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p roduction d ata s heet t he i nfinite p ower of i nnovation l in f inity m icroelectronics i nc . 11861 w estern a venue , g arden g rove , ca. 92841, 714-898-8121, f ax : 714-893-2570 1 copyright ? 1994 rev. 1.0b,2005-03-01 lx1552/3/4/5 u ltra -l ow s tart -u p c urrent , c urrent -m ode pwm description the lx155x family of ultra-low start-up current (250a max), current mode control ics offer new levels of energy efficiency for offline converter applications. they are ideally optimized for personal computer and crt power supplies although they can be used in any number of off-line applications where energy e fficiency is critical. coupled with the fact that the lx155x series requires a minimal set of external components, the series offers an excellent value for cost conscious consumer applications. optimizing energy efficiency, the lx155x series demonstrates a significant power reduction as compared with other similar off-line controllers. table 1 compares the sg384x, uc384xa and the lx155x start-up resistor power dissipation. the lx155x offers an overall 4x reduction in power dissipation. additionally, the precise oscillator discharge current gives the power supply designer considerable flexibility in optimizing system duty cycle consistency. the current mode architecture demonstrates improved load regulation, pulse by pulse current limiting and inherent protection of the power supply output switch. the lx155x includes a bandgap reference trimmed to 1%, an error amplifier, a current sense comparator internally clamped to 1v, a high current totem pole output stage for fast switching of power mosfets, and an externally programmable oscillator to set operating frequency and maximum duty cycle. the under voltage lock-out circuitry is designed to operate with as little as 250a of supply current permitting very efficient bootstrap designs. important: for the most current data, consult microsemi ?s website: http://www.microsemi.com product highlight typical application of lx155x usi ng its micropower start-up feature i st r st v cc ac i nput lx1552 or lx1554 design using sg384x uc384xa lx155x max. start-up current specification (i st ) 1000a 500a 250a typical start-up resistor value (r st ) 62k ? 124k ? 248k ? max. start-up resistor power dissipation (p r ) 2.26w 1.13w 0.56w note: calculation is done for universal ac input specification of v acmin = 90v rms to v acmax = 256v rms using the following equation: (resistor cu rrent is selected to be 2 * i st @ v acmin ) 2 a cmin ac max st r st st 2v v r= , p= r 2?i key features ? ultra-low start-up current (150a typical) ? trimmed oscillator discharge current (2% typical) ? initial oscillator frequency better than 4% ? output pulldown during uvlo ? precision 2.5v reference (2 maximum) ? current sense delay to output (150ns typical) ? automatic feed forward compensation ? pulse-by-pulse current limiting ? enhanced load response characteristics ? under-voltage lockout with hysteresis ? double pulse suppression ? high current totem pole output (1a peak) ? 500khz operation applications ? economy off-line flyback or forward converters ? dc-dc buck or boost converters ? low cost dc motor control available options per part# part # start-up voltage hysteresis max. duty cycle lx1552 16v 6v <100% lx1553 8.4v 0.8v <100% lx1554 16v 6v <50% lx1555 8.4v 0.8v <50% package order info m plastic dip 8-pin dm plastic soic 8-pin d plastic soic 14-pin y ceramic dip 8-pin pw plastic tssop 20-pin t a ( c) rohs compliant / pb-free transition dc: 0503 rohs compliant / pb-free transition dc: 0440 rohs compliant / pb-free transition dc: 0440 rohs compliant / pb-free transition dc: 0442 0 to 70 lx155xcm lx155xcdm lx155xcd - lx155xcpw -40 to 85 lx155xim lx155xidm lx155xid - - -55 to 125 - - - lx155xmy - note: available in tape & reel. append the letters ?tr? to the part number (i.e. lx1552cdm-tr).
u l tr a -l o w s t a r t - u p c urren t , c urren t - m od e pwm lx1552/3/4/5 product d a t abook 1996/1997 copyright ? 1994 rev. 1.0b 2 p roductio n d ata s heet absolute maximum r a tings (note 1) supply voltage (low impedance source)..................................................................30v supply voltage (i cc < 30ma).........................................................................self limiting output current............................................................................................................. 1a output energy (capacitive load)................................................................................5j analog inputs (pins 2, 3)...........................................................................-0.3v to +6.3v error amp output sink current...............................................................................10ma power dissipation at t a = 25c (dil-8)......................................................................1w operating junction temperature ceramic (y package)............................................................................................150c plastic (m, dm, d, pw packages)........................................................................150c storage temperature range....................................................................-65c to +150c lead temperature (soldering, 10 seconds)............................................................300c p ackage pin outs v ref v cc output gnd comp v fb i sense r t /c t 1 8 27 36 45 m & y package (top view) dm package (top view) v ref v cc output gnd comp v fb i sense r t /c t 1 8 27 36 45 v ref n.c. v cc v c output gnd pwr gnd comp n.c. v fb n.c. i sense n.c. r t /c t 1 1 4 2 1 3 3 1 2 4 1 1 5 1 0 69 78 d package (top view) pw package (top view) 1 2 0 2 1 9 3 1 8 4 1 7 5 1 6 6 1 5 7 1 4 8 1 3 9 1 2 10 11 n.c. n.c. comp v fb n.c. i sense n.c. r t /c t n.c. n.c. n.c. n.c. v ref n.c. v cc v c output gnd pwr gnd n.c. m package: thermal resistance-junction to ambient, q q q q q ja 95c/w dm package: thermal resistance-junction to ambient, q q q q q ja 165c/w d package: thermal resistance-junction to ambient, q q q q q ja 120c/w y package: thermal resistance-junction to ambient, q q q q q ja 130c/w pw package: thermal resistance-junction to ambient, q q q q q ja 144c/w junction temperature calculation: t j = t a + (p d x q ja ). the q ja numbers are guidelines for the thermal performance of the device/pc-board system. all of the above assume no ambient airflow thermal d a t a note 1.exceeding these ratings could cause damage to the device. all voltages are with respect to ground. currents are positive into, negative out of the specified terminal. pin numbers refer to dil packages only. pb-free / rohs peak package solder reflow temp. (40 second max. exposure)................ 260c (+0, -5) rohs / pb-free 100% matte tin lead finish rohs / pb-free 100% matte tin lead finish rohs / pb-free 100% matte tin lead finish m package rohs / pb-free 100% matte tin lead finish
u l tr a -l o w s t a r t - u p c urren t , c urren t - m od e pwm lx1552/3/4/5 product d a t abook 1996/1997 3 copyright ? 1994 rev. 1.0b p roduction d ata s heet electrical characteristics (unless otherwise specified, these specifications apply over the operating ambient temperatures for lx155xc with 0 c t a 70c, lx155xi with -40 c t a 85c, lx155xm with -55 c t a 125c; v cc =15v (note 5); r t =10k; c t =3.3nf. low duty cycle pulse testing techniques are used which maintains junction and case temperatures equal to the ambient temperature.) reference section parameter symbol test conditions output voltage v ref t a = 25c, i l = 1ma line regulation 1 2 v in 25v load regulation 1 i o 20ma temperature stability (note 2 & 7) total output variation over line, load, and temperature output noise voltage (note 2) v n 10hz f 10khz, t a = 25c long term stability (note 2) t a = 125c, t = 1000hrs output short circuit i sc lx155xc units min . typ . max . min . typ . max. lx155xi/155xm 4.9 5 5.0 0 5.0 5 4.9 5 5.0 0 5.05 v 6 2 0 6 2 0 m v 6 2 5 6 2 5 m v 0.2 0.4 0.2 0.4 mv/c 4.9 5.1 4.9 5.1 v 50 50 v 5 2 5 5 2 5 m v -3 0 -10 0 -180 -3 0 -10 0 -180 m a oscillator section initial accuracy (note 6) t a = 25c t a = 25c, r t = 698 w , c t = 22nf, lx1552/3 only voltage stability 1 2 v cc 25v temperature stability (note 2) t min t a t max amplitude (note 2) v pin 4 peak to peak discharge current i d t a = 25c, v pin 4 = 2v v pin 4 = 2v, t min t a t max 48. 5 50. 5 52. 5 48. 5 50. 