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_______________general description the max253 monolithic oscillator/power-driver is specifically designed to provide isolated power for an isolated rs-485 or rs-232 data interface. the device drives a center-tapped transformer primary from a 5v or 3.3v dc power supply. the secondary can be wound to provide any isolated voltage needed at power levels up to 1w. the max253 consists of a cmos oscillator driving a pair of n-channel power switches. the oscillator runs at double the output frequency, driving a toggle flip-flop to ensure 50% duty cycle to each of the switches. internal delays are arranged to ensure break-before- make action between the two switches. the sd pin puts the entire device into a low-power shutdown state, disabling both the power switches and oscillator. ________________________applications isolated rs-485/rs-232 power-supply transformer driver high noise-immunity communications interface isolated and/or high-voltage power supplies bridge ground differentials medical equipment process control ____________________________features ? ? power-supply transformer driver for isolated rs-485/rs-232 data-interface applications ? ? single 5v or 3.3v supply ? ? low-current shutdown mode: 0.4 a ? ? pin-selectable frequency: 350khz or 200khz ? ? 8-pin dip, so, and max ? packages ______________ordering information max253 transformer driver for isolated rs-485 interface max253 d1 d2 fs gnd1 gnd2 v cc 1 8 46 27 3 v in sd frequency switch c3 c1 c2 5v @ 200ma output 5v on / off __________typical operating circuit part temp range pin-package max253cpa 0c to +70c 8 plastic dip max253csa 0c to +70c 8 so max253cua 0c to +70c 8 max max253c/d 0c to +70c dice* max253epa -40c to +85c 8 plastic dip max253esa -40c to +85c 8 so max253esa/v -40c to +85c 8 so max253mja -55c to +125c 8 cerdip** * contact factory for dice specifications. ** contact factory for availability and processing to mil-std-883. devices are also available in a lead(pb)-free/rohs-compliant package. specify lead-free by adding a (+) to the part number when ordering. /v denotes an automotive qualified part. ?ax is a registered trademark of maxim integrated products, inc. __________typical operating circuit 19-0226; rev 2; 4/10 functional diagrams pin configurations appear at end of data sheet. functional diagrams continued at end of data sheet. ucsp is a trademark of maxim integrated products, inc. available for pricing, delivery, and ordering information, please contact maxim direct at 1-888-629-4642, or visit maxim?s website at www.maxim integrated.com. max253
transformer driver for isolated rs-485 interface absolute maximum ratings electrical characteristics (v cc = 5v ?0%, t a = t min to t max , unless otherwise noted. typical values are at t a = +25?.) stresses beyond those listed under ?bsolute maximum ratings?may cause permanent damage to the device. these are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. exposure to absolute maximum rating conditions for extended periods may affect device reliability. note 1: operating supply current is the current used by the max253 only, not including load current. note 2: shutdown supply current includes output switch-leakage currents. parameter min typ max units shutdown input threshold 0.8 ? 2.4 v shutdown supply current (note 2) 0.4 ? operating supply current (note 1) 0.45 5.0 ma shutdown input leakage current 10 pa 2.4 v fs input threshold 0.8 50 ? switch on-resistance 1.5 4.0 250 350 500 khz switch frequency 150 200 300 fs input leakage current 10 pa start-up voltage 2.5 2.2 v conditions fs = v cc low high sd = v cc no load, v sd = 0v, fs low high low v fs = 0v d1, d2; 100ma fs = v cc or open v fs = 0v supply voltage (v cc ) ...............................................-0.3v to +7v control input voltages (sd, fs) .................