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lx3005 p roduction d atasheet microsemi analog mixed signal group 11861 western avenue, garden grove, ca. 92841, 800-8776458, 714-898-8121, fax: 714-893-2570 page 1 copyright ? 2010 rev.1.0, 2010-02-05 2a s tep -d own c onverter tm ? www. microsemi . com l l x x 3 3 0 0 0 0 5 5 descri pt i on k ey feat u res ? 2a constant output current ? 130m r dson internal power mosfet ? up to 94% efficiency ? fixed 420khz frequency ? wide 4.75v to 25v input voltage range ? output voltage adjustable from 0.8v to 21v ? built-in thermal shutdown function ? built-in current limit function ? built-in soft-start function ? support ceramic or electrolytic capacitors ? pb-free and rohs compliant applications the lx3005 is a 420khz fixed frequency pwm buck (step-down) dc-dc converter, capable of driving a 2a load with high efficiency, low ripple and excellent line and load regulation. the device operates over a wide input voltage range of 4.75v to 25v, and the output voltage can be externally set from 0.8v to a voltage near vin, as the pwm control circuit is able to adjust the duty ratio linearly from 0% to close to 100%. the lx3005 device integrates a high-side low rds on pmos for a low cost and high efficiency solution. an internal transconduct ance error amplifier is used in the control loop allowing flexibility to compensate the system using an all ceramic capacitor system. the lx3005 also f eatures an enable function, internal circuitry for soft start, and protection schemes such as thermal shutdown, over-current protection, and short-circuit protection. when ocp or scp is triggered, the device operating frequency will be reduced from typically 420khz to typically 40khz, limiting the output power capability. the lx3005 serves as an ideal power supply device for portable devices, especially for chipset power in portable systems. its widely used for pdvd, lcd monitor and dpf chipset power sources. the lx3005 is available in soic8 package and is functional from an ambient temperature range of 0 ? c to 85 ? c. ? portable dvd ? lcd monitor/lcd tv ? digital photo frame ? adsl ? set-top box important: : for the most current data, consult microsemi s website: http://www.microsemi.com patents pending produ ct h i gh li gh t on off l1 c o r2 r1 lx3005 vin en gnd fb sw c in c c v out @ 2a v in c1 r c comp d1 pack age order i n fo t h erm al dat a dm plastic soic 8-pin j a = 100 c/w t a ( c) rohs compliant / pb-free thermal resistance - junction to ambient 0 to 85 LX3005CDM note: available in tape & reel. append the letters tr to the part number. (i.e. LX3005CDM-tr) junction temperature calculation: t j = t a + (p d x ja ). the ja numbers are guidelines for the thermal performance of the device/pc-board system. all of th e above assume no ambient airflow. downloaded from: http:///
lx3005 p roduction d atasheet microsemi analog mixed signal group 11861 western avenue, garden grove, ca. 92841, 800-8776458, 714-898-8121, fax: 714-893-2570 page 2 copyright ? 2010 rev.1.0, 2010-02-05 2a s tep -d own c onverter tm ? www. microsemi . com l l x x 3 3 0 0 0 0 5 5 absolu t e m ax i m u m rat i n gs pack age pi n ou t supply input voltage (v in )................................................................-0.3v to 30v fb pin voltage (v fb )...........................................................................-0.3v to 6v en pin voltage (v en )......................................................................... -0.3v to v in comp pin voltage (v comp ) ................................................................-0.3v to 6v sw pin voltage (v sw )........................................................................ -0.3v to v in power dissipation (p d ) ............................................................... internally limited maximum operating junc tion temperat ure.................................................150 c storage temperat ure range .......................................................... -65 c to 150 c lead temperature (soldering, 10 seconds) ..................................................260 c note: exceeding these ratings could cause damage to the device. all voltages are with respect to ground. currents are positive into, ne gative out of specified terminal. msc 3005cdm xxxx 12 3 45 6 7 8 nc vin sw gnd fb nc en comp dm p ackage (top view) xxxx = date/lot code rohs / pb-free 100% matte tin pin finish fu n ct i on al pi n descri pt i on name pin # description nc 1 pin not used. vin 2 supply voltage pin. the lx3005 operates from a 4.75v to 25v dc voltage. bypass vin to gnd with a suitable large capacitor to eliminate noise on the input. sw 3 power switch output pin. sw is the swit ch node that supplies power to the output. gnd 4 ground for ic. fb 5 feedback pin. through an external resistor di vider network, fb senses the output voltage and regulates it. to prevent current limit run away in a short circ uit fault condition, the frequency feedback comparator lowers the oscillator fre quency to 40khz when the fb voltage is below 0.52v. the feedback threshold voltage is 0.8v. comp 6 compensation pin. this pin is the output of the error amplifier. frequen cy compensation is done at this pin by connecting a series rc to ground(parallel a capacitor if necessary) en 7 enable pin. drive en pin high to turn on the device, drive it low to turn off. default of this pin is high level. nc 8 pin not used. downloaded from: http:///
lx3005 p roduction d atasheet microsemi analog mixed signal group 11861 western avenue, garden grove, ca. 92841, 800-8776458, 714-898-8121, fax: 714-893-2570 page 3 copyright ? 2010 rev.1.0, 2010-02-05 2a s tep -d own c onverter tm ? www. microsemi . com l l x x 3 3 0 0 0 0 5 5 elect ri cal ch aract eri st i cs unless otherwise listed, the following specifica tions at the operating ambient temperature 25 c and vin = 12v, vout = 5v. parameter symbol test conditions / comment min typ max units vin recommended voltage range v in 4.75 25 v shut-down quiescent current i shdn ven = 0.4v 44 60 a operating quiescent current i q ven = 2v, v fb = 1.3v 1.3 2 ma enable v h 1.5 en pin threshold v l 0.7 v en pin input leakage current i fb v en = 2.5v -5 -10 a fb voltage internal fb voltage v fb vin = 5v to 25v 0.784 0.8 0.816 v input bias current i fb v fb = 1.3v -0.1 -0.5 a error amp error amplifier voltage gain g v 1000 v/v error amplifier transconductance g s 700 a/v oscillator operating frequency f osc 336 420 504 khz high side driver internal pmos on resistance r dson v fb = 065v, v en = 12v, i out = 2a, t c = 25c 130 150 ohm maximum duty cycle d max v fb = 0.65v, isw = 0.1a 100 % current limit switch current limit i lim t c = 25c 2.5 3.4 a frequency of current limit or short circuit protection gs 40 khz thermal shutdown threshold t otsd note1 155 c hysteresis t hys 20 c note1 : this parameter is guaranteed by design but not tested in production (gbnt). downloaded from: http:///
lx3005 p roduction d atasheet microsemi analog mixed signal group 11861 western avenue, garden grove, ca. 92841, 800-8776458, 714-898-8121, fax: 714-893-2570 page 4 copyright ? 2010 rev.1.0, 2010-02-05 2a s tep -d own c onverter tm ? www. microsemi . com l l x x 3 3 0 0 0 0 5 5 fu n ct i on al block di agram vin ocp 420khz/40khz clock and ramp pwm ea gnd bg ref 800mv comp latch sw driver otp osp fb bias generator en soft-start uvlo 0.52v + - + - + - + - figure 1. block diagram t y pi cal appli cat i on on off l1 22h c o 22f 16v r2 107k r1 20k lx3005cmd vin en gnd fb sw c in 22f 25v c c 10nf 5v/2a 12v c1 100pf r c 8.2k comp d1 dfls230l 30v 2a c optional 8 35 4 6 2 figure 2. v in = 12v, v out = 5v, ceramic capacitors input & output downloaded from: http:///
