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1 ltc1484 low power rs485 transceiver with receiver fail-safe n no damage or latchup to 15kv esd (human body model), iec-1000-4-2 level 4 contact ( 8kv) and level 3 ( 8kv) air gap specifications n guaranteed high receiver output state for floating, shorted or terminated inputs with no signal present n drives low cost residential telephone wires n low power: i cc = 700 m a max with driver disabled n i cc = 900 m a max for driver enable with no load n 20 m a max quiescent current in shutdown mode n single 5v supply n C 7v to 12v common mode range permits 7v ground difference between devices on the data line n power up/down glitch-free driver outputs n up to 32 transceivers on the bus n pin compatible with the ltc485 n available in 8-lead msop , pdip and so packages the ltc ? 1484 is a low power rs485 compatible trans- ceiver. in receiver mode, it offers a fail-safe feature which guarantees a high receiver output state when the inputs are left open, shorted together or terminated with no signal present. no external components are required to ensure the high receiver output state. both driver and receiver feature three-state outputs with separate receiver and driver control pins. the driver outputs maintain high impedance over the entire com- mon mode range when three-stated. excessive power dissipation caused by bus contention or faults is pre- vented by a thermal shutdown circuit that forces the driver outputs into a high impedance state. enhanced esd protection allows the ltc1484 to with- stand 15kv (human body model), iec-1000-4-2 level 4 ( 8kv) contact and level 3 ( 8kv) air discharge esd without latchup or damage. the ltc1484 is fully specified over the commercial and industrial temperature ranges and is available in 8-lead msop, pdip and so packages. n battery-powered rs485/rs422 applications n low power rs485/rs422 transceiver n level translator , ltc and lt are registered trademarks of linear technology corporation. ro1 re1 de1 di1 r v cc1 ltc1484 gnd1 b1 a1 b2 a2 120 w 120 w d v cc2 gnd2 1484 ta01 r d ro2 re2 de2 di2 ltc1484 rs485 interface applicatio s u features typical applicatio u descriptio u driving a 2000 foot stp cable dl1 - de1 = v cc 1484 ta01a dl2 = 0 de2 = 0 re1 = re2 = 0 dl1 b2 a2 ro2
2 ltc1484 absolute m axi m u m ratings w ww u (note 1) supply voltage (v cc )............................................... 6.5v control input voltages ................. C 0.3v to (v cc + 0.3v) driver input voltage ..................... C 0.3v to (v cc + 0.3v) driver output voltages ................................. C 7v to 10v receiver input voltages (driver disabled) .. C12v to 14v receiver output voltage ............... C 0.3v to (v cc + 0.3v) junction temperature .......................................... 125 c operating temperature range LTC1484C ......................................... 0 c t a 70 c ltc1484i ...................................... C 40 c t a 85 c storage temperature range ................. C 65 c to 150 c lead temperature (soldering, 10 sec).................. 300 c the l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at t a = 25 c. v cc = 5v 5% (notes 2 and 3) unless otherwise noted. symbol parameter conditions min typ max units v od1 differential driver output voltage (unloaded) i out = 0 l v cc v 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 r = 22 w , figure 1 l 1.5 5 v v od3 differential driver output voltage v tst = C 7v to 12v, figure 2 l 1.5 5 v (with common mode) d v od change in magnitude of driver differential r = 22 w , 27 w or r = 50 w , figure 1 l 0.2 v output voltage for complementary output states v tst = C 7v to 12v, figure 2 v oc driver common mode output voltage r = 22 w , 27 w or