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an 007 march 1999 1 high efficiency lcd monitor power design using aic1578 description the aic1578 is a high performance step - down dc/dc converter, designed to drive an external p - channel mosfet to generate programmable output voltages. two main schematics o f pulse - skipping and pulse - frequency modulation are employed to maintain low quiescent current and high conversion efficiency under wide ranges of input voltage and loading condition. a current sense comparator with both inverting and non - inverting input u ncommitted is included to provide the crucial function of either current limit protection or constant output current control. when the aic1578 is used in a high - side current source step - down constant current source, the efficiency is typically greater than 90%. duty cycle can be adjusted to greater than 90% by connecting a resistor from duty pin to vin. switching frequency being in around 90khz to 280khz range small size switching components are ideal for portable equipment. in order to maintain good conve rsion efficiency form light loads to full loads, the aic1578 uses the intermittent switch control method of pfm (pulse - frequency modulation) rather than the conventional pwm control method, fig1. shows its basic structure. when the feedback voltage is grea ter than the reference voltage (1.22v) , the err amp. output is low, and dri ( pin6 ) is hi level, turn off outside drive device(p mosfet), whereas when the feedback voltage is lower than the reference voltage, the err amp is hi, and dri( pin6 ) is low level , turn off outside drive device. the kind of control method works similar to pwm at full load, with a stable switch waveform, whereas when at light load it uses intermittent switching to efficiently sustain output loading requirements.
an 007 2 6 0 m v 8 7 6 5 4 3 2 1 c u r r e n t l i m i t c o m p a r a t o r o u t p u t d r i v e r g n d d r i c s + c s - v i n p f m o s c 1 . 2 2 v r e f e r e n c e v o l t a g e e r r o r c o m p a r a t o r d u t y f b s h d n v i n + - l a t c h - + fig1. aic1578 function block in addition , the aic1578 converter has the following feature: 1. it can operate under an input voltage of 4v to 20v. 2. output voltage can be adjusted externally. 3. it has a pfm design and automatically adjusted s witching frequency and duty cycle, which makes it possible to obtain highly efficient conversion over a wide input and output voltage range. 4. it has a shutdown mode control 5. it works in the high frequency range of 90khz to 280khz , and only requires sm all size inductors. 6. it has complementary push - pull output , and can drive external p - channel mosfet or pnp transistor. 7. low cost. buck switching regulator topology basic operation: fig. 2 shows the basic structure of an buck dc/dc converter (switchin g regulator).
an 007 3 sw drive signal v g t sw current signal i sw t il waveform i l t output voltage v out t v in v out i out control circuit sw load v g c + d + - v l l - v o u t t o f f t o n v i n - v o u t t t v l fig2. typical buck converter topology the basic operation principle is to use feedback to control the on - and - off of the power switch to obtain the specif ied output voltage, for low power applications, conventional pwm control schemes are not ideal, because of, first, the low conversion efficiency due to high switching losses as compared to low output power, and second , the fact that the pwm controller requ ires a minimum load to maintain its stability. the most efficient and reliable control method is then to use a pulse - skipping - modulation switching control with the control waveforms shown in fig. 3. this switching control method can put the dc/dc converte r into quasi - sleeping mode under no load or light load condition, which reduces switching losses while maintaining high conversion efficiency and good stability. in order to choose the appropriate switching converter for an electronic product, therefore, 4 key factors need to be considered: (1) the current capacity and regulation of the output current should meet what the product demands. (2) high conversion efficiency. (3) low power consumption. (4) small size and light weight.
an 007 4 t off t on output oscillator output comparator error output driver v ref sensed output input comparator error fig 3. psm time sequence waveform typical application the circuit shown in fig.4 is an output power for lcd monitor , when v in is 10v ~ 14v , a high efficiency of 86% can be obtