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for free samples & the latest literature: http://www.maxim-ic.com, or phone 1-800-998-8800 _______________general description the MAX845 provides an isolated power supply small enough to fit in thin pcmcia cards and space-sensitive applications. it drives a low-profile center-tapped trans- former primary from a 5v or 3.3v dc power supply. the secondary can be wound to provide any isolated posi- tive or negative voltage at powers up to 750mw. the MAX845 consists of an oscillator followed by a tog- gle flip-flop. the flip-flop generates two 50% duty-cycle square waves, which are complementary at half the oscillator frequency (450khz, min). these two signals drive the ground-referenced n-channel power switch- es. internal circuitry ensures break-before-make action between the two switches. a low-power shutdown disables both the switches and the oscillator, reducing power consumption. an evalua- tion kit (MAX845evkit-mm) is available to evaluate low- profile 5v 40ma and 5v 100ma applications. ________________________applications pcmcia modem cards isolated data acquisition isolated interface power supply noise-immunity communications interface bridging ground differences medical equipment process control low-power lan networks ____________________________features ? transformer driver for ultra-thin 5v-? transformers ? isolated dc-to-dc power supply for pcmcia applications ? 450khz minimum switching frequency ? ultra-low input supply current ripple ? single +5v or +3.3v supply ? 5? low-power shutdown mode ? 8-pin so and ?ax packages ? low output ripple permits miniature output capacitors MAX845 isolated transformer driver for pcmcia applications ________________________________________________________________ maxim integrated products 1 1 2 3 4 8 7 6 5 d2 gnd2 v cc n.c. sd fs gnd1 d1 so/ m max top view MAX845 __________________pin configuration MAX845 d1 d2 fs gnd1 gnd2 v cc 1 8 46 27 3 v in sd frequency select c2 c1 c3 5v @ 150ma output 5v on / off t1 cr2 cr1 __________typical operating circuit 19-0372; rev 3; 8/96 part MAX845c/d MAX845eua -40? to +85? 0? to +70? temp. range pin-package dice* 8 ?ax evaluation kit manual follows data sheet *contact factory for dice specifications. ______________ordering information MAX845esa -40? to +85? 8 so
MAX845 isolated transformer driver for pcmcia applications 2 _______________________________________________________________________________________ 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 MAX845 only. load current is not included. note 2: shutdown supply current includes output switch leakage currents. supply voltage (v cc ) ...............................................-0.3v to +7v control input voltage (sd, fs) ...................-0.3v to (v cc + 0.3v) peak output switch current (d1, d2) ......................................1a output switch voltage (d1, d2) .............................................12v average output switch current (d1, d2) .........................200ma continuous power dissipation (t a = +70?) so (derate 5.88mw/? above +70?) .........................471mw ?ax (derate 4.10mw/? above +70?) ....................330mw operating temperature range ...........................-40? to +85? storage temperature range .............................-65? to +160? junction temperature ......................................................+150? lead temperature (soldering, 10sec) .............................+300? fs = v cc = 5.5v fs = v cc = 4.5v d1, d2; 100ma low fs = v cc fs = 0v high sd = v cc fs = 0v, v cc = 4.5v fs = 0v, v cc = 5.5v low no load, sd = 0v, fs = v cc high conditions v 2.5 2.2 ? 10 fs input current 50 v 0.8 fs input threshold 2.4 550 860 1100 450 675 900 1.5 4.0 switch on-resistance pa 10 shutdown input leakage current v 0.8 shutdown input threshold 2.4 ? 0.4 shutdown supply current (note 2) 500 khz 575 switch frequency ma 1.1 5.0 operating supply current (note 1) units min typ max parameter minimum start-up voltage