r = 50 w , figure 1 l 3v d |v oc | change in magnitude of driver common mode r = 22 w , 27 w or r = 50 w , figure 1 l 0.2 v output voltage for complementary output states v ih input high voltage de, di, re l 2.0 v v il input low voltage de, di, re l 0.8 v i in1 input current de, di, re l 2 m a i in2 input current (a, b) de = 0, v cc = 0 or 5v, v in = 12v l 1.0 ma de = 0, v cc = 0 or 5v, v in = C 7v l C 0.8 ma v th differential input threshold voltage for receiver C 7v v cm 12v, de = 0 l C 0.20 C 0.015 v electrical characteristics package/order i n for m atio n w u u order part number order part number LTC1484Cms8 LTC1484Cn8 LTC1484Cs8 ltc1484in8 ltc1484is8 s8 part marking 1484 1484i ms8 part marking ltdx t jmax = 125 c, q ja = 200 c/ w t jmax = 125 c, q ja = 130 c/ w (n8) t jmax = 125 c, q ja = 135 c/ w (s8) consult factory for military grade parts. 1 2 3 4 ro re de di 8 7 6 5 v cc b a gnd top view ms8 package 8-lead plastic msop 1 2 3 4 8 7 6 5 top view ro re de di v cc b a gnd n8 package 8-lead pdip s8 package 8-lead plastic so r d
3 ltc1484 symbol parameter conditions min typ max units d v th receiver input hysteresis v cm = 0v, de = 0 l 30 mv v oh receiver output high voltage i out = C 4ma, (v a C v b ) = 200mv l 3.5 v v ol receiver output low voltage i out = 4ma, (v a C v b ) = C 200mv l 0.4 v i ozr three-state (high impedance) output current v cc = max, 0.4v v out 2.4v, l 1 m a at receiver de = 0 r in receiver input resistance C7v v cm 12v l 12 22 k w i cc supply current no load, output enabled (de = v cc ) l 600 900 m a no load, output disabled (de = 0) l 400 700 m a i shdn supply current in shutdown mode de = 0, re = v cc , di = 0 l 120 m a i osd1 driver short-circuit current, v out = high (note 4) C 7v v out 10v 35 250 ma i osd2 driver short-circuit current, v out = low (note 4) C 7v v out 10v 35 250 ma i osr receiver short-circuit current 0v v out v cc l 785ma the l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at t a = 25 c. v cc = 5v 5% (notes 2 and 3) unless otherwise noted. electrical characteristics switchi n g characteristics u symbol parameter conditions min typ max units t plh driver input to output r diff = 54 w , c l1 = c l2 = 100pf l 10 28.5 60 ns (figures 4, 6) t phl driver input to output r diff = 54 w , c l1 = c l2 = 100pf l 10 31 60 ns (figures 4, 6) t skew driver output to output r diff = 54 w , c l1 = c l2 = 100pf l 2.5 10 ns (figures 4, 6) t r , t f driver rise or fall time r diff = 54 w , c l1 = c l2 = 100pf l 31540 ns (figures 4, 6) t zh driver enable to output high c l = 100pf (figures 5, 7) s2 closed l 40 70 ns t zl driver enable to output low c l = 100pf (figures 5, 7) s1 closed l 40 100 ns t lz driver disable time from low c l = 15pf (figures 5, 7) s1 closed l 40 70 ns t hz driver disable time from high c l = 15pf (figures 5, 7) s2 closed l 40 70 ns t plh receiver input to output r diff = 54 w , c l1 = c l2 = 100pf, l 30 160 200 ns (figures 4, 8) t phl receiver input to output r diff = 54 w , c l1 = c l2 = 100pf, l 30 140 200 ns (figures 4, 8) t skd |t plh C t phl | differential receiver skew r diff = 54 w , c l1 = c l2 = 100pf, 20 ns (figures 4, 8) t zl receiver enable to output low c rl = 15pf (figures 3, 9) s1 closed l 20 50 ns t zh receiver enable to output high c rl = 15pf (figures 3, 9) s2 closed l 20 50 ns t lz receiver disable from low c rl = 15pf (figures 3, 9) s1 closed l 20 50 ns t hz receiver disable from high c rl = 15pf (figures 3, 9) s2 closed l 20 50 ns t dzr driver enable to receiver valid r diff = 54 w , c l1 = c l2 = 100pf l 1600 3000 ns (figures 4, 10) f max maximum data rate (note 5) l 4 5 mbps t shdn time to shutdown (note 6) de = 0, re - l 50 300 600 ns the l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at t a = 25 c.
4 ltc1484 symbol parameter conditions min typ max units t zh(shdn) driver enable from shutdown to output high c l = 100pf (figures 5, 7) s2 closed, l 40 100 ns di = de t zl(shdn) driver enable from shutdown to output low c l = 100pf (figures 5, 7) s1 closed, l 40 100 ns di = 0 t zh(shdn) receiver enable from shutdown to output high c l = 15pf (figures 3, 9) s2 closed, l 10 m s de = 0 t zl(shdn) receiver enable from shutdown to output low c l = 15pf (figures 3, 9) s1 closed, l 10 m s de = 0 note 1: absolute maximum ratings are those values beyond which the life of a device may be impaired. note 2: all typicals are given for v cc = 5v and t a = 25 c. note 3: 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 4: for higher ambient temperatures, the part may enter thermal shutdown during short-circuit conditions. note 5: guaranteed by design. note 6: time for i cc to drop to i cc /2 when the receiver is disabled. switchi n g characteristics u the l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at t a = 25 c. v cc = 5v 5% (notes 2 and 3) unless otherwise noted. typical perfor a ce characteristics uw input voltage (v) ?.2 ?.16 ?.12 ?.08 ?.04 0 receiver output voltage (v) 1484 g01 6 5 4 3 2 1 0 t a = 25 c v cc = 5v v th(high) v th(low) temperature ( c) 55 35 15 5 25 45 65 85 105 125 receiver input threshold voltage (v) 1484 g02 0 0.05 0.10 0.15 0.20 0.25 v cc = 5v v th(high) v cm = 7v v cm = 12v v cm = 0v temperature ( c) 55 35 15 5 25 45 65 85 105 125 receiver input threshold voltage (v) 1484 g03 0 0.05 0.10 0.15 0.20 0.25 v cc = 5v v th(low) v cm = 7v v cm = 12v v cm = 0v receiver output voltage vs input voltage receiver input threshold voltage (output high) vs temperature receiver input threshold voltage (output low) vs temperature
5 ltc1484 typical perfor a ce characteristics uw temperature ( c) 55 35 15 5 25 45 65 85 105 125 receiver input offset voltage (mv) 1484 g04 0 ?0 ?0 ?0 ?0 100 120 140 160 180 200 v cc = 5v v cm = 7v v cm = 12v v cm = 0v temperature ( c) 55 35 15 5 25 45 65 85 105 125 receiver hysteresis (mv) 1484 g05 100 90 80 70 60 50 40 30 20 10 0 v cc = 5v v th(high) ?v th(low) v cm = 7v to 12v supply voltage (v) 4.5 4.75 5 5.25 receiver input threshold voltage (v) 1484 g06 0 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20 t a = 25 c v cm = 0v v th(high) v th(low) receiver input offset voltage vs temperature receiver hysteresis vs temperature receiver input threshold voltage vs supply voltage receiver output high voltage vs output current receiver output low voltage vs output current receiver output high voltage vs temperature output current (ma) ?5 ?0 ?5 ?0 5 0 receiver output high voltage (v) 1484 g07 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0 v cc = 4.75v output current (ma) 0 5 10 15 20 25 receiver output low voltage (v) 1484 g08 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 v cc = 4.75v temperature ( c) 55 35 15 5 25 45 65 85 105 125 receiver output high voltage (v) 1484 g09 4.5 4.4 4.3 4.2 4.1 4.0 3.9 3.8 3.7 3.6 3.5 v cc = 4.75v i out = 8ma receiver output low voltage vs temperature input current (a, b) vs temperature receiver input resistance vs temperature temperature ( c) 55 35 15 5 25 45 65 85 105 125 receiver output low voltage (v) 1484 g10 0.50 0.45 0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05 0 v cc = 4.75v i out = 8ma temperature ( c) 55 35 15 5 25 45 65 85 105 125 input current ( a) 1484 g11 600 500 400 300 200 100 0 100 200 300 400 v cc = 0v or 5v v cm = 12v v cm = 7v temperature ( c) 55 35 15 5 25 45 65 85 105 125 receiver input resistance (k ) 1484 g12 26.0 25.5 25.0 24.5 24.0 23.5 23.0 22.5 22.0 v cc = 0v or 5v v cm = 12v v cm = 7v
6 ltc1484 typical perfor a ce characteristics uw receiver short-circuit current vs temperature receiver propagation delay vs temperature receiver skew vs temperature receiver propagation delay vs supply voltage shutdown supply current vs temperature shutdown supply current vs supply voltage supply current vs temperature supply current vs supply voltage logic input threshold vs temperature temperature ( c) 55 35 15 5 25 45 65 85 105 125 receiver short-circuit current (ma) 1484 g13 100 90 80 70 60 50 40 30 20 10 0 v cc = 5.25v output low short to v cc output high short to ground temperature ( c) 55 35 15 5 25 45 65 85 105 125 receiver propagation delay (ns) 1484 g14 200 180 160 140 120 100 80 60 40 20 0 v cc = 5v t plh t phl temperature ( c) 55 35 15 5 25 45 65 85 105 125 receiver skew (ns) 1484 g15 30 25 20 15 10 5 0 v cc = 5v supply voltage (v) 4.5 4.75 5 5.25 5.5 receiver propagation delay (ns) 1484 g16 200 180 160 140 120 100 t a = 25 c t plh t phl temperature ( c) 55 35 15 5 25 45 65 85 105 125 shutdown supply current ( a) 1484 g17 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 v cc = 5v de = di = 0 re = 5v supply voltage (v) 4.5 4.6 4.7 4.8 4.9 5 5.1 5.2 5.3 5.4 5.5 shutdown supply current ( a) 1484 g18 1.00 0.95 0.90 0.85 0.80 0.75 0.70 0.65 0.60 0.55 0.50 t a = 25 c temperature ( c) ?5 ?0 ? 20 45 70 95 120 145 170 supply current ( a) 1484 g19 1000 900 800 700 600 500 400 300 200 100 0 v cc = 5v driver enabled no load thermal shutdown with driver enabled driver disabled supply voltage (v) 4.5 4.6 4.7 4.8 4.9 5 5.1 5.2 5.3 5.4 5.5 supply current ( a) 1484 g20 700 600 500 400 300 200 100 0 t a = 25 c driver enabled no load driver disabled temperature ( c) ?5 logic input threshold voltage (v) 1484 g21 2.00 1.95 1.90 1.85 1.80 1.75 1.70 1.65 1.60 1.55 1.50 v cc = 5.25v v cc = 4.75v v cc = 5v 35 15 5 25 45 65 85 105 125
7 ltc1484 typical perfor a ce characteristics uw driver differential output voltage vs temperature driver differential output voltage vs temperature driver differential output voltage vs temperature driver common mode output voltage vs temperature driver common mode output voltage vs temperature driver common mode output voltage vs temperature driver differential output voltage vs temperature driver differential output voltage vs temperature driver differential output voltage vs output current temperature ( c) ?5 driver differential output voltage (v) 1484 g22 3.0 2.5 2.0 1.5 1.0 0.5 0 0.5 ? v od , v cc = 4.5v to 5.25v r l = 44 v cc = 5v v cc = 4.5v v cc = 4.75v v cc = 5.25v 35 15 5 25 45 65 85 105 125 temperature ( c) ?5 driver differential output voltage (v) 1484 g23 3.0 2.5 2.0 1.5 1.0 0.5 0 0.5 ? v od , v cc = 4.5v to 5.25v r l = 54 v cc = 5v v cc = 4.5v v cc = 4.75v v cc = 5.25v 35 15 5 25 45 65 85 105 125 temperature ( c) ?5 driver differential output voltage (v) 1484 g24 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0 0.5 ? v od , v cc = 4.5v to 5.25v r l = 100 v cc = 5v v cc = 4.5v v cc = 4.75v v cc = 5.25v 35 15 5 25 45 65 85 105 125 temperature ( c) ?5 driver common mode voltage (v) 1484 g25 3.0 2.5 2.0 1.5 1.0 0.5 0 ? v oc , v cc = 4.5v to 5.25v r l = 44 v cc = 5v v cc = 4.5v v cc = 4.75v v cc = 5.25v 35 15 5 25 45 65 85 105 125 temperature ( c) ?5 driver common mode voltage (v) 1484 g26 3.0 2.5 2.0 1.5 1.0 0.5 0 ? v oc , v cc = 4.5v to 5.25v r l = 54 v cc = 5v v cc = 4.5v v cc = 4.75v v cc = 5.25v 35 15 5 25 45 65 85 105 125 temperature ( c) ?5 driver common mode voltage (v) 1484 g27 3.0 2.5 2.0 1.5 1.0 0.5 0 ? v oc , v cc = 4.5v to 5.25v r l = 100 v cc = 5v v cc = 4.5v v cc = 4.75v v cc = 5.25v 35 15 5 25 45 65 85 105 125 temperature ( c) ?5 driver differential output voltage (v) 1484 g28 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0 0.5 ? v od3 for v cc = 4.5v to 5.25v see figure 2 v cm = 7v v od3 di high v cc = 5v v cc = 4.5v v cc = 4.75v v cc = 5.25v 35 15 5 25 45 65 85 105 125 temperature ( c) ?5 driver differential output voltage (v) 1484 g29 3.0 2.5 2.0 1.5 1.0 0.5 0 0.5 ? v od3 for v cc = 4.5v to 5.25v v cm = 12v v od3 di high see figure 2 v cc = 5v v cc = 4.5v v cc = 4.75v v cc = 5.25v 35 15 5 25 45 65 85 105 125 output current (ma) 0 10 20 30 40 50 60 70 80 90 100 driver differential output voltage (v) 1484 g30 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0 v cc = 5v t a = 25 c
8 ltc1484 ro (pin 1): receiver output. if the receiver output is en- abled (re low) and the part is not in shutdown, ro is high if (a C b) > v th(max) and low if (a C b) < v th(min) . ro is also high if the receiver inputs are open or shorted to- gether, with or without a termination resistor. re (pin 2): receiver output enabled. a high on this pin three-states the receiver output (ro) and a low enables it. de (pin 3): driver enable input. de = high enables the output of the driver with the driver outputs determined by typical perfor a ce characteristics uw driver output high voltage vs output current driver output low voltage vs output current driver propagation delay vs temperature driver short-circuit current vs temperature driver skew vs temperature driver propagation delay vs supply voltage output current (ma) 100 ?0 ?0 ?0 ?0 ?0 ?0 ?0 ?0 ?0 0 driver output high voltage (v) 1484 g31 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0 v cc = 4.75v output current (ma) 0 10 20 30 40 50 60 70 80 90 100 driver output low voltage (v) 1484 g32 3.0 2.5 2.0 1.5 1.0 0.5 0 v cc = 4.75v temperature ( c) ?5 driver propagation delay (ns) 1484 g33 50 45 40 35 30 25 20 15 10 5 0 v cc = 5v t phl t plh 35 15 5 25 45 65 85 105 125 temperature ( c) ?5 driver short-circuit current (ma) 1484 g34 250 200 150 100 50 0 v cc = 5.25v driver output high short to 7v driver output low short to 10v 35 15 5 25 45 65 85 105 125 temperature ( c) ?5 driver skew (ns) 1484 g35 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0 35 15 5 25 45 65 85 105 125 supply voltage (v) 4.5 4.75 5 5.25 5.5 driver propagation delay (ns) 1484 g36 40 35 30 25 20 15 10 5 0 t a = 25 c t phl t plh pi n fu n ctio n s uuu the di pin. de = low forces the driver outputs into a high impedance state. the ltc1484 enters shutdown when both receiver and driver outputs are disabled (re is high and de is low). di (pin 4): driver input. when the driver outputs are enabled (de high), di high takes the a output high and the b output low. di low takes the a output low and the b output high. gnd (pin 5): ground.
9 ltc1484 pi n fu n ctio n s uuu a (pin 6): driver output/receiver input. the input resis- tance is typically 22k when the driver is disabled (de = 0). when the driver is enabled, the a output follows the logic level at the di pin. b (pin 7): driver output/receiver input. the input resis- tance is typically 22k when the driver is disabled (de = 0). when the driver is enabled, the b output is inverted from the logic level at the di pin. v cc (pin 8): positive supply. 4.75v v cc 5.25v. a 0.1 m f bypass capacitor is recommended. driver inputs outputs re de di b a x1101 x1010 o0xzz 10xz*z* fu ctio tables u u receiver inputs outputs re de a C b ro 00 3 v th(max) 1 00 v th(min) 0 0 0 inputs open 1 0 0 inputs shorted 1 1x x z ? test circuits output under test c l s1 s2 v cc 500 w 1484 f05 output under test c rl 1k s1 s2 v cc 1k 1484 f03 figure 3 figure 5 de re a b di c l1 r diff c l2 ro 15pf a b 1484 f04 figure 4 note: z = high impedance, x = dont care *shutdown mode for ltc1484 ? shutdown mode for ltc1484 if de = 0. table valid with or without termination resistors. v od1 v od2 a b r r v oc 1484 f01 figure 1 v od3 a b 375 w 375 w 60 w ?v to 12v 1484 f02 figure 2
10 ltc1484 switchi g ti e wavefor s uw w figure 6. driver propagation delays figure 9. receiver enable and shutdown timing figure 8. receiver propagation delays 1.5v 2.3v 2.3v t zh(shdn) , t zh note: a, b are three-stated when de = 0, 1k w pull-up or 1k w pull-down t zl(shdn) , t zl 1.5v 0.5v 0.5v t hz t lz output normally low output normally high 3v 0v de 5v v ol v oh 0v a, b a, b 1484 f07 f = 1mhz, t r 10ns, t f 10ns figure 7. driver enable and disable timing 1.5v 1.5v 1.5v t zh(shdn), t zh note: de = 0, ro is three-stated in shutdown, 1k w pull-up for normally low output, 1k w pull-down for normally high output t zl(shdn), t zl 1.5v 0.5v 0.5v t hz t lz output normally low output normally high re 5v 0v 5v 0v ro ro 1484 f09 f = 1mhz, t r 10ns, t f 10ns 0v t phl a ?b v ol ro 1.5v note: t skd = |t phl ?t plh |, re = 0 1.5v 0v t plh input output 5v ? od2 v od2 1484 f08 f = 1mhz, t r 10ns, t f 10ns di 3v 1.5v t plh t r t skew note: de = 1 1/2 v o v o f = 1mhz, t r 10ns, t f 10ns 90% 50% 50% 10% 0v b a v o ? o 90% 1.5v t phl t skew 10% t f v o = v(a) ?v(b) 1484 f06
11 ltc1484 applicatio n s i n for m atio n wu u u switchi g ti e wavefor s uw w 1.5v 1.5v note: di = 0, re = 0, a and b are three-stated when de = 0 t dzr output normally low output normally high 3v 0v de v(a) ?v(b) ro 1484 f10 f = 1mhz, t r 10ns, t f 10ns figure 10. driver enable to receiver valid timing low power operation the ltc1484 has a quiescent current of 900 m a max when the driver is enabled. with the driver in three-state, the supply current drops to 700 m a max. the difference in these supply currents is due to the additional current drawn by the internal 22k receiver input resistors when the driver is enabled. under normal operating conditions, the additional current is overshadowed by the 50ma current drawn by the external termination resistor. receiver open-circuit fail-safe some encoding schemes require that the output of the receiver maintain a known state (usually a logic 1) when data transmission ends and all drivers on the line are forced into three-state. earlier rs485 receivers with a weak pull-up at the a input will give a high output only when the inputs are floated. when terminated or shorted together, the weak pull-up is easily defeated causing the receiver output to go low. external components are needed if a high receiver output is mandatory. the receiver of the ltc1484 has a fail-safe feature which guarantees the output to be in a logic 1 when the receiver inputs are left open or shorted together, regardless of whether the termi- nation resistor is present or not. in encoding schemes where the required known state is a low, external components are needed for the ltc1484 and other rs485 parts. fail-safe is achieved by making the receiver trip points fall within the v th(min) to v th(max) range. when any of the listed receiver input conditions exist, the receiver inputs are effectively at 0v and the receiver output goes high. the receiver fail-safe mechanism is designed to reject fast common mode steps (C 7v to 12v in 10ns) switching at 100khz typ. this is achieved through an internal carrier detect circuit similar to the ltc1482. this circuit has built- in delays to prevent glitches while the input swings be- tween v th(max) levels. when all the drivers connected to the receiver inputs are three-stated, the internal carrier detect signal goes low to indicate that no differential signal is present. when any driver is taken out of three-state, the carrier detect signal takes 1.6 m s typ (see t dzr ) to detect the enabled driver. during this interval, the transceiver output (ro) is forced to the fail-safe high state. after 1.6 m s, the receiver will respond normally to changes in driver output. if the part is taken out of shutdown mode with the receiver inputs floating, the receiver output takes about 10 m s to leave three-state (see t zl(shdn) ). if the receiver inputs are actively driven to a high state, the outputs go high after about 5.5 m s.
12 ltc1484 applicatio n s i n for m atio n wu u u shutdown mode the receiver output (ro) and the driver outputs (a, b) can be three-stated by taking the re and de pins high and low respectively. taking re high and de low at the same time puts the ltc1484 into shutdown mode and i cc drops to 20 m a max. in some applications (see cdma), the a and b lines are pulled to v cc or gnd through external resistors to force the line to a high or low state when all connected drivers are disabled. in shutdown, the supply current will be higher than 20 m a due to the additional current drawn through the external pull-up and the 22k input resistance of the ltc1484. esd protection the esd performance of the ltc1484 a and b pins is characterized to meet 15kv using the human body model (100pf, 1.5k w ), iec-1000-4-2 level ( 8kv) contact mode and iec-1000-4-2 level 3 ( 8kv) air discharge mode. this means that external voltage suppressors are not required in many applications when compared with parts that are only protected to 2kv. pins other than the a and b pins are protected to 4.5kv typical per the human body model. when powered up, the ltc1484 does not latch up or sustain damage when the a and b pins are tested using any of the three conditions listed. the data during the esd event may be corrupted, but after the event the ltc1484 continues to operate normally. the additional esd protec- tion at the a and b pins is important in applications where these pins are exposed to the external world via connec- tions to sockets. fault protection when shorted to C7v or 10v at room temperature, the short-circuit current in the driver pins is limited by internal resistance or protection circuitry to 250ma. over the industrial temperature range, the absolute maximum positive voltage at any driver pin should be limited to 10v to avoid damage to the driver pins. at higher ambient temperatures, the rise in die temperature due to the short-circuit current may trip the thermal shutdown circuit. when the driver is disabled, the receiver inputs can withstand the entire C 7v to 12v rs485 common mode range without damage. the ltc1484 includes a thermal shutdown circuit which protects the part against prolonged shorts at the driver outputs. if a driver output is shorted to another