lx3005 p roduction d atasheet microsemi analog mixed signal group 11861 western avenue, garden grove, ca. 92841, 800-8776458, 714-898-8121, fax: 714-893-2570 page 5 copyright ? 2010 rev.1.0, 2010-02-05 2a s tep -d own c onverter tm ? www. microsemi . com l l x x 3 3 0 0 0 0 5 5 t y pi cal appli cat i on on off l1 22h c o 100f 16v r2 107k r1 20k lx3005cmd vin en gnd fb sw c in 100f 25v c c 10nf 5v/2a 12v c1 100pf r c 8.2k comp d1 dfls230l 30v 2a c optional 8 35 4 6 2 figure 3. v in = 12v, v out = 5v, electrolytic capacitors input & output ou t pu t ri pple 2 a load ou t pu t ri pple di scon t i n ou s m ode v in = 12v, v out = 5v, i out = 2a, 10mvpp 22uf ceramic output capacitor and a 22 h inductor channel 1 C switch node channel 2 C vout ac coupled channel 3 C vout channel 4 C inductor current v in = 12v, v out = 5v, i out = 50ma 22uf ceramic output capacitors and a 22 h inductor channel 1 C switch node channel 2 C vout ac coupled channel 3 C vout channel 4 C inductor current downloaded from: http:///
lx3005 p roduction d atasheet microsemi analog mixed signal group 11861 western avenue, garden grove, ca. 92841, 800-8776458, 714-898-8121, fax: 714-893-2570 page 6 copyright ? 2010 rev.1.0, 2010-02-05 2a s tep -d own c onverter tm ? www. microsemi . com l l x x 3 3 0 0 0 0 5 5 case t em perat u re v s i ou t & v ou t case t em perat u re v s v i n & i ou t vin - 12v 20 30 40 50 60 70 80 90 0.2 0.6 1 1.4 1.8 output current (a) case temperature (c) vout = 5 v vout = 3.3v vout = 1a vout = 2a 40 50 60 70 80 90 81 21 62 02 4 input voltge (v) case temperature (c) ou t pu t v olt age v ou t pu t cu rren t fb v olt age v s t em perat u re 4.80 4.85 4.90 4.95 5.0 0 5.0 5 5.10 5.15 5.2 0 00 . 40 . 81 . 21 . 6 22 . 4 output current (a) output voltage (v) 0.796 0.798 0.800 0.802 0.804 -40 0 40 80 120 junction temperature (c) fb voltage (v) m ax i m u m i ou t v s i n pu t v olt age op & ocp frequ ency v s t em perat ure 0.25 0.75 1. 2 5 1. 7 5 2.25 2.75 0 5 10 15 2 0 2 5 input voltage (v) maximum output current (a) vout = 2.5 vout = 3.3v vout = 5v 300 350 400 450 500 -40 0 40 80 120 junction temperature (c) operation frequency (khz) operating frequency ocp frequency * 10 downloaded from: http:///
lx3005 p roduction d atasheet microsemi analog mixed signal group 11861 western avenue, garden grove, ca. 92841, 800-8776458, 714-898-8121, fax: 714-893-2570 page 7 copyright ? 2010 rev.1.0, 2010-02-05 2a s tep -d own c onverter tm ? www. microsemi . com l l x x 3 3 0 0 0 0 5 5 su pply qu i escen t cu rren t sh u t down qu i escen t cu rren t 0.4 0.8 1.2 1.6 2 2.4 4 8 12 16 20 24 input voltage (v) supply quiescent current (ma) t j = 125c t j = 75c t j = 25c t j = -50c 0 20 40 60 80 100 4 8 12 16 20 24 input voltage (v) shutdown quiescent current (ua) tj = 125c tj = 75c tj = 25c tj = -50c ocp cu rren t li m i t v s t em perat u re ocp cu rren t li m i t v s i n pu t su pply vin = 12v 1 2 3 4 5 -40 0 40 80 120 junction temperature (c) ocp current limit (a) 1 2 3 4 5 4 8 12 16 2 0 2 4 input voltge (v) ocp current limit (a) effi ci en cy v s v ou t & i ou t effi ci en cy v s v i n & i ou t vin - 12v vout = 5v vout = 3.3v vout = 2.5v 78 82 86 90 94 0.4 0.8 1.2 1.6 2 output current (a) efficiency (%) vout = 1a vout = 2a 80 85 90 95 10 0 8 1 2 1 62 02 4 input supply (v) efficiency (%) downloaded from: http:///
lx3005 p roduction d atasheet microsemi analog mixed signal group 11861 western avenue, garden grove, ca. 92841, 800-8776458, 714-898-8121, fax: 714-893-2570 page 8 copyright ? 2010 rev.1.0, 2010-02-05 2a s tep -d own c onverter tm ? www. microsemi . com l l x x 3 3 0 0 0 0 5 5 appli cat i on i n form at i on setting the o utput v oltage to set the output voltage, connect a resistive divider from the output to the fb pin to signal ground. note that the feedback voltage is 0.8v. for the desired output voltage vout, r2 is calculated by the following equation: ? ? ? ? ? ? ? = 1 vfb vout 1r 2r r1 is selected to be 10k ? to 20k ? and vfb=0.8v. refer to figure 2 or figure 3. o utput i nductor s election the value of inductor is decided by the input and output voltage, inductor ripple current and operating frequency. a larger inductor value means smaller ripple current. however if the inductance is chosen too large, it results in a slower response and possibly lower efficiency if the losses from the