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ltc3803-5 1 38035fd typical application description constant frequency current mode flyback dc/dc controller in thinsot the ltc ? 3803-5 is a constant frequency current mode ? yback controller optimized for driving n-channel mosfets in high input voltage applications. the ltc3803-5 operates from inputs as low as 5v. constant frequency operation is maintained down to very light loads, resulting in less low frequency noise generation over a wide range of load currents. slope compensation can be programmed with an external resistor. the ltc3803-5 provides 1.5% output voltage accuracy and consumes only 240a of quiescent current. ground- referenced current sensing allows ltc3803-5-based con- verters to accept input supplies beyond the ltc3803-5s absolute maximum v cc . for simplicity, the ltc3803-5 can be powered from a high v in through a resistor, due to its internal shunt regulator. an internal undervoltage lockout shuts down the ic when the input voltage is too low to provide suf? cient gate drive to the external mosfet. the ltc3803-5 is available in a low pro? le (1mm) 6-lead sot-23 (thinsot?) package. dual output wide input range converter features applications n v in and v out limited only by external components n 4.8v undervoltage lockout threshold n operating junction temperature from C55c to 150c n adjustable slope compensation n internal soft-start n constant frequency 200khz operation n 1.5% reference accuracy n current mode operation for excellent line and load transient response n no minimum load requirement n low quiescent current: 240a n low pro? le (1mm) sot-23 package n 42v and 12v automotive power supplies n telecom power supplies n auxiliary/housekeeping power supplies n power over ethernet ef? ciency and power loss vs output power i th /run gnd v fb ltc3803-5 ngate v cc sense 10mq100n b3100 1f 100v 3 13v/0.3a 20ma min load 6.5v/1.2a 38035 ta01 0.012 0.1f 22k 7.5k 57.6k 8.06k 4.7k 10nf 1f 100v 1f 100v phm25nq10t all capacitors are x7r, tdk v in 6v to 50v mmbta42 22f 10v 47f 10v pdz6.8b vph5-0155 output power (w) 02 efficiency (%) power loss (w) 90 85 80 75 70 65 60 3.0 2.5 2.0 1.5 1.0 0.5 0 4 6810 38035 ta01b 12 v in = 8v v in = 12v v in = 12v v in = 24v v in = 48v l , lt, ltc, ltm, burst mode, linear technology and the linear logo are registered trademarks and thinsot and no r sense are trademarks of linear technology corporation. all other trademarks are the property of their respective owners.
ltc3803-5 2 38035fd pin configuration absolute maximum ratings v cc to gnd (current fed) ..................... 25ma into v cc * ngate voltage .......................................... C 0.3v to v cc v fb , i th /run voltages............................... C0.3v to 3.5v sense voltage ............................................ C0.3v to 1v ngate peak output current (<10s) ......................... 1a operating junction temperature range (notes 2, 3) ltc3803e-5 .......................................C 40c to 125c ltc3803i-5 ........................................C 40c to 125c ltc3803h-5 (note 3) ......................... C40c to 150c ltc3803mp-5 (note 3) ...................... C55c to 150c storage temperature range ...................C 65c to 150c lead temperature (soldering, 10 sec) .................. 