output or to v cc , the current will be limited to 250ma. if the die temperature rises above 150 c, the thermal shutdown circuit three-states the driver outputs to open the current path. when the die cools down to about 130 c, the driver outputs are taken out of three-state. if the short persists, the part will heat again and the cycle will repeat. this thermal oscillation occurs at about 10hz and protects the part from excessive power dissipation. the average fault current drops as the driver cycles between active and three-state. when the short is removed, the part will return to normal operation. carrier detect multiple access (cdma) application in normal half-duplex rs485 systems, only one node can transmit at a time. if an idle node suddenly needs to gain access to the twisted pair while other communications are in progress, it must wait its turn. this delay is unaccept- able in safety-related applications. a scheme known as carrier detect multiple access (cdma) solves this prob- lem by allowing any node to interrupt on-going communi- cations. figure 11 shows four nodes in a typical cdma communi- cations system. in the absence of any active drivers, bias resistors (1.2k) force a 1 across the twisted pair. all drivers in the system are connected so that when enabled, they transmit a 0. this is accomplished by tying di low and using de as the driver data input. a 1 is transmitted by disabling the drivers 0 output and allowing the bias resistors to reestablish a 1 on the twisted pair. control over communications is achieved by asserting a 0 during the time an active transmitter is sending a 1. any node that is transmitting data watches its own
13 ltc1484 r d 1 1k ro4 de4 67 2 5 8 5v 34 r d 1 1k ro2 de2 6 7 120 w 2 5 8 5v 5v 3 4 1.2k 1484 f11 1.2k 120 w 5v 1.2k 1.2k r d 1 1k ro3 de3 ro1 de1 6 7 2 5 8 5v 3 4 r d 1 1k 67 2 5 8 5v 34 receiver output and expects to see perfect agreement between the two data streams. (note that the driver inverts the data, so the transmitted and received data streams are actually opposites.) if the simultaneously transmitted and received data streams differ (usually detected by compar- ing ro and de with an xor), it signals the presence of a second, active driver. the first driver falls silent, and the second driver seizes control. if the ltc1484 is connected as shown in figure 11, the overhead of xoring the transmitted and received data in hardware or software is eliminated. de and re are con- nected together so the receiver is disabled and its output three-stated whenever a 0 is transmitted. a 1k pull-up ensures a 1 at the receiver output during this condition. the receiver is enabled when the driver is disabled. during this interval the receiver output should also be 1. thus, under normal operation the receiver output is always 1. if a 0 is detected, it indicates the presence of a second active driver attempting to seize control of communica- tions. the maximum frequency at which the system in figure 11 can operate is determined by the cable capacitance, the values of the pull-up and pull-down resistors and receiver propagation delay. the external resistors take a longer time to pull the line to a 1 state due to higher source resistance compared to an active driver, thereby affecting the duty cycle of the receiver output at the far end of the line. figure 11. transmit 0 cdma application applicatio n s i n for m atio n wu u u (b) figure 12a shows a 100khz de1 waveform for an ltc1484 driving a 1000-foot shielded twisted-pair (stp) cable and the a2, b2 and ro2 waveforms of a receiving ltc1484 at the far end of the cable. the propagation delay between de1 of the driver and ro2 at the far end of the line is 1.8 m s at the rising edge and 3.7 m s at the falling edge of de1. the (a) figure 12. ltc1484 driving a 1000 foot stp cable 1484 f12a 1484 f12b b2 a2 ro2 de1 de1 b2 a2 ro2