5 52.5 kh z 56 58 60 56 58 60 kh z 0.2 1 0.2 1 % 5 5 % 1.7 1.7 v 8.0 8.3 8.6 8.0 8.3 8.6 ma 7.6 8.8 7.8 8.8 ma output voltage low level v ol i sink = 20ma i sink = 200ma output voltage high level v oh i source = 20ma i source = 200ma rise time (note 2) t r t a = 25c, c l = 1nf fall time (note 2) t f t a = 25c, c l = 1nf uvlo saturation v sat v cc = 5v, i sink = 10ma error amp section current sense section gain (note 3 & 4) a vol maximum input signal (note 3) v pin 1 = 5v power supply rejection ratio (note 3) psrr 12 v cc 25v input bias current i b delay to output (note 2) t pd v pin 3 = 0 to 2v output section 2.4 5 2.5 0 2.5 5 2.4 5 2.5 0 2.55 v -0.1 -1 -0.1 -0.5 a 65 90 65 90 d b 0.6 0.6 mhz 60 70 60 70 d b 24 24 m a -0.5 -0.8 -0.5 -0.8 ma 5 6.5 5 6.5 v 0.7 1.1 0.7 1.1 v 2.85 3 3.1 5 2.85 3 3.15 v/ v 0.9 1 1.1 0.9 1 1.1 v 70 70 d b -2 -10 -2 -5 a 150 300 150 300 n s 0.1 0.4 0.1 0.4 v 1.5 2.2 1.5 2.2 v 1 3 13.5 1 3 13.5 v 1 2 13.5 1 2 13.5 v 5 0 100 5 0 100 n s 5 0 100 5 0 100 n s 0.7 1.2 0.7 1.2 v (electrical characteristics continue next page.) input voltage v pin 1 = 2.5v input bias current i b open loop gain a vol 2 v o 4v unity gain bandwidth (note 2) ugbw t a = 25c power supply rejection ratio (note 3) psrr 12 v cc 25v output sink current i ol v pin 2 = 2.7v, v pin 1 = 1.1v output source current i oh v pin 2 = 2.3v, v pin 1 = 5v output voltage high level v oh v pin 2 = 2.3v, r l = 15k to ground output voltage low level v ol v pin 2 = 2.7v, r l = 15k to v ref
u l tr a -l o w s t a r t - u p c urren t , c urren t - m od e pwm lx1552/3/4/5 product d a t abook 1996/1997 copyright ? 1994 rev. 1.0b 4 p roductio n d ata s heet electrical characteristic s (con't.) under-voltage lockout section parameter symbol test conditions start threshold v st 1552/1554 1553/1555 min. operation voltage after turn-on 1552/1554 1553/1555 lx155xc units min . typ . max . min . typ . max. lx155xi/155xm 15 16 17 15 16 17 v 7.8 8.4 9.0 7.8 8.4 9.0 v 9 10 11 9 10 11 v 7.0 7.6 8.2 7.0 7.6 8.2 v pwm section maximum duty cycle 1552/1553 1552/1553, r t = 698 w , c t = 22nf 1554/1555 minimum duty cycle 94 96 94 96 % 50 50 % 47 48 47 48 % 0 0 % power consumption section start-up current i st operating supply current i cc v cc zener voltage v z i cc = 25ma 150 250 150 250 a 11 17 11 17 m a 30 35 30 35 v notes : 2 . these parameters, although guaranteed, are not 100% tested in production. 3 . parameter measured at trip point of latch with v fb = 0. 4 . gain defined as : a = ; 0 v isense 0.8v. 5 . adjust v cc above the start threshold before setting at 15v. 6 . output frequency equals oscillator frequency for the lx1552 and lx1553. output frequency is one half oscillator frequency for the lx1554 and lx1555. 7 . temperature stability, sometimes referred to as average temperature coefficient, is described by the equation: temp stability = v ref (max.) & v ref (min.) are the maximum & minimum reference voltage measured over the appropriate temperature range. note that the extremes in voltage do not necessarily occur at the extremes in temperature. v ref (max.) - v ref (min.) t a (max.) - t a (min.) d v comp d v isense block diagram * - v cc and v c are internally connected for 8 pin packages. ** - power ground and ground are internally connected for 8 pin packages. *** - toggle flip flop used only in 1554 and 1555. oscillator s r *** v ref good logic internal bias s / r 5v ref pwm latch current sense comparator 1v r 2r error amp 16v (1552/1554) 8.4v (1553/1555) 16v (1552/1554) 8.4v (1553/1555) uvlo 34v g round** v cc * r t /c t v fb t comp i sense power ground** output v c * v ref
u l tr a -l o w s t a r t - u p c urren t , c urren t - m od e pwm lx1552/3/4/5 product d a t abook 1996/1997 5 copyright ? 1994 rev. 1.0b p roduction d ata s heet graph / cu r ve index characteristic curves figure # 1. oscill a tor frequency vs. timing resistor 2. maximum duty cycle vs. timing resistor 3. oscill a tor discharge current vs. temper a ture 4. oscill a tor frequency vs. temper a ture 5. output initial accuracy vs. temper a ture 6. output duty cycle vs. temper a ture 7. reference vo l t age vs. temper a ture 8. reference short circuit current vs. temper a ture 9. e.a. input vo l t age vs. temper a ture 10 . s t a r t -up current vs. temper a ture 11 . s t a r t -up current vs. supp l y vo l t age 12 . s t a r t -up current vs. supp l y vo l t age 13 . dynamic supp l y current vs. oscill a tor frequency 14 . current sense delay to output vs. temper a ture 15 . current sense threshold vs. error amplifier output 16 . s t a r t -up threshold vs. temper a ture 17 . s t a r t -up threshold vs. temper a ture 18 . minimum oper a ting vo l t age vs. temper a ture 19 . minimum oper a ting vo l t age vs. temper a ture 20 . low level output