-0.3v to (v cc + 0.3v) output switch voltage (d1, d2) .............................................12v peak output switch current (d1, d2) ......................................1a average output switch current (d1, d2) .........................200ma continuous power dissipation (t a = +70?) plastic dip (derate 9.09mw/? above +70?) .............727mw so (derate 5.88mw/? above +70?) ..........................471mw ?ax (derate 4.10mw/? above +70?) .....................330mw cerdip (derate 8.00mw/? above +70?) ..................640mw operating temperature ranges max253c_ _ ........................................................0? to +70? max253e_ _ .....................................................-40? to +85? max253mja ...................................................-55? to +125? junction temperatures max253c_ _/e_ _..........................................................+150? max253mja .................................................................+175? storage temperature range .............................-65? to +160? lead temperature (soldering, 10s) .................................+300? soldering temperature (reflow) pdip, so, ?ax lead(pb)-free .....................................+260? pdip, so, ?ax, cerdip containing lead(pb) ............+240? max253 2 maxim integrated
transformer driver for isolated rs-485 interface 10.5 6.0 -60 -20 60 140 output resistance vs. temperature (fs = low) 7.0 9.5 plot01 temperature (c) output resistance () 20 100 8.5 8.0 6.5 7.5 10.0 9.0 -40 0 8040 120 v in = 4.5v v in = 5.0v measured at tp1 15 6 -60 -20 60 140 output resistance vs. temperature (fs = high) plot02 temperature (c) output resistance () 20 100 9 12 -40 0 8040 120 v in = 4.5v v in = 5.0v measured at tp1 1.0 0 -60 -20 60 140 shutdown supply current vs. temperature plot03 temperature (c) shutdown current (a) 20 100 0.4 0.8 -40 0 8040 120 0.6 0.2 includes switch leakage currents 260 160 -60 -20 60 140 d1, d2 frequency vs. temperature (fs = low) plot04 temperature (c) frequency (khz) 20 100 200 240 -40 0 8040 120 220 180 v in = 5.5v v in = 4.5v v in = 5.0v v in = 6.0v 850 400 -60 -20 60 140 supply current vs. temperature (fs = high) 500 750 plot07 temperature (c) supply current (a) 20 100 650 600 450 550 800 700 -40 0 8040 120 v in = 5.5v v in = 6.0v v in = 4.5v v in = 5.0v 480 280 -60 -20 60 140 d1, d2 frequency vs. temperature (fs = high) plot05 temperature (c) frequency (khz) 20 100 360 440 -40 0 8040 120 400 320 v in = 4.5v v in = 5.0v v in = 6.0v v in = 5.5v 550 250 -60 -20 60 140 supply current vs. temperature (fs = low) plot06 temperature (c) supply current (a) 20 100 350 -40 0 8040 120 300 400 500 450 600 v in = 4.5v v in = 5.0v v in = 5.5v v in = 6.0v 100 0 0 40 120 200 efficiency vs. load current (fs = low) 20 80 plot08 load current (ma) efficiency (%) 80 160 60 40 10 30 90 70 50 20 60 140100 180 v in = 5.5v v in = 4.5v __________________________________________typical operating characteristics (circuit of figure 6, v in = 5v ?0%, t a = +25?, unless otherwise noted.) max253 maxim integrated 3
transformer driver for isolated rs-485 interface 10 0 0 40 220 output voltage vs. load current (fs = low) 2 8 plot10 load current (ma) output voltage (v) 80 6 4 1 3 9 7 5 20 60 140 12 0 100 160 180 200 circuit of figure 6 v in = 5.0v turns ratio = 1:1 circuit of figure 6 v in = 5.0v turns ratio = 1:1.3 circuit of figure 7 v in = 3.3v turns ratio = 1:2.1 measured at tp1 10 0 0 40 220 output voltage vs. load current (fs = high) 2 8 plot11 load current (ma) output voltage (v) 80 6 4 1 3 9 7 5 20 60 140 12 0 100 160 180 200 circuit of figure 6 v in = 5.0v turns ratio = 1:1 circuit of figure 6 v in = 5.0v turns ratio = 1:1.3 circuit of figure 7 v in = 3.3v turns ratio = 1:2.1 measured at tp1 circuit of figure 1 switching waveforms (two cycles) d1 d2 circuit of figure 1 switching waveforms (break before make) d1 d2 circuit of figure 6 time from shutdown to power-up sd tp1 (output voltage) ____________________________typical operating characteristics (continued) (circuit of figure 6, v in = 5v ?0%, t a = +25?, unless otherwise noted.) 100 0 0 40 120 200 efficiency vs. load current (fs = high) 20 80 plot09 load current (ma) efficiency (%) 80 160 60 40 10 30 90 70 50 20 60 140100 180 v in = 5.5v v in = 4.5v max253 4 maxim integrated