MAX845 isolated transformer driver for pcmcia applications _______________________________________________________________________________________ 3 40 -20 60 output resistance vs. temperature 30 MAX845-01 temperature (?) output resistance ( w ) 20 100 20 15 10 35 25 -40 0 80 40 v in = 4.5v v in = 5.5v figure 11c 7.5 2.5 -20 60 output resistance vs. temperature 3.5 6.0 MAX845-02 temperature (?) output resistance ( w ) 20 100 5.0 4.5 3.0 4.0 7.0 6.5 5.5 -40 0 80 40 figure 11b 1.6 1.4 1.2 0.2 -20 60 shutdown supply current vs. temperature MAX845-03 temperature (?) shutdown current (?) 20 100 0.6 1.0 -40 0 80 40 0.8 0.4 sd = v cc 1000 950 600 -20 60 d1, d2 frequency vs. temperature MAX845-04 temperature (?) frequency (khz) 20 100 750 900 850 -40 0 80 40 800 700 650 v in = 5.5v v in = 4.5v v in = 5.0v v in = 6.0v 90 100 0 60 140 160 efficiency vs. load current 20 70 MAX845-07 load current (ma) efficiency (%) 20 100 50 40 10 30 80 60 0 80 40 120 figure 11b figure 11c 850 800 500 550 -20 60 d1, d2 frequency vs. temperature MAX845-05 temperature (?) frequency (khz) 20 100 650 750 -40 080 40 700 600 fs high v in = 5.0v fs low 1.5 1.4 0.8 -20 60 supply current vs. temperature MAX845-06 temperature (?) supply current (ma) 20 100 1.0 -40 0 80 40 0.9 1.1 1.3 1.2 1.7 1.6 v in = 4.5v v in = 5.0v v in = 5.5v v in = 6.0v 7.5 2.5 040 output voltage vs. load current 3.5 6.5 MAX845-08 load current (ma) output voltage (v) 80 5.5 4.5 3.0 4.0 7.0 6.0 5.0 20 60 140 12 0 100 160 transformers used in figure 11b tgm-010p3 tgm-030p3 tgm-020p3 15 5 040 output voltage vs. load current 7 13 MAX845-09 load current (ma) output voltage (v) 80 11 9 6 8 14 12 10 20 60 140 12 0 100 160 transformers used in figure 11c tgm-010p3 tgm-030p3 tgm-020p3 __________________________________________typical operating characteristics ( typical operating circuit , v in = 5v, c1 = 0.1?, c2 = c3 = 0.33?, t1 = halo tgm-010p3, cr1 = cr2 = mbr0520, fs = v cc , t a = +25?, unless otherwise noted.)
MAX845 isolated transformer driver for pcmcia applications 4 _______________________________________________________________________________________ ____________________________typical operating characteristics (continued) ( typical operating circuit , v in = 5v, c1 = 0.1?, c2 = c3 = 0.33?, t1 = halo tgm-010p3, cr1 = cr2 = mbr0520, fs = v cc , t a = +25?, unless otherwise noted.) switching waveforms (two cycles) 5v/div 400ns/div d1 d2 switching waveform (break-before-make) 500mv/div 200ns/div d2off d1off circuit of fig. 1 d2on d1on time from shutdown to power-up 2v/div 5 m s/div sd output _____________________pin description no connect. not internally connected. n.c. 5 +5v supply voltage v cc 6 ground. connect both gnd1 and gnd2 to ground. gnd2 7 open drain of n-channel transformer drive 2 d2 8 shutdown. ground for normal operation, connect to v cc for shutdown. sd 4 frequency select (internal pull-up). if fs = v cc or open, switch frequency = 725khz; if fs = 0v, switch frequency = 535khz. fs 3 pin ground. connect both gnd1 and gnd2 to ground. gnd1 2 open drain of n-channel transformer drive 1 d1 1 function name MAX845 d1 d2 fs gnd1 gnd2 v cc 1 8 4 6 27 3 sd frequency select on / off r2 50 w r1 50 w v in 5v c1 0.1? figure 1. test circuit
_______________detailed description the MAX845 is a transformer driver specifically designed to provide isolated power for pcmcia and other height- and/or space-sensitive applications. it drives a center-tapped transformer primary from a 5v or 3.3v dc power supply. the secondary can be wound to provide any isolated dc voltage needed at power levels up to 750mw. the 450khz minimum switching frequency allows the use of very thin transformers, making the MAX845 ideal for pcmcia and other space-limited applications. the MAX845 is designed to drive a single transformer less than 0.09 inches (2.3mm) in height, including package. further reduction down to 0.050 inches (1.27mm) can be achieved using a transformer without a package. the MAX845 consists of an rc 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 circuitry ensures break-before-make action between the two switches. a low-current shutdown mode disables all internal cir- cuitry, including the oscillator and both power switches. drive the shutdown pin (sd) high to shut down the part; drive sd low