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1 for more information www.linear.com/lt8309 typical a pplica t ion fea t ures descrip t ion secondary-side synchronous rectifier driver the lt ? 8309 is a secondary-side synchronous rectifier driver that replaces the output rectifier diode in a flyback topology. by replacing the diode with a n- channel mosfet, applications are no longer restricted by the heat constraints of the output diode. the ic replicates the behavior of a diode by sensing the drain-to-source voltage to determine when the current becomes negative. the lt8309's low minimum on- and off-times help improve noise immunity. the fast propagation delay of 26 ns allows applications to operate in discontinuous conduction mode ( dcm) and critical conduction mode ( crcm). the gate driver features a 0.8 pull-down device for fast turn-offs. the 40 v v cc rating allows the part to be driven from the output volt- age or the rectified drain voltage of the mosfet. a low quiescent current of 400 a maximizes efficiency at low output currents. 40w, 5v isolated telecom supply efficiency vs load current a pplica t ions n works with dcm and bcm/crcm conduction mode flyback topologies n v cc : 4.5v to 40v n supports up to 150v mosfets n 26ns turn-off propagation delay n accurate minimum on and off timers for reliable operation n adjustable and accurate trip point: +5mv to C30mv n 1 gate driver pull-down n sot-23 5-lead package n high output current flybacks n high efficiency flybacks l, lt , lt c , lt m , linear technology and the linear logo are registered trademarks of linear technology corporation. all other trademarks are the property of their respective owners. intv cc v cc v out + 5v, 8a v out ? 6.2v v in 36v to 72v gate 8309 ta01a drain gnd 100pf 60 6.04k 147k lt8309 ? ? 0.1f 30.9k 12.1k 15nf gate sense r ref intv cc v c ss tc en/uvlo r fb gnd lt3748 5.33:1 10 2.2k 4.7f 4.7f 400f 412k 15.4k 12m 10f 1f v in load current (a) 0 65 efficiency (%) 70 80 90 1 2 3 4 65 7 75 85 100 95 8 8309 ta01b v in = 48v with pds760 diode v in = 48v with lt8309 lt 8309 8309fa
2 for more information www.linear.com/lt8309 p in c on f igura t ion a bsolu t e maxi m u m r a t ings v cc ............................................................................ 40 v int v cc gate ............................................................ 10 v d rain ..................................................................... 15 0 v maximum junction temperature .......................... 12 5 c operating temperature range ( note 2) lt 8 309 e ............................................ C4 0 c to 125 c lt 8 309 i ............................................. C 40 c to 125 c lt 8 309 h ............................................ C 40 c to 150 c storage temperature range .................. C 65 c to 150 c (note 1) gate 1 gnd 2 top view s5 package 5-lead plastic tsot-23 intv cc 3 5 drain 4 v cc ja = 215c/w o r d er i n f or m a t ion lead free finish tape and reel part marking* package description temperature range lt8309es5#pbf lt8309es5#trpbf ltgfz 5-lead plastic tsot-23 C40c to 125c lt8309is5#pbf lt8309is5#trpbf ltgfz 5-lead plastic tsot-23 C40c to 125c lt8309hs5#pbf lt8309hs5#trpbf ltgfz 5-lead plastic tsot-23 C40c to 150c consult lt c marketing for parts specified with wider operating temperature ranges. *the temperature grade is identified by a label on the shipping container. for more information on lead free part marking, go to: http://www.linear.com/leadfree/ for more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/ lt 8309 8309fa
3 for more information www.linear.com/lt8309 e lec t rical c harac t eris t ics note 1: stresses beyond those listed under absolute maximum ratings may cause permanent damage to the device. exposure to any absolute maximum rating condition for extended periods may affect device reliability and lifetime. note 2: the lt8309e is guaranteed to meet performance specifications from 0c to 125c operating junction temperature. specifications over the C40c to 125c operating junction temperature range are assured by design, characterization and correlation with statistical process controls. the lt8309i is guaranteed over the full C40c to 125c operating junction parameter conditions min typ max units input voltage range l 4.5 40 v v cc quiescent current not switching, intv cc = 8v 300 345 a comparator turn-on threshold r s = 0 l C69 C57 C45 mv turn-off threshold r s = 0 l 17 16 21 25 28 mv mv drain voltage latch reset r s = 0 1.21 v minimum off-time 75 95 115 ns minimum on-time 310 360 410 ns drain current out of pin l 9.5 10 10.5 a intv cc linear regulator intv cc quiescent current not switching, intv cc = 8v 100 117 a intv cc voltage range 4.5 10 v intv cc regulation voltage l 6.8 7 7.2 v intv cc uvlo 4.03 v dropout (v cc to intv cc ) i intvcc = C10ma, v in = 7v 1 1.3 1.6 v current limit l 30 42 55 ma gate driver turn-on propagation delay (t d(on) ) r s = 0, C100mv od , v ds to v gate 31 40 ns turn-off propagation delay (t d(off) ) r s = 0, C100mv od , v ds to v gate 26 36 ns t r gate driver output rise time c l = 3300pf 21 ns t f gate driver output fall time c l = 3300pf 11 ns pull-up resistance 2.7 pull-down resistance 0.8 the l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at t a = 25c. temperature range. the lt8309h is guaranteed over the full C40c to 150c operating junction temperature range. high junction temperatures degrade operating lifetimes. operating lifetime is derated at junction temperatures greater than 125c. note 3: the lt8309 includes overtemperature protection that is intended to protect the device during momentary overload conditions. junction temperature will exceed 150c when overtemperature protection is active. continuous operation above the specified maximum operating junction temperature may impair device reliability. lt 8309 8309fa
4 for more information www.linear.com/lt8309 temperature (c) ?50 0 v cc i q current (a) 50 150 250 ?25 0 25 50 10075 125 100 200 350 450 300 400 150 8309 g01 temperature (c) ?50 intv cc i q current (a) ?25 0 25 50 10075 125 150 8309 g02 0 40 80 120 160 200 20 60 100 140 180 0 intv cc voltage (v) 2 4 6 8 1 3 5 7 v cc voltage (v) 0 10 20 30 40 8309 g04 temperature (c) ?50 3.0 threshold voltage (v) 3.5 4.5 ?25 0 25 50 10075 125 5.0 4.0 150 8309 g05 falling threshold rising threshold intv cc current (ma) 0 0 intv cc regulator dropout (v) 1.5 10 20 30 2.5 1.0 0.5 2.0 40 8309 g06 v cc = 7v temp = ?50c temp = 25c temp = 100c temp = 150c temperature (c) ?50 0 intv cc dropout (v) 1.0 ?25 0 25 50 10075 125 2.0 0.5 1.5 150 8309 g07 v cc = 7v i load = 0ma i load = 5ma i load = 10ma i load = 20ma temperature (c) ?50 0 intv cc current limit (ma) 10 30 50 ?25 0 25 50 10075 125 20 40 60 150 8309 g08 typical p er f or m ance c harac t eris t ics intv cc voltage vs v cc voltage intv cc undervoltage lockout vs temperature intv cc regulator dropout vs intv cc current intv cc dropout vs temperature intv cc current limit vs temperature minimum on-time and off-time vs temperature i vcc vs temperature i intvcc vs temperature intv cc vs temperature temperature (c) ?100 ?50 time (ns) 0 50 100 150 200 8309 g09 minimum on-time minimum off-time 50 150 250 100 200 350 300 400 temperature (c) ?50 6.0 intv cc voltage (v) 6.4 6.8 7.2 ?25 0 25 50 10075 125 8.0 7.8 6.2 6.6 7.0 7.4 7.6 150 8309 g03 lt 8309 8309fa