transformer driver for isolated rs-485 interface 1 2 3 4 8 7 6 5 d2 gnd2 v cc n.c. sd fs gnd1 d1 dip/so/ top view max253 + __________________pin configuration _____________________pin description not internally connected. n.c. 5 5v supply voltage. v cc 6 gnd2 7 open drain of n-channel transformer drive 2. d2 8 sd 4 fs 3 pin gnd1 2 open drain of n-channel transformer drive 1. d1 1 function name ground. connect both gnd1 and gnd2 to ground. frequency switch. if fs = v cc or open, switch frequency = 350khz; if v fs = 0v, switch frequency = 200khz. shutdown. ground for normal operation, connect high for shutdown. ground. connect both gnd1 and gnd2 to ground. max253 maxim integrated 5
transformer driver for isolated rs-485 interface _______________detailed description the max253 is an isolated power-supply transformer driver specifically designed to form the heart of a fully isolated rs-485 data interface. completely isolated communications are obtained by combining the max253 with a linear regulator, a center-tapped trans- former, optocouplers, and the appropriate maxim inter- face product (as described in the isolated rs-485/rs- 232 data interface section). the max253 consists of an rc oscillator followed by a toggle flip-flop, which generates two 50% duty-cycle square waves, out-of-phase at half the oscillator fre- quency (figure 2). these two signals drive the ground- referenced output switches. internal delays ensure break-before-make action between the two switches. ground sd for normal operation. when high, sd dis- ables all internal circuitry, including the oscillator and both power switches. pulling fs low reduces the oscillator frequency and low- ers the supply current (see supply current vs. temperature in the typical operating characteristics ). fs includes a weak pull-up, so it will be set to the high- frequency state if not connected. max253 d1 d2 fs gnd1 gnd2 v cc 1 8 4 6 27 3 sd frequency switch on / off r2 50 r1 50 v in 5v c1 0.1f figure 1. test circuit max253 d1 d2 fs gnd2 gnd1 v cc frequency switch c3 c1 c2 5v @ 200ma iso output 5v n n q q osc f / f v in sd on / off 400khz/ 700khz t iso gnd figure 2. block diagram max253 6 maxim integrated
transformer driver for isolated rs-485 interface c3 0.1f c1 0.1f c2 22f iso 5v isolation barrier 5v v in max253 d1 d2 fs gnd1 gnd2 v cc 1 8 6 27 3 1n5817 sd 4 on / off 390 * 74hc04 de ro 390 * 74hc04 di 3.3k 1 3 5 4 6 1 2 4 3 5 4 1 3 6 * 74hc04 * 74hc04 or equivalent ** see table 2 max667 5 64 shdn set gnd max481 max483 max485 max487 52 re gnd c4 22f in out 82 1n5817 390 4 3.3k 3.3k 3 1 8 6 7 di de ro a b pc410 / 417 pc357t pc410 / 417 1ct:1.3ct** v cc 485 i/o figure 3. typical rs-485 application circuit, 5v configuration max253 maxim integrated 7
transformer driver for isolated rs-485 interface c3 0.1f c1 0.1f c2 22f iso 5v isolation barrier 3.3v v in max253 d1 d2 gnd1 gnd2 v cc 1 8 6 27 n.c. 5 1n5817 1n5817 c5 0.1f sd 4 on / off 390 * 74hc04 de ro 390 * 74hc04 di 3.3k 1 3 5 4 6 1 2 4 3 5 4 1 3 6 * 74hc04 * 74hc04 or equivalent ** see table 2 max667 5 64 shdn set gnd max481 max483 max485 max487 52 re gnd c4 22f in out 82 1n5817 390 4 3 1 8 6 7 di de ro a b pc410 / 417 pc357t pc410 / 417 1n5817 fs 3 1ct:2.1ct** v cc 3.3k 3.3k 485 i/o figure 4. typical rs-485 application circuit, 3.3v configuration max253 8 maxim integrated
transformer driver for isolated rs-485 interface c3 0.1f c2 22f iso 5v isolation barrier 5v v in 1 8 5 c1 0.1f 1n5817 max253 d1 d2 gnd1 gnd2 v cc 6 27 n.c. sd 4 on / off 10 x pc417 *74hc04 or equivalent ** see table 2 max205 ensd c4 22f 82 1n5817 max667 5 64 shdn set gnd in out 390 83 t1 in t1 out 390 *74hc04 t1 in 1 2 4 5 6 1 2 4 5 6 390 74hc04 t2 in 390 74hc04 t3 in 390 74hc04 t4 in 390 74hc04 t5 in 74hc04 r1 out 74hc04 74hc04 74hc04 74hc04 r2 out r3 out r4 out r5 out 5 x 3.3k 390 390 390 390 74 t2 in t2 out 15 2 t3 in t3 out 16 1 t4 in t4 out 22 19 t5 in t5 out 91 0 r1 out r1 in 65 r2 out r2 in 23 24 r3 out r3 in 17 18 r4 out r4 in 14 13 r5 out r5 in v cc 20 21 gnd 5 x 3.3k 1n5711 6 5 4 1 2 390 3.3k v cc iso r out r out 1n5711 1 2 6 5 4 390 3.3k v cc iso t in t in *74hc04 4n25 lower speed, lower cost alternate optocoupler configurations (for data rates below 9.6kbps) fs 3 74hco4 4n25 iso gnd 4n25 iso gnd 1ct:1.3ct** figure 5. typical rs-232 application circuit max253 maxim integrated 9
