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MAX13256 36v h-bridge transformer driver for isolated supplies ????????????????????????????????????????????????????????????????? maxim integrated products 1 general description the MAX13256 h-bridge transformer driver provides a simple solution for making isolated power supplies up to 10w. the device drives a transformers primary coil with up to 300ma of current from a wide 8v to 36v dc sup - ply. the transformers secondary-to-primary winding ratio defines the output voltage, allowing selection of virtually any isolated output voltage. the device features adjustable current limiting, allowing indirect limiting of secondary-side load currents. the cur - rent limit of the MAX13256 is set by an external resistor. a fault output asserts when the device detects an overtem - perature or overcurrent condition. in addition, the device features a low-power mode to reduce the overall supply current to 0.65ma (typ) when the driver is not in use. the device can be operated using the internal oscillator or driven by an external clock to synchronize multiple MAX13256 devices and precisely set the switching fre - quency. internal circuitry guarantees a fixed 50% duty cycle to prevent dc current flow through the transformer, regardless of which clock source is used. the device is available in a small 10-pin (3mm x 3mm) tdfn package and is specified over the -40 n c to +125 n c automotive temperature range. benefits and features s simple, flexible design ? 8v to 36v supply range ? up to 90% efficiency ? provides up to 10w to the transformer ? undervoltage lockout ? 2.5v to 5v compatible logic interface ? internal or external clock source ? adjustable overcurrent threshold s integrated system protection ? fault detection and indication ? overcurrent limiting ? overtemperature protection s saves space on board ? small 10-pin tdfn package (3mm x 3mm) applications power meters isolated fieldbus interfaces 24v plc supply isolation medical equipment motor controls typical operating circuit 19-5847; rev 0; 6/11 ordering information appears at end of data sheet. for related parts and recommended products to use with this part, refer to: www.maxim-ic.com/MAX13256.related e v a l u a t i o n k i t a v a i l a b l e fault 1f 0.1f +24v 4.6k i r lim v dd en clk st2 gnd st1 ith isolated v out MAX13256 for pricing, delivery, and ordering information, please contact maxim direct at 1-888-629-4642, or visit maxims website at www.maxim-ic.com.
????????????????????????????????????????????????????????????????? maxim integrated products 2 MAX13256 36v h-bridge transformer driver for isolated supplies (voltages referenced to gnd.) v dd , fault ........................................................... -0.3v to +40v st1, st2 ................................................... -0.3v to (v dd + 0.3v) clk, ith, en ........................................................... -0.3v to +6v fault continuous current ............................................. q 50ma st1, st2 continuous current ........................................ q 850ma continuous power dissipation (t a = +70 n c) tdfn (four-layer board) (derate 24.4mw/ n c above +70 n c) ......................... 1951.2mw tdfn (single-layer board) (derate 18.5mw/ n c above +70 n c) ......................... 1481.5mw operating temperature range ........................ -40 n c to +125 n c junction temperature ..................................................... +150 n c storage temperature range ............................ -65 n c to +150 n c lead temperature (soldering, 10s) ............................... +300 n c soldering temperature (reflow) ...................................... +260 n c tdfn (four-layer board) junction-to-ambient thermal resistance ( b ja ) .......... 41 n c/w junction-to-case thermal resistance ( b jc ) ................. 9 n c/w tdfn (single-layer board) junction-to-ambient thermal resistance ( b ja ) .......... 54 n c/w junction-to-case thermal resistance ( b jc ) ................. 