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1 lt1681 1681f applicatio s u descriptio u features typical applicatio u dual transistor synchronous forward controller the lt ? 1681 controller simplifies the design of high power synchronous dual transistor forward dc/dc converters. the part employs fixed frequency current mode control and supports both isolated and nonisolated topologies. the ic drives external n-channel power mosfets and operates with input voltages up to 72v. the lt1681s operating frequency is programmable and can be synchronized up to 350khz. switch phase is also con- trolled during synchronized operation to accommodate mul- tiple converter systems. internal logic guarantees 50% maxi- mum duty cycle operation to prevent transformer saturation. the lt1681 incorporates a soft-start feature that provides a controlled increase in supplied current during start-up and after an undervoltage lockout or overvoltage/overcurrent event. the part is available in a 20-lead wide so package to support high voltage pin-to-pin clearance. n isolated telecommunication systems n personal computers and peripherals n lead acid battery backup systems n automotive and heavy equipment n high voltage: operation up to 72v n synchronizable operating frequency and output switch phase for multiple controller systems n fixed frequency operation to 350khz n adaptive and adjustable blanking n synchronous rectifier driver n local 1% voltage reference n undervoltage lockout protection with hysteresis n input overvoltage protection n programmable start inhibit n transformer primary saturation protection n optocoupler feedback support n soft-start control , ltc and lt are registered trademarks of linear technology corporation. 36v-72v dc to 5v/7a synchronous forward converter (half-brick footprint) v cc 14 2 1 5 1 f 150pf ovlo shdn 73.2k 1% 20k 10k 270k 0.25w 68 f 20v 5v ref 6 f set 4.7nf 8 ss 10 15 bat54 zvn3310f 9 v c pgnd v fb 374 therm lt1681 sync sgnd 52.3k 100 1k 1% 100 2k fzt690 4.7 f 16v 0.22 f 50v cmpz5242b 12v 3.3 10k 0.01 f 13 sg + 0.1 f 11 sense 16 bg 18 bstref 19 tg blksens t max 20 bas21 0.1 f 100v v bst c4 1.5 f 100v c3 1.5 f 100v l1 4.7 h 0.025 1/2w q3 1nf 100v 1nf 100v q5 q6 murs120t3 murs120t3 q1 v in + v in c2 22 f 100v v out + v out c5 330 f 10v 10 0.25w 10 0.25w l2 4.1 h 330pf + 1.24k 1% v cc1 in2 in1 gnd2 v cc2 out2 out1 gnd1 8 3 1 4 6 5 7 2 ltc1693-2 1ov bias 1ov bias mbr- 0540t1 mmbd914lt1 5v out 3.01k 1% 51 0.047 f 6 7 5 8 4 9 1 2 3 10 11 12 1nf 4.7 + 1681 ta01 56k 12 17 cmpz5248b 18v cmpz- 5248b 15v q10 24k t1 c2:sanyo 100mv22ax c3, c4: vitramon vj1825y155mxb c5: 4x kemet t510x337ko10as l1: coilcraft do1608c-472 l2: panasonic etqp6f4r1lf4 q1,q3:100v siliconix sud40n10-25 q5,q6: siliconix si4450 t1:coiltronics vp5-1200 q10: on semi mmbt3906lti v out = 5v i out = 7a
2 lt1681 1681f supply voltages power supply (v cc ) ............................. C 0.3v to 20v topside supply (v bst ) ................... v bstref C 0.3v to v bstref + 20v (v bst(max) = 90v) topside reference pin (v bstref ) .......... C 0.6v to 75v input voltages shdn pin .................................. C 0.3v to v cc + 0.3v all other inputs ..................... C 0.3v to 5v ref + 0.3v maximum currents 5v ref pin ........................................ C 85ma to 10ma fset pin ............................................. C 2ma to 5ma all other inputs .................................. C 2ma to 2ma operating ambient temperature range lt1681e (note 4) .............................. C 40 c to 85 c lt1681i ............................................. C 40 c to 85 c storage temperature range ................ C 65 c to 150 c lead temperature (soldering, 10 sec)................. 300 c (note 1) order part number lt1681esw lt1681isw t jmax = 125 c, q ja = 85 c/ w the l denotes specifications which apply over the full operating temperature range, otherwise specifications are t a = 25 c. v cc = v bst = 12v, v bstref = 0v, v vc = 2v, v fb = v ref = 1.25v, c tg = c bg = c sg = 1000pf. absolute m axi m u m ratings w ww u package/order i n for m atio n w u u electrical characteristics 1 2 3 4 5 6 7 8 9 10 top view sw package 20-lead plastic so 20 19 18 17 16 15 14 13 12 11 shdn ovlo therm sgnd 5v ref fset sync ss v fb v c v bst tg bstref blksens bg pwrgnd v cc sg i max sense symbol parameter conditions min typ max units supply and protection v cc operating supply voltage range l 91218 v i cc dc active supply current (note 2) 17 22 ma l 25 ma dc active uvl supply current v shdn > 1.35v, v cc = 8v l 800 1200 m a dc standby supply current v shdn < 0.3v 0.5 m a i bst dc active supply current tg logic high (note 2) l 5 8.5 ma dc standby supply current v shdn < 0.3v 0.1 m a v shdn shutdown rising threshold l 1.15 1.25 1.35 v shutdown threshold hysteresis l 100 150 200 mv i ss soft-start charge current v ss = 2v l C14 C 10 C 6 m a v ss soft-start reset threshold 225 mv v ccuvlo undervoltage lockout threshold falling edge l 8.0 8.40 8.60 v rising edge l 8.3 8.75 8.95 v undervoltage lockout hysteresis l 0.25 0.35 v v bstuvlo boost undervoltage lockout falling edge l 5.7 6.4 7.1 v (v bst -bstref) rising edge l 6.5 7.0 7.5 v boost uvlo hysteresis l 0.3 0.6 v consult ltc marketing for parts specified with wider operating temperature ranges.
3 lt1681 1681f the l denotes specifications which apply over the full operating temperature range, otherwise specifications are t a = 25 c. v cc = v bst = 12v, v bstref = 0v, v vc = 2v, v fb = v ref = 1.25v, c tg = c bg = c sg = 1000pf. electrical characteristics symbol parameter conditions min typ max units 5v external reference v 5vref 5v reference voltage 0 (i 5vref C i vc ) < 20ma 4.85 5 5.10 v l 4.80 5.15 v i 5vrefsc short-circuit current source, i vc = 0 l 20 45 ma r 5vref output impedance 0 (i 5vref C i vc ) < 20ma 1 w error amp v fb error amplifier reference voltage measured at feedback pin 1.242 1.250 1.258 v l 1.225 1.265 v i fb feedback input current v fb = v ref C50 na a v error amplifier voltage gain 72 db i vc error amplifier current limit source l 10 25 ma sink l 0.5 1 ma v vc zero current output voltage 1.4 v maximum output voltage 3.2 v gbw gain bandwidth product (note 3) 1 mhz current sense and blanking a v amplifier dc gain 12 v/v i sense input bias current C 275 m a v sense current limit threshold measured at sense pin 135 150 165 mv l 130 170 mv t d current sense to switch delay 175 ns v blksens blanking input threshold l 4.5 5 5.5 v i blksens blanking input bias current C2 m a t min switch minimum on time v blksens = v bg , measured at bg output 250 ns i max sense i imax input bias current C 250 m a v imax i max threshold (rising edge) measured at i max input l 320 360 400 mv i max threshold hysteresis measured at i max input 140 mv t p i max output switch disable delay measured at bg output 130 ns therm and ovlo fault detectors v therm / threshold (rising edge) l 1.2 1.25 1.3 v v ovlo threshold hysteresis l 20 40 60 mv t p fault delay to output disable 50mv overdrive 650 ns oscillator and synchronization decoder f osc oscillator frequency, free run measured at fset pin 700 khz frequency programming error, free run f osc 500khz (note 3) l C10 5 % i fset fset input bias current fset charging, v fset = 2v 50 na v sync sync logic high input threshold positive-going edge l 1.4 2 v sync logic low input threshold negative-going edge l 0.8 1.4 v f sync sync frequency l f osc /2 350 khz t h, l maximum sync pulse width f osc = oscillator free-run frequency 1/f osc s (logic high or logic low)
4 lt1681 1681f the l denotes specifications which apply over the full operating temperature range, otherwise specifications are t a = 25 c. v cc = v bst = 12v, v bstref = 0v, v vc = 2v, v ts = 0v, v fb = v ref = 1.25v, c tg = c bg = c sg = 1000pf. electrical characteristics symbol parameter conditions min typ max units output drivers v tg tg on voltage l 11 11.5 v tg off voltage l 0.1 0.5 v t tgr/f tg rise/fall times 10% to 90%/90% to 10% 35 ns v bg bg on voltage l 11 11.5 v bg off voltage l 0.1 0.5 v t bgr/f bg rise/fall times 10% to 90%/90% to 10% 35 ns v sg sg on voltage l 11 11.5 v sg off voltage l 0.1 0.5 v t sgr/f sg rise/fall times 10% to 90%/90% to 10% 35 ns t sg-bg sg to bg enable lag time 4v on/off thresholds l 80 150 300 ns t tg-bg tg to bg enable lag time 4v on/off thresholds 100 ns note 1: absolute maximum ratings are those values beyond which the life of a device may be impaired. note 2: supply current specification does not include external fet gate charge currents. actual supply currents will be higher and vary with operating frequency, operating voltages and the type of external switch elements used. see applications information. note 3: guaranteed but not tested. note 4: the lt1681e is guaranteed to meet performance specifications from 0 c to 70 c. specifications over the C 40 c to 85 c operating temperature range are assured by design, characterization and correlation with statistical process controls. for guaranteed performance to specifications over the C40 c to 85 c range, the lt1681i is available. typical perfor a ce characteristics uw i cc supply current vs temperature temperature ( c) ?5 i cc supply current (ma) 15 125 80 1681 g01 ?0 0 40 20 19 18 17 16 v cc = 12v shdn pin voltage (mv) 500 i cc supply current (na) 700 900 1100 100 0 200 300 400 1681 g02 500 t a = 25 c supply voltage (v) 9 i cc supply current (ma) 16 18 16 1681 g03 15 10 12 14 18 17 t a = 25 c i cc supply current vs shdn pin voltage i cc supply current vs v cc supply voltage
5 lt1681 1681f typical perfor a ce characteristics uw i bst boost supply current vs temperature i cc supply current vs shdn pin voltage temperature ( c) ?5 i bst boost supply current (ma) 5.0 4.9 125 80 1681 g04 4.8 ?0 0 40 5.2 5.1 shdn pin current (v) 0 0 i cc supply current ( a) 20 40 60 0.2 0.4 0.6 0.8 1681 g05 1.0 1.2 t a = 25 c uvlo i cc supply current vs temperature 5v ref voltage vs temperature 5v ref short-circuit current limit vs temperature error amp reference vs temperature temperature ( c) ?5 uvlo i cc supply current (ma) 0.8 125 80 1681 g06 0.5 0.6 ?0 0 40 1 temperature ( c) ?5 5v ref voltage (v) 5.00 4.95 125 80 1681 g07 4.90 ?0 0 40 5.10 5.05 temperature ( c) ?5 5v ref short-circuit current limit (ma) 40 125 80 1681 g08 30 ?0 0 40 60 50 temperature ( c) ?5 error amp reference (v) 1.250 1.245 125 80 1681 g09 1.240 ?0 0 40 1.260 1.255 v c pin short-circuit current limit vs temperature soft-start output current vs temperature soft-start output current vs soft-start pin voltage temperature ( c) ?5 v c pin short-circuit current limit (ma) 15 125 80 1681 g10 10 ?0 0 40 25 20 temperature ( c) ?5 soft-start output current ( a) 10 9 125 80 1681 g11 8 ?0 0 40 12 11 v ss = 2v soft-start pin voltage (mv) 0 soft-start output current ( a) 20 40 60 100 0 200 300 400 1681 g12 500 t a = 25 c
6 lt1681 1681f typical perfor a ce characteristics uw soft-start output current vs soft-start pin voltage current sense amplifier bandwidth vs temperature soft-start pin voltage (v) 0 soft-start output current ( a) 20 40 60 1 0234 1681 g13 5 t a = 25 c temperature ( c) ?5 2 current sense amp bandwidth (mhz) 3 5 6 7 ?5 25 45 125 1681 g14 4 ?5 5 65 85 105 8 shdn (pin 1): shutdown pin. pin voltages exceeding positive-going threshold of 1.25v enables the lt1681. 150mv of input hysteresis resists mode switching insta- bility. the shdn pin can be controlled by either a logic-level input or with an analog signal. this shutdown feature is typically used for input supply undervoltage protection. a resistor divider from the converter input supply to the shdn pin monitors that supply for control of system power-up sequencing, etc. all internal functions are dis- abled during shutdown. ovlo (pin 2): overvoltage shutdown sense. typically connected to input supply through a resistor divider. if pin voltage exceeds 1.25v, the lt1681 switching function is disabled to protect boosted circuitry from exceeding ab- solute maximum voltage. 40mv of input hysteresis resists mode switching instability. exceeding the ovlo threshold also triggers soft-start reset, resulting in a graceful recov- ery from an input transient event. therm (pin 3): system thermal shutdown. auxiliary shutdown pin that is typically used for system thermal protection. if pin voltage exceeds 1.25v, the lt1681 switching function is disabled. 40mv of input hysteresis uu u pi fu ctio s resists mode switching instability. exceeding the therm threshold also triggers soft-start reset, resulting in a graceful recovery. sgnd (pin 4): signal ground reference. careful board layout techniques must be used to prevent corruption of the signal ground reference. high current switching paths must be oriented on the converter ground plane such that currents to/from the switches do not affect the integrity of the lt1681 signal ground reference. 5v ref (pin 5): 5v local reference. allows connection of external loads up to 20ma dc. typically bypassed with 1 m f ceramic capacitor to sgnd. reference output is current limit protected to a typical value of 45ma. if the load on the 5v reference exceeds the current limit value, lt1681 switching function is disabled and the soft-start function is reset. fset (pin 6): oscillator timing pin. connect a resistor (r fset ) from the 5v ref pin to this pin and a capacitor (c fset ) from this pin to ground. the lt1681 oscillator operates by monitoring the voltage on c fset as it is charged via r fset . when the voltage on the fset pin reaches 2.5v, the oscillator rapidly discharges the capacitor with an average current of 0.8ma. once the
7 lt1681 1681f uu u pi fu ctio s voltage on the pin is reduced to 1.5v, the pin becomes high impedance and the charging cycle repeats. the oscillator operates at twice the switching frequency of the controller. oscillator frequency f osc can be approximated by the relation: fc r r osc fset fset fset @+ ++ ? ? ? ? ? ? ? ? ? y ? t ? 05 10 3 810 2 64 1 1 . sync (pin 7): oscillator synchronization input pin with ttl-level compatible input. the sync input signal (at the desired synchronized operating frequency) controls both the internal oscillator (running at twice the sync fre- quency) and the output switch phase. if the synchroniza- tion function is not desired, this pin may be shorted to ground. the lt1681 internal oscillator drives a toggle flip-flop that assures 50% duty cycle operation during oscillator free- run. the oscillator, therefore, runs at twice the operating frequency of the converter. the sync input decoder incorporates a frequency doubling circuit for oscillator synchronization, resetting the internal oscillator on both the rising and falling edges of the input signal. the sync input decoder also differentiates transition phase and forces the toggle flip-flop to phase-lock with the sync input. a transition to logic high on the sync input signal corresponds to the initiation of a new switching cycle (primary switches turning on pending current con- trol) and a transition to logic low forces a primary switch off state. as such, the maximum operating duty cycle is equal to the duty cycle of the sync signal. the sync input can therefore be used to reduce the maximum duty cycle of the converter by reducing the duty cycle of the sync input. ss (pin 8): soft-start. connect a capacitor (c ss ) from this pin to ground. the output voltage of the lt1681 error amplifier corre- sponds to the peak current sense amplifier output de- tected before resetting the switch outputs. the soft-start circuit forces the error amplifier output to a zero sense current for start-up. a 10 m a current is forced from this pin onto an external capacitor. as the ss pin voltage ramps up, so does the lt1681 internally sensed current limit. this effectively forces the internal current limit to ramp from zero, allowing overall converter current to slowly increase until normal output regulation is achieved. this function reduces output overshoot on converter start-up. the soft-start function incorporates a 1v be dead zone such that a zero current condition is maintained on the v c pin until the ss pin rises to 1v be above ground. the ss pin voltage is reset to start-up condition during shutdown, undervoltage lockout and overvoltage or overcurrent events, yielding a graceful converter output recovery from these events. v fb (pin 9): error amplifier inverting input. typically connected to a resistor divider from the output and com- pensation components to the v c pin. the v fb pin is the converter output voltage feedback node. input bias current of ~50na forces the pin high in the event of an open-feedback path condition. the error amplifier is internally referenced to 1.25v. values for the v out to v fb feedback resistor (r fb1 ) and the v fb to ground resistor (r fb2 ) can be calculated to program converter output voltage (v out ) via the following relation: v out = 1.25 ? (r fb1 + r fb2 )/r fb2 v c (pin 10): error amplifier output. the lt1681 error amplifier is a low impedance output inverting gain stage. the amplifier has ample current source capability to allow easy integration of isolation optocouplers that require bias currents up to 10ma. external dc loading of the v c pin reduces the external current sourcing capacity of the 5v ref pin by the same amount as the load on the v c pin. the error amplifier is typically configured using a feedback rc network to realize an integrator circuit. this circuit creates the dominant pole for the converter regulation feedback loop. integrator characteristics are dominated by the value of the capacitor connected from the v c pin to the v fb pin and the feedback resistor connected to the v fb pin. specific integrator characteristics can be configured to optimize transient response.
8 lt1681 1681f uu u pi fu ctio s the error amplifier can also be configured as a transimpedance amplifier for use in secondary-side con- troller applications. (see applications information section for configuration and compensation details) sense (pin 11): current sense amplifier (csa) nonin- verting input. current is monitored via a ground refer- enced current sense resistor, typically in series with the source of the bottom-side switch fet. internal limit cir- cuitry provides for a maximum peak value of 150mv across the sense resistor during normal operation. i max (pin 12): primary current runaway protection. the i max pin is used to detect primary-side switch currents and shuts down the primary switches if a current runaway condition is detected. the i max function is not disabled during the current sense blanking interval. the pin is typically connected to the primary bottom-side switch source and monitors switch current via a ground-refer- enced current sense resistor. if the pin voltage exceeds 360mv, lt1681 switching function is disabled in 130ns. exceeding the i max threshold also triggers a soft-start reset, resulting in a graceful recovery from a current runaway event. for single-sense resistor systems, this pin can be shorted to sense for protection during the blank- ing interval or shorted to sgnd if not used. sg (pin 13): synchronous switch output driver. this pin can be connected directly to the gate of the synchronous switch if small fets are used (c gate < 5000pf), however, the use of a gate drive buffer is recommended for peak efficiencies. the sg pin output is synchronized and out-of-phase with the bg output. the control timing of the sg output causes its transition to lead the primary switch path during turn- on by 150ns. v cc (pin 14): ic local power supply input. bypass with a capacitor at least 10 times greater than c 5vref to pgnd. the lt1681 incorporates undervoltage lockout that dis- ables switching functions if v cc is below 8.4v. the lt1681 supports operational v cc power supply voltages from 9v to 18v (20v absolute maximum). pwrgnd (pin 15): output driver ground reference. connect through low impedance trace to v in decoupling capacitor. bg (pin 16): bottom-side primary switch/forward switch output driver. pin can be connected directly to gate of primary bottom-side and forward switches if small fets are used (c gate total < 5000pf), however, the use of a gate drive buffer is recommended for peak efficiencies. the bg output is enabled at the start of each oscillator cycle in phase with the tg pin but is timed to lag the tg output during turn-on and lead the tg output during turn-off. these delays force the concentration of transi- tional losses onto the bottom-side primary switch. blksens (pin 17): blanking sense input. the current sense function (via sense pin) is disabled while the blksens pin is below 5v. blksens is typically con- nected to the gate of the bottom-side primary switch mosfet. bstref (pin 18): v bst supply reference. typically con- nects to source of topside external power fet switch. tg (pin 19): topside (boosted) primary output driver. pin can be connected directly to gate of primary topside switch if small fets are used (c gate < 5000pf), however, the use of a gate drive buffer is recommended for peak efficiencies. v bst (pin 20): topside primary driver bootstrapped sup- ply. this boosted supply rail is referenced to the bstref pin. supply voltage is maintained by a bootstrap capacitor tied from the v bst pin to the boosted supply reference (bstref) pin. the charge on the capacitor is refreshed each switch cycle through a schottky diode connected from the v cc supply (cathode) to the v bst pin (anode). the bootstrap capacitor (c boost ) must be at least 100 times greater than the total load capacitance on the tg pin. a capacitor in the range of 0.1 m f to 1 m f is generally adequate for most applications. the bootstrap diode must have a reverse- breakdown voltage greater than the converter v in . the lt1681 supports operational v bst supply voltages up to 90v (absolute maximum) referenced to ground. undervoltage lockout disables the topside switch until v bst -bstref > 7.0v for start-up protection of the topside switch.
9 lt1681 1681f block diagra w + + + + + + + 350mv error amp 5v ref r sq r s 225mv q 10 a s q t phase detect nol logic f = 2 12 20 19 18 16 13 v bst tg bstref bg sg 15 pwrgnd 17 blksens + 1.25v 1.25v 1.25v 1.25v reference generator 4 uvl (<8v) i lim 14 v cc 6 fset 7 sync 10 v c 11 sense 12 i max 9 v fb 1 shdn 3 therm 5 5v ref 2 ovlo 4 sgnd 8 ss 1681 bd
10 lt1681 1681f overview the lt1681 is a high voltage, high current synchronous regulator controller, optimized for use with dual transistor forward topologies. the ic uses a constant frequency, current mode architecture with internal logic that prevents operation over 50% duty cycle. a unique synchronization scheme allows the system clock to be synchronized up to an operational frequency of 350khz, along with phase control for easy integration of multicontroller systems. a local precision 5v supply is available for external support circuitry and can be loaded up to 20ma. internal fault detection circuitry disables switching when a variety of system faults are detected such as: input supply overvoltage or undervoltage faults, excessive sys- tem temperature, transformer primary-side saturation and local supply overcurrent conditions. the lt1681 has a current limit soft-start feature that gradually increases the current drive capability of a converter system to yield a smooth start-up with minimal overshoot. the soft-start circuitry is also used for smooth recoveries from system fault conditions. external fet switches are employed for the switch ele- ments, and hearty switch drivers allow implementation of high current designs. an adaptive blanking scheme built into the lt1681 allows for correct current-sense blanking regardless of switch size and even while using external switch drive buffers. the lt1681 employs a voltage output error amplifier, providing superior integrator linearity and allowing easy high bandwidth integration of optocoupler feedback for fully isolated solutions. theory of operation (see block diagram) the lt1681 senses the output voltage of its associated converter via the v fb pin. the difference between the voltage on this pin and an internal 1.25v reference is amplified to generate an error voltage on the v c pin, which is used as a threshold for the current sense comparator. the current sense comparator gets its information from the sense pin, which monitors the voltage drop across an external current sense resistor. when the detected switch current increases to the level corresponding to the error applicatio s i for atio wu uu voltage on the v c pin, the switches are disabled until the next switch cycle. during normal operation, the lt1681 internal oscillator runs at twice the switching frequency. the oscillator output toggles a t flip-flop, generating a 50% duty cycle pulse that is used internally as the system clock for the ic. when the output of this flip-flop transitions high, the primary switches are enabled. the primary-side switches stay enabled until the transformer primary current, sensed via the sense pin, connected to a ground-referenced resistor in series with the bottom-side switch fet, is sufficient to trip the current sense comparator and, in turn, reset the rs latch. when the rs latch resets, the primary switches are disabled and the synchronous switch is enabled. the adaptive blanking circuit senses the bottom- side gate voltage via the blksens pin and prevents current sensing until the fet is fully enabled, preventing false triggering due to a turn-on transition glitch. if the current comparator threshold is not obtained when the flip-flop output transitions low, the rs latch is bypassed and the primary switches are disabled until the next flip- flop output transition, forcing a maximum switch duty cycle less than 50%. system fault detectionthe general fault condition (gfc) the lt1681 contains circuitry for detecting internal and system faults. detection of a fault triggers a general fault condition or gfc. when a gfc is detected, the lt1681 disables switching and discharges the soft-start capaci- tor. when the gfc subsides, the lt1681 initiates a start- up cycle via the soft-start circuitry to assure a graceful recovery. recovery from a gfc is gated by the soft-start capacitor discharge. the capacitor must be discharged to a threshold of 225mv before the gfc can be concluded. as the zero output current threshold of the ss pin is typically a transistor v be , or 0.7v, latching the gfc until a 225mv threshold is achieved assures a zero output current state is obtained in the event of a short-duration fault. a gfc is also triggered during a system state change event, such as entering shutdown mode, to prevent any mode transition abnormalities.