transformer driver for isolated rs-485 interface * pc-series optocouplers, sharp electronics usa phone: (206) 834-2500 fax: (206) 834-8903 sharp electronics, europe gmbh germany phone: (040) 2376-0 fax: (040) 230764 __________applications information figures 3? are typical isolated rs-485/rs-232 data-inter- face circuits. these circuits withstand 1800v rms (1sec) and are intended for industrial communications and control applications where very high voltage transients, differential ground potentials, or high noise may be encountered. table 2 lists transformer characteristics for the applica- tions of figures 3?0. some suggested manufacturers of transformers, transformer cores, and optocouplers are listed in table 3, along with their respective phone and fax numbers. important layout considerations include: ? ? for maximum isolation, the ?solation barrier?should not be breached. connections and components from one side should not be located near those of the other side. ? ? since the optocoupler outputs are relatively high- impedance nodes, they should be located as close as possible to the maxim interface ic. this mini- mizes stray capacitance and maximizes data rate. refer to the ?ax package information for pin spacing and physical dimensions. isolated rs-485 data interface the max253 power-supply transformer driver is designed specifically for isolated rs-485 data-interface applications. the application circuits of figures 3 and 4 combine the max253 with a low-dropout linear regulator, a transformer, several high-speed optocouplers, and a maxim rs-485 interface device. with a few modifica- tions to these circuits, full-duplex communications can be implemented by substituting the max481/max485 with the max490/max491 (for data rates up to 2.5mbps) or substituting the max483/max487 with the max488/max489 (for data rates up to 250kbps). the data transfer rates of the application circuits in figures 3 and 4 are critically limited by the optocou- plers. table 1 lists suggested optocouplers and the appropriate maxim interface device for data-transfer rates up to 2.5mbps. refer to the max1480 data sheet for a complete isolat- ed rs-485 solution in one package. isolated rs-232 data interface the max253 is ideal for isolated rs-232 data-interface applications requiring more than four transceivers. the 1w power output capability of the max253 enables it to drive more than 10 transceivers simultaneously. figure 5 shows the typical application circuit for a complete 120kbps isolated rs-232 data interface. the figure also shows how the sharp pc417 optocouplers can be replaced by the lower-cost 4n25 devices to achieve data-transfer rates up to 9.6kbps. for 3.3v operation, substitute the primary portion of figure 5 with the circuit of figure 7. for applications requiring two transceivers or fewer, refer to the max250/max251 or max252 data sheet. isolated power supplies the max253 is a versatile isolated power driver, capa- ble of driving a center-tapped transformer primary from a 5v or a 3.3v dc power supply (figures 6 and 7). the secondary can be wound to provide any isolated volt- age needed at power levels up to 1w with a 5v supply, or 600mw with a 3.3v supply. figure 6 shows a typical 5v to isolated 5v application circuit that delivers up to 200ma of isolated 5v power. in figure 7, the max253 is configured to operate from a 3.3v supply, deriving a ?oost?v cc for the max253 by connecting diodes to both ends of the transformer pri- mary. this produces nearly double the input supply, and may be useful for other applications, as shown in figure 4. the average current in each max253 switch must still be limited to less than 200ma, so the total power available is approximately 600mw. table 1. optocouplers and rs-485 interface ics for various data rates data rate full duplex rs-485 ic half duplex rs-485 ic optocoupler for di / ro optocoupler for de 250kbps max488/max489 max483/max487 pc417* pc357t* 2.5mbps max490/max491 max481/max485 pc410* pc357t max253 10 maxim integrated