s a tur a tion vo l t age during under- vo l t age lockout 21 . output s a tur a tion vo l t age vs. output current and temper a ture 22 . output s a tur a tion vo l t age vs. output current and temper a ture figure index theory of operation section figure # 23 . typical applic a tion of s t a r t -up circuitry 24 . reference vo l t age vs. temper a ture 25 . simplified schem a tic of oscill a tor section 26 . duty cycle v ari a tion vs. discharge current 27 . oscill a tor frequency vs. timing resistor 28 . maximum duty cycle vs. timing resistor 29 . current sense threshold vs. error amplifier output t ypical applications section figure # 30 . current sense spike suppression 31 . mosfet p arasitic oscill a tions 32 . adjus t able buffered reduction of clamp level with sof t - s t a rt 33 . external duty cycle clamp and mu l ti-unit sychroniz a tion 34 . slope compens a tion 35 . open loop labor a to r y fixture 36 . off-line f l yback regula to r
u l tr a -l o w s t a r t - u p c urren t , c urren t - m od e pwm lx1552/3/4/5 product d a t abook 1996/1997 copyright ? 1994 rev. 1.0b 6 p roductio n d ata s heet characteristic cu r ves figure 2 . maximum duty cycle vs. timing resistor figure 3. oscill a tor discharge current vs. temper a ture figure 4. oscill a tor frequency vs. temper a ture figure 1. oscill a tor frequency vs. timing resistor 0.1 0 40 (r t ) timing resistor - ( k ) 100 maximum duty cycle - (%) 20 50 80 1 1 0 1 0 0 10 60 70 90 30 v cc = 15v t a = 25c 100 0.1 0.1 1 1000 oscillator frequency - (khz) (r t ) timing resistor - ( k ) 100 10 1 10 v cc = 15v t a = 25c c t = 3.3nf c t = 1nf c t = 6.8nf c t = 22nf c t = 47nf c t = 0.1f 7.70 8.10 (t a ) ambient temperature - (c) (i d ) oscillator discharge current - (ma) 7.90 8.20 7.80 8.30 8.40 8.00 -75 -50 -25 0 25 50 75 100 125 8.50 v cc = 15v v pin4 = 2v 45 49 (t a ) ambient temperature - (c) oscillator frequency - (khz) 47 50 46 51 52 48 -75 -50 -25 0 25 50 75 100 125 53 v cc = 15v r t = 10k c t = 3.3nf 54 55
u l tr a -l o w s t a r t - u p c urren t , c urren t - m od e pwm lx1552/3/4/5 product d a t abook 1996/1997 7 copyright ? 1994 rev. 1.0b p roduction d ata s heet characteristic cu r ves figure 6 . output duty cycle vs. temper a ture figure 7. reference vo l t age vs. temper a ture figure 8. reference short circuit current vs. temper a ture figure 5. output initial accuracy vs. temper a ture 40 44 (t a ) ambien t temperature - (c) output duty cycle - (%) 42 45 41 46 47 43 -75 -50 -25 0 25 50 75 100 125 48 v cc = 15v r t = 698 w c t = 22nf 50.0 56.0 (t a ) ambien t temperature - (c) output initial accuracy - (khz) 53.0 57.5 51.5 59.0 60.5 54.5 -75 -50 -25 0 25 50 75 100 125 62.0 v cc = 15v r t = 698 w c t = 22nf 63.5 65.0 lx1552 and lx1553 only 4.95 4.99 (t a ) ambien t temperature - (c) (v ref ) reference voltage - (v) 4.97 5.00 4.96 5.01 5.02 4.98 -75 -50 -25 0 25 50 75 100 125 5.03 v cc = 15v i l = 1ma 30 90 (t a ) ambient temperature - (c) (i sc ) reference short circuit current - (ma) 60 105 45 120 135 75 -75 -50 -25 0 25 50 75 100 125 180 150 165
u l tr a -l o w s t a r t - u p c urren t , c urren t - m od e pwm lx1552/3/4/5 product d a t abook 1996/1997 copyright ? 1994 rev. 1.0b 8 p roductio n d ata s heet characteristic cu r ves figure 10 . s t ar t -up current vs. temper a ture figure 11. s t ar t -up current vs. supp l y vo l t age figure 12. s t ar t -up current vs. supp l y vo l t age figure 9. e.a. input vo l t age vs. temper a ture 0 100 (t a ) ambient temperature - (c) (i st ) start-up current - (a) 50 125 25 150 175 75 -75 -50 -25 0 25 50 75 100 125 250 200 225 lx1552/lx1554 lx1553/lx1555 2.45 2.49 (t a ) ambient temperature - (c) e.a. input voltage - (v) 2.47 2.50 2.46 2.51 2.52 2.48 -75 -50 -25 0 25 50 75 100 125 2.55 2.53 2.54 v cc = 15v 0 100 (v cc ) supply voltage - (v) (i st ) start-up current - (a) 50 125 25 150 175 75 0 2 4 6 8 10 12 14 20 250 200 225 16 18 lx1553/lx1555 t a = 25c 0 100 (v cc ) supply voltage - (v) (i st ) start-up current - (a) 50 125 25 150 175 75 0 123 4 5 6 7 1 0 250 200 225 89 lx1552/lx1554 t a = 25c
u l tr a -l o w s t a r t - u p c urren t , c urren t - m od e pwm lx1552/3/4/5 product d a t abook 1996/1997 9 copyright ? 1994 rev. 1.0b p roduction d ata s heet characteristic cu r ves figure 14 . current sense del a y to output vs. temper a ture figure 15. current sense threshold vs. error amplifier output figure 16. s t ar t -up threshold vs. temper a ture figure 13. dynamic supp l y current vs. oscill a tor frequency 0 120 (t a ) ambient temperature - (c) (t pd ) c.s. delay to output - (ns) 60 150 30 180 210 90 -75 -50 -25 0 25 50 75 100 125 300 240 270 v cc = 15v v pin3 = 0v to 2v c l = 1nf 100 10 0 12 oscillator frequency - (khz) 30 (i cc ) dynamic supply current - (ma) 6 15 24 1000 3 18 21 27 9 t a = 25c r t = 10k c l = 1000pf v in = 16v v in = 12v v in = 10v 7.8 8.2 (t a ) ambient temperature - (c) start-up trheshold - (v) 8.0 8.3 7.9 8.4 8.5 8.1 -75 -50 -25 0 25 50 75 100 125 8.6 lx1553 lx1555 8.7 8.8 0 0.4 error amplifier output voltage - (v) current sense threshold - (v) 0.2 0.5 0.1 0.6 0.7 0.3 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 5.0 1.0 0.8 0.9 4.0 4.5 t a = 25c 1.1 t a = 125c t a = -55c
u l tr a -l o w s t a r t - u p c urren t , c urren t - m od e pwm lx1552/3/4/5 product d a t abook 1996/1997 copyright ? 1994 rev. 1.0b 10 p roductio n d ata s heet characteristic cu r ves figure 18 . minimu m oper a tin g vo l t ag e vs. temper a ture figure 19. minimum oper a ting vo l t age vs. temper a ture figure 20. low level output s a tur a tion vo l t age during under-vo l t age lockout figure 17. s t ar t -up threshold vs. temper a ture 15.0 15.8 (t a ) ambient temperature - (c) start-up trheshold - (v) 15.4 16.0 15.2 16.2 16.4 15.6 -75 -50 -25 0 25 50 75 100 125 16.6 lx1552 lx1554 16.8 17.0 7.0 7.4 (t a ) ambient temperature - (c) minimum operating voltage - (v) 7.2 7.5 7.1 7.6 7.7 7.3 -75 -50 -25 0 25 50 75 100 125 7.8 lx1553 lx1555 7.9 8.0 1 0.1 0.00 0.48 output sink current - ( ma) 1.20 (v sat ) output saturation voltage - (v) 0.24 0.60 0.96 10 0.12 0.72 0.84 1.08 0.36 v cc = 5v t a = -55c t a = 25c t a = 125c 9.0 9.8 (t a ) ambient temperature - (c) minimum operating voltage - (v) 9.4 10.0 9.2 10.2 10.4 9.6 -75 -50 -25 0 25 50 75 100 125 10.6 lx1552 lx1554 10.8 11.0
u l tr a -l o w s t a r t - u p c urren t , c urren t - m od e pwm lx1552/3/4/5 product d a t abook 1996/1997 11 copyright ? 1994 rev. 1.0b p roduction d ata s heet characteristic cu r ves figure 21. output s a tur a tion vo l t age vs. output current and temper a ture figure 22. output s a tur a tion vo l t age vs. output current and temper a ture 100 10 0.00 output sink current - (ma) 6.0 (v sat ) output saturation voltage - (v) 3.0 100 0 1.0 4.0 2.0 v cc = 5v sink transistor t a = -55c t a = 25c t a = 125c 5.0 100 10 0.00 2.40 output source current - (ma) 6.00 (v sat ) output saturation voltage - (v) 1.20 3.00 4.80 1000 0.60 3.60 4.20 5.40 1.80 v cc = 15v source transistor t a = -55c t a = 25c t a = 125c
u l tr a -l o w s t a r t - u p c urren t , c urren t - m od e pwm lx1552/3/4/5 product d a t abook 1996/1997 copyright ? 1994 rev. 1.0b 12 p roductio n d ata s heet theo r y of oper a tion ic description the lx1552/3/4/5 series of current mode pwm controller ic's are designed to offer substantial improvements in the areas of start- up current and oscillator accuracy when compared to the first generation products, the uc184x series. while they can be used in most dc-dc applications, they are optimized for single-ended designs such as flyback and forward converters. the lx1552/ 54 series are best suited for off-line applications, whereas the 1553/55 series are mostly used in power supplies with low input voltages. the ic can be divided into six main sections as shown in the block diagram (page 4): undervoltage lockout and start- up circuit; voltage reference; oscillator; current sense comparator and pwm latch; error amplifier; and the output stage. the operation of each section is described in the following sections. the differences between the members of this family are summa- rized in table 1. the start-up capacitor (c1) is charged by current through resistor (r1) minus the start-up current. resistor (r1) is designed such that it provides more than 250a of current (typically 2x i st(max) ). once this voltage reaches the start-up threshold, the ic turns on, starting the switching cycle. this causes an increase in ic operating current, resulting in discharging the start-up capacitor. during this time, the auxiliary winding flyback voltage gets rectified & filtered via (d1) and (c1) and provides sufficient voltage to continue to operate the ic and support its required supply current. the start-up capacitor must be large enough such that during the discharge period, the bootsrap voltage exceeds the shutdown threshold of the ic. table 2 below shows a comparison of start-up resistor power dissipation vs. maximum start-up current for different devices. max. start-u p current specificatio n (i st ) typical start-up resisto r valu e ( r st ) max. start-up resistor power dissipatio n ( p r ) design using sg38 4 x uc38 4 x a lx15 5 x 2.26w 1.13w 0.56w 62 k w w w w w 124 k w w w w w 248 k w w w w w 1000a 500a 250a (resistor r1 is designed