300c (note 1) order information i th /run 1 gnd 2 v fb 3 6 ngate 5 v cc 4 sense top view s6 package 6-lead plastic tsot-23 t jmax = 150c, ja = 192c/w lead free finish tape and reel part marking* package description temperature range ltc3803es6-5#pbf ltc3803es6-5#trpbf ltbpf 6-lead low pro? le (1mm) sot-23 C40c to 125c ltc3803is6-5#pbf ltc3803is6-5#trpbf ltbmh 6-lead low pro? le (1mm) sot-23 C40c to 125c ltc3803hs6-5#pbf ltc3803hs6-5#trpbf ltbmh 6-lead low pro? le (1mm) sot-23 C40c to 150c ltc3803mps6-5#pbf ltc3803mps6-5#trpbf ltbmh 6-lead low pro? le (1mm) sot-23 C55c to 150c lead based finish tape and reel part marking* package description temperature range ltc3803es6-5 ltc3803es6-5#tr ltbpf 6-lead low pro? le (1mm) sot-23 C40c to 125c ltc3803is6-5 ltc3803is6-5#tr ltbmh 6-lead low pro? le (1mm) sot-23 C40c to 125c ltc3803hs6-5 ltc3803hs6-5#tr ltbmh 6-lead low pro? le (1mm) sot-23 C40c to 150c ltc3803mps6-5 ltc3803mps6-5#tr ltbmh 6-lead low pro? le (1mm) sot-23 C55c to 150c consult ltc marketing for parts speci? ed with wider operating temperature ranges. *the temperature grade is identi? ed 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 speci? cations, go to: http://www.linear.com/tapeandreel/ electrical characteristics the l denotes the speci? cations which apply over the full operating junction temperature range, otherwise speci? cations are at t a = 25c. v cc = 5v, unless otherwise noted. (notes 2, 3) symbol parameter conditions min typ max units v turnon v cc turn on voltage ltc3803e-5 ltc3803i-5, ltc3803h-5, ltc3803mp-5 l l 4 3.9 4.8 4.8 5.7 5.7 v v v turnoff v cc turn off voltage ltc3803e-5 ltc3803i-5, ltc3803h-5, ltc3803mp-5 l l 3.3 3.2 4 4 4.9 4.9 v v v hyst v cc hysteresis v turnon C v turnoff l 0.05 0.8 v v clamp1ma v cc shunt regulator voltage at 1ma i cc = 1ma, v ith/run = 0v ltc3803e-5 ltc3803i-5, ltc3803h-5 ltc3803mp-5 l l l 6.2 6.2 5.9 8 8 8 9.9 10.4 10.4 v v v *ltc3803-5 internal clamp circuit self regulates v cc voltage to 8.1v.
ltc3803-5 3 38035fd electrical characteristics the l denotes the speci? cations which apply over the full operating junction temperature range, otherwise speci? cations are at t a = 25c. v cc = 5v, unless otherwise noted. (notes 2, 3) symbol parameter conditions min typ max units v clamp25ma v cc shunt regulator voltage at 25ma i cc = 25ma, v ith/run = 0v ltc3803e-5 ltc3803i-5, ltc3803h-5 ltc3803mp-5 l l l 6.3 6.3 6 8.1 8.1 8.1 10.3 10.7 10.7 v v v i cc input dc supply current in normal operation (note 4) v ith/run = 1.3v 240 350 a i cc,(uv) input dc supply current in undervoltage v cc = v turnon C 100mv ltc3803e-5 ltc3803i-5, ltc3803h-5, ltc3803mp-5 l l 40 40 90 100 a a v ithshdn shutdown threshold (at i th /run) v cc > v turnon , v ith/run falling ltc3803e-5 ltc3803i-5, ltc3803h-5 ltc3803mp-5 l l l 0.12 0.08 0.08 0.28 0.28 0.28 0.45 0.45 0.47 v v v i ithstart start-up current source v ith/run = 0v ltc3803e-5 ltc3803i-5, ltc3803h-5, ltc3803mp-5 l l 0.07 0.07 0.34 0.34 0.8 1 a a v fb regulated feedback voltage (note 5) ltc3803e-5: 0c t j 85c C40c t j 85c ltc3803i-5: 0c t j 85c C40c t j 125c ltc3803h-5: 0c t j 85c C40c t j 150c ltc3803mp-5: 0c t j 85c C55c t j 150c l l l l 0.788 0.780 0.788 0.780 0.788 0.780 0.788 0.780 0.800 0.800 0.800 0.800 0.800 0.800 0.800 0.800 0.812 0.816 0.812 0.820 0.812 0.820 0.812 0.820 v v v v v v v v g m error ampli? er transconductance i th/run pin load = 5a (note 5) 200 333 500 a/v v o(line) output voltage line regulation (note 5) 0.1 mv/v v o(load) output voltage load regulation i th/run sinking 5a (note 5) i th/run sourcing 5a (note 5) 3 3 mv/a mv/a i fb v fb input current (note 5) 10 50 na f osc oscillator frequency v ith/run = 1.3v 170 200 230 khz dc on(min) minimum switch on duty cycle v ith/run = 1.3v, v fb = 0.8v 6.5 8.5 % dc on(max) maximum switch on duty cycle v ith/run = 1.3v, v fb = 0.8v 70 80 90 % t rise gate drive rise time c load = 3000pf 40 ns t fall gate drive fall time c load = 3000pf (note 7) 40 ns v imax peak current sense voltage r sl = 0 (note 6) ltc3803e-5 ltc3803i-5, ltc3803h-5 ltc3803mp-5 l l l 90 85 85 100 100 100 115 115 120 mv mv mv i slmax peak slope compensation output current (note 7) 5 a t sfst soft-start time 0.7 ms
ltc3803-5 4 38035fd typical performance characteristics temperature (c) C60 780 785 790 795 800 805 810 815 820 v fb voltage (mv) 60 90 38035 g01 0 C30 30 150 120 v cc = 5v v cc supply voltage (v) 4.0 v fb voltage (mv) 792 796 812 800 5.0 6.0 6.5 38035 g02 788 804 808 4.5 5.5 7.0 7.5 t a = 25c v cc v clamp1ma i cc (ma) 0 v fb voltage (mv) 792 796 800 15 25 38035 g03 788 510 20 804 808 812 t a = 25c temperature (c) C60 180 190 200 210 220 230 240 oscillator frequency (khz) 0 30 150 120 38035 g04 C30 60 90 v cc = 5v v cc supply voltage (v) 4.0 oscillator frequency (khz) 180 190 200 4.5 5.0 5.5 6.0 38035 g05 6.5 7.0 210 220 185 195 205 215 7.5 t a = 25c i cc (ma) 0 oscillator frequency (khz) 180 190 200 5 10 15 20 38035 g06 210 220 185 195 205 215 25 t a = 25c reference voltage vs v cc shunt regulator current reference voltage vs supply voltage reference voltage vs temperature oscillator frequency vs v cc shunt regulator current oscillator frequency vs supply voltage oscillator frequency vs temperature electrical characteristics 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 ltc3803-5 is tested under pulsed load conditions such that t j t a . the ltc3803e-5 is guaranteed to meet speci? cations from 0c to 85c junction temperature. speci? cations over the C40c to 125c operating junction temperature range are assured by design, characterization and correlation with statistical process controls. the ltc3803i-5 is guaranteed over the C40c to 125c operating junction temperature range, the ltc3803h-5 is guaranteed over the C40c to 150c operating junction temperature range and the ltc3803mp-5 is tested and guaranteed over the full C55c to 150c operating junction temperature range. note that the maximum ambient temperature consistent with these speci? cations is determined by speci? c operating conditions in conjunction with board layout, the rated package thermal impedance and other environmental factors. junction temperature (t j ) is calculated from the ambient temperature t a and the power dissipation p d in the ltc3803-5 using the formula: t j = t a + (p d ? 230c/w) note 3: high junction temperatures degrade operating lifetimes; operating lifetime is derated for junction temperatures greater than 125c. note 4: dynamic supply current is higher due to the gate charge being delivered at the switching frequency. note 5: the ltc3803-5 is tested in a feedback loop that servos v fb to the output of the error ampli? er while maintaining i th /run at the midpoint of the current limit range. note 6: peak current sense voltage is reduced dependent on duty cycle and an optional external resistor in series with the sense pin (r sl ). for details, refer to the programmable slope compensation feature in the applications information section. note 7: guaranteed by design.