for normal operation. the sd pin has no internal default condition and must not be allowed to float. most MAX845 applications will operate at high frequen- cies. the frequency-select pin (fs) is pulled high or left open (fs is internally pulled up to v cc ) to operate at a minimum of 450khz. pulling fs low selects the low-fre- quency state. theory of operation figure 2 shows the MAX845 driving both a tgm-010p3 transformer with a center-tapped primary, and a sec- ondary with a voltage-doubler rectifier topology. all of the transformers driven by the MAX845 must have a center tap with v in applied. whenever one of the MAX845 out- puts (d1 or d2) goes low, the other goes to approximate- ly double the supply voltage. a voltage is induced in the secondary and the rectifier diodes steer the currents into the appropriate output capacitor. on alternate half cycles, each capacitor is charged. the output voltage is the sum of the voltages from each output capacitor. this topology yields the simplest and smallest transformer because the least number of secondary turns is required for a given voltage. __________applications information with the MAX845 transformer driver, designers have the advantages of push/pull converter topology in space-sensitive applications. the push/pull dc-dc converter topology allows isolated multiple outputs, step-up/step-down or inverted outputs, easier filtering on the input and the output, and lower overall noise. isolated power for pcmcia applications medical instrumentation, modems, and lan-interface cards often require isolated power supplies. one of the best switching-regulator topologies for this application is the push/pull forward-converting dc-dc power sup- ply shown in figures 3 and 4. because the transformer works in the forward mode (rather than the flyback mode), its core does not store energy and, therefore, can be small. input and output capacitors can be small because of the high-frequency and continuous-current waveforms. MAX845 isolated transformer driver for pcmcia applications _______________________________________________________________________________________ 5 MAX845 d1 d2 fs gnd2 gnd1 v cc frequency select c2 c3 c1 output 5v @ 150ma 5v n n q q osc f / f v in sd on / off 400khz/ 700khz t iso gnd v cc cr1 cr2 figure 2. detailed block diagram
MAX845 the MAX845 is a versatile transformer driver, capable of driving a center-tapped transformer primary from a 5v or 3.3v dc power supply (figures 3 and 4). the secondary can be wound to provide any isolated volt- age needed at power levels up to 750mw with a 5v supply or up to 500mw with a 3.3v supply. figure 3 shows a typical 5v to isolated 5v application circuit that delivers up to 150ma of isolated 5v power. 3.3v supply any of the application circuits shown may be converted to 3.3v operation by changing the turns ratio of the trans- former and operating the MAX845 from a boost supply, as shown in figure 4. in normal operation, whenever one of the MAX845 outputs goes low, the other goes to approximately double the supply voltage. since the cir- cuit is symmetrical, the two outputs can be combined with diodes, lightly filtered, then used to power the MAX845, and possibly other light loads as well. the diodes on the primary side may be any fast-switch- ing small-signal diodes, such as the 1n914, 1n4148, or cmpd2838. the value of the primary filter capacitor is not critical and can be very small, since it only needs to supply current to the MAX845 during the break-before- make interval. the transformer could be any of the same ones used for 5v operation, but for optimum performance it should have fewer primary turns, as the et product required is now only 3.3v-?. for a given power level, the currents will be higher at 3.3v, so transformer winding resistance will be more critical and efficiencies will be lower. the MAX845 output current must still be limited to 200ma (see absolute maximum ratings ), so the available out- put power will be less than with a 5v power source. low-noise power supply the MAX845 topology is inherently low noise, in that either one or the other of the two power devices is on at