5 for more information www.linear.com/lt8309 temperature (c) ?50 0 drain pin current (a) 2 6 12 ?25 0 25 50 10075 125 10 4 8 14 150 8309 g10 temperature (c) ?50 turn-on threshold (mv) ?25 0 25 50 10075 125 150 8309 g11 ?80 ?70 ?60 ?50 ?40 ?75 ?65 ?55 ?45 temperature (c) ?50 turn-off threshold (mv) ?25 0 25 50 10075 125 150 8309 g12 0 10 20 30 40 5 15 25 35 temperature (c) ?50 1.00 threshold voltage (v) 1.10 1.20 1.30 ?25 0 25 50 10075 125 1.40 1.05 1.15 1.25 1.35 150 8309 g13 typical p er f or m ance c harac t eris t ics latch reset threshold vs temperature gate rise and fall time vs charge gate current vs charge drain pin current vs temperature comparator turn-on threshold vs temperature comparator turn-off threshold vs temperature 0 time (ns) 50 150 200 100 fall time rise time charge (nc) 0 60 120 30 90 150 8309 g14 0 time (ns) 1.0 2.0 3.0 4.0 0.5 1.5 2.5 3.5 charge (nc) 8309 g15 0 60 120 30 90 150 gate source current gate sink current lt 8309 8309fa
6 for more information www.linear.com/lt8309 b lock diagra m p in func t ions gate (pin 1): n-channel mosfet gate driver output. switches between intv cc and gnd. driven to gnd during undervoltage lockout of intv cc . gnd (pin 2): ground. intv cc (pin 3): regulated supply for internal loads, and gate driver. supplied from v cc and regulates to 7v (typical). intv cc must be bypassed with a 4.7 f capacitor placed close to the pin. v cc (pin 4): input voltage. this pin supplies current to the internal start-up circuitry and to the intv cc ldo. this pin must be locally bypassed with a capacitor. drain ( pin 5): current sense pin. this pin senses the voltage across the drain-to-source of the external n-channel mosfet . a series resistor is needed to set the offset voltage and needs to be at least 800. the offset is equal to: 20mv ? 1 ? r 2000 ? ? ? ? ? ? v bias 8309 bd ? + ? + intv cc intv cc v bias v cc gate gnd drain 1.25v 10a 10a logic 3 4 2 5 1 lt 8309 8309fa
7 for more information www.linear.com/lt8309 8309 f01 gnd v offset t d(on) t d(off) t min(off) 80mv t min(on) o pera t ion the lt8309 is a synchronous rectifier designed for boundary conduction mode/ critical conduction mode and discontinuous mode flyback converters. existing solutions use a pulse signal from the primary side to control the synchronous rectifier, but the lt8309 senses the drain-source voltage of the mosfet to control the synchronous rectifier. this technique not only elimi - nates a transformer, but allows it to work with linear t echnologys line of no-opto critical conduction flyback converters. synchronous rectification improves ef - ficiency and more importantly increases the maximum output current when compared to nonsynchronous designs. the lt8309 imitates the behavior of a diode but re- places the forward voltage with the r ds(on) of an external n-channel mosfet . the lt8309 includes an ldo, a very fast comparator and a powerful gate driver. an external resistor connects the parts drain pin to the drain of the mosfet. this resistor sets the trip point of the comparator with a precise internal current source. to set the trip point to C10 mv, a 3000 resistor is needed. the trip point decreases by 1 mv for every 100 added to this drain resistor. this trip point will be referred to as v offset . figure 1 shows the drain waveform on top and the gate waveform on the bottom. the gate node goes high when the drain node goes 74 mv below the v offset . the comparators output is ignored for a minimum on-time to eliminate the chance that ringing triggers the comparator. after the minimum on-time, the comparator is ready to trigger at the v offset voltage. once the drain voltage goes above v offset , the gate turns off after a very fast propaga- tion delay , t d(off) . the body diode begins to conduct again before the current reaches ?0 a. the drain voltage needs to go above 1.21 v and then wait for the minimum off-time before the comparator is re-enabled. figure 1. drain and gate waveforms lt 8309 8309fa