9 n c/w absolute maximum ratings note 1: package thermal resistances were obtained using the method described in jedec specification jesd51-7. for detailed information on package thermal considerations, refer to www.maxim-ic.com/thermal-tutorial . stresses beyond those listed under absolute maximum ratings may cause permanent damage to the device. these are stress ratings only, and functional opera - tion 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. package thermal characteristics (note 1) electrical characteristics (v dd = 8v to 36v, v en = 0v, t a = -40 n c to +125 n c, unless otherwise noted. typical values are at t a = +25 n c.) (note 2) parameter symbol conditions min typ max units dc characteristics supply voltage range v dd (note 3) 8 36 v supply current i dd v en = 0v, v clk = 0v, r lim = 1000 i , st1/st2 not connected 6 9 ma disable supply current i dis v en = 3.3v, v clk = 0v 0.65 1.1 ma driver output resistance r oh st1 = st2 = high, i st1, st2 = +300ma, r lim = 1000 i 1 1.5 i r ol st1 = st2 = low, i st1, st2 = -300ma, r lim = 1000 i 0.6 1.0 undervoltage-lockout threshold v uvlo v dd rising 5.9 6.3 6.9 v undervoltage-lockout threshold hysteresis v uvlo_hyst 300 mv
????????????????????????????????????????????????????????????????? maxim integrated products 3 MAX13256 36v h-bridge transformer driver for isolated supplies electrical characteristics ( continued ) (v dd = 8v to 36v, v en = 0v, t a = -40 n c to +125 n c, unless otherwise noted. typical values are at t a = +25 n c.) (note 2) note 2: all units are production tested at t a = +25 n c. specifications over temperature are guaranteed by design. note 3: if v dd is greater than 27v, see the snubber section. parameter symbol conditions min typ max units st1, st2 current limit i lim r lim = 1000 i 500 650 800 ma r lim = 3010 i 165 215 265 st1, st2 leakage current i lkg v en = 3.3v, v clk = 0v, v st1 = v st2 = 0v or v dd -1 +1 f a logic signals (clk, en , fault ) input logic-high voltage v ih 2 v input logic-low voltage v il 0.8 v input leakage current i il v clk = v en = 5.5v or 0v -1 +1 f a fault output logic-low voltage v ol i fault = 10ma 1 v fault leakage current i lkgf v fault = 36v, fault deasserted 10 f a ac characteristics switching frequency f sw v clk = 0v, measured at st1/st2 outputs 255 425 700 khz clk input frequency f ext external clocking 200 2000 khz st1/st2 duty cycle d tc internal or external clocking 49 50 51 % st1/st2 rise time t rise st1/ st2 = 20% to 80% of v dd , r l = 1k i , c l = 50pf, figure 1a 100 ns st1/st2 fall time t fall st1/st2 = 80% to 20% of v dd , r l = 1k i , c l = 50pf, figure 1a 100 ns crossover dead time t dead r l = 200 i , figure 1b 30 ns watchdog timeout t wdog 20 32 55 f s current-limit blanking time t blank figure 2 0.73 1.2 2.0 ms current-limit autoretry time t retry figure 2 23.4 38.4 64.0 ms protection thermal-shutdown threshold t shdn +160 n c thermal-shutdown hysteresis t shdn_hys 10 n c
????????????????????????????????????????????????????????????????? maxim integrated products 4 MAX13256 36v h-bridge transformer driver for isolated supplies figure 1. test circuits (a and b) and timing diagram (c) for rise, fall, and dead times figure 2. timing diagram for current limiting test circuits/timing diagrams (c) (a) (b) v dd v dd 0v st1 st2 0v 80 % 20 % 80 % 20 % t rise t fall t dead st1/st2 st1 st2 r l r l c l i lim 0ma i st1, st2 t blank t retry 50% 50% 50%
????????????????????????????????????????????????????????????????? maxim integrated products 5 MAX13256 36v h-bridge transformer driver for isolated supplies typical operating characteristics (v dd = 24v, t a = +25 n c, unless otherwise noted.) supply current vs. external clock frequency MAX13256 toc01 external clock frequency (khz) i dd (ma) 1700 1400 1100 800 500 1 2 3 4 5 6 7 8 9 10 0 200 2000 no load normalized current-limit threshold vs. temperature i lim 0.92 0.94 0.96 0.98 1.02 1.04 1.06 1.08 1.10 0.90 MAX13256 toc04 1.00 110 95 80 65 50 35 20 5 -10 -25 -40 125 t a (c) st1/ st2 switching frequency vs. temperature MAX13256 toc02 t a (c) f sw (khz) 110 95 80 65 50 35 20 5 -10 -25 400 350 450 500 550 600 300 -40 125 clk = gnd st1/ st2 output-voltage low vs. sink current MAX13256 toc05 v ol (mv) 600 500 400 300 200 100 0 i sink (ma) 100 200 300 400 500 600 700 0 800 current-limit threshold vs. r lim MAX13256 toc03 r lim ( i ) i lim (ma) 2600 2200 1800 1400 200 300 400 500 600 700 100 1000 3000 st1/ st2 output-voltage high vs. source current MAX13256 toc06 i source (ma) v oh (v) 700 600 400 500 200 300 100 23.2 23.3 23.4 23.5 23.6 23.7 23.8 23.9 24.0 24.1 23.1 0 800 MAX13256 toc07 v ol (mv) 50 100 150 200 250 300 350 400 0 i sink (ma) 8 6 4 2 0 10 fault output-voltage low vs. sink current isolated output voltage vs. load current MAX13256 toc08 i load (ma) 400 300 200 100 0 500 v out (v) 21 22 23 24 25 26 27 28 29 30 20 v dd = 24v 1:1 transformer full-wave rectifier no snubber