such that it provides 2x maximum start-up current under low line conditions. maximum power dissipation is calculated under maximum line conditions. ex- ample assumes 90 to 265vac universal input application.) figure 23 typical application of start-up circuitry undervoltage lockout the lx155x undervoltage lock-out is designed to maintain an ultra low quiescent current of less than 250a, while guarantee- ing the ic is fully functional before the output stage is activated. comparing this to the sg384x series, a 4x reduction in start-up current is achieved resulting in 75% less power dissipation in the start-up resistor. this is especially important in off-line power supplies which are designed to operate for universal input voltages of 90 to 265v ac. figure 23 shows an efficient supply voltage using the ultra low start-up current of the lx1554 in conjunction with a bootstrap winding off of the power transformer. circuit operation is as follows. hysterise s voltage (v hys ) part # start-up voltage (v st ) lx1552 lx1553 lx1554 lx1555 16v 8.4v 16v 8.4v 6v 0.8v 6v 0.8v <100% <100% <50% <50% uvlo maximum duty cycle table 1 r s gnd dc bus c 1 d 1 i 1 > 250a 1 st < 250a v i n ref r t /c t v o gnd r t c t lx1554 table 2
u l tr a -l o w s t a r t - u p c urren t , c urren t - m od e pwm lx1552/3/4/5 product d a t abook 1996/1997 13 copyright ? 1994 rev. 1.0b p roduction d ata s heet theo r y of oper a tion voltage reference the voltage reference is a low drift bandgap design which provides +5.0v to supply charging current to the oscillator timing capacitor, as well as supporting internal circuitries. initial accuracy for all devices are specified at 1% max., which is a 2x improvement for the commercial product when compared to the sg384x series. the reference is capable of providing in excess of 20ma for powering any external control circuitries and has built-in short circuit protection. figure 25 simplified schematic of oscillator section oscillator the oscillator circuit is designed such that discharge current and valley voltage are trimmed independently. this results in more accurate initial oscillator frequency and maximum output duty cycle, especially important in lx1552/53 applications. the oscillator is programmed by the values selected for the timing components (r t ) and (c t ). a simplified schematic of the oscillator is shown in figure 25. the operation is as follows; capacitor (c t ) is charged from the 5v reference thru resistor (r t ) to a peak voltage of 2.7v nominally. once the voltage reaches this threshold, comparator (a1) changes state, causing (s1) to switch to position (2) and (s2) to (v v ) position. this will allow the capacitor to discharge with a current equal to the difference between a constant discharge current (i d ) and current through charging resistor (i r ), until the voltage drops down to 1v nominally and the comparator changes state again, repeating the cycle. oscillator charge time results in the output to be in a high state (on time) and discharge time sets it to a low state (off time). since the oscillator period is the sum of the charge and discharge time, any variations in either of them will ultimately affect stability of the output frequency and the maximum duty cycle. in fact, this figure 2 4 reference voltage vs. temperature 4.95 4.99 (t a ) ambien t temperature - (c) (v ref ) reference voltage - (v) 4.97 5.00 4.96 5.01 5.02 4.98 -75 -50 -25 0 25 50 75 100 125 5.03 v cc = 15v i l = 1ma figure 26 duty cycle variatio n vs. discharge current 20 60 (r t ) timing resistor - ( ) 100 oscillator duty cycle - (%) 40 70 600 700 800 900 1000 t a = 25c v p = 2.7v v = 1 v v ref = 5v 30 80 90 50 i d = 7.5ma i d = 8.0ma i d = 8.6ma i d = 9.3ma sg384x lower limit lx155x limits sg384x upper limit c t r t i r ref 5v r t /c t i d = 8.3ma 2 1 open 2.8v 1.1v s2 v p v v s1 a1 to output stage variation is more pronounced when maximum duty cycle has to be limited to 50% or less. this is due to the fact that for longer output off time, capacitor discharge current (i d - i r ) must be decreased by increasing i r . consequently, this increases the sensitivity of the frequency and duty cycle to any small variations of the internal current source (i d ), making this parameter more critical under those conditions. because this is a desired feature in many applications, this parameter is trimmed to a nominal current value of 8.30.3ma at room temperature, and guaranteed to a maximum range of 7.8 to 8.8ma over the specified ambient temperature range . figure 26 shows variation of oscillator duty cycle versus discharge current for lx155x and sg384x series devices.
