xian l ongtium m icroelectronics technology developing co., ltd www.longtiumic.com 1 / 10 general description LT1548 is a high performance offline pwm power switch for low power ac/dc charger and adaptor applications. it operates in primary - side sensing and regulation. consequently, opto - coupler and tl431 could be eliminated. proprietar y constant voltage (cv) and constant current (cc) control is integrated as shown in the figure below. in cc control, the current and output power setting can be adjusted externally by the sense resistor rs at cs pin. in cv control, multi - mode operations ar e utilized to achieve high performance and high efficiency. in addition, good load regulation is achieved by the built - in cable drop compensation. device operates in pfm in cc mode as well at large load condition and it operates in pwm with frequency reduc tion at light/medium load. LT1548 offers power on soft start control and protection coverage with auto - recovery features including cycle - by - cycle current limiting, vdd ovp, vdd clamp and uvlo. excellent emi performance is achieved with on - bright proprietar y frequency shuffling technique. high precision constant voltage (cv) and constant current (cc) can be achieved by LT1548 . features ? 5% constant voltage regulation, 5% constant current regulation at universal ac input ? primary - side sensing and regulation without tl431 and opto - coupler ? programmable cv and cc regulation ? adjustable constant current and output power setting ? built - in secondary constant current control with primary side feedback ? built - in adaptive current peak regulation ? built - in primary winding inductance compensation ? programmable cable drop compensation ? power on soft - start ? built - in leading edge blanking (leb) ? cycle - by - cycle current limiting ? vdd under voltage lockout with hysteresis (uvlo) ? vdd ovp , vdd clamp ? o f fered in dip8 package appllications l ow power ac/dc offline smps for : ? cell phone charger digital cameras charger ? small power adaptor ? auxiliary power for pc, tv etc. ? linear regulator/rcc replacement LT1548 high pre c i s ion cc/cv primar y - side pwm po w er s w itch ? led ? 2
LT1548 xian l ongtium m icroelectronics technology developing co., ltd w ww.longtiumic.com 2 / 10 typical application general information pin configuration terminal assignme nts pin num pin name i/o description 1 vdd p p ower supply 2 comp i l oop compensation for cv stability 3 gnd p ground 4 inv i t he voltage feedback from auxiliary winding. connected to resisto r driver from auxiliary winding reflecting output voltage. pwm duty cycle is determined by ea output and current sense signal at pin 4. 5 cs i c urrent sense input 6 nc no connect 7/8 drain o hv mosfet drain pin. t he drain pin is connected to the primary lead of the transformer v d d 1 c o m p 2 3 g n d 4 i n v d r a i n d r a i n c s 8 7 6 5 a c ~ r s n s n p n a u x v o . . . n c 7 1 2 3 4 5 6 8 o v d d c o m p g n d i n v c s d r a i n d r a i n n c
LT1548 xian l ongtium m icroelectronics technology developing co., ltd w ww.longtiumic.com 3 / 10 absolute maximum ratings paramet e r value drain voltage (off state) - 0.3v to bvdss vdd voltage - 0.3v to vdd clamp vdd zener cl a m p contin u ous current 1 0 m a comp voltage - 0.3 to 7v cs input vol t age - 0.3 to 7v inv input voltage - 0.3 to 7v min/max o p erating junction temperature t j - 20 t o 150 o c min/max storage t e m perature t s t g - 55 to 150 o c lead t e m pe r ature (soldering, 10secs) 260 o c note: stresses b e y o nd those listed under ab s olute maximum ratings m a y c a use perman e nt damage to the d evice. these a r e stress ratings onl y , functional o p eration of the d e vice at these or a n y oth e r conditions b e y ond tho s e indicated under recommen d ed operating cond i tions is not i m plied. exposure to absolute maximum - rated conditions for e x tended periods m a y affect device reliabilit y output power table prod uct 230vac15% 85 - 264vac adapter adapter LT1548 1 5 w 12 w package dissipation rating device mosfet rdson(ohm) package r ja ( /w) max pout(w)(85~264v) frequency LT1548 5 dip8 90 12 72khz n ote: drain pin connected 1 0 0m m 2 pcb copper clad. order ing information part numb e r des c ription LT1548 a p dip 8, pb - free
LT1548 xian l ongtium m icroelectronics technology developing co., ltd w ww.longtiumic.com 4 / 10 block diagram 1 u v l o 4 5 v c c c o n t r o l l e r d r o p c o m p s a m p l e r s a m p l i n g c o n t r o l l e r + e a p w m g e n e r a t o r c l k g e n e r a t o r f r e q u e n c y s h u f f l i n g 2 g a t e d r i v e r 7 / 8 o c p s o f t s t a r t l e b 5 3 p o r 2 v i n v c o m p c s g n d p o w e r m o s f e t v d d d r a i n
LT1548 xian l ongtium m icroelectronics technology developing co., ltd w ww.longtiumic.com 5 / 10 elec t r ical characteristics ( t a = 25 o c, v d d =v d d g = 1 6v , if not o t he r w i se not e d) note: freq max indicates ic internal maximum clock frequency. in system application, the maximum operation frequency of 60k h z nominal occurs at maximum output power or the transition point from cv to cc. symbol parameter test conditions min typ max unit supply voltage (vdd) section i dd st sta n d b y c u rre n t vdd= 13 v 5 20 ua i d d op operat i o n cu r rent operat i o n supply curr e nt inv= 2 v, cs = 0 v, vdd= v dd g = 20 v - 2.5 3.5 ma uvlo ( on) vdd un d er volta g e loc k o u t e n ter vdd falling 7.5 8.5 10 v uvlo ( of f ) vdd un d er volta g e loc k o u t e x it vdd rsi n g 13 . 5 14 . 5 16 . 0 v v dd _cla m p ma x i m u m v d d op e rat i on vo l t a ge i dd =1 0 m a 30 . 5 32 . 5 34 current sense input section tleb leb t i m e 540 ns vth _ oc o v er curr e nt t h resh o ld 8 7 0 9 0 0 9 3 0 m v td _ o c o cp propag at ion d e lay 1 5 0 ns z sense _in inp u t i m pe d a nce 50 k oh m t_ss soft start ti m e 10 m s frequency section freq_m a x ic ma x i m u m frequ e ncy 66 72 78 khz freq_nom s y st e m n o m i nal swit c h frequ e ncy 60 khz freq_star t up i nv =0 v , c om p =5v 14 kh z err or ampli f ier s ection v ref_ea reference v ol t a g e for ea 1. 9 7 2 2. 0 3 v gain dc ga i n of e a 60 db i_comp_m a x max. c a b l e c o m p ensa t i o n current inv=2v, c om p =0v 42 ua power mo s fet sect i o n bvdss m o sfet d rain - sour c e breakdown vol t age 6 0 0 v rds o n o n resi s t a n ce stati c , i d =0 .4 a 5 ?