????????????????????????????????????????????????????????????????? maxim integrated products 6 MAX13256 36v h-bridge transformer driver for isolated supplies typical operating characteristics (continued) (v dd = 24v, t a = +25 n c, unless otherwise noted.) isolated output voltage vs. load current MAX13256 toc09 i load (ma) 1600 1200 800 400 0 2000 v out (v) 1 2 3 4 5 6 7 8 9 10 0 v dd = 24v 4:1 transformer full-wave rectifier no snubber isolated output voltage vs. load current MAX13256 toc10 i load (ma) 400 300 200 100 0 500 v out (v) 2 4 6 8 10 12 14 16 18 20 0 v dd = 24v 4:1 transformer voltage doubler no snubber efficiency vs. load current MAX13256 toc11 i load (ma) efficiency (%) 350 300 200 250 100 150 50 10 20 30 40 50 60 70 80 90 100 0 0 400 v dd = 24v v dd = 16v v dd = 12v v dd = 8v 1:1 transformer full-wave rectifier no snubber 1250 1000 750 500 250 efficiency vs. load current MAX13256 toc12 i load (ma) efficiency (%) 10 20 30 40 50 60 70 80 90 100 0 0 1500 v dd = 24v v dd = 12v v dd = 16v v dd = 8v 4:1 transformer full-wave rectifier no snubber 350 300 200 250 100 150 50 efficiency vs. load current MAX13256 toc13 i load (ma) efficiency (%) 10 20 30 40 50 60 70 80 90 100 0 0 400 v dd = 32v v dd = 28v v dd = 36v 1:1 transformer full-wave rectifier with snubber 1250 1000 750 500 250 efficiency vs. load current MAX13256 toc14 i load (ma) efficiency (%) 10 20 30 40 50 60 70 80 90 100 0 0 1500 v dd = 36v v dd = 32v v dd = 28v 4:1 transformer full-wave rectifier with snubber maximum output current vs. temperature MAX13256 toc15 t a (c) i st1, st2 (ma) 110 95 80 65 50 35 20 5 -10 -25 500 600 700 800 900 1000 400 -40 125 multilayer board v dd = 24v v dd = 36v v dd = 8v v dd = 16v maximum output current vs. temperature MAX13256 toc16 t a (c) i st1, st2 (ma) 110 95 80 65 50 35 20 5 -10 -25 400 500 600 700 800 900 300 -40 125 single-layer board v dd = 24v v dd = 36v v dd = 8v v dd = 16v
????????????????????????????????????????????????????????????????? maxim integrated products 7 MAX13256 36v h-bridge transformer driver for isolated supplies pin configuration pin description pin name function 1, 2 v dd power supply. bypass v dd to ground with a 1 f f capacitor as close as possible to the device. 3 clk clock input. connect clk to gnd to enable internal clocking. apply a clock signal to clk to enable exter - nal clocking. 4 en enable input. drive en low to enable the device. drive en high to disable the device. 5 ith overcurrent threshold adjustment input. connect a resistor (r lim ) from ith to gnd to set the overcurrent threshold for the st1 and st2 outputs. do not exceed 10pf of capacitance to gnd on ith. 6 fault fault open-drain output. the fault open-drain transistor turns on when there is either an overtemperature or overcurrent condition. 7, 9 gnd ground 8 st2 transformer drive output 2 10 st1 transformer drive output 1 ep exposed pad. internally connected to gnd. connect ep to a large ground plane to maximize thermal perfor - mance; not intended as an electrical connection point. 1 3 4 10 8 7 st1 st2 gnd 2 9 gnd 5 + 6 faul t v dd clk en v dd ith *ep tdfn top view *exposed pad?connect to gnd MAX13256
????????????????????????????????????????????????????????????????? maxim integrated products 8 MAX13256 36v h-bridge transformer driver for isolated supplies detailed description the MAX13256 is an integrated primary-side controller and h-bridge driver for isolated power-supply circuits. the device contains an on-board oscillator, protec - tion circuitry, and internal mosfets to provide up to 300ma of current to the primary winding of a trans - former. the device can be operated using the internal oscillator or driven by an external clock to synchronize multiple MAX13256 devices and control emi behavior. regardless of the clock source being used, an internal flip-flop stage guarantees a fixed 50% duty cycle to prevent dc current flow in the transformer as long as the period of the clock is constant. the device operates from a wide single-supply voltage of 8v to 36v, and includes undervoltage lockout for con - trolled startup. the device features break-before-make switching to prevent cross conduction of the h-bridge mosfets. an external resistor sets an overcurrent limit, allowing primary-side limiting of load currents on the transformers secondary side. thermal-shutdown cir - cuitry provides additional protection against excessive power dissipation. isolated power supply the MAX13256 allows a versatile range of secondary- side rectification circuits (see figure 3 ). the primary-to- secondary transformer winding ratio can be chosen to adjust the isolated output voltage. the device delivers up to 300ma of current to the transformer with a supply up to +36v. the MAX13256 provides the advantages of the h-bridge converter topology, including multiple isolated outputs, step-up/step-down or inverted output, relaxed filtering requirements, and low output ripple. functional diagram clk st1 v dd st2 en ith mosfet h-bridge driver uvlo mux flip- flop v uvlo v dd p n current limit osc watchdog v dd p n fault gnd MAX13256