u l tr a -l o w s t a r t - u p c urren t , c urren t - m od e pwm lx1552/3/4/5 product d a t abook 1996/1997 copyright ? 1994 rev. 1.0b 14 p roductio n d ata s heet given: frequency @ f; maximum duty-cycle @ dm calculate: 1) r t = 277 ( w ), 0.3 dm 0.95 note: r t must always be greater than 520 w for proper operation of oscillator circuit. 2) c t = (f) for duty cycles above 95% use: 3) f ? where r t 3 5k w theo r y of oper a tion oscillato r (continued) the oscillator is designed such that many values of r t and c t will give the same frequency, but only one combination will yield a specific duty cycle at a given frequency. a set of charts as well as the timing equations are given to determine approximate values of timing components for a given frequency and duty cycle. 1-dm dm (1.74) -1 1 dm 1.81 r t c t (1.74) -1 example: a flyback power supply design requires the duty cycle to be limited to less than 45%. if the output switching frequency is selected to be 100khz, what are the values of r t and c t for the a) lx1552/53 , and th e b) lx1554/55 ? figure 2 8 maximum duty cycle vs. timing resistor 0.1 0 40 (r t ) timing resistor - ( k ) 100 maximum duty cycle - (%) 20 50 80 1 1 0 1 0 0 10 60 70 90 30 v cc = 15v t a = 25c 1.81 * dm f * r t 100 0.1 0.1 1 1000 oscillator frequency - (khz) (r t ) timing resistor - ( k ) 100 10 1 10 v cc = 15v t a = 25c c t = 3.3nf c t = 1nf c t = 6.8nf c t = 22nf c t = 47nf c t = 0.1f figure 27 oscillator frequency vs. timing resistor a) lx1552/53 given: f = 100khz dm = 0.45 r t = 267 = 66 9 w c t = = .012 m f b) lx1554/55 f out = ? f osc (due to internal flip flop) f osc = 200khz select c t = 1000pf using figure 27 or equation 3: r t = 9.1k (1.74) -1 (1.74) -1 1 .45 .55 .45 1.81 * 0.45 100x10 3 * 669
u l tr a -l o w s t a r t - u p c urren t , c urren t - m od e pwm lx1552/3/4/5 product d a t abook 1996/1997 15 copyright ? 1994 rev. 1.0b p roduction d ata s heet theo r y of oper a tion current sense comparator and pwm latch switch current is sensed by an external sense resistor (or a current transformer), monitored by the c.s. pin and compared internally with voltage from error amplifier output. the comparator output resets the pwm latch ensuring that a single pulse appears at the output for any given oscillator cycle. the lx1554/55 series has an additional flip flop stage that limits the output to less than 50% duty cycle range as well as dividing its output frequency to half of the oscillator frequency. the current sense comparator threshold is internally clamped to 1v nominally which would limit peak switch current to: equation 1 is used to calculate the value of sense resistor during the current limit condition where switch current reaches its maximum level. in normal operation of the converter, the relationship between peak switch current and error voltage (voltage at pin 1) is given by: the above equation is plotted in figure 29. notice that the gain becomes non-linear above current sense voltages greater tha n ? 0.95 volts. it is therefore recommended to operate below this range during normal operation. this would insure that the overall closed loop gain of the system will not be affected by the change in the gain of the current sense stage. 0 0.4 error amplifier output voltage - (v) current sense threshold - (v) 0.2 0.5 0.1 0.6 0.7 0.3 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 5.0 1.0 0.8 0.9 4.0 4.5 t a = 25c 1.1 t a = 125c t a = -55c figure 2 9 current sense threshol d vs. error amplifier output error amplifier the error amplifier has a pnp input differential stage with access to the inverting input and the output pin. the n.i. input is internally biased to 2.5 volts and is not available for any external connections. the maximum input bias current for the lx155xc series is 0.5a, while lx155xi/155xm devices are rated for 1a maximum over their specified range of ambient temperature. low value resistor dividers should be used in order to avoid output voltage errors caused by the input bias current. the error amplifier can source 0.5ma and sink 2ma of current. a minimum feedback resistor (r f ) value of is given by: output stage the output section has been specifically designed for direct drive of power mosfets. it has a totempole configuration which is capable of high peak current for fast charging and discharging of external mosfet gate capacitance. this typically results in a rise and fall time of 50ns for a 1000pf capacitive load. each output transistor (source and sink) is capable of supplying 200ma of continuous current with typical saturation voltages versus tem- perature as shown in figures 21 & 22 of the characteristic curve section. all devices are designed to minimize the amount of shoot-thru current which is a result of momentary overlap of output transistors. this allows more efficient usage of the ic at higher frequencies, as well as improving the noise susceptibility of the device. internal circuitry insures that the outputs are held off during v cc ramp-up. figure 20, in the characteristic curves section, shows output sink saturation voltage vs. current at 5v. v z r s (1) i sp = where: i sp o peak switch current v z o internal zener 0.9 v v z 1.1v (1) i sp = where: v e o voltage at pin 1 v f o diode - forward voltage 0.7v at t a = 25 c v e - 2v f 3 * r s r fmin = ? 10k 3(1.1) + 1.8 0.5ma