transformer driver for isolated rs-485 interface c3 0.1f c1 0.1f c2 22f 5v @ 200ma iso output 5v v in l2 25h max253 d1 d2 fs gnd1 gnd2 v cc 1 8 6 27 3 frequency switch sd 4 c7 2.2f filter output output optional 21khz lowpass output filter tp1 1n5817 1ct:1.3ct* 1n5817 on / off *see table 2 figure 6. 5v to isolated 5v application circuit c3 0.1? c1 0.1? c2 22? 5v @ 100ma iso output 3.3v v in l2 25? max253 d1 d2 fs gnd1 gnd2 v cc 1 8 6 27 3 frequency switch sd 4 c7 2.2f filter output output optional 21khz lowpass output filter tp1 1n5817 1ct:2.1ct* 1n5817 on / off 1n5817 1n5817 c4 0.1f *see table 2 figure 7. 3.3v to isolated 5v application circuit max253 maxim integrated 11
transformer driver for isolated rs-485 interface 10f 5v max253 d1 d2 gnd1 gnd2 v cc 1 8 6 27 1n5817 sd 4 1n5817 1ct:5ct* 49.9k 7 6 3 2 4 7 6 3 2 4 10k 24.9 2n3904 max480 max480 5v 0.1v to 0.5v iso 5v 78l05 il300 4 3 2 1 5 6 24v unregulated 49.9k r l 0k to 1k 2n3904 *see table 2 isolation barrier v in figure 8. typical 4ma to 20ma application circuit output-ripple filtering a simple lowpass pi-filter (figures 6 and 7) can be added to the output to reduce output ripple noise to approximately 10mvp-p. the cutoff frequency shown is 21khz. since the filter inductor is in series with the circuit output, minimize its resistance so the voltage drop across it is not excessive. isolated 4ma to 20ma analog interface the 4ma to 20ma current loop is a standard analog signal range that is widely used in the process-control industry for transducer and actuator control signals. these signals are commonly referred to a distant ground that may be at a considerably higher voltage with respect to the local ground. an analog signal in the range of 0.1v to 0.5v is applied to the first max480 to generate a signal current in the range of 20? to 100?. this low-level signal is trans- ferred across the barrier by the siemens il300 linear optocoupler. this device is unique in that it corrects the dominant nonlinearity present in most optocou- plersthe led efficiency variation. the il300 is really two optocouplers in the same package sharing the same led; one detector is across the isolation barrier, the other is on the same side as the led (figure 8). the lat- ter detector is used to generate a feedback signal identi- cal to the signal on the isolated side of the barrier. the current signal transferred across the barrier is converted back to a voltage that matches the input in the 100mv to 500mv range. this voltage is then transformed to the final 4ma to 20ma current signal range by the second max480, darlington stage, and the 20 resistor. isolated adc almost any serial-interface device is a candidate for operation across an isolation barrier; figure 10 illus- trates one example. the max176 analog-to-digital converter (adc) operates from 5v and -12v supplies, provided by the multiple-tapped secondary and linear regulators. if some additional isolated power is needed for signal conditioning, multiplexing, or possibly for a max253 12 maxim integrated
transformer driver for isolated rs-485 interface -v out output -2v in v in input max253 gnd1 gnd2 v cc 1 8 6 27 1n5817 1ct:1ct* 1n5817 d1 d2 +v out output 2v in r l + r l - r l + ? r l - *see table 2 figure 9a. half-wave rectifier?ipolar v out -v in output v in input max253 gnd1 gnd2 v cc 1 8 6 27 1ct:1ct* d1 d2 v out +v in output 4 x 1n5817 *see table 2 figure 9b. full-wave rectifier?ipolar v in input max253 gnd1 gnd2 v cc 1 8 6 27 1ct:1ct* d1 d2 v out 2 x v in output 4 x 1n5817 *see table 2 figure 9c. full-wave rectifier?nipolar max253 maxim integrated 13