????????????????????????????????????????????????????????????????? maxim integrated products 9 MAX13256 36v h-bridge transformer driver for isolated supplies clock source either the internal oscillator or an external clock provides the switching signal for the MAX13256. connect clk to ground to select the internal oscillator. provide a clock signal to clk to automatically select external clocking. internal oscillator mode the MAX13256 includes an internal oscillator that drives the h-bridge when a watchdog timeout is detected on clk. the outputs switch at 425khz (typ) with a guaran - teed 50% duty cycle in the internal oscillator mode. external clock mode the MAX13256 provides an external clock mode. when an external clock source is applied to clk, the external clock drives the h-bridge. an internal flip-flop divides the external clock by two in order to generate a switching signal with a guaranteed 50% duty cycle. as a result, the device outputs switch at one-half of the external clock frequency. the device switches on the rising edge of the external clock signal. watchdog a stalled clock could cause excessive dc current to flow through the primary winding of the transformer. the MAX13256 features an internal watchdog circuit to pre - vent damage from this condition. the internal oscillator provides the switching signal to the h-bridge whenever the period between edges on clk exceeds the watch - dog timeout period of 20 f s (min). transients on st1/st2 during t dead when the MAX13256 switches, there is a period of time when both st1 and st2 are high impedance to ensure that there are no shoot-through currents in the h-bridge. during this dead time, the voltage at these pins may temporarily exceed the absolute maximum ratings due to the inductive load presented by the transformer. this transient voltage will not damage the device. disable mode the MAX13256 provides a disable mode to reduce cur - rent consumption. the st1 and st2 outputs are high impedance in disable mode. power-up and undervoltage lockout the MAX13256 provides an undervoltage-lockout feature to both ensure a controlled power-up state and prevent operation before the oscillator has stabilized. on power- up and during normal operation if the supply voltage drops below v uvlo , the undervoltage-lockout circuit forces the device into disable mode. the st1 and st2 outputs are high impedance in disable mode. overcurrent limiting the MAX13256 limits the st1/st2 output current. connect an external resistor (r lim ) to ith to set the current limit. when the current reaches the limit for longer than the blanking time of 1.2ms (typ), the drivers are disabled and fault is asserted low. the drivers are reenabled after the autoretry time of 38.4ms (typ). if a continuous fault condition is present, the duty cycle of the fault current is approximately 3%. to set the current-limit threshold, use the following equation: r lim ( i ) = 650mv/i st1, st2 (ma) where i st1, st2 is the desired current threshold in the range of 215ma < i st1, st2 < 650ma (typ). for example, a 1k i resistor sets the current limit to 650ma. use a 1% resistor for r lim for increased accuracy. ensure that the overcurrent threshold is set to at least twice the expected maximum operating current. for an expected maximum operating current of 300ma, set the i lim to 650ma. for an expected operating current of 100ma, set the i lim to 215ma. fault output the fault output is asserted low whenever the device is disabled due to a fault condition. fault is automati - cally deasserted when the device is enabled after the autoretry time following an overcurrent fault, resulting in fault toggling during a continuous overcurrent condi - tion. fault is asserted for the entire duration of an over - temperature fault. fault is an open-drain output. thermal shutdown the MAX13256 is protected from overtemperature dam - age by a thermal-shutdown circuit. when the junction temperature (t j ) exceeds +160 n c, the device is disabled and fault is asserted low. fault stays low for the dura - tion of an overtemperature fault. the device resumes normal operation when t j falls below +150 n c.
