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ltc486 1 486fb typical application description quad low power rs485 driver the ltc ? 486 is a low power differential bus/line driver designed for multipoint data transmission standard rs485 applications with extended common-mode range (12v to C7v). it also meets rs422 requirements. the cmos design offers signi? cant power savings over its bipolar counterpart without sacri? cing 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 speci? cations are guaranteed from 0c to 70c (commercial), C40c to 85c (industrial), over the 4.75v to 5.25v supply voltage range. rs485 length speci? cation l , lt, ltc and ltm are registered trademarks of linear technology corporation. all other trademarks are the property of their respective owners. features applications n very low power: i cc = 110a typ n designed for rs485 or rs422 applications n single 5v supply n C7v 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 sn75172, ds96172, a96172, and ds96f172 n low power rs485/rs422 drivers n level translator 486 ta01a receiver en di en 44 12 1 1/4 ltc486 120 120 ro 3 en 2 1 1/4 ltc488 4000 ft belden 9841 driver data rate (bps) 10k * applies for 24 gauge, polyethylene dielectric twisted pair 10 cable length (ft) 100 1k 10k 100k 1m 10m 486 ta01b 2.5m
ltc486 2 486fb pin configuration absolute maximum ratings supply voltage (v cc ) ................................................12v control input voltages .......................0.5v 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 ltc486c .................................................. 0c to 70c ltc486i................................................ C40c to 85c storage temperature range ................... C65c to 150c lead temperature (soldering, 10 sec) .................. 300c (note 1) 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 di1 do1a do1b en do2b do2a di2 gnd di3 do3a do3b en do4b do4a di4 v n package 16-lead plastic dip top view cc sw package 16-lead plastic sol t jmax = 125c, ja = 70c/w (n) t jmax = 150c, ja = 95c/w (sw) consult factory for military grade parts. order information lead free finish tape and reel part marking package description temperature range ltc486cn#pbf ltc486cn#trpbf ltc486cn 16-lead plastic dip 0c to 70c ltc486csw#pbf ltc486csw#trpbf ltc486csw 16-lead plastic dip 0c to 70c ltc486in#pbf ltc486in#trpbf ltc486in 16-lead plastic sol C40c to 85c ltc486isw#pbf ltc486isw#trpbf ltc486isw 16-lead plastic sol C40c to 85c consult ltc marketing for parts speci? ed with wider operating temperature ranges. for more information on lead free part marking, go to: http://www.linear.com/leadfree/ for more information on tape and reel speci? cations, go to: http://www.linear.com/tapeandreel/
ltc486 3 486fb dc electrical characteristics v cc = 5v 5%, 0c temperature 70c (commercial), C40c temperature 85c (industrial) (notes 2, 3) symbol parameter conditions min typ max units v od1 differential driver output voltage (unloaded) i out = 0 5 v v od2 differential driver output voltage (with load) r = 50; (rs422) 2 v r = 27; (rs485) (figure 1) 1.5 5 v v od change in magnitude of driver differential output voltage for complementary output states r = 27 or r = 50 (figure 1) 0.2 v v oc driver common-mode output voltage 3v |v oc | change in magnitude of driver common-mode output voltage for complementary output states 0.2 v v ih input high voltage di, en, en 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 output disabled 110 110 200 200 a a i osd1 driver short-circuit current, v out = high v out = C7v 100 250 ma i osd2 driver short-circuit current, v out = low v out = 12v 100 250 ma i oz high impedance state output current v out = C7v to 12v 10 200 a switching characteristics v cc = 5v 5%, 0c temperature 70c (commercial), C40c temperature 85c (industrial) (notes 2, 3) symbol parameter conditions min typ max units t plh driver input to output r diff = 54, c l1 = c l2 = 100pf (figures 2, 4) 10 30 50 ns t phl driver input to output 10 30 50 ns t skew driver output to output 515 ns t r , t f driver rise or fall time 5 15 25 ns t zh driver enable to output high c l = 100pf (figures 3, 5) s2 closed 35 70 ns t zl driver enable to output low c l = 100pf (figures 3, 5) s1 closed 35 70 ns t lz driver disable time from low c l = 15pf (figures 3, 5) s1 closed 35 70 ns t hz driver disable time from high c l = 15pf (figures 3, 5) s2 closed 35 70 ns 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. 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 speci? ed. note 3: all typicals are given for v cc = 5v and temperature = 25c.
ltc486 4 486fb switching time waveforms driver output high voltage vs output current t a = 25c driver differential output voltage vs output current t a = 25c driver output low voltage vs output current t a = 25c figure 1. driver propagation delays figure 2. driver enable and disable times Cv o 486 f01 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) C v(b) v o 80% 20% t f 90% diff 10% t skew t r f = 1mhz : t 10ns : t 10ns << rf 486 f02 a, b en 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 rf typical performance characteristics output voltage (v) 0 output current (ma) 0 C24 C 4 8 C72 C96 1234 486 g01 output voltage (v) 0 output current (ma) 0 16 32 48 64 1234 486 g02 output voltage (v) 0 output current (ma) 0 20 40 60 80 1234 486 g03
ltc486 5 486fb typical performance characteristics ttl input threshold vs temperarue driver skew vs temperature supply current vs temperature driver differential output voltage vs temperature r o = 54 function table temperature (c ) C50 input threshold voltage (v) 1.55 1.57 1.59 1.61 1.63 0 50 100 486 g04 temperature (c ) C50 time (ns) 1 2 3 4 5 0 50 100 486 g05 temperature (c ) C50 supply current (a) 90 100 110 120 130 0 50 100 486 g06 temperature (c ) C50 differential voltage (v) 1.5 1.7 1.9 2.1 2.3 0 50 100 486 g07 input enables outputs 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 h: high level l: low level x: irrelevant z: high impedance (off)
ltc486 6 486fb pin functions 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 fordetails. do2b (pin 5): driver 2 output. do2a (pin 6): driver 2 output. di2 (pin 7): driver 2 input. refer to di1 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. v cc (pin 16): positive supply; 4.75v < v cc < 5.25v test circuits figure 3. driver dc test load 486 f03 a b r r od v oc v figure 4. driver timing test circuit figure 5. driver timing test load #2 driver 486 f04 en di a b en r diff ci1 ci2 486 f05 output under test c l s1 500 cc v s2
ltc486 7 486fb 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 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 impedance, typically 120. 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 ? ow 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 ltc486 drivers are shorted directly, the driver outputs cannot 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 ? exible, more bulky, and more costly than twisted pairs. many cable manufacturers offer a broad range of 120 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, with relatively low overall loss (figure 7). applications information figure 7. attenuation vs frequency for belden 9841 figure 6. typical connection en 12 en 4 486 f06 120 dx 1 2 3 shield 120 rx rx shield 3 dx en 12 en 4 2 en en 4 1 2 3 rx rx 3 dx en 12 en 4 1 dx 1/4 ltc488 1/4 ltc486 1/4 ltc488 1/4 ltc486 2 1 12 frequency (mhz) 0.1 0.1 loss per 100 ft (db) 1 10 1 10 100 486 f07
ltc486 8 486fb applications information 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 how- ever, 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 imped- ance, 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 re? ections 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., 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 re? ected back out of phase because of the mister- mination. when the re? ected 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., 470), the signal re? ects 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 un- wanted re? ections, but they consume power. the typical differential output voltage of the driver is 2v when the cable is terminated with two 120 resistors. when no data is being sent 33ma of dc current ? ows 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. figure 8. cable length vs data rate data rate (bps) 10k 10 cable length (ft) 100 1k 10k 100k 1m 10m 486 f08 2.5m figure 9. termination effects rt driver dx receiver rx rt = 120 rt = 47 rt = 470 486 f09 probe here 486 f10 c = line length (ft) s 16.3pf 120 receiver rx c figure 10. ac coupled termination
ltc486 9 486fb applications information the coupling capacitor allows high frequency energy to ? ow 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 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, 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). 486 f11 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 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 ? nished 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 ? oating (open-circuit). however, when the cable is terminated with 120, the differential inputs to the receiver are shorted together, not left ? oating. 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 ? rst 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 ltcs rs485 products are protected against esd transients up to 2kv using the human body model (100pf, 1.5k). 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 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. 486 f12 120 driver z y figure 12. esd protection transzorb is a registered trademark of general instruments, gsi.
ltc486 10 486fb package description n package 16-lead pdip (narrow .300 inch) (reference ltc dwg # 05-08-1510) n16 1002 .255 .015* (6.477 0.381) .770* (19.558) max 16 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 .020 (0.508) min .120 (3.048) min .130 .005 (3.302 0.127) .065 (1.651) typ .045 ?.065 (1.143 ?1.651) .018 .003 (0.457 0.076) .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
ltc486 11 486fb 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. package description sw package 16-lead plastic small outline (wide .300 inch) (reference ltc dwg # 05-08-1620) s16 (wide) 0502 note 3 .398 ?.413 (10.109 ?10.490) note 4 16 15 14 13 12 11 10 9 1 n 23 4 5 6 78 n/2 .394 ?.419 (10.007 ?10.643) .037 ?.045 (0.940 ?1.143) .004 ?.012 (0.102 ?0.305) .093 ?.104 (2.362 ?2.642) .050 (1.270) bsc .014 ?.019 (0.356 ?0.482) typ 0 ?8 typ note 3 .009 ?.013 (0.229 ?0.330) .005 (0.127) rad min .016 ?.050 (0.406 ?1.270) .291 ?.299 (7.391 ?7.595) note 4 45 .010 ?.029 (0.254 ?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
ltc486 12 486fb 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 0409 rev b ? printed in usa typical application rs232 to rs485 level translator with hysteresis 486 ta14 120 driver y z r = 220k 10k rs232 in 5.6k hysteresis = 10k s |vy - vz| r 19k r 1/4 ltc486

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