8 for more information www.linear.com/lt8309 intv cc v cc v out v in gate 8309 f02 drain gnd lt8309 ? ? o pera t ion undervoltage lockout the part features a intv cc undervoltage lockout (uvlo) to prevent switching until the intv cc voltage is above 4v. intv cc ldo an internal ldo regulator provides a regulated 7 v output from the v in pin to the intv cc pin. an output capacitor is needed to provide the current needed for the gate driver. a 4.7 f capacitor is recommended and must be placed as close as possible to the intv cc pin. the current limit for the ldo is 42ma. mosfet selection a mosfets r ds(on) is important to the operation of the lt8309. the drain-source voltage is used to determine when to turn off the mosfet. the peak current through the mosfet times the mosfets r ds(on) should be above 75 mv. when this voltage is too low, the high speed comparator may trip early due to ringing on the drain pin. when this voltage is too high, the mosfet dissipates a large amount of power which causes efficiency to go down and may cause thermal issues with the mosfet. setting the drain pin resistor the drain pin resistor sets when the lt8309 turns off the mosfet. the trip point, v offset , is set with the fol- lowing equation: v offset = 20mv C 10a tr drain r drain is the resistor connected between the drain of the mosfet and the drain pin of the part. r drain needs to be at least 800 to operate correctly. for most applications, v offset should be set at C5 mv. high r ds(on) mosfets may require a more negative v offset voltage to keep the drain- to- source current from reversing. if the current is reversing, decrease v offset in 5 mv steps to eliminate the cross-conduction. short-circuit operation in the typical application diagram on page 1, the v cc pin is connected to the output of the flyback converter. during an output short-circuit condition, the lt8309 is off and the body diode of the mosfet must handle the short-circuit condition. this puts additional thermal requirements on the mosfet. the drain voltage of the mosfet is equal to v in / n in short-circuit and capable of powering the lt8309 with the circuit in figure 2. this allows the lt8309 figure 2. short-circuit application circuit lt 8309 8309fa
9 for more information www.linear.com/lt8309 o pera t ion figure 3. demo board topside silkscreen figure 4. demo board topside metal to operate during a short, and the current will flow through the low resistance channel of the mosfet instead of its body diode. make sure to use a resistor in series with the diode to keep v cc below 40v. layout considerations the main current loop is the mosfets drain-to-source current. this should not share the same ground path as the lt8309. the drain resistor needs to sense directly at the drain of the mosfet and not have any current of the drain current flow through its metal trace. the drain node of the mosfet is used as the heat sink and will need to be sized according to the power dissipation requirements. figure 3 is an example layout of the lt8309. effects of the body diode when the discontinuous ring voltage goes below ground, the body diode of the primary-side mosfet turns on and begins to conduct current. this diode clamps the voltage to a diode drop below ground. when the current reverses direction, the diode does not turn off immediately, and conducts current in the opposite direction for a small period of time. this is known as the reverse - re covery time. during this time, the magnetizing inductance of the transformer stores energy just like it does when the mosfet turns on. when the primary-side body diode finally turns off, the energy is transferred to the secondary side and may be enough to turn on the body diode of the secondary mosfet. then, the lt8309 turns on its mosfet for a second time. since this happens during the discontinuous ring, the primary side may turn on during this time and cause cross conduction. if this problem does occur, one way to improve the reverse-recovery time of the primary sides mosfet is to add a parallel schottky diode, which will conduct most of the current and turn off much faster than the body diode. another way of eliminating this issue is by sizing the turns ratio so that the discontinuous ring never goes below ground. lt 8309 8309fa