11 lt1681 1681f applicatio s i for atio wu uu events that trigger a gfc are: a) exceeding the current limit of the 5v ref pin b) detecting an undervoltage condition on v cc c) detecting an undervoltage condition on 5v ref d) pulling the shdn pin below the shutdown threshold e) exceeding the i max pin threshold f) exceeding the 1.25v fault detector threshold on either the ovlo or therm pins the ovlo and therm pins are used to directly trigger a gfc. if either of these pins are not used, they can be disabled by connecting the pin to sgnd. the intention of the olvo pin is to allow monitoring of the input supply to protect from an overvoltage condition. monitoring of system temperature (therm) is possible through use of a resistor divider using a thermistor as a resistor divider component. the 5v ref pin can provide the precision supply required for these applications. when these fault detection circuits are disabled during shutdown or v cc pin uvlo conditions, a reduction in ovlo and therm pin input impedance to ground will occur. to prevent exces- sive pin input currents, low impedance pull-up devices must not be used on these pins. undervoltage lockout the lt1681 maintains a low current operational mode when an undervoltage condition is detected on the v cc supply pin, or when v cc is below the undervoltage lockout (uvlo) threshold. during a uvlo condition on the v cc pin, the lt1681 disables all internal functions with the exception of the shutdown and uvlo circuitry. the exter- nal 5v ref supply is also disabled during this condition. disabling of all switching control circuity reduces the lt1681 supply current to < 1ma, simplifying integration of trickle charging in systems that employ output feedback supply generation. the function of the high side switch output (tg) is also gated by uvlo circuitry monitoring the bootstrap supply (v bst -bstref). switching of the tg pin is disabled until the voltage across the bootstrap supply is greater than 7.4v. this helps prevent the possibility of forcing the high side switch into a linear operational region, potentially causing excessive power dissipation due to inadequate gate drive during start-up. error amplifier configurations the converter output voltage information is fed back to the lt1681 onto the v fb pin where it is transformed into an output current control voltage by the error amplifier. the error amplifier is generally configured as an integrator and is used to create the dominant pole for the main converter feedback loop. the lt1681 error amplifier is a true high gain voltage amplifier. the amplifier noninverting input is internally referenced to 1.25v; the inverting input is the v fb pin and the output is the v c pin. because both low frequency gain and integrator frequency characteristics can be controlled with external components, this amplifier allows far greater flexibility and precision compared with use of a transconductance error amplifier. in a nonisolated converter configuration where a resistor divider is used to program the desired output voltage, the error amplifier can be configured as a simple active integrator, forming the system dominant pole (see fig- ure 1). placing a capacitor c err from the v fb pin to the v c pin will set the single-pole crossover frequency at (2 p r fb c err ) C1 . additional poles and zeros can be added by increasing the complexity of the rc network. v fb r fb c err v out v c 1.25v 1681 f01 lt1681 9 + 10 figure 1. nonisolated error amp configuration another common error amplifier configuration is for optocoupler use in fully isolated converters with second- ary-side control (see figure 2). in such a system, the dominant pole for the feedback loop is created at the sec- ondary-side controller, so the error amplifier needs only to
12 lt1681 1681f translate the optocoupler information. the bandwidths of the optocoupler and amplifier should be as high as pos- sible to simplify system compensation. this high band- width operation is accomplished by using the error ampli- fier as a transimpedance amplifier, with the optocoupler transistor emitter providing feedback information directly into the v fb pin. a resistor from v fb to ground provides the dc bias condition for the optocoupler. connecting the optocoupler transistor collector to the local 5v ref supply reduces miller capacitance effects and maximizes the band- width of the optocoupler. higher optocoupler current also means higher bandwidth, and the 5v ref supply can pro- vide collector currents up to 10ma. applicatio s i for atio wu uu figure 3 is a plot of oscillator frequency vs c fset and r fset . typical values for 300khz operation (150khz sys- tem frequency) are c fset = 150pf and r fset = 51k. v fb 5v ref v out sense v c 1.25v 1681 f01 lt1681 9 5 10 + 5v figure 2. optocoupler high bw configuration oscillator frequency programming and synchronization the lt1681 internal oscillator runs at twice the system switching frequency. the oscillator output toggles a t flip- flop, generating a 50% duty cycle pulse that is used internally as the system clock for the ic. free-run fre- quency for the internal oscillator is programmed via an rc timing network connected to the fset pin. a pull-up resistor r fset , connected from the 5v ref pin to fset, provides current to charge a timing capacitor c fset con- nected from the fset pin to ground. the oscillator oper- ates by allowing r fset to charge c fset up to 2.5v at which point r fset is pulled back toward ground by a 2.5k resistor internal to the lt1681. when the voltage across c fset is pulled down to 1.5v, the fset pin becomes high imped- ance, once again allowing r fset to charge c fset . timing resistor (k ) 20 100 oscillator frequency (khz) 150 250 300 350 600 450 40 60 70 1681 f03 200 500 550 400 30 50 80 90 100 330pf 150pf 100pf 200pf figure 3. oscillator frequency vs timing components due the relatively fast fall time of the oscillator waveform, the fset pin is held at its 1.5v threshold by an internal low- impedance clamp to reduce undershoot error. if this pin is externally forced low for any reason, external current limiting is required to prevent damage to the lt1681. continuous source current from the fset pin should not exceed 1ma. putting a 2k resistor in series with any low impedance pull-down device will assure proper function and protect the ic from damage. oscillator synchronization synchronization of the lt1681 system clock is accom- plished by driving a ttl level logic pulse train at the desired system switching frequency into the sync pin. in order to assure proper synchronization, each phase of the synchronization signal must be less then an oscillator free-run cycle. the sync input pulse controls the phasing as well as the frequency of controller switching. the sync circuit func- tions by forcing the phase of the oscillator output flip-flop to match the phase of the sync pulse and prematurely ending the oscillator charge cycle on each transition edge. at the sync low-to-high transition, the lt1681 starts a switch-on cycle and the minimum switch-off period is forced during the sync logic low period. because the sync logic low period corresponds directly
13 lt1681 1681f to the mini mum off time, the converter maximum duty cycle can be forced using the sync input. for example, a 30% duty cycle sync pulse forces 30% maximum duty cycle operation for the converter. because the logic low pulse width exceeds the logic high pulse width in < 50% duty cycle operation, the oscillator free-run cycle time must be programmed to exceed the logic low duration. the lt1681 enters an ultralow current shutdown mode when the shdn pin is below 350mv. during this mode, total supply current drops to a typical value of less than 1 m a. when shdn rises above 350mv, the ic will draw increasing amounts of supply current until just before the 1.25v turn-on threshold is achieved, when the typical supply current reaches 60 m a. the shutdown function can be disabled by connecting the shdn pin to v cc . this pin is internally clamped to 2.5v through a 20k series input resistance and can therefore draw almost 1ma when tied directly to the v cc supply. this additional current can be minimized by making the con- nection through an external series resistor (100k is typi- cally used). soft-start the lt1681 current control pin (v c ) limits sensed current to zero at voltages less than 1.4v through full current limit at v c = 3.2v, yielding 1.8v over the full regulation range. the voltage on the v c pin is internally forced to be less than or equal to ss + 0.7v. as such, the ss pin has a dead zone between 0v and 0.7v, where a zero sensed current condition is maintained. at ss voltages above 0.7v, the sensed current limit threshold on pin v c may rise as needed up to the ss maintained current limit value. once the ss pin rises to the v c pin maximum value less 0.7v, or 2.5v, the ss circuit has no effect. the ss pin sources a typical current of 10 m a. placing a capacitor (c ss ) from the ss pin to ground will cause the voltage on the ss pin to ramp up at a controlled rate, allowing a graceful increase of maximum converter output current during a start-up condition. the start-up delay time to full available current limit is: t ss = 2.5 ? 10 5 ? c ss (sec) the lt1681 internally pulls the ss pin below the zero current threshold during any fault condition to assure graceful recovery. the ss circuit also acts as a fault control latch to assure a full-range recovery from a short duration fault. once a fault condition is detected, the lt1681 will suspend switching until the ss pin has discharged to approximately 225mv. applicatio s i for atio wu uu 1.5v fset sync 1681 f04 system clock (internal) 2.5v figure 4. oscillator/sync waveforms 5v ref fset 75k 50k 100pf 1681 f05 lt1681 5 6 figure 5. oscillator connection for sync-only mode operation shutdown the lt1681 shdn pin will support ttl and cmos logic signals and also analog inputs. the shdn pin turn-on (rising) threshold is 1.25v with 150mv of hysteresis. a common use of the shdn pin is for undervoltage detec- tion on the input supply. driving the shdn pin with a resistor divider connected from the input supply to ground will prevent switching until the desired input supply volt- age is achieved. it is also possible to run the lt1681 in a sync-only mode by disabling the oscillator completely. connecting a resis- tor divider from the 5v ref pin to the fset pin, forcing a voltage within the charge range of 1.5v to 2.5v, will allow the oscillator to follow the sync input exclusively with no provision for free-run. setting values to force a voltage as close to 2v as possible is recommended.
14 lt1681 1681f layout considerationsgrounding the lt1681 is typically used in high current converter designs that involve substantial switching transients. the switch drivers on the ic are designed to drive large capacitances and, as such, generate significant transient currents. careful consideration must be made regarding input and local power-supply bypassing to avoid corrupt- ing the ground references used by the error amplifier and current sense circuitry. effective grounding of the two-transistor synchronous forward topology where the lt1681 is used is inherently difficult. the situation is complicated further by the num- ber of bypass elements that must be considered. typically, high current paths and transients from the input supply and any local drive supplies must be kept isolated from sgnd, to which sensitive circuits such as the error amp reference and the current sense circuits, as well as the local 5v ref supply, are referred. by virtue of the topologies used in lt1681 applications, the large currents from the primary switches, as well as the switch drive transients, lt1681 5v ref sgnd v bst v bst v in bstref v cc v cc pgnd 1681 f06 pass through the sense resistor to ground. this defines the ground connection of the sense resistor as the refer- ence point for both sgnd and pgnd. in nonisolated applications where sgnd is the output reference, we now have a condition where every bypass capacitor in the converter is referenced to the same point. effective grounding can be achieved by considering the return current paths from the sense resistor to each respective bypass capacitor. dont be tempted to run small traces to separate the grounds. a power ground plane is important as always in high-power converters, but bypass elements must be oriented such that transient currents in the return paths of v in and v cc do not mix. care must be taken to keep these transients away from the sgnd reference. an effective approach is to use a 2-layer ground plane, reserving an entire layer for sgnd. the 5v ref and non-isolated converter output bypasses can then be directly connected to the sgnd plane. figure 6. high current transient return paths applicatio s i for atio wu uu
15 lt1681 1681f c1: murata erie ghm3045x7r222k-gc c2, c3, c4: vitramon vj1825y155mxb c5 to c8: 330 f 10v kemet t510x337k010as or 330 f 6.3v kemet t520d337m006as iso1: fairchild moc207 l1: coilcraft do1608c-472 l2: panasonic etqpaf4r8hfa l3: coilcraft do1608c-105 q1, q3: siliconix si4486ey q5, q6, q14,q15: fairchild fds6680a t1: midcom 31267r or coiltronics ctx02-14675 (functional insulation) or midcom 31322r (basic insulation) t2: midcom 31264r (functional insulation) or midcom 31323r (basic insulation) v cc 14 2 1 5 1 f 82pf ovlo shdn 1.24k 73.2k 20k 24k 10k 267k 0.25w 68 f 20v 56k 5v ref 6 f set 4700pf 8 ss 10 15 bas21 bat54 t2 s s bas21 bat54 bat54 zvn3310f 9 v c pgnd 12 i max v fb 374 therm lt1681sw sync sgnd 52.3k 10 1k 3k 1k 3.3k 100 0.25w 10k 2k fzt690b 4.7 f 0.22 f s mmbz5240blt1 10v 1 3.3 10k 5v ref iso1 moc207 7 1 43 3 1 4 6 5 14 15 6 5 82 3300pf 4700pf 0.1 f 5v ref 13 sg + 0.1 f 1nf 11 sense 16 bg 18 bstref 19 tg 17 20 bas21 0.1 f l3 1mh blksens mmbz5245lt1 15v v bst 220pf c4 1.5 f 100v c3 1.5 f 100v l1 4.7 h 0.022 f 1000pf sync v fb ovpin margin i comp v dd optodrv v aux 0.1 f 12 i sns 11 i snsgnd 16 fg 2 cg pgnd gnd ltc1698 pwrgd 6 8 9 7 13 1.24k 1% 976 4.22k 1% 10 4 3 v comp 1681 f07 3.01k 1k 0.1 f mbr0530 0.030 1/2w 1 2 5 4 q3 nc 1000pf 100v 2.2nf 250v 1nf 100v q5, q6 fds6680a 2 q14, q15 fds6680a 2 murs120t3 murs120t3 10 6 11 10 8 7 9 12 3 q1 t1 10 mmbt3906lt1 mmbt3906lt1 v in + v in c2 1.5 f 100v v out + v out s c5 to c8 330 f 10v 4 10 0.25w 10 0.25w 4.7 l2 4.8 h 330pf + mmbt3906lt1 mmbz5248b-7 18v figure 7. 36v-72v dc in to 5v/10a isolated synchronous forward converter typical applicatio s u
16 lt1681 1681f v cc 14 2 1 5 1 f 82pf ovlo shdn 1.24k 1% 73.2k 1% 20k 24k 10k 267k 0.25w c26 68 f 20v 56k 5v ref 6 f set 4700pf 8 ss 10 15 bas21 bat54 t2 s s bas21 bat54 bat54 q12 zvn3310f 9 v c pgnd 12 i max v fb 374 therm lt1681sw sync sgnd 52.3k 1% 10 1k 3k 1k 1k 100 10k 2k 0.25w q13 fzt690b 4.7 f 16v 0.22 f 50v s mmbz5240blt1 10v 1 3.3 10k 5v ref iso1 moc207 7 1 43 3 1 4 6 5 14 15 6 5 82 3300pf 4700pf 0.1 f 5v ref 13 sg + 0.1 f 1nf 11 sense 16 bg 18 bstref 19 tg 17 20 bas21 0.1 f 100v l3 1mh blksens mmbz5245lt1 15v mmbz5248b-7 18v v bst 220pf c4 1.5 f 100v c3 1.5 f 100v l1 3.3 h 0.022 f 1000pf sync v fb ovpin margin i comp v dd optodrv v aux 0.1 f 12 i sns 11 i snsgnd 16 fg 2 cg pgnd gnd ltc1698 pwrgd 6 8 9 7 trim 13 1.24k 1% 1.78k 1% 2.43k 1% 10 4 3 v comp 1698 f11 3.01k 1k 0.33 f mbr0530 0.025 1/2w 3 4 q3 1000pf 100v c1 2200pf 250v 1000pf 100v q5, q14 fds6680a 2 q6, q15, q17 fds6680a 3 murs120t3 murs120t3 10 2 7 v sec 5 1 q1 t1 10 mmbt3906lt1 mmbt3906lt1 v in + v in c2 1.5 f 100v v out + v out s c5 to c8 330 f 10v 4 10 0.25w 10 0.25w 4.7 l2 2.35 h 330pf + mmbt3906lt1 c1: murata erie ghm3045x7r222k-gc c2, c3, c4: vitramon vj1825y155mxb c5 to c8: 330 f 10v kemet t510x337k010as or 330 f 6.3v kemet t520d337m006as c26: avx tpse686m020r0150 iso1: fairchild moc207 l1: coilcraft do1608c-332 l2: pulse p1977 planar inductor l3: coilcraft do1608c-105 q1, q3: siliconix si4486ey q5, q6, q14,q15,q17: fairchild fds6680a q7: fairchild ndt410el q12: zetex zvn3310f q13: zetex fzt690 t1: pulse p1976 planar transformer (functional insulation) or pulse pa-0191 (basic insulation) t2: midcom 31264r (functional insulation) or midcom 31323r (basic insulation) figure 8. 36v-72v dc in to 3.3v/20a isolated synchronous forward converter typical applicatio s u
17 lt1681 1681f c1: murata erie ghm3045x7r222k-gc c2, c3, c4: vitramon vj1825y155mxb c5 to c8: 330 f 10v kemet t510x337k010as or 330 f 6.3v kemet t520d337m006as c26: avx tpse686m020r0150 iso1: fairchild moc207 l1: coilcraft do1608c-332 l2: pulse p1977 planar inductor l3: coilcraft do1608c-105 q1, q3: siliconix si4486ey q5, q6, q14,q15,q17: fairchild fds6680a q7: fairchild ndt410el q12: zetex zvn3310f q13: zetex fzt690 t1: pulse p1976 planar transformer (functional insulation) or pulse pa-0191 (basic insulation) t2: midcom 31264r (functional insulation) or midcom 31323r (basic insulation) v cc 14 2 1 5 1 f 82pf ovlo shdn 1.24k 1% 73.2k 1% 20k 24k 10k 267k 0.25w c26 68 f 20v 56k 5v ref 6 f set 4700pf 8 ss 10 15 bas21 bat54 t2 s s bas21 bat54 bat54 q12 zvn3310f 9 v c pgnd 12 i max v fb 374 therm lt1681sw sync sgnd 52.3k 1% 10 1k 3k 1k 1k 100 9v 10k 2k 0.25w q13 fzt690 4.7 f 16v 0.22 f 50v s mmbz5240blt1 10v 1 3.3 10k 5v ref iso1 moc207 7 1 43 3 1 4 6 5 14 15 6 5 82 3300pf 4700pf 0.1 f 5v ref 13 sg + 0.1 f 1nf 11 sense 16 bg 18 bstref 19 tg 17 20 bas21 0.1 f 100v l3 1mh blksens mmbz5245lt1 15v v bst 220pf c4 1.5 f 100v c3 1.5 f 100v l1 3.3 h 0.022 f 1000pf sync v fb ovpin margin i comp v dd optodrv v aux 0.1 f 12 i sns 11 i snsgnd 16 fg 2 cg pgnd gnd ltc1698 pwrgd 6 8 9 7 13 1.24k 1% 1.78k 1% 3.01k 1% sense + sense trim 100 0.25w 5 6 4 2 3 1 7 9v v out + 2.43k 1% 10 4 3 v comp 1698 f12 3.01k 1% 3.01k 1% 3.01k 1% 3.01k 1% 100 0.25w 1k 0.33 f mbr0530 0.025 1/2w 3 4 q3 1000pf 100v c1 2200pf 250v 1000pf 100v q5, q14 fds6680a 2 q6, q15, q17 fds6680a 3 murs120t3 murs120t3 10 2 7 v sec 5 1 q1 t1 10 mmbt3906lt1 mmbt3906lt1 v in + v in c2 1.5 f 100v v out + v out s c5 to c8 330 f 10v 4 10 0.25w 10 0.25w 4.7 l2 2.35 h 330pf + mmbt3906lt1 62k 0.25w mmbz5248lt1 18v mmbt3904lt1 47k 4.7 f 5v ref mmbd914lt1 q7 ndt410el 1.5k 0.25w 1.5k 0.25w + lt1006s8 figure 9. 36v-72v dc in to 3.3v/20a isolated synchronous forward converter with fast start and differential sense typical applicatio s u
18 lt1681 1681f current (a) 1 efficiency (%) 80 85 90 36v 48v 72v 5678910 1681 ta04 75 70 60 2 3 4 65 100 95 current (a) 2 70 efficiency (%) 75 85 90 36v 48v 72v 95 6 10 12 20 1681 ta05 80 48 14 16 18 100 typical applicatio s u lt1681/ltc1698 36v-72v v in to 5v/10a module (see figure 7 for application schematic) lt1681/ltc1698 isolated 5v/10a converter efficiency vs load current lt1681/ltc1698 36v-72v v in to 3.3v/20a module (see figure 9 for application schematic) lt1681/ltc1698 isolated 3.3v/20a converter efficiency vs load current
19 lt1681 1681f package descriptio n u information furnished by linear technology corporation is believed to be accurate and reliable. however, no responsibility is assumed for its use. linear technology corporation makes no represen- tation that the interconnection of its circuits as described herein will not infringe on existing patent rights. sw package 20-lead plastic small outline (wide .300 inch) (reference ltc dwg # 05-08-1620) s20 (wide) 1098 note 1 0.496 ?0.512* (12.598 ?13.005) 20 19 18 17 16 15 14 13 1 23 4 5 6 78 0.394 ?0.419 (10.007 ?10.643) 910 11 12 0.037 ?0.045 (0.940 ?1.143) 0.004 ?0.012 (0.102 ?0.305) 0.093 ?0.104 (2.362 ?2.642) 0.050 (1.270) bsc 0.014 ?0.019 (0.356 ?0.482) typ 0 ?8 typ note 1 0.009 ?0.013 (0.229 ?0.330) 0.016 ?0.050 (0.406 ?1.270) 0.291 ?0.299** (7.391 ?7.595) 45 0.010 ?0.029 (0.254 ?0.737) note: 1. pin 1 ident, notch on top and cavities on the bottom of packages are the manufacturing options. the part may be supplied with or without any of the options dimension does not include mold flash. mold flash shall not exceed 0.006" (0.152mm) per side dimension does not include interlead flash. interlead flash shall not exceed 0.010" (0.254mm) per side * **
20 lt1681 1681f ? linear technology corporation 2001 linear technology corporation 1630 mccarthy blvd., milpitas, ca 95035-7417 (408) 432-1900 l fax: (408) 434-0507 l www.linear-tech.com lt/tp 0302 2k ? printed in usa part number description comments lt1158 half-bridge n-channel mosfet driver current limit protection, 100% of duty cycle lt1160 half-bridge n-channel mosfet driver up to 60v input supply, no shoot-through lt1162 dual half-bridge n-channel mosfet driver v in to 60v, good for full-bridge applications lt1336 half-bridge n-channel mosfet driver smooth operation at high duty cycle (95% to 100%) lt1339 high power synchronous dc/dc controller 60v dual n-channel mosfet controller ltc ? 1530 high power step-down switching regulator controller excellent for 5v to 3.x up to 50a ltc1622 550khz step-down controller 8-pin msop; synchronizable; soft-start; current mode ltc1625/ltc1775 no r sense tm current mode synchronous step-down controller 97% efficiency; no sense resistor; 16-pin ssop ltc1628-pg dual, 2-phase synchronous step-down controller power good output; minimum input/output capacitors; 3.5v v in 36v ltc1628-sync dual, 2-phase synchronous step-down controller synchronizable 150khz to 300khz, v in to 36v lt1680 high power dc/dc current mode step-up controller high side current sense, up to 60v input ltc1698 secondary synchronous rectifier controller use with the lt1681, isolated power supplies, contains voltage margining, optocoupler driver, synchronization circuit with the primary side ltc1709-7 high efficiency, 2-phase synchronous step-down controller up to 42a output; 0.925v v out 2v with 5-bit vid ltc1709-8 high efficiency, 2-phase synchronous step-down controller up to 42a output; vrm 8.4; 1.3v v out 3.5v ltc1735 high efficiency, synchronous step-down controller burst mode ? operation; 16-pin narrow ssop; 3.5v v in 36v ltc1736 high efficiency, synchronous step-down controller with 5-bit vid mobile vid; 0.925v v out 2v; 3.5v v in 36v ltc1772 thinsot tm step-down controller current mode; 550khz; very small solution size ltc1773 synchronous step-down controller up to 95% efficiency, 550khz, 2.65v v in 8.5v, 0.8v v out v in , synchronizable to 750khz ltc1778 wide operating range, no r sense step-down controller gn16-pin, 0.8v fb reference ltc1874 dual, step-down controller current mode; 550khz; small 16-pin ssop, v in < 9.8v ltc1876 2-phase, dual synchronous step-down controller with 3.5v v in 36v, power good output, 300khz operation step-up regulator ltc1922-1 synchronous phase modulated full-bridge controller 50w to 2kw power supply design, adaptive direct sense zvs ltc1929 2-phase 42a synchronous controller minimizes c in and c out , 4v v in 36v, 300khz ltc3714 intel compatible, wide operating range, no r sense step-down g28 package, v out = 0.6v to 1.75v 5-bit mobile vid, controller with internal op amp active voltage positioning i mvp2 , v in to 36v ltc3716 high efficiency, 2-phase synchronous step-down controller v out = 0.6v to 1.75v, active voltage positioning i mvp2 , with 5-bit mobile vid v in to 36v related parts no r sense and thinsot are trademarks of linear technology corporation. burst mode is a registered trademark of linear technolgy corporation.

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