any given time. by alternating between two identical states with one side on and the other off, the input cur- rent is nearly constant and secondary output power is available at all times. there is an intentional break- before-make action to prevent any possibility of both power switches conducting at the same time. during this 100ns non-overlap interval, the input current goes to zero. this adds a small high-frequency component to the input current waveform. this ripple current can easily be absorbed by a small input bypass capacitor (0.33?) from v cc to ground. figure 5 shows a low- noise bias supply using the MAX845 transformer driver. when using the two-diode push-pull (figure 11a) rectifier or the four-diode bridge (figure 11b), the out- put voltage tends to be more constant than in most alternative topologies. as described above, the circuit alternates between two identical states that both pro- vide power to the load. the only part of the cycle that produces output ripple is the 100ns non-overlap inter- val, which can easily be filtered by a small ceramic output capacitor (0.33?). isolated transformer driver for pcmcia applications 6 _______________________________________________________________________________________ c2 0.33? c1 0.1? 5v @ 150ma iso output 5v v in MAX845 d1 d2 fs gnd1 gnd2 v cc 1 8 6 27 3 frequency select sd 4 iso gnd mbr0520 1ct : 1.3ct mbr0520 on / off figure 3. 5v to isolated 5v application circuit MAX845 gnd1 gnd2 v cc 1 8 6 27 1n4148 1n4148 d1 d2 3.3v supply see figure 11 for rectifier configurations 0.01? figure 4. 3.3v input to isolated output application circuit
isolated data conversion almost any serial-interface device is a candidate for operation across an isolation barrier; figure 6 illustrates one example. the max176 analog-to-digital converter (adc) operates from +5v and -12v supplies, provided by the multiple-tapped secondary and linear regulators. this circuit easily supplies several hundred milliwatts of additional isolated power for signal conditioning, multi- plexing, or sensors. a +12v supply can be generated by adding two more diodes from the ends of the sec- ondary, and a -5v supply can be generated by con- necting additional diodes to the 1 4 and 3 4 tap points on the secondary. the MAX845 supplies sufficient power for almost any maxim adc. telephone-subscriber-line power supply the standard telephone system is placed in the ?ff hook?state by placing a load on the line to signal the central office that service is requested. normally, most of this power is wasted in a load resistor, but some systems can benefit from utilizing this free power. figure 7 shows one way to transform the wasted telephone power to an isolated, regulated 5v at currents up to 50ma. because the telephone line is a high-impedance source, there can be a start-up problem with any dc- to-dc converter; when the line voltage is low during start-up, the frequency can be too low for the trans- former, causing it to saturate. this excess saturation current can keep the voltage from climbing to normal operating levels. thus the purpose of q1, q2, and the associated resistors is to ensure that the MAX845 remains in the shutdown mode until the voltage is high enough to allow proper operation. isolated 4ma to 20ma analog interface the 4ma to 20ma current loop 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. the circuit in figure 8 generates an isolated 4ma to 20ma current from a 5v supply. isolated rs-485 data interface the MAX845 power-supply transformer driver also pro- vides isolated power for rs-485 data-interface applica- tions. the application circuit of figure 9 combines the MAX845 with a low-dropout linear regulator, a trans- former, several high-speed optocouplers, and a maxim rs-485 interface device. isolated rs-232 data interface the MAX845 is ideal for isolated rs-232 data-interface applications requiring more than four transceivers. its 750mw output power capability enables it to drive 10 transceivers simultaneously. figure 10 shows the typi- cal application circuit for a complete 120kbps isolated rs-232 data interface. this figure also shows how the sharp pc417 optocouplers can be replaced by the lower-cost quality technologies 4n25 devices to achieve data transfer rates up to 19.2kbps. MAX845 isolated transformer driver for pcmcia applications _______________________________________________________________________________________ 7 0.33? MAX845 d1 d2 fs sd gnd1 gnd2 n.c. 1 8 5 27 3 v cc 6 4 mbr0520l* 5v in *1n914 possible for lower currents 0.33? in out gnd 78l05 -5v 100ma halo tgm-030p3 n.c. figure 5. low-noise supply
MAX845 isolated transformer driver for pcmcia applications 8 _______________________________________________________________________________________ 10? 6n136 MAX845 d1 fs d2 gnd1 gnd2 v cc 4 27 8 1 sd 6 3 6n136 6n136 10? 79l12 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 w 0.1? 10? 0.1? 10? 0.1? 10? 200 w 200 w 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 input 0.1? d11(msb) d10 d9 d8 5v input 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 input 0.1? d7 d6 d5 d4 d3 d2 d1 d0 (lsb) 5v input 74hc04 on/off start input clock 1ct : 1.5ct : 3ct 4 x 1n5817 v in 5v input signal ground analog input iso 5v iso -12v 8 qh isolation barrier figure 6. typical isolated data-conversion application
MAX845 isolated transformer driver for pcmcia applications _______________________________________________________________________________________ 9 0.1? iso gnd c1 0.1? 5v @ 50ma iso output d1 telephone subscriber line 2m 100k 1k 100k 100k 100k 680k q1 2n3906 q2 2n3904 6.8v 2w MAX845 d1 d2 fs sd gnd1 gnd2 v cc 1 t1 1:2:1 8 3 22k 22k n.c. 6 27 4 d2 1n5817 ic2 tl431 isolation barrier d3 1n5817 ic1 figure 7. 5v from telephone-subscriber line 10? 5v MAX845 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 w 2n3904 max480 max480 0.1v to 0.5v iso 5v i out 4ma to 20ma 78l05 4 3 2 1 5 6 24v unregulated 49.9k r l 0k to 1k 2n3904 isolation barrier v in in gnd out il300 figure 8. typical 4ma/20ma application circuit
MAX845 isolated transformer driver for pcmcia applications 10 ______________________________________________________________________________________ c3 0.1? c1 0.1? c2 2.2? iso 5v isolation barrier 5v v in MAX845 d1 d2 fs gnd1 gnd2 v cc 1 8 6 27 3 1n5817 sd 4 on / off 390 w * 74hc04 de ro 390 w * 74hc04 di 3.3k 1 3 5 4 6 1 2 4 3 5 4 1 3 6 * 74hc04 * 74hc04 or equivalent max883 5 64 shdn set gnd max481 max483 max485 max487 5 2 re gnd c4 2.2? in out 82 1n5817 390 w 4 3.3k 3.3k 3 1 8 6 7 di de ro a b pc410 / 417 pc357t pc410 / 417 ict : 1.3ct v cc 485 i/o n.c. figure 9. typical rs-485 application circuit
MAX845 isolated transformer driver for pcmcia applications ______________________________________________________________________________________ 11 c3 0.1? c2 2.2? iso 5v isolation barrier 5v v in 1 8 5 c1 0.1? mbr0520 MAX845 d1 d2 gnd1 gnd2 v cc 6 27 n.c. sd 4 on / off 10 x pc417 *74hc04 or equivalent max225 en sd c4 2.2? 82 mbr0520 max883 5 64 shdn set gnd in out 390 w 311 13, 14 27, 28 t1 in t1 out 390 w *74hc04 t1 in 1 2 4 5 6 1 2 4 5 6 390 w 74hc04 t2 in 390 w 74hc04 t3 in 390 w 74hc04 t4 in 390 w 74hc04 t5 in 74hc04 r1 out 74hc04 74hc04 74hc04 74hc04 r2 out r3 out r4 out r5 out 5 x 3.3k 390 w 390 w 390 w 390 w 412 t2 in t2 out 25 18 t3 in t3 out 24 17 t4 in t4 out 23 16 t5 in t5 out 510 r1 out r1 in 69 r2 out r2 in 78 r3 out r3 in 22 19 r4 out r4 in 21 20 r5 out r5 in v cc 2 1 gnd 5 x 3.3k 1n5711 6 5 4 1 2 390 w 3.3k v cc iso r out r out 1ct : 1.3ct 1n5711 1 2 6 5 4 390 w 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 n.c. figure 10. typical rs-232 application circuit
______________component selection transformer the MAX845 drives any transformer that has a center- tapped primary and a saturation rating of at least 5v-? (et product) per side. the oscillator frequency varies linearly with v cc . the transformer is most vulnerable to saturation at the minimum frequency, because the switches are on for the longest period. at v cc = 4.5v, the transformer must withstand at least: 11 4.5v x ?x = 5v-? 450khz min 2 and at v cc = 5.5v, the transformer must withstand at least: 11 5.5v x ?x ?= 5v-? 