transformer driver for isolated rs-485 interface 10f 6n136 max253 d1 d2 gnd1 gnd2 v cc 4 27 8 1 sd 6 6n136 6n136 10f 79 l12 78l05 1 2 3 4 8 7 6 5 1 2 3 4 8 7 6 5 1 2 3 4 8 7 6 5 1 2 3 4 8 7 6 5 convst v dd v ss clock data ain vref gnd max176 3k 3k 470 0.1 f10 f 0.1f 10f 0.1f 10f 5v 200 200 8.2k 74hc04 7 6 5 4 3 2 1 15 16 qh qg qf qe qd qc qb qa ser sck rck sclr 14 11 12 10 74hc595 13 8 5v 0.1f d11(msb) d10 d9 d8 5v 7 6 5 4 3 2 1 15 16 qh qg qf qe qd qc qb qa ser sck rck sclr 14 11 12 10 74hc595 13 8 5v 0.1f d7 d6 d5 d4 d3 d2 d1 d0(lsb) 5v 74hc04 on/off start input clock 1ct : 1.5ct : 3ct* 4 x 1n5817 v in 5v signal ground analog input iso 5v iso -12v 8 qh isolation barrier *see table 2 figure 10. typical isolated adc application max253 14 maxim integrated
transformer driver for isolated rs-485 interface sensor, an extra several hundred milliwatts could easily be supplied by the circuit, as shown. a 12v supply could be generated by adding two more diodes to the ends of the secondary, and a -5v supply could be gen- erated by connecting additional diodes to the 1/4 and 3/4 tap points on the secondary. for 5v only applica- tions, the max187 is recommended. ______________component selection transformer selection the transformer primary used with the max253 must be a center-tapped winding with sufficient et product to prevent saturation at the worst-case lowest selected frequency. the max253? guaranteed minimum fre- quency with the fs pin held low is 150khz, equating to a maximum period of 6.67?. the required et product for half the primary is simply the product of the maxi- mum supply voltage and half the maximum period. with fs connected high, the guaranteed minimum fre- quency is 250khz, giving a maximum period of 4?. the secondary winding may or may not be center tapped, depending on the rectifier topology used. the phasing of the secondary winding is not critical. in some applications, multiple secondaries might be required. half-wave rectification could be used, but is discouraged because it normally adds a dc imbalance to the magnetic flux in the core, reducing the et prod- uct. if the dc load is imbalanced, full-wave rectification is recommended, as shown in figure 9b. the transformer turns ratio must be set to provide the minimum required output voltage at the maximum anticipated load with the minimum expected input volt- table 2. typical transformer characteristics table 3. transformer, transformer core, and optocoupler suppliers characteristic 5v to 10v 5v to 5v 3.3v to 5v 5v to 24v 5v to 5v; 12v figure 9a 2, 3, 5, 6 4, 7 8 10 turns ratio 1ct*:1 1ct:1.3ct 1ct:2.1ct 1ct:5ct 1ct:1.5ct:3ct primary 44ct 44ct 28ct 44ct 44ct typical windings secondary 44 56ct 56ct 220ct 66ct, 132ct fs low 18.3v-? 18.3v-? 12v-? 18.3v-? 18.3v-? primary et product fs high 11v-? 11v-? 7.2v-? 11v-? 11v-? transformers transformer cores optocouplers bh electronics phone: (507) 532-3211 fax: (507) 532-3705 philips components phone: (407) 881-3200 fax: (407) 881-3300 quality technology phone: (408) 720-1440 fax: (408) 720-0848 coilcraft phone: (708) 639-6400 fax: (708) 639-1469 magnetics inc. phone: (412) 282-8282 fax: (412) 282-6955 sharp electronics phone: (206) 834-2500 fax: (206) 834-8903 coiltronics phone: (516) 241-7876 fax: (516) 241-9339 fair-rite products phone: (914) 895-2055 fax: (914) 895-2629 siemens components phone: (408) 777-4500 fax: (408) 777-4983 *ct = center tapped max253 maxim integrated 15
transformer driver for isolated rs-485 interface age. in addition, include in the calculations an allowance for worst-case losses in the rectifiers. since the turns ratio determined in this manner will ordinarily produce a much higher voltage at the secondary under conditions of high input voltage and/or light loading, be careful to prevent an overvoltage condition from occur- ring (see output voltage vs. load current in the typical operating characteristics ). transformers used with the max253 will ordinarily be wound on high-permeability magnetic material. to min- imize radiated noise, use common closed-magnetic- path physical shapes (e.g., pot cores, toroids, e/i/u cores). a