???????????????????????????????????????????????????????????????? maxim integrated products 10 MAX13256 36v h-bridge transformer driver for isolated supplies applications information snubber for v dd greater than 27v, use a simple rc snubber circuit on st1 and st2 to ensure that the peak voltage is less than 40v during switching ( figure 4 ). recommended values for the snubber are r = 91 i and c = 330pf. power dissipation the power dissipation of the device is approximated by: p d = (r ohl x i pri 2 ) + (i dd x v dd ) where r ohl is the combined high-side and low-side on- resistance of the internal fet drivers, and i pri is the load current flowing through st1 and st2. high-temperature operation when the MAX13256 is operated under high ambient temperatures, the power dissipated in the package can raise the junction temperature close to thermal shutdown. under such temperature conditions, the power dissipa - tion should be held low enough so that that junction tem - perature observes a factor of safety margin. the maximum junction temperature should be held below +140c. use the packages thermal resistances to calculate the junc - tion temperature. alternatively use the maximum output current vs. temperature curves shown in the typical operating characteristics section to determine the maxi - mum st1/st2 load currents. hot insertion if the MAX13256 is inserted into a live backplane, it is possible to damage the device. damage is caused by overshoot on v dd exceeding the absolute maximum rating. limit the transient input voltage to the MAX13256 with an external protection device. output-ripple filtering output-voltage ripple can be reduced with a lowpass lc filter (see figure 5 ). the component values shown give a cutoff frequency of 21.5khz by the equation: 3db 1 f 2 lc = use an inductor with low dc resistance and sufficient sat - uration current rating to minimize filter power dissipation. power-supply decoupling bypass v dd to ground with a 1 f f ceramic capacitor as close as possible to the device. figure 4. output snubber figure 3. secondary-side rectification topologies figure 5. output ripple filtering n:1 ct figure 3a. push-pull rectification n:1 n:1 + v in - v out = 1/( 2 x n) x v in - v d + - v d = diode forward voltage v out = 2(v in /n - v d ) + - v out = v in /n - 2v d + - + v in - + v in - figure 3b. voltage doubler figure 3c. full-wave rectifier filter output c 2.2f l 25h r 91 i st1 st2 c 330pf r 91 i c 330pf
???????????????????????????????????????????????????????????????? maxim integrated products 11 MAX13256 36v h-bridge transformer driver for isolated supplies output-voltage regulation for many applications, the unregulated output of the MAX13256 meets the output-voltage tolerances. this configuration represents the highest efficiency possible with the device. for applications requiring a regulated output voltage, maxim provides several solutions. in the following examples, assume a tolerance of q 10% for the input voltage. when the load currents on the transformers secondary side are low, the output voltage can strongly increase. if operation under low load currents is expected, output- voltage limiting should be used to keep the voltage within the tolerance range of the subsequent circuitry. if the min - imum output load current is less than approximately 5ma, connect a zener diode from the output node to ground as shown in figure 6 to limit the output voltage to a safe value. example 1: +24v to isolated, regulated +3.3v in figure 6 , the MAX13256 feeds approximately +4.4v to the input of an ldo through a tgmr-502v6lf 4:1 transformer and 4-diode bridge rectifier (see figure 3c ). from this, a max604 ldo produces a regulated +3.3v output at up to 500ma. example 2: +24v to isolated, regulated +12v in the circuit of figure 7 , the MAX13256 feeds approxi - mately +14.2v through a 1.5:1 transformer and a 4-diode bridge rectifier (see figure 3c ). from this, a max1659 ldo produces a regulated +12v output at up to 350ma. figure 6. +24v to isolated, regulated +3.3v figure 7. +24v to isolated, regulated +12v mbrs140 x 4 gnd st2 st1 en clk +24v tgmr-502v6lf 4:1 + +3.3v - 10f v dd 10f 1f MAX13256 max604 mbrs140 x 4 gnd st2 st1 en clk +24v 1.5:1 + +12v - 10f v dd 1.0f 1f MAX13256 max1659
???????????????????????????????????????????????????????????????? maxim integrated products 12 MAX13256 36v h-bridge transformer driver for isolated supplies example 3: +24v to isolated, regulated 15v in figure 8 , the MAX13256 is used with a 1:1.5 center tapped transformer and a 4-diode bridge rectifier net - work (see figure 3c ) to supply q 17.1v to a max8719 ldo and a 7915 ldo. the circuit produces regulated q 15v outputs at up to 100ma. isolated dac/adc interface for industrial process control the MAX13256 provides isolated power for data con - verters in industrial process control applications (see figure 9 ). the 300ma output current capability allows for multiple data converters operating across an isola - tion barrier. the power output capability also supports circuitry for signal conditioning and multiplexing. figure 8. +24v to isolated, regulated 15v 0.1f 7915 0.1f 1:1.5 ct 4 x mbrs140 10f +24v st1 st2 gnd MAX13256 r1 +15v -15v 10f common r2 en clk 1f max8719