550khz min 2 thus, the required et product is constant over the entire 5v ?0% range. select either a toroid or a gapped core. although some applications will require custom transformers, many can use standard transformer designs, such as those listed in table 1. some of these manufacturers have standard products designed for the MAX845, while some have standard products that can be adapted for specific customer requirements. table 1 also lists some suppliers of suitable magnetic cores. an ungapped toroid core must never be allowed to sat- urate. an empirical way to measure a toroid? et prod- uct is to wind 20 turns on the bare core and observe the current waveform on an oscilloscope while driving the winding with a function generator. generate a 50% duty-cycle square wave at a test frequency of 500khz, with no dc offset. gradually increase the driving volt- age until the waveform suddenly begins to draw more current. at this point, the core is saturating, so reduce the driving voltage until the core just barely stops satu- rating. the et product indicated is simply the maxi- mum voltage that can be applied without saturation, multiplied by 1? (the time of half of the period of the input signal). because the et product varies linearly with the number of turns, this test winding can be scaled up or down to act as a suitable primary for that particular core. a gapped core, such as a bobbin or drum core, is not limited by et product, but rather by inductance and winding resistance. the primary inductance must be high enough to prevent excessive current flow under light-load conditions, yet low enough that it can be wound on the core. good results can be achieved by using a primary inductance between 50? and 200?. calculate the number of turns required by using the manufacturer? a l (inductance per turn squared) value, or measure a test winding with an inductance meter. inductance varies with the square of the number of turns. while most MAX845 applications will use a toroid trans- former for highest efficiency and lowest emi, there may be applications that can utilize less expensive trans- formers, such as e, i, or u-shaped cores, magnetic bobbins, or etched windings on a printed circuit board. table 1 lists some transformer and core suppliers who can assist with your magnetics design. the secondary or secondaries can be scaled to produce whatever output is required for the application at hand, taking into account the rectifier topology to be used and the forward voltage loss of the diodes selected. step-by-step transformer design procedure before starting the design, determine the minimum and maximum output voltage requirement, the minimum and maximum load current, the physical size con- straints, and the cost budget. 1) select an appropriate core shape and material from core vendors?data sheets; trade-off emi vs. space and cost. since the MAX845? output waveform is a square wave, it is rich in harmonics, so choose a material with low losses at up to several mhz. MAX845 isolated transformer driver for pcmcia applications 12 ______________________________________________________________________________________ table 1. transformer and transformer-core suppliers transformers transformer cores tdk corporation phone: (408) 437-9585 fax: (408) 437-9591 ask for MAX845 transformer magnetics inc. phone: (412) 282-8282 fax: (412) 282-6955 halo electronics phone: (415) 969-7313 fax: (415) 367-7158 ask for MAX845 transformer fair-rite products phone: (914) 895-2055 fax: (914) 895-2629 coilcraft phone: (708) 639-6400 fax: (708) 639-1469 ask for MAX845 transformer philips components phone: (401) 762-3800 fax: (401) 762-3805, ext. 324 bh electronics phone: (612) 894-9590 fax: (612) 894-9380 ask for MAX845 transformer mmg (magnetic materials group) phone: (201) 345-8900 fax: (201) 345-1172 amidon associates phone: (714) 850-4660 fax: (714) 850-1163 sumida usa phone: (708) 956-0666 fax: (708) 956-0702
MAX845 isolated transformer driver for pcmcia applications ______________________________________________________________________________________ 13 2) use a test winding to measure et product (if using an ungapped toroid) and/or a l value for the core. 3) determine the number of turns required for the pri- mary winding. for an ungapped toroid, et product from center-tap to d1 must be at least 5v-?. other core types must have sufficient inductance to limit d1 and d2 output current under minimum load con- ditions, and must not be allowed to saturate. 