typical core is the philips 213ct050-3b7, which is a toroid 0.190?in diameter and 0.05?thick. for operation with this core at 5.5v maximum supply voltage, the primary should have approximately 22 turns on each side of the center tap, or 44 turns total. this will result in a nominal primary inductance of approximately 832?. the secondary can be scaled to produce the required dc output. diode selection the max253? high switching frequency demands high-speed rectifiers. schottky diodes are recom- mended. ensure that the schottky diode average cur- rent rating exceeds the load-current level. the 1n5817 is a good choice for through-hole applications, and the niec* sb05w05c dual in an sot-23 package is rec- ommended for surface-mount applications. use the higher frequency setting to reduce ripple. output filter capacitor in applications sensitive to output-ripple noise, the out- put filter capacitor c2 should have a low effective series resistance (esr), and its capacitance should remain fairly constant over temperature. sprague 595d surface-mount solid tantalum capacitors and sanyo os-con through-hole capacitors are recommended due to their extremely low esr. capacitor esr usually rises at low temperatures, but os-con capacitors pro- vide very low esr below 0?. in applications where output ripple is not critical, a 0.1? chip or ceramic capacitor is sufficient. refer to table 4 for suggested capacitor suppliers. use the higher frequency setting to reduce ripple. input bypass capacitor the input bypass capacitor c1 is not critical. unlike switching regulators, the max253? supply current is fairly constant, and is therefore less dependent on the input bypass capacitor. a low-cost 0.1? chip or ceramic capacitor is normally sufficient for input bypassing. * nihon inter electronics corp. usa phone: (805) 867-2555 fax: (805) 867-2556 japan phone: 81-3-3494-7411 fax: 81-3-3494-7414 table 4. suggested capacitor suppliers production method capacitors surface mount matsuo 267 series (low esr) usa phone: (714) 969-2491, fax: (714) 960-6492 sprague electric co. 595d/293d series (very low esr) usa phone: (603) 224-1961, fax: (603) 224-1430 murata erie ceramic usa phone: (800) 831-9172, fax: (404) 436-3030 high-performance through hole sanyo os-con series (very low esr) usa phone: (619) 661-6835, fax: (619) 661-1055 japan phone: 81-7-2070-1005, fax: 81-7-2070-1174 through hole nichicon pl series (low esr) usa phone: (708) 843-7500, fax: (708) 843-2798 japan phone: 81-7-5231-8461, fax: 81-7-5256-4158 max253 16 maxim integrated
transformer driver for isolated rs-485 interface ___________________chip information process: cmos package type package code document no. 8 ?ax u8+1 21-0036 8 pdip p8+1 21-0043 8 so s8+4 21-0041 8 cdip j8-2 21-0045 package information for the latest package outline information and land patterns, go to www.maxim-ic.com/packages . note that a ?? ?? or ??in the package code indicates rohs status only. package drawings may show a different suffix character, but the drawing pertains to the package regardless of rohs status. max253 maxim integrated 17
revision history revision number revision date description pages changed 0 1/94 initial release 1 8/09 deleted the max253eua part number from the ordering information table 1 2 4/10 added automotive qualified part number to the ordering information table 1 transformer driver for isolated rs-485 interface max253 18 maxim integrated 160 rio robles, san jose, ca 95134 usa 1-408-601-1000 maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a maxim product. no circuit patent licenses are implied. maxim reserves the right to change the circuitry and specifications without notice at any time. the parametric values (min and max limits) shown in the electrical characteristics table are guaranteed. other parametric values quoted in this data sheet are provided for guidance. ? 2010 maxim integrated the maxim logo and maxim integrated are trademarks of maxim integrated products, inc.

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