u l tr a -l o w s t a r t - u p c urren t , c urren t - m od e pwm lx1552/3/4/5 product d a t abook 1996/1997 copyright ? 1994 rev. 1.0b 16 p roductio n d ata s heet typical applic a tion circuits figure 33. external duty cycle clamp and mu l ti-unit synchroniz a tion figure 32. adjus t able buffered reduction of clamp level with sof t -s t a rt precision duty cycle limiting as well as synchronizing several parts is possible with the above circuitry. soft start and adjustable peak current can be done with the external circuitry shown above. f = (r a + 2r b )c 1.44 f = r a + 2r b r b v cs r s v eao - 1.3 5 ( ) r 1 r 2 r 1 +r 2 t softstart = -ln 1 - ( ) c where; v eao o voltage at the error amp output under minimum line and maximum load conditions. r 1 r 1 +r 2 r 1 r 1 +r 2 i pk = where: v cs = 1.67 ( ) and v c.s.max = 1v (typ.) unless otherwise specified, pin numbers refer to 8-pin package. figure 30. ? current sense spike suppression figure 31. ? mosfet p arasitic oscill a tions a resistor (r 1 ) in series with the mosfet gate reduces overshoot & ringing caused by the mosfet input capacitance and any inductance in series with the gate drive. (note: it is very important to have a low inductance ground path to insure correct operation of the i.c. this can be done by making the ground paths as short and as wide as possible.) the rc low pass filter will eliminate the leading edge current spike caused by parasitics of power mosfet. lx155x 3 5 6 7 r s c q1 v cc dc bus i pk i pk(max) = 1.0v r s lx155x 6 7 q1 v cc dc bus 5 r s r 1 mpsa63 r 1 r 2 c 1n4148 1 2 4 8 lx155x 5 3 6 7 q 1 i pk v cs r s v cc v in 2 6 7 r b r a 5 1 84 3 555 timer 4 5 8 lx155x to other lx155x devices 0.01
u l tr a -l o w s t a r t - u p c urren t , c urren t - m od e pwm lx1552/3/4/5 product d a t abook 1996/1997 17 copyright ? 1994 rev. 1.0b p roduction d ata s heet typical applic a tion circuits (continued) figure 34. slope compens a tion high peak currents associated with capacitive loads necessitate careful grounding techniques. timing and bypass capacitors should be connected to pin 5 in a single point ground. the transistor and 5k potentiometer are used to sample the oscillator waveform a n d apply an adjustable ramp to pin 3. due to inherent instability of fixed frequency current mode converters running above 50% duty cycle, slope compensation should be added to either the current sense pin or the error amplifier. figure 34 shows a typical slope compensation technique. pin numbers inside parenthesis refer to 14-pin package. oscillator v ref good logic s r 5v ref internal bias 8(14) 4(7) 2(3) 1(1) r f c f r d r i from v o r slope 2n222a r t 5v uvlo 2.5v error amp c t 1v 2r r c.s. comp pwm latch 5(9) 3(5) 5(8) cr s r 6(10) 7(11) 7(12) v cc dc bus v o q1 lx155x figure 35. open loop labor a to r y fixture 2 3 4 8 7 6 5 comp v fb i sense r t c t v ref v cc output ground 0.1f 0.1f a lx155x r t 2n2222 100k 4.7k 1k 4.7k 5k i sense adjust error amp a djust c t 1k ground output v cc v ref 1
u l tr a -l o w s t a r t - u p c urren t , c urren t - m od e pwm lx1552/3/4/5 product d a t abook 1996/1997 copyright ? 1994 rev. 1.0b 18 p roductio n d ata s heet typical applic a tion circuit s (continued) figure 36. ? off-line f l yback regul a tor 7 150k w 100pf v fb comp v ref r t /c t 4700f 10v 5v 2-5a isolation boundary 3600pf 400v 1n4935 820pf 2.5k w 1n4935 irf830 27k w 0.01f 10f 20v 1n4935 1k w 470pf 0.85k w mbr73 5 ti 4.7k w 2w 250k w 1/2w 220f 250v 4.7 w 1w 1n4004 1n400 4 1n4004 1n4004 ac input v cc out cur sen gnd lx155 4 20k w 3.6k w 10k w .0022 f 0.01f 16v 3 6 2 1 8 4 5 specifications input line voltage: 90vac to 130vac inpu t frequency: 50 or 60hz switching frequency: 40kh z 10% output power: 25w maximum output voltage: 5 v +5% outpu t current: 2 to 5a line regulation: 0.01%/v load regulation: 8%/a* efficiency @ 25 watts, v in = 90vac: 70 % v in = 130vac: 65 % output short-circuit current: 2.5amp average * this circuit uses a low-cost feedback scheme in which the dc voltage developed from the primary-side control winding is sensed by the lx1554 error amplifier. load regulation is therefore dependent on the coupling between secondary and control windings, and on transformer leakage inductance.

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