ltc3803-5 5 38035fd typical performance characteristics temperature (c) C60 5.0 volts 5.5 4.5 3.5 6.0 0 30 60 38035 g07 3.0 4.0 C30 90 150 120 v turnon v turnoff temperature (c) C60 v cc (v) 7.0 7.5 8.0 10.5 9.0 0 30 38035 g08 9.5 10.0 8.5 C30 60 90 150 120 i cc = 25ma i cc = 1ma temperature (c) C60 supply current (a) 200 300 240 0 30 60 38035 g09 260 280 220 C30 90 120 150 v cc = 5v v ith/run = 1.3v temperature (c) C60 0 start-up supply current (a) 10 20 30 40 0 30 120 150 38035 g10 50 60 70 C30 60 90 v cc = v turnon C 0.1v temperature (c) C60 shutdown threshold (mv) 300 350 400 60 90 250 200 0 C30 30 150 120 150 100 50 0 450 500 38035 g11 temperature (c) C60 0 i th /run pin current source (na) 100 200 300 400 0 30 120 150 38035 g12 500 600 700 800 900 1000 C30 60 90 v cc = v turnon + 0.1v v ith/run = 0v temperature (c) C60 sense pin voltage (mv) 100 110 150 120 38035 g13 90 80 0 30 C30 60 90 120 95 105 85 115 v cc = 5v temperature (c) C60 soft-start time (ms) 0.6 1.0 150 38035 g14 0.2 0 0 30 C30 60 120 90 1.4 0.4 0.8 1.2 v cc = 5v i cc supply current vs temperature v cc shunt regulator voltage vs temperature v cc undervoltage lockout thresholds vs temperature i th /run shutdown threshold vs temperature start-up i cc supply current vs temperature i th /run start-up current source vs temperature soft-start time vs temperature peak current sense voltage vs temperature
ltc3803-5 6 38035fd block diagram pin functions i th /run (pin 1): this pin performs two functions. it serves as the error ampli? er compensation point as well as the run/shutdown control input. nominal voltage range is 0.7v to 1.9v. forcing this pin below the shutdown threshold (v ithshdn ) causes the ltc3803-5 to shut down. in shutdown mode, the ngate pin is held low. gnd (pin 2): ground pin. v fb (pin 3): receives the feedback voltage from an external resistive divider across the output. sense (pin 4): this pin performs two functions. it monitors switch current by reading the voltage across an external current sense resistor to ground. it also injects a current ramp that develops slope compensation voltage across an optional external programming resistor. v cc (pin 5): supply pin. must be closely decoupled to gnd (pin 2). ngate (pin 6): gate drive for the external n-channel mosfet. this pin swings from 0v to v cc . C + C + slope comp current ramp v cc gate driver ngate 4 sense 38035 bd 200khz oscillator undervoltage lockout q r current comparator shutdown comparator shutdown s 20mv i th /run error amplifier v fb soft- start clamp v cc shunt regulator switching logic and blanking circuit 6 0.28v v cc < v turnon 0.3a v cc C + 5 3 gnd 1.2v 2 1 800mv reference
ltc3803-5 7 38035fd operation the ltc3803-5 is a constant frequency current mode controller for ? yback, sepic and dc/dc boost converter applications in a tiny thinsot package. the ltc3803-5 is designed so that none of its pins need to come in contact with the input or output voltages of the power supply circuit of which it is a part, allowing the conversion of voltages well beyond the ltc3803-5s absolute maximum ratings. main control loop due to space limitations, the basics of current mode dc/dc conversion will not be discussed here; instead, the reader is referred to the detailed treatment in application note 19, or in texts such as abraham pressmans switching power supply design. please refer to the block diagram and the typical ap- plication on the front page of this data sheet. an external resistive voltage divider presents a fraction of the output voltage to the v fb pin. the divider must be designed so that when the output is at the desired voltage, the v fb pin voltage will equal the 800mv from the internal reference. if the load current increases, the output voltage will de- crease slightly, causing the v fb pin voltage to fall below 800mv. the error ampli? er responds by feeding current into the i th /run pin. if the load current decreases, the v fb voltage will rise above 800mv and the error ampli? er will sink current away from the i th /run pin. the voltage at the i th /run pin commands the pulse-width modulator formed by the oscillator, current comparator and rs latch. speci? cally, the voltage at the i th /run pin sets the current comparators trip threshold. the current comparator monitors the voltage across a current sense resistor in series with the source terminal of the external mosfet. the ltc3803-5 turns on the external power mosfet when the internal free-running 200khz oscillator sets the rs latch. it turns off the mosfet when the cur- rent comparator resets the latch or when 80% duty cycle is reached, whichever happens ? rst. in this way, the peak current levels through the ? yback transformers primary and secondary are controlled by the i th /run voltage. since the i th /run voltage is increased by the error ampli- ? er whenever the output voltage is below nominal, and decreased whenever output voltage exceeds nominal, the voltage regulation loop is closed. for example, whenever the load current increases, output voltage will decrease slightly, and sensing this, the error ampli? er raises the i th /run voltage by sourcing current into the i th /run pin, raising the current comparator threshold, thus increasing the peak currents through the transformer primary and secondary. this delivers more current to the load, bringing the output voltage back up. the i th /run pin serves as the compensation point for the control loop. typically, an external series rc network is connected from i th /run to ground and is chosen for optimal response to load and line transients. the impedance of this rc network converts the output current of the error ampli? er to the i th /run voltage which sets the current comparator threshold and commands considerable in? u- ence over the dynamics of the voltage regulation loop. start-up/shutdown the ltc3803-5 has two shutdown mechanisms to disable and enable operation: an undervoltage lockout on the v cc supply pin voltage, and a forced shutdown whenever ex- ternal circuitry drives the i th /run pin low. the ltc3803-5 transitions into and out of shutdown according to the state diagram (figure 1). figure 1. start-up/shutdown state diagram ltc3803-5 shut down v ith/run < v ithshdn (nominally 0.28v) 38035 f01 v cc < v turnoff (nominally 4v) v ith/run > v ithshdn and v cc > v turnon (nominally 4.8v) ltc3803-5 enabled
ltc3803-5 8 38035fd operation the undervoltage lockout (uvlo) mechanism prevents the ltc3803-5 from trying to drive a mosfet with in- suf? cient v gs . the voltage at the v cc pin must exceed v turnon (nominally 4.8v) at least momentarily to enable ltc3803-5 operation. the v cc voltage is then allowed to fall to v turnoff (nominally 4v) before undervoltage lockout disables the ltc3803-5. the i th /run pin can be driven below v ithshdn (nominally 0.28v) to force the ltc3803-5 into shutdown. an internal 0.3a current source always tries to pull this pin towards v cc . when the i th /run pin voltage is allowed to exceed v ithshdn , and v cc exceeds v turnon , the ltc3803-5 begins to operate and an internal clamp immediately pulls the i th /run pin up to about 0.7v. in operation, the i th /run pin voltage will vary from roughly 0.7v to 1.9v to represent current comparator thresholds from zero to maximum. internal soft-start an internal soft-start feature is enabled whenever the ltc3803-5 comes out of shutdown. speci? cally, the i th / run voltage is clamped and is prevented from reaching maximum until roughly 0.7ms has passed. this allows the input and output currents of ltc3803-5-based power supplies to rise in a smooth and controlled manner on start-up. powering the ltc3803-5 in the simplest case, the ltc3803-5 can be powered from a high voltage supply through a resistor. a built-in shunt regulator from the v cc pin to gnd will draw as much current as needed through this resistor to regulate the v cc voltage to around 8.1v as long as the v cc pin is not forced to sink more than 25ma. this shunt regulator is always active, even when the ltc3803-5 is in shutdown, since it serves the vital function of protecting the v cc pin from seeing too much voltage. the v cc pin must be bypassed to ground immediately ad- jacent to the ic pins with a ceramic or tantalum capacitor. proper supply bypassing is necessary to supply the high transient currents required by the mosfet gate driver. 10f is a good starting point. adjustable slope compensation the ltc3803-5 injects a 5a peak current ramp out through its sense pin which can be used for slope compensation in designs that require it. this current ramp is approximately linear and begins at zero current at 6.5% duty cycle, reach- ing peak current at 80% duty cycle. additional details are provided in the applications information section.
ltc3803-5 9 38035fd applications information many ltc3803-5 application circuits can be derived from the topology shown in figure 2. the ltc3803-5 itself imposes no limits on allowed power output, input voltage v in or desired regulated output voltage v out ; these are all determined by the ratings on the external power components. the key factors are: q1s maximum drain-source voltage (bv dss ), on-resistance (r ds(on) ) and maximum drain current, t1s saturation ? ux level and winding insulation breakdown voltages, c in and c out s maximum working voltage, esr, and maximum ripple current ratings, and d1 and r sense s power ratings. choose resistance values for r1 and r2 to be as large as possible in order to minimize any ef? ciency loss due to the static current drawn from v out , but just small enough so that when v out is in regulation, the error caused by the nonzero input current to the v fb pin is less than 1%. a good rule of thumb is to choose r1 to be 80k or less. transformer design considerations transformer speci? cation and design is perhaps the most critical part of applying the ltc3803-5 successfully. in addition to the usual list of caveats dealing with high fre- quency power transformer design, the following should prove useful. turns ratios due to the use of the external feedback resistor divider ratio to set output voltage, the user has relative freedom in selecting transformer turns ratio to suit a given appli- cation. simple ratios of small integers, e.g., 1:1, 2:1, 3:2, etc. can be employed which yield more freedom in setting total turns and mutual inductance. simple integer turns ratios also facilitate the use of off-the-shelf con? gu- rable transformers such as the coiltronics versa-pac? series in applications with high input to output voltage ratios. for example, if a 6-winding versa-pac is used with three windings in series on the primary and three windings in parallel on the secondary, a 3:1 turns ratio will be achieved. turns ratio can be chosen on the basis of desired duty cycle. however, remember that the input supply voltage plus the secondary-to-primary referred version of the ? yback pulse (including leakage spike) must not exceed the allowed external mosfet breakdown rating. figure 2. typical ltc3803-5 application circuit selecting feedback resistor divider values the regulated output voltage is determined by the resistor divider across v out (r1 and r2 in figure 2). the ratio of r2 to r1 needed to produce a desired v out can be calculated: r2 = v out ? 0.8v 0.8v ?r1 v cc i th /run ltc3803-5 gnd ngate sense v fb 5 6 4 1 2 3 d1 ? ? c out c in l sec l pri c vcc c c v out 38035 f02 r sense r sl r1 r vcc r2 q1 t1 v in
ltc3803-5 10 38035fd leakage inductance transformer leakage inductance (on either the primary or secondary) causes a voltage spike to occur after the output switch (q1) turn-off. this is increasingly prominent at higher load currents, where more stored energy must be dissipated. in some cases a snubber circuit will be required to avoid overvoltage breakdown at the mosfets drain node. application note 19 is a good reference on snubber design. a bi? lar or similar winding technique is a good way to minimize troublesome leakage inductances. however, remember that this will limit the primary-to-secondary breakdown voltage, so bi? lar winding is not always practical. current sense resistor considerations the external current sense resistor (r sense in figure 2) allows the user to optimize the current limit behavior for the particular application. as the current sense resistor is varied from several ohms down to tens of milliohms, peak switch current goes from a fraction of an ampere to several amperes. care must be taken to ensure proper circuit operation, especially with small current sense resistor values. for example, a peak switch current of 5a requires a sense resistor of 0.020. note that the instantaneous peak power in the sense resistor is 0.5w and it must be rated accord- ingly. the ltc3803-5 has only a single sense line to this resistor. therefore, any parasitic resistance in the ground side connection of the sense resistor will increase its ap- parent value. in the case of a 0.020 sense resistor, one milliohm of parasitic resistance will cause a 5% reduction in peak switch current. so the resistance of printed circuit copper traces and vias cannot necessarily be ignored. programmable slope compensation the ltc3803-5 injects a ramping current through its sense pin into an external slope compensation resistor (r sl in figure 2). this current ramp starts at zero right after the ngate pin has been high for the ltc3803-5s minimum duty cycle of 6.5%. the current rises linearly towards a peak of 5a at the maximum duty cycle of 80%, shutting off once the ngate pin goes low. a series resistor (r sl ) connecting the sense pin to the current sense resistor (r sense ) thus develops a ramping voltage drop. from the perspective of the sense pin, this ramping voltage adds to the voltage across the sense resistor, effectively reducing the current comparator threshold in proportion applications information
ltc3803-5 11 38035fd applications information to duty cycle. this stabilizes the control loop against subharmonic oscillation. the amount of reduction in the current comparator threshold (v sense ) can be calculated using the following equation: v sense = duty cycle ? 6.5% 73.5% ?5 a?r sl note: ltc3803-5 enforces 6.5% < duty cycle < 80%. a good starting value for r sl is 5.9k, which gives a 30mv drop in current comparator threshold at 80% duty cycle. designs not needing slope compensation may replace r sl with a short circuit. v cc shunt regulator an internal shunt regulator allows the ltc3803-5 to be powered through a single dropping resistor from v in to v cc , in conjunction with a bypass capacitor, c vcc , that closely decouples v cc to gnd (see figure 3). the shunt regulator can draw up to 25ma through the v cc pin to gnd to drop enough voltage across r vcc to regulate v cc to around 8.1v. for applications where v in is low enough such that the static power dissipation in r vcc is acceptable, using the v cc shunt regulator is the simplest way to power the ltc3803-5. external preregulator the circuit in figure 4 shows another way to power the ltc3803-5. an external series preregulator consisting of series pass transistor q1, zener diode d1, and bias resistor r b brings v cc above the v cc turn-on threshold, enabling the ltc3803-5. figure 3. powering the ltc3803-5 via the internal shunt regulator figure 4. powering the ltc3803-5 with an external preregulator ltc3803-5 v cc r vcc c vcc 38035 f03 v in gnd ltc3803-5 v cc r b 100k d1 6.8v q1 mmbta42 c vcc 0.1f 38035 f04 8v to 75 v in gnd
ltc3803-5 12 38035fd typical applications 2w isolated housekeeping telecom converter i th /run ltc3803-5 gnd ngate v cc v fb sense 6 5 1 2 4 3 primary ground 2.2f 1f 2.2f 1f 38035 ta03 0.1 22k 9.09k 1nf primary side 10v, 100ma output secondary side 10v, 100ma output v in 36v to 75v 5.6k fdc2512 t1: pulse engineering pa0648 or tyco tti8698 1k 130 220k mmbta42 pdz6.8b bas516 t1 secondary side ground bas516 ? ? ? 787 bas516
ltc3803-5 13 38035fd typical applications 4:1 input range 3.3v output isolated flyback dc/dc converter ef? ciency vs load current 38035 ta05 fdc2512 t1 pa1277nl ? ? ? 0.1f 0.1f 100f 6.3v 3 0.33f 2.2nf 47pf 100k 56k 22.1k v in + v in C v cc v cc v out + v out + 3.3v 3a gnd v out + mmbta42 pdz6.8b 0.1f 2.2f 4.7k 680 22 6.8k 270 1 1 2 3 6 5 4 2 bas516 bas516 pds1040 10 bas516 ps2801-1 130 68 150pf 220k 0.040 1 2 3 6 5 4 18 v to 72v i th /run ltc3803-5 gnd gate sense v cc v fb gnd bas516 bat760 lt4430 oc v in comp fb opto i out (a) 0 70 efficiency (%) 72 74 76 78 80 82 84 1234 38035 ta05a v in = 48v v in = 24v
ltc3803-5 14 38035fd package description 1.50 ? 1.75 (note 4) 2.80 bsc 0.30 ? 0.45 6 plcs (note 3) datum ?a? 0.09 ? 0.20 (note 3) s6 tsot-23 0302 rev b 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 pin one id 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 s6 package 6-lead plastic tsot-23 (reference ltc dwg # 05-08-1636)
ltc3803-5 15 38035fd revision history rev date description page number d 6/10 mp-grade part added. re? ected throughout the data sheet. 1 to 16 (revision history begins at rev d) 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.
ltc3803-5 16 38035fd linear technology corporation 1630 mccarthy blvd., milpitas, ca 95035-7417 (408) 432-1900 fax: (408) 434-0507 www.linear.com ? linear technology corporation 2004 lt 0610 rev d ? printed in usa typical application synchronous flyback converter related parts part number description comments lt3573 isolated flyback switching regulator with 60v integrated switch 3v v in 40v, no opto-isolator or third winding required, up to 7w output power, msop-16e ltc3805/ ltc3805-5 adjustable constant frequency flyback, boost, sepic dc/dc controller v in and v out limited only by external components, 3mm 3mm dfn-10, msop-10e packages ltc3873/ ltc3873-5 no r sense ? constant frequency flyback, boost, sepic controller v in and v out limited only by external components, 8-pin thinsot or 2mm 3mm dfn-8 packages lt3757 boost, flyback, sepic and inverting controller 2.9v v in 40v, 100khz to 1mhz programmable operating frequency, 3mm 3mm dfn-10 and msop-10e package lt3758 boost, flyback, sepic and inverting controller 5.5v v in 100v, 100khz to 1mhz programmable operating frequency, 3mm 3mm dfn-10 and msop-10e ltc1871/ltc1871-1/ ltc1871-7 wide input range, no r sense low quiescent current flyback, boost and sepic controller programmable operating frequency, 2.5v v in 36v, burst mode ? operation at light load, msop-10 i th /run ltc3803-5 gnd gate sense v out * 3.3v 1.5a 38035 ta04 d1 c in 33k q1 q2 t1: pulse engineering pa1006 q1: fairchild fdc2512 q2: vishay si9803 t1 ? ? 0.1f ? 1n 8.06k 25.5k* r fb v out v cc v in 36v to 72v v fb 1f 10v 560 5k c o r cs 1 2 3 6 5 4 d1: philips bas516 c in : tdk 1f, 100v, x5r c o : tdk 100f, 6.3v, x5r r cs : vishay or irc, 80m *for 5v output change r fb to 42.2k 130 220k mmbta42 pdz6.8b

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