LT1548 xian l ongtium m icroelectronics technology developing co., ltd w ww.longtiumic.com 6 / 10 characterizat i on plots u v l o ( o n ) ( v ) v s t e m p e r a t u r e ( c ) t e m p e r a t u r e ( c ) u v l o ( o n ) ( v ) - 4 0 - 1 0 2 0 5 0 8 0 1 1 0 7 . 5 0 8 . 0 0 8 . 5 0 9 . 0 0 9 . 5 0 1 0 . 0 0 - 4 0 - 1 0 2 0 5 0 8 0 1 1 0 1 3 . 0 0 1 3 . 5 0 1 4 . 0 0 1 4 . 5 0 1 5 . 0 0 1 5 . 5 0 1 6 . 0 0 u v l o ( o f f ) ( v ) v s t e m p e r a t u r e ( c ) t e m p e r a t u r e ( c ) u v l o ( o f f ) ( v ) - 4 0 - 1 0 2 0 5 0 8 0 1 1 0 1 . 0 1 . 5 2 . 0 2 . 5 3 . 0 i s t a r t u p ( u a ) v s t e m p e r a t u r e ( c ) t e m p e r a t u r e ( c ) i s t a r t u p ( u a ) - 4 0 - 1 0 2 0 5 0 8 0 1 1 0 5 5 6 0 6 5 7 0 7 5 t e m p e r a t u r e ( c ) f r e q _ m a x ( k h z ) v s t e m p e r a t u r e ( c ) f r e q _ m a x ( k h z ) t e m p e r a t u r e ( c ) 2 5 5 0 7 5 1 0 0 1 2 5 1 5 0 0 . 0 0 6 . 0 0 1 2 . 0 0 1 8 . 0 0 2 4 . 0 0 3 0 . 0 0 r d s o n ( o h m ) v s t e m p e r a t u r e ( c ) r d s o n ( o h m ) 0 1 2 3 4 0 5 1 0 1 5 2 0 2 5 3 0 3 5 4 0 4 5 5 0 v c o m p ( v ) i _ c a b l e _ c o m p e n s a t i o n ( u a ) v s v c o m p ( v ) i _ c a b l e _ c o m p e n s a t i o n ( u a )
LT1548 xian l ongtium m icroelectronics technology developing co., ltd w ww.longtiumic.com 7 / 10 operation d escription LT1548 is a cost effective pwm power switch optimized for off - line low power ac/dc applications including battery chargers and adaptors. it operates in primary side sensing and regulation, thus opto - coupler and tl431 are not required. proprietary built - in cv and cc control can achieve high precision cc/cv control mee ting most adaptor and charger application requirements. ? startup current and start up control startup current of LT1548 is designed to be very low so that vdd could be charged up above uvlo threshold and starts up quickly. a large value startup resistor can therefore be used to minimize the power loss in application. ? operating current the operating current of LT1548 is as low as 2.5ma. good efficiency is achieved with the low operating current together with ?muti - mode? control features. ? soft start LT1548 fe atures an internal soft start to minimize the component electrical over - stress during power on startup. as soon as vdd reaches uvlo (off), the control algorithm will ramp peak current voltage threshold gradually from nearly zero to normal setting of 0.90v. every restart is a soft start. ? cc/cv operation LT1548 is designed to produce good cc/cv control characteristic as shown in the fig.1. in charger applications, a discharged battery charging starts in the cc portion of the curve until it is nearly full char ged and smoothly switches to operate in cv portion of the curve. in an ac/dc adapter, the normal operation occurs only on the cv portion of the curve. the cc portion provides output current limiting. in cv operation, the output voltage is regulated through the primary side control. in cc operation mode, LT1548 will regulate the output current constant regardless of the output voltage drop. fig. 1 typical cc/cv curve ? principle of operation to support LT1548 proprietary cc/cv con trol, system needs to be designed in dcm mode for flyback system (refer to typical application diagram on page1). in the dcm flyback converter, the output voltage can be sensed via the auxiliary winding. during mosfet turn - on time, the load current is supp lied from the output filter capacitor co. the current in the primary winding ramps up. when mosfet turns off, the primary current transfers to the secondary at the amplitude of (1) t he auxiliary voltage reflects th e output voltage as shown in fig.2 and it is given by (2) w here v indicates the drop voltage of the output diode. fig.2 auxiliary voltage wavefor m 5 % 5 % v o i o i c c p s p s i n n i ? ? ? ? v v n n v o s aux aux ? ? ? ? t o n t d e m a g v a u x t o f f s a m p l i n g i n s t a n c e t v
LT1548 xian l ongtium m icroelectronics technology developing co., ltd w ww.longtiumic.com 8 / 10 via a resistor divider connected between the auxiliary winding and inv (pin 3), the auxiliary voltage is sampled at the end of the de - magnetization and it is hold until the next sampling. the sampled voltage is compared with vref (2.0v) and the error is amplified. the error amplifier output comp re flects the load condition and controls the pwm switching frequency to regulate the output voltage, thus constant output voltage can be achieved. when sampled voltage is below vref and the error amplifier output comp reaches its maximum, the switching frequ ency is controlled by the sampled voltage thus the output voltage to regulate the output current, thus the constant output current can be achieved. ? adjustable cc point and output power in LT1548 , the cc point and maximum output power can be externally adju sted by external current sense resistor rs at cs pin as illustrated in typical application diagram. the output power is adjusted through cc point change. the larger rs, the smaller cc point is, and the smaller output power becomes, and vice versa as shown in fig.3 . fig.3 adjustable output power by changing rs ? operation switching frequency the switching frequency of LT1548 is adaptively controlled according to the load conditions and the operation modes. no external frequency s etting components are required. the operation switching frequency at maximum output power is set to 60k hz internally. for flyback operating in dcm, the maximum output power is given by (3) where lp indicate the inductance of prim ary winding and ip is the peak current of primary winding. refer to the equation 3, the change of the primary winding inductance results in the change of the maximum output power and the constant output current in cc mode. to compensate the change from var iations of primary winding inductance, the switching frequency is locked by an internal loop such that the switching frequency is (4) since tdemag is inversely proportional to the inductance, as a result, the product lp and f sw is constant, thus the maximum output power and constant current in cc mode will not change as primary winding inductance changes. up to +/ - 10% variation of the primary winding inductance can be compensated. ? frequency shuffling for emi improvement the fr equency shuffling (switching frequency modulation) is implemented in LT1548 . the oscillation frequency is modulated so that the tone energy is spread out. the spread spectrum minimizes the conduction band emi and therefore eases the system design. ? current sensing and leading edge blanking cycle - by - cycle current limiting is offered in LT1548 current mode pwm control. the switch current is detected by a sense resistor into the cs pin. an internal leading edge blanking circuit chops off the sensed voltage spik e at initial internal power mosfet on state so that the external rc filtering on sense input is no longer needed. the pwm duty cycle is determined by the current sense input voltage v o i o l a r g e r s s m a l l r s 2 2 1 p sw p max i f l po ? demag sw t f 2 1 ?
LT1548 xian l ongtium m icroelectronics technology developing co., ltd w ww.longtiumic.com 9 / 10 and the ea output voltage. ? gate drive the internal power mosfet in LT1548 is driven by a dedicated gate driver for power switch control. too weak the gate drive strength results in higher conduction and switch loss of mosfet while too strong gate drive compromises emi. a good tradeoff is achieved through the built - in totem pole gate design with right output strength control. ? programmable cable drop compensation in LT1548 , cable drop compensation is implemented to achieve good load regulation. an offset voltage is generate d at inv by an internal current flowing into the resister divider. the current is inversely proportional to the voltage across pin comp, as a result, it is inversely proportional to the output load current, thus the drop due to the cable loss can be compen sated. as the load current decreases from full - load to no - load, the offset voltage at inv will increase. it can also be programmed by adjusting the resistance of the divider to compensate the drop for various cable lines used. ? protection control good power supply system reliability is achieved with its rich protection features including cycle - by - cycle current limiting (ocp), vdd clamp, power on soft start, and under voltage lockout on vdd (uvlo). vdd is supplied by transformer auxiliary winding output. the output of LT1548 is shut down when vdd drops below uvlo (on) limit and switcher enters power on start - up sequence thereafter .
LT1548 xian l ongtium m icroelectronics technology developing co., ltd w ww.longtiumic.com 10 / 10 dip 8 package outline dimensions symbol dimensions in millimeters dimensions in inches min max min max a 3.710 4.310 0.146 0. 170 a1 0. 500 0 0.0 20 a2 3.200 3.600 0. 126 0. 142 b 0.3 5 0 0. 65 0 0.01 4 0.02 6 b 1 1.524(bsc) 0.060(bsc) c 0.200 0.360 0. 008 0. 014 d 9.000 9.500 0. 354 0. 374 e 6.200 6.600 0.2 44 0.2 60 e1 7.320 7.920 0.288 0.312 e 2.540(bsc) 0.100(bsc) l 3 .000 3.600 0.118 0.142 e 2 8.200 9.000 0.323 0.354
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