4) select a rectifier topology based on performance requirements (ripple vs. loss, and space required for secondary winding). refer to table 2, rectifier topology trade-offs. 5) work backward from v out requirements to deter- mine the secondary to primary turns ratio. include losses in the rectifier diodes, and estimate resistive losses in the windings. for load currents exceed- ing 150ma, use a voltage step-down transformer to step up the output current from the MAX845. do not exceed the MAX845? absolute maximum out- put current rating (200ma). 6) wind the transformer with the largest diameter wire that will fit the winding area. select a wire gauge to fill the winding aperture as much as possible. larger diameter wire has lower resistance per unit length. doubling the wire diameter reduces resis- tive losses by a factor of four. bobbin or drum cores suffer from low coupling between windings. this usually requires bifilar winding for the two halves of the primary. due to the inherent complexity of magnetic circuit design, it will be necessary to build a prototype and re- iterate the design. if necessary, adjust the design by altering the number of primary or secondary turns, or the wire gauge. if using a different core material or geome- try, evaluate its et product or a l as described above. rectifier topology figure 11 shows various rectifier topologies. refer to table 2 for selection criteria. the turns ratio of the trans- former must be set to provide the minimum required out- put voltage at the maximum anticipated load, with the minimum expected input voltage. in addition, the calcu- lations should allow for worst-case losses in the recti- fiers. since the turns ratio determined in this manner will ordinarily produce a much higher voltage at the sec- ondary under conditions of high input voltage and/or light loading, be careful to prevent an overvoltage con- dition from occurring (see the output voltage vs. load current graph in the typical operating characteristics ). diodes use fast-switching diode rectifiers. ordinary silicon sig- nal diodes like the 1n914 or 1n4148 may be used for low output current levels (less than 50ma), but schottky diodes have a lower forward voltage drop and should be used for higher-current applications. central semiconductor has low-current schottky diodes as duals in sot-23 packages (cmpsh-3 series). the nihon sb05w05c is a common-cathode dual in a sot- 23; it works well in the two-diode full-wave configura- tion. the motorola mbr0520 is an excellent choice for all configurations. figure 11c. voltage doubler figure 11a. 2-diode push-pull figure 11b. 4-diode bridge v in 1 8 MAX845 gnd1 gnd2 v cc 6 27 d1 d2 v in 1 8 MAX845 gnd1 gnd2 v cc 6 27 d1 d2 v in MAX845 gnd1 gnd2 v cc 1 8 6 27 d1 d2
output regulator since the output voltage is not regulated against changes in the input voltage or load current, an output voltage regulator may be needed. a series linear regu- lator gives good performance and reasonably good efficiency at low cost. a shunt regulator costs less, occupies less space, and gives adequate performance for some applications. series regulators such as the max666, max667, max882/max883/max884, or max603/max604 simpli- fy designs. just select one with the desired output volt- age and current capability, and connect it. the simplest voltage regulator is the shunt zener shown in figure 12. the series resistor (r s ) value should be as high as possible to still deliver the maximum expected load current with minimum input voltage. be sure that no ratings are exceeded at maximum input voltage and minimum load current conditions; under such conditions, the zener diode may have to dissipate much more power than the load. alternatively, start with the maximum allow- able zener dissipation and select the series resistor under light-load, high-line conditions. then verify that there is sufficient output current