increased dcr outweigh the losses eliminated from a smaller ripple current. likewise, a smaller inductor reduces the inductor size and cost, improves large signal response, but increases inductor ripple current which leads to lower efficiency and also an increase output ripple voltage. the magnitude of ripple current is a design freedom which can be decided by the design engineer according to various application requirements. the inductor value can be calculated by using the following equations: () load s ik f vin vout vout vin l ? = load ripple ik i = the inductor ripple current can be calculated by: () l f vin vout vout vin i s ripple ? = where f s is the switching frequency (420khz), i load is the output load current; k is percentage of output current. a good design rule is to choose the inductor value such that k=0.3, which means that th e inductor ripple current is 30% of the nominal output load current. o utput c apacitor s election the output capacitor value is basically decided by the amount of the output voltage ripple allowed during the steady state (dc) load cond ition as well as the load transient response requirement. the optimum design may require a couple of iterations to satisfy both conditions. the output ripple voltage is due to the esr of the output capacitor and the output capac itor charge and discharge. for aluminum electrolytic capacitors, the output ripple is largely caused by the capac itor esr, where the output ripple is: ripple i esr ripple v = however, if ceramic capacitors are used, the output ripple voltage is a combination of both the esr and the capacitor charge and discharge, and can be approximated by: ? ? ? ? ? ? ? ? + = s f out c 8 ripple i ripple i esr ripple v c out is the output capacitance used, and f s is the switching frequency. the desirable output voltage change during a load transient dictates the output capacitance requirement. for a given output voltage change vout, the output capacitance can be calculated by: () 2 2 2 load load vout vout vout 2 i i l cout ? + ? ? ? ? ? ? + = where i load is the amount of change in the load current. based on the desired output ripple voltage and output voltage deviation during load transients, the output capacitance and its esr can be approximated by the equations listed above. i nput c apacitor s election input capacitors are usually a mix of high frequency ceramic capacitors and bulk ca pacitors. ceramic capacitors bypass the high frequency noise, and bulk capacitors supply switching current to the mosfet while keeping the dc input voltage steady. usually a 1 f ceramic capacitor is chosen to decouple the high frequency noise. the bulk input capacitor is selected to support the input voltage rating and input rms current rating, and can be a ceramic type. downloaded from: http:///
lx3005 p roduction d atasheet microsemi analog mixed signal group 11861 western avenue, garden grove, ca. 92841, 800-8776458, 714-898-8121, fax: 714-893-2570 page 9 copyright ? 2010 rev.1.0, 2010-02-05 2a s tep -d own c onverter tm ? www. microsemi . com l l x x 3 3 0 0 0 0 5 5 d evice p ower d issipation the lx3005 will enter thermal shutdown when the die temperature reaches close to 150 ? c. the device junction temperature is a function of the devices total power dissipation, the junction to ambient thermal resistance, and the ambient temperature: ( ) ja total p a t j t ? + = the total power dissipated by the lx3005 device, p total , is comprised of the power diss ipated by the rms current flowing through the internal high-side fet, the switching or transitioning of the fet, and the power dissipated by the device quiescent supply current the power dissipated due to the rms input current flowing through the high side fet during the on time is: on rds rms i rdson p = ? ? ? ? ? ? ? ? ? ? + = 12 2 ripple i 2 iout d irms the power dissipated during the switching or transition of the internal fet is: s f r sw f 4 )t t( iout vin p + = where t r and t f are the rise and fall time of the switch node or