10 for more information www.linear.com/lt8309 typical a pplica t ions 60w, 12v output, isolated telecom supply intv cc v cc v out + 12v, 5a v out ? 13v central semi: cmz5928b v in 36v to 72v gate 8309 ta02 drain gnd 220pf 100 6.04k 158k lt8309 ? ? 0.1f 57.6k 5k 47nf 9m 470pf gate sense r ref intv cc v c ss tc en/uvlo r fb gnd lt3748 2.67:1 pulse: pa1736nl 10 2.37k 4.7f infineon: bsc320n20ns3 infineon: bsc047n08ns3 4.7f 300f 412k 15.4k 10f 1f v in lt 8309 8309fa
11 for more information www.linear.com/lt8309 40w, 5v isolated telecom supply typical a pplica t ions intv cc v cc v out + 5v, 8a v out ? 6.2v central semi: cmz5920b v in 36v to 72v gate 8309 ta03 drain gnd 100pf 60 6.04k 147k lt8309 ? ? 0.1f 30.9k 12.1k 15nf 12m gate sense r ref intv cc v c ss tc en/uvlo r fb gnd lt3748 5.33:1 pulse: pa1735nl 10 2.15k 4.7f infineon: bsc320n20ns3 infineon: bsc028n06ls 4.7f 400f 412k 15.4k 10f 1f v in lt 8309 8309fa
12 for more information www.linear.com/lt8309 p ackage descrip t ion please refer to http://www .linear.com/designtools/packaging/ for the most recent package drawings. 1.50 ? 1.75 (note 4) 2.80 bsc 0.30 ? 0.45 typ 5 plcs (note 3) datum ?a? 0.09 ? 0.20 (note 3) s5 tsot-23 0302 pin one 2.90 bsc (note 4) 0.95 bsc 1.90 bsc 0.80 ? 0.90 1.00 max 0.01 ? 0.10 0.20 bsc 0.30 ? 0.50 ref note: 1. dimensions are in millimeters 2. drawing not to scale 3. dimensions are inclusive of plating 4. dimensions are exclusive of mold flash and metal burr 5. mold flash shall not exceed 0.254mm 6. jedec package reference is mo-193 3.85 max 0.62 max 0.95 ref recommended solder pad layout per ipc calculator 1.4 min 2.62 ref 1.22 ref s5 package 3-lead plastic tsot-23 (reference ltc dwg # 05-08-1635) lt 8309 8309fa
13 for more information www.linear.com/lt8309 information furnished by linear technology corporation is believed to be accurate and reliable. however, no responsibility is assumed for its use. linear technology corporation makes no representa- tion that the interconnection of its circuits as described herein will not infringe on existing patent rights. r evision h is t ory rev date description page number a 11/14 added h-grade version 2, 3 lt 8309 8309fa
14 for more information www.linear.com/lt8309 ? linear technology corporation 2014 lt 1114 rev a ? printed in usa linear technology corporation 1630 mccarthy blvd., milpitas, ca 95035-7417 (408) 432-1900 fax : (408) 434-0507 www.linear.com/lt8309 r ela t e d p ar t s typical a pplica t ion part number description comments lt3748 100v isolated flyback controller 5v v in 100v, no-opto flyback, msop-16 package lt3798 offline isolated no-opto-coupler flyback controller with active pfc v in and v out limited only by external components, msop-16 package lt3799/lt3799-1 offline isolated flyback led controller with active pfc v in and v out limited only by external components, msop-16 package lt3957a/lt3958 40v/80v flyback/boost/inverting/sepic converter monolithic with integrated 5a/3.3a switch lt3573/lt3574/ lt3575 40v isolated flyback converters monolithic no-opto flybacks with integrated 1.25a/0.65a/2.5a switch lt3757a/lt3759/ lt3758 40v/100v boost, flyback, sepic and inverting controllers universal controllers with small package and powerful gate drive lt8302 micropower no-opto isolated flyback converter 2.8v v in 42v, with integrated 3.6a, 65v dmos power switch, 106a quiescent current, so-8 package 33w, 3.3v isolated telecom supply intv cc v cc v out + 3.3v, 10a v out ? 3.9v central semi: cmz5915b v in 36v to 72v gate 8309 ta04 drain gnd 150pf 60 6.04k 150k lt8309 ? ? 0.1f 19.1k 2.5k 22nf 15m gate sense r ref intv cc v c ss tc en/uvlo r fb gnd lt3748 8:1.4 pulse: pa1477nl 20 2k 4.7f infineon: bsc320n20ns3 infineon: bsc016n04ls 4.7f 1000f 412k 15.4k 10f 1f v in ? 40 lt 8309 8309fa

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