14 ltc1484 longer delay for the falling edge is due to the larger voltage range the line must swing (typically > 2v compared to 370mv) before the receiver trips high again. the differ- ence in delay affects the duty cycle of the received data and depends on cable capacitance. for a 1-foot stp cable, the delays drop to 0.13 m s and 0.4 m s. using smaller valued pull-up and pull-down resistors to equalize the positive and negative voltage swings needed to trip the receivers will reduce the difference in delay and increase the maxi- mum data rate. with 220 w resistors, both rising and falling edge delays are 2.2 m s when driving a 1000-foot stp cable as shown in figure 12b. the fail-safe feature of the ltc1484 receiver allows a cdma system to function without the a and b pull-up and pull-down resistors. however, if the resistors are left out, noise margin will be reduced to as low as 15mv and propagation delays will increase significantly. operation in this mode is not recommended. since de and re are tied together, the part never shuts down. the receiver inputs are never floating (due to the external bias resistors) so that the t dzr timing does not apply to this application. the whole system can be changed to actively transmit only a 1 by swapping the pull-up and pull-down resistors in figure 11, shorting di to v cc and connecting the 1k resistor as a pull-down. in this configu- ration the driver is noninverting and the receiver output ro truly follows de. applicatio n s i n for m atio n wu u u package descriptio n u dimensions in inches (millimeters), unless otherwise noted. ms8 package 8-lead plastic msop (ltc dwg # 05-08-1660) msop (ms8) 1098 * dimension does not include mold flash, protrusions or gate burrs. mold flash, protrusions or gate burrs shall not exceed 0.006" (0.152mm) per side ** dimension does not include interlead flash or protrusions. interlead flash or protrusions shall not exceed 0.006" (0.152mm) per side 0.021 0.006 (0.53 0.015) 0 ?6 typ seating plane 0.007 (0.18) 0.040 0.006 (1.02 0.15) 0.012 (0.30) ref 0.006 0.004 (0.15 0.102) 0.034 0.004 (0.86 0.102) 0.0256 (0.65) bsc 12 3 4 0.193 0.006 (4.90 0.15) 8 7 6 5 0.118 0.004* (3.00 0.102) 0.118 0.004** (3.00 0.102)
15 ltc1484 package descriptio n u dimensions in inches (millimeters), unless otherwise noted. n8 package 8-lead pdip (narrow 0.300) (ltc dwg # 05-08-1510) n8 1098 0.100 (2.54) bsc 0.065 (1.651) typ 0.045 ?0.065 (1.143 ?1.651) 0.130 0.005 (3.302 0.127) 0.020 (0.508) min 0.018 0.003 (0.457 0.076) 0.125 (3.175) min 12 3 4 87 6 5 0.255 0.015* (6.477 0.381) 0.400* (10.160) max 0.009 ?0.015 (0.229 ?0.381) 0.300 ?0.325 (7.620 ?8.255) 0.325 +0.035 0.015 +0.889 0.381 8.255 () *these dimensions do not include mold flash or protrusions. mold flash or protrusions shall not exceed 0.010 inch (0.254mm) 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. s8 package 8-lead plastic small outline (narrow 0.150) (ltc dwg # 05-08-1610) 0.016 ?0.050 (0.406 ?1.270) 0.010 ?0.020 (0.254 ?0.508) 45 0 ?8 typ 0.008 ?0.010 (0.203 ?0.254) so8 1298 0.053 ?0.069 (1.346 ?1.752) 0.014 ?0.019 (0.355 ?0.483) typ 0.004 ?0.010 (0.101 ?0.254) 0.050 (1.270) bsc 1 2 3 4 0.150 ?0.157** (3.810 ?3.988) 8 7 6 5 0.189 ?0.197* (4.801 ?5.004) 0.228 ?0.244 (5.791 ?6.197) dimension does not include mold flash. mold flash shall not exceed 0.006" (0.152mm) per side dimension does not include interlead flash. interlead flash shall not exceed 0.010" (0.254mm) per side * **
16 ltc1484 1484f lt/tp 0400 4k ? printed in usa ? linear technology corporation 1998 linear technology corporation 1630 mccarthy blvd., milpitas, ca 95035-7417 (408) 432-1900 l fax: (408) 434-0507 l www.linear-tech.com related parts part number description comments ltc485 5v low power rs485 interface transceiver low power ltc1480 3.3v ultralow power rs485 transceiver with shutdown lower supply voltage ltc1481 5v ultralow power rs485 transceiver with shutdown lowest power ltc1482 5v low power rs485 transceiver with carrier detect output low power, high output state when inputs are open, shorted or terminated, 15kv esd protection ltc1483 5v ultralow power rs485 low emi transceiver with shutdown low emi, lowest power ltc1485 5v rs485 transceiver high speed, 10mbps, 15kv esd protection ltc1487 5v ultralow power rs485 with low emi, shutdown and highest input impedance, low emi, lowest power high input impedance ltc1535 isolated rs485 transceiver 2500v rms isolation ltc1685 52mbps rs485 transceiver propagation delay skew 500ps (typ) ltc1690 5v differential driver and receiver pair with fail-safe receiver output low power, 15kv esd protection lt1785 60v fault protected rs485 transceiver 15kv esd protection, industry standard pinout typical applicatio u ro re de di r d ro re de di v cc b a gnd 1484 ta02 ? ? 5v ltc1484 i1 i2 fail-safe 0 application (idle state = logic 0)

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