???????????????????????????????????????????????????????????????? maxim integrated products 13 MAX13256 36v h-bridge transformer driver for isolated supplies figure 9. isolated power supply for industrial control applications -15v common +15v v dd v dd rs485 mpu m u x dac/adc optoisolators optoisolators MAX13256
???????????????????????????????????????????????????????????????? maxim integrated products 14 MAX13256 36v h-bridge transformer driver for isolated supplies isolated rs-485/rs-232 data interfaces the MAX13256 provides power for multiple transceivers in isolated rs-485/rs-232 data interface applications. the 300ma output current capability of the MAX13256 allows multiple rs-485/rs-232 transceivers to operate simultaneously. pcb layout guidelines as with all power-supply circuits, careful pcb lay - out is important to achieve low switching losses and stable operation. for thermal performance, connect the exposed pad to a solid copper ground plane. the traces from st1 and st2 to the transformer must be low resistance and inductance paths. place the trans - former as close as possible to the MAX13256 using short, wide traces. when the device is operating with the internal oscillator, it is possible for high-frequency switching components on st1 and st2 to couple into the clk circuitry through pcb parasitic capacitance. this capacitive coupling can induce duty-cycle errors in the oscillator, resulting in a dc current through the transformer. to ensure proper opera - tion, ensure that clk has a solid ground connection. exposed pad ensure that the exposed pad has a solid connection to the ground plane for best thermal performance. failure to provide a low thermal impedance path to the ground plane results in excessive junction temperatures when delivering maximum output power. component selection transformer selection transformer selection for the MAX13256 can be simpli - fied by the use of a design metric, the et product. the et product relates the maximum allowable magnetic flux density in a transformer core to the voltage across a wind - ing and switching period. inductor magnetizing current in the primary winding changes linearly with time during the switching period of the device. transformer manufactur - ers specify a minimum et product for each transformer. the transformers et product must be larger than: et = v dd /(2 x f sw ) where f sw is the minimum switching frequency of the st1/st2 outputs (255khz (min)) when the internal oscil - lator is used or one-half of the clock frequency when an external clock source is used. choose a transformer with sufficient et product in the primary winding to ensure that the transformer does not saturate during operation. saturation of the magnetic core results in significantly reduced inductance of the primary, and therefore a large increase in current flow. this can cause the current limit to be reached even when the load is not high. for example, when the internal oscillator is used to drive the h-bridge, the required transformer et product for an application with v dd (max) = 36v is 70.6v f s. an applica - tion with v dd (max) = 8.8v has a transformer et product requirement of 17.3v f s. in addition to the constraint on et product, choose a transformer with a low dc-winding resistance. power dissipation of the transformer due to the copper loss is approximated as: p d_tx = i load 2 x (r pri /n 2 + r sec ) where r pri is the dc winding resistance of the primary, and r sec is the dc winding resistance of the second - ary. in most cases, an optimum is reached when r sec = r pri /n 2 . for this condition, the power dissipation is equal for the primary and secondary windings. as with all power-supply designs, it is important to opti - mize efficiency. in designs incorporating small trans - formers, the possibility of thermal runaway makes low transformer efficiencies problematic. transformer losses produce a temperature rise that reduces the efficiency of the transformer. the lower efficiency, in turn, produces an even larger temperature rise. to ensure that the transformer meets these require - ments under all operating conditions, the design should focus on the worst-case conditions. the most stringent demands on et product arise for minimum input volt - age, switching frequency, and maximum temperature and load current. additionally, the worst-case values for transformer and rectifier losses should be considered. the primary should be a single winding; however, the secondary can be center-tapped, depending on the desired rectifier topology. in most applications, the phas - ing between primary and secondary windings is not sig - nificant. half-wave rectification architectures are possible with the MAX13256; however, these are discouraged. if a net dc current results due to an imbalanced load, the average magnetic flux in the core is increased. this reduces the effective et product and can lead to satura - tion of the transformer core.
???????????????????????????????????????????????????????????????? maxim integrated products 15 MAX13256 36v h-bridge transformer driver for isolated supplies transformers for use with the device are typically wound on a high-permeability magnetic core. to minimize radi - ated electromagnetic emissions, select a toroid, pot core, e/i/u core, or equivalent. low-voltage operation the MAX13256 can be operated from an +8v supply by decreasing the turns ratio of the transformer, or by design - ing a voltage doubler circuit as shown in figure 3b . optimum performance at +8v is obtained with fewer turns on the primary winding since the et product requirement is lower than for a +24v supply. however, any of the transformers for use with a +24v supply can operate properly with a +8v supply. for a given power level, the transformer currents are higher with a +8v supply than with a +24v supply. therefore, the dc resis - tance of the transformer windings has a larger impact on the circuit efficiency. diode selection the high switching speed of the MAX13256 necessitates high-speed rectifiers. ordinary silicon signal diodes such as 1n914 or 1n4148 can be used for low-output current levels (less than 50ma.) but at higher output cur - rent levels, their reverse recovery times might degrade efficiency. at higher output currents, select low forward- voltage schottky diodes to improve efficiency. ensure that the average forward current rating for the rectifier diodes exceeds the maximum load current of the circuit. for surface-mount applications, schottky diodes such as the bat54, mbrs140, and mbrs340 are recommended. capacitor selection input bypass capacitor bypass the supply pin to gnd with a 1 f f ceramic capacitor as close as possible to the device. the equiva - lent series resistance (esr) of the input capacitors is not as critical as for the output filter capacitors. typically ceramic x7r capacitors are adequate. output filter capacitor in most applications, the actual capacitance rating of the output filter capacitors is less critical than the capacitors esr. in applications sensitive to output-voltage ripple, the output filter capacitor must have low esr. for optimal performance, the capacitance should meet or exceed the specified value over the entire operating temperature range. capacitor esr typically rises at low temperatures; however, os-con capacitors can be used at tempera - tures below 0 n c to help reduce output-voltage ripple in sensitive applications. in applications where low output- voltage ripple is not critical, standard ceramic 0.1 f f capacitors are sufficient. table 1. component manufacturers suggested external component manufacturers manufacturer component website central semiconductor diodes www.centralsemi.com halo electronics transformers www.haloelectronics.com kemet capacitors www.kemet.com sanyo capacitors www.sanyo.com taiyo yuden capacitors www.t-yuden.com tdk capacitors www.component.tdk.com
???????????????????????????????????????????????????????????????? maxim integrated products 16 MAX13256 36v h-bridge transformer driver for isolated supplies ordering information package information for the latest package outline information and land patterns (footprints), 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. + denotes a lead(pb)-free/rohs-compliant package. * ep = exposed pad chip information process: bicmos package type package code outline no. land pattern no. 10 tdfn-ep t1033+1 21-0137 90-0003 part temp range pin-package MAX13256atb+ -40 n c to +125 n c 10 tdfn-ep*
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. maxim integrated products, 120 san gabriel drive, sunnyvale, ca 94086 408-737-7600 17 ? 2011 maxim integrated products maxim is a registered trademark of maxim integrated products, inc. MAX13256 36v h-bridge transformer driver for isolated supplies revision history revision number revision date description pages changed 0 6/11 initial release

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