available with worst- case low input voltage. for better regulation than the simple shunt zener, con- sider a shunt regulator ic such as the tl431. this device behaves like a zener diode whose voltage can be programmed by a resistor ratio. it can be used as a stand-alone device or can be boosted above its 150ma maximum rating without compromising its accuracy by adding a discrete pnp transistor, as shown in figure 12. the input power of a shunt regulator is nearly indepen- dent of load, so efficiency at light loads tends to be worse than it would be with a series regulator. output filter capacitor ceramic capacitors can be used as output capacitors because of the lower level of output ripple current. in applications where output ripple is not critical, a 0.33? chip or ceramic capacitor is normally sufficient. refer to table 3 for suggested capacitor suppliers. in applications sensitive to output-ripple noise, the out- put filter capacitor (c2) should have a low equivalent series resistance (esr) and a low equivalent series inductance (esl), and its capacitance should remain fairly constant over temperature. sprague 595d surface-mount solid tantalum capacitors and sanyo os-con through-hole capacitors are recom- mended, if space allows, due to their extremely low esr. capacitor esr usually rises at low temperatures, but os- con capacitors provide very low esr below 0?. input bypass capacitor the input bypass capacitor (c1) is not critical. unlike switching regulators, the MAX845? supply current is fairly constant, and is therefore less dependent on the input bypass capacitor. a low-cost 0.33? chip or ceramic capacitor is normally sufficient for input bypassing. MAX845 isolated transformer driver for pcmcia applications 14 ______________________________________________________________________________________ r s simple shunt zener r s tl431 22k 22k programmable-ic shunt regulator (stand alone) programmable-ic shunt regulator with discrete pnp r s tl431 22k 22k 1k 2n2907 5v output 5v output figure 12. shunt-regulator circuits
MAX845 isolated transformer driver for pcmcia applications ______________________________________________________________________________________ 15 table 2. rectifier topology trade-offs topology advantage disadvantage 2-diode push/pull (figure 11a) ?only 3 external components ?low output ripple ?single diode drop ?more turns on transformer 4-diode bridge (figure 11b) ?simpler transformer winding requirements ?low output ripple ?5 external components ?higher cost ?2 diode drops voltage doubler (figure 11c) ?fewest turns on transformer ?4 external components ?higher output ripple ?2 diode drops ___________________chip topography v cc gnd2 fs 0.085" (2.159mm) 0.058" (1.4732mm) sd d1 d2 gnd1 supplier capacitor low-esr 267 series matsuo usa phone: (714) 969-2491 fax: (714) 960-6492 ceramic murata erie usa phone: (800) 831-9172 fax: (404) 436-3030 very low-esr 595d/293d series sprague electric co. usa phone: (603) 224-1961 fax: (603) 224-1430 table 3. suggested capacitor suppliers substrate connected to v cc transistor count: 31
MAX845 isolated transformer driver for pcmcia applications 16 ______________________________________________________________________________________ ________________________________________________________package information l a c a1 b dim a a1 b c d e e h l a min 0.036 0.004 0.010 0.005 0.116 0.116 0.188 0.016 0 max 0.044 0.008 0.014 0.007 0.120 0.120 0.198 0.026 6 min 0.91 0.10 0.25 0.13 2.95 2.95 4.78 0.41 0 max 1.11 0.20 0.36 0.18 3.05 3.05 5.03 0.66 6 inches millimeters 8-pin m max micromax small-outline package 0.65 0.0256 a e e h d 0.101mm 0.004 in 21-0036d dim a a1 b c e e h l min 0.053 0.004 0.014 0.007 0.150 0.228 0.016 max 0.069 0.010 0.019 0.010 0.157 0.244 0.050 min 1.35 0.10 0.35 0.19 3.80 5.80 0.40 max 1.75 0.25 0.49 0.25 4.00 6.20 1.27 inches millimeters 21-0041a narrow so small-outline package (0.150 in.) dim d d d min 0.189 0.337 0.386 max 0.197 0.344 0.394 min 4.80 8.55 9.80 max 5.00 8.75 10.00 inches millimeters pins 8 14 16 1.27 0.050 l 0?8 h e d e a a1 c 0.101mm 0.004in. b

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