20 40 60 80 1 2 3 4 487 g03 t a = 25c temperature (c ) ?50 input threshold voltage (v) 1.55 1.57 1.59 1.61 1.63 0 50 100 487 g04 temperature (c ) ?50 time (ns) 1.0 2.0 3.0 4.0 5.0 0 50 100 487 g05 temperature (c ) ?50 supply current (a) 90 100 110 120 130 0 50 100 487 g06 temperature (c ) ?50 differential voltage (v) 1.5 1.7 1.9 2.1 2.3 0 50 100 487 g07 r o = 54
ltc487 487fc p in func t ions 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 func- tion table for details. do2b (pin 5): driver 2 output. do2a (pin 6): driver 2 output. di2 (pin 7): driver 2 input. refer to di1. 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 func- tion table for details. do4b (pin 13): driver 4 output. do4a (pin 14): driver 4 output. di4 (pin 15): driver 4 input. refer to di1. v cc (pin 16): positive supply; 4.75 < v cc < 5.25. h: high level l: low level x: irrelevant z: high impedance (off) input enables outputs di en12 or en34 out a out b h l x h h l h l z l h z func t ion table s wi t ching ti m e w ave f or m s ?v o ltc487 ? ta05 b a di v o 1/2 v o 3v 0v t skew 1.5v t plh 1.5v t phl 1/2 v o v = v(a) ? v(b) v o 80% 20% t f 90% diff 10% t skew t r f = 1mhz : t 10ns : t 10ns < < r f ltc487 ? ta06 a, b en12 3v 0v f = 1mhz : t 10ns : t 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 r f figure 2. driver enable and disable times figure 1. driver propagation delays
ltc487 487fc tes t c ircui t a pplica t ions i n f or m a t ion ltc487 ta02 a b r r od v oc v driver 1 ltc487 ta03 di a b en12 r diff c l1 c l2 ltc487 ta04 output under test c l s1 500 cc v s2 figure 5. driver timing test load #2 figure 4. driver timing test circuit figure 3. driver dc test load 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 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 im- pedance, typically 120?. 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 through the part. the thermal shutdown circuit disables the driver outputs when the internal temperature reaches 150c and turns them back on when the temperature cools to 130c. if the outputs of two or more ltc487 drivers are shorted directly, the driver outputs can not supply enough cur- rent 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 120? cables designed for rs485 applications. en12 4 ltc487 ? ta07 120 dx 1 2 3 shield 120 rx rx shield 3 dx en12 4 2 en12 4 1 2 3 rx rx 3 dx en12 4 1 dx 1/4 ltc489 1/4 ltc487 1/4 ltc489 1/4 ltc487 2 1 figure 6. typical connection
ltc487 487fc a pplica t ions i n f or m a t ion 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). frequency (mhz) 0.1 0.1 loss per 100 ft (db) 1.0 10 1.0 10 100 ltc487 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. data rate (bps) 10k 10 cable length (ft) 100 1k 10k 100k 1m 10m ltc487 ta09 2.5m figure 8. cable length vs data rate 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 termi- nated properly, the waveform will look like a square wave (figure 9). rt driver dx receiver rx rt = 120 rt = 47 rt = 470 ltc487 ta10 probe here figure 9. termination effects if the cable is loaded excessively (47?), 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?), 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 487fc a pplica t ions i n f or m a t ion 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? 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. ltc487 ta11 c = line length (ft) s 16.3pf 120 receiver rx c figure 10. 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 ca- pacitor 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? 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? ? 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 120?, 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. if the receiver output must be forced to a known state, the circuits of figure 11 can be used. ltc487 ta12 140 receiver rx 5v 1.5k receiver rx 5v 110 130 110 130 120 receiver rx c 5v 100k 1.5k 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 resis- tor. simply swap the receiver inputs for data protocols ending in logic 1. fault protection all of ltcs rs485 products are protected against esd transients up to 2kv using the human body model (100pf, 1.5k?). 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).
ltc487 487fc a pplica t ions i n f or m a t ion 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, dont forget to check how much the added parasitic capacitance will load down the bus. ltc487 ta13 120 driver z y figure 11. forcing 0 when all drivers are off p ackage descrip t ion n16 1002 .255 .015* (6.477 0.381) .770* (19.558) max 16 1 2 3 4 5 6 7 8 910 11 12 13 14 15 .020 (0.508) min .120 (3.048) min .130 .005 (3.302 0.127) .065 (1.651) typ .045 C .065 (1.143 C 1.651) .018 .003 (0.457 0.076) .008 C .015 (0.203 C 0.381) .300 C .325 (7.620 C 8.255) .325 +.035 C.015 +0.889 C0.381 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 n package 16-lead pdip (narrow .300 inch) (reference ltc dwg # 05-08-1510)
ltc487 0 487fc p ackage descrip t ion s16 (wide) 0502 note 3 .398 C .413 (10.109 C 10.490) note 4 16 15 14 13 12 11 10 9 1 n 2 3 4 5 6 7 8 n/2 .394 C .419 (10.007 C 10.643) .037 C .045 (0.940 C 1.143) .004 C .012 (0.102 C 0.305) .093 C .104 (2.362 C 2.642) .050 (1.270) bsc .014 C .019 (0.356 C 0.482) typ 0 C 8 typ note 3 .009 C .013 (0.229 C 0.330) .005 (0.127) rad min .016 C .050 (0.406 C 1.270) .291 C .299 (7.391 C 7.595) note 4 45 .010 C .029 (0.254 C 0.737) inches (millimeters) note: 1. dimensions in 2. drawing not to scale 3. 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 4. these dimensions do not include mold flash or protrusions. mold flash or protrusions shall not exceed .006" (0.15mm) .420 min .325 .005 recommended solder pad layout .045 .005 n 1 2 3 n/2 .050 bsc .030 .005 typ sw package 16-lead plastic small outline (wide .300 inch) (reference ltc dwg # 05-08-1620)
ltc487 487fc 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 representa- tion that the interconnection of its circuits as described herein will not infringe on existing patent rights. r evision h is t ory rev date description page number c 8/10 reversed temperature ranges for ltc487csw#pbf and ltc487in#pbf. 2 (revision history begins at rev c)
ltc487 487fc linear technology corporation 1630 mccarthy blvd., milpitas, ca 95035-7417 (408) 432-1900 fax : (408) 434-0507 www.linear.com ? linear technology corporation 1994 lt 0810 rev c ? printed in usa typical a pplica t ion ltc487 ta14 hysteresis = 10k ? |vy - vz | ???? r 19k ???? r 120 driver y z r = 220k 10k rs232 in 5.6k 1/4 ltc487 rs232 to rs485 level translator with hysteresis

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