the internal fet source node. finally, the power dissipated due to the device switching supply current is: iq vin iq p = where iq, the device supply current when the device is switching, can be approximat ed by the devices supply quiescent current. the total power dissipated by the device is therefore: iq p sw p rdson p total p + + = based on the device total power dissipation, the ambient temperature, thermal resistance ja , the device junction temperature can be determined. c ompensation the lx3005 uses external compensation components that allow for flexibility in designing the converter, since the compensation can be optimized after the output filter components (i.e. inductor and output capacitor) are selected for the required application. normally for low cost applications, electrolytic capacitors that have high esr are used. for applica tions where board space is critical, ceramics capacitors which have very low esr are used. the lx3005 incorporates a tr ansconductance amplifier in its feedback control path. the inverting input to the amplifier is at the fb pin, and the output of the amplifier at the comp pin. for compensating the device, a simple zero C pole pair can be used if the frequency of the zero created by the output capacitor and its esr is lower than the chosen unity gain cross-over frequency f c . this is known as type ii compensation. see figure 2. if the zero of the output capacitor is located above the cross-over frequency f c , as with ceramic capacitors that have very low esr, use a 2 zero 2 pole compensation, or a type iii compensation network. for the lx3005, set the cross-over frequency to be approximately 20khz to 40khz. t ype ii c ompensation if the output capacitor zero is located below the cross-over frequency, use the following procedure for type ii compensation. the following is an explanation of how to design a type ii compensation network for the lx3005 converter: estimate the lc output filter double pole and zero: cout l 2 1 lc f = cout esr 2 1 zesr f = next, select the cross-over frequency of the closed loop bandwidth to be 40khz or below. in order for the overall closed loop bandwidth to cross over at the desired frequency f c , the gain of the error or transconductance amplifier should be adjusted such that at the cross-over frequency f c , the product of the error amplifier gain and the gain of the feed-forward modulator path equals to 1. the feed-forward modulator gain consists of the internal pwm modulator gain, the lc output filter response, and the external resistive divider gain. this feed-forward modulator response can be approximated by the following equation: downloaded from: http:///
lx3005 p roduction d atasheet microsemi analog mixed signal group 11861 western avenue, garden grove, ca. 92841, 800-8776458, 714-898-8121, fax: 714-893-2570 page 10 copyright ? 2010 rev.1.0, 2010-02-05 2a s tep -d own c onverter tm ? www. microsemi . com l l x x 3 3 0 0 0 0 5 5 appli cat i on i n form at i on () l c f 2 esr vout vfb ramp v vin mod g = the gain of the transconducta nce amplifier near the cross over frequency is: c r gm ea g = the product of the modulator gain and error amplifier gain should equal to 1 at the cross-over frequency. thus, g mod xg ea = 1 and solve for r c . esr vfb vin gm l c f 2 vout ramp v c r = , where v ramp 1.2v. type ii compensation places a zero at or below the frequency of the lc double pole, and a high frequency pole at ? the switching frequency or lower. the zero is created by r c and c c while the pole is from r c and c optional . see figure 2. where c c z c r 2 1 f = and opt c c r 2 1 fp = based on the calculated r c value, we can solve for c c such that the zero is placed at or below the double pole frequency. z f c r c c 2 1 lc f = , with r c and f lc , solve for c c place a pole at high frequency at or below ? the switch frequency, and solve for c opt . p f c r opt c 2 1 s f 2 1 = note that the high frequency pole is optional. the purpose of the high frequency pole is to close or attenuate the overall loop response rejecting high frequency noise. if the high frequency pole is not used, the overall loop at high frequency will be determined by the high frequency response of the error amplifier. a good rule of thumb in terms of placing the zero-pole pair for approximately 60 degrees phase margin is to satisfy the following condition: 5 c f p f z f c f = = however, while satisfying this condition, it is important that the zero is placed at or below the double frequency pole to ensure stability. t ype iii c ompensation when using low esr ceramic output capacitors, the frequency of the zero produced by the output capacitor is usually above the cross-over frequency. in this case type iii compensation should be used. in traditional type iii compensation, two zeros and two poles, in addition to the pole at the origin, are introduced by the error amplifier, and the overall amplifier response is set via a feedback network from the comp output to the non-inverting input fb. a cost effective method minimizing the number of compensation components is to directly place a second zero f z2 , at the frequency of the double pole. this zero is created by r2 and c1 as shown in figure 2. 2 r 1 c 2 1 lc f = and solve for c 1 . note that in this case, the second zero is added by c 1 and r 2 in addition to the zero already introduced by r c and c c and also the pole from r c and c opt as defined in type ii compensation. g eneral l ayout g uidelines the following are general but good practices for pcb layout to prevent noise related issues, and achieve stable operation of the converter: 1) place all filtering capacitors as close to the ic as possible. use a power gr ound plane for the input (c in ) and output (c out ) capacitors. all other capacitors such as for compensation should use signal ground. 2) while having separate power ground and signal ground planes, the two grounds should be connected at one common point near the input bypass capacitor ground. 3) make high current traces short and wide. this includes the input current path and the inductor current path. minimize the loop path that consist of the switch node (sw), the output filter components, and the input capacitor. 4) keep the switch node (sw) , which is noisy, away from sensitive analog paths to prevent noise coupling onto sensitive signals such as at fb and comp pins. 5) place all compensation components and feedback resistors as close to the ic as possible, minimizing trace lengths. note that a lx3005 evaluation board or demo board is available. please contact the factory for availability. downloaded from: http:///
lx3005 p roduction d atasheet microsemi analog mixed signal group 11861 western avenue, garden grove, ca. 92841, 800-8776458, 714-898-8121, fax: 714-893-2570 page 11 copyright ? 2010 rev.1.0, 2010-02-05 2a s tep -d own c onverter tm ? www. microsemi . com l l x x 3 3 0 0 0 0 5 5 pack age di m en si on s dm 8-pin plastic soic d e e h a1 l a2 b c a *lead coplanarity note: 1. controlled dimensions are in mm, inches are for reference only. 2. dimensions do not include mold flash or protrusions; these shall not exceed 0.155mm (.006) on any side. lead dimension shall not include solder coverage. m illimeters i nches dim min max min max a 1.35 1.75 0.053 0.069 a1 0.10 0.30 0.004 0.012 a2 1.25 1.45 0.049 0.057 b 0.33 0.51 0.013 0.020 c 0.19 0.25 0.007 0.010 d 4.70 5.10 0.185 0.201 e 5.79 6.20 0.228 0.244 e 1.27 bsc 0.050 bsc h 3.80 4.01 0.150 0.158 l 0.40 1.27 0.016 0.050 0 8 0 8 *lc .010 0.004 production data C information contained in this document is proprietary to microsemi and is current as of publication date. this document may not be modified in any way without the express written consent of microsemi. product processing does not necessarily include testing of all parameters. microsemi reserves the right to change the configuration and performance of the product and to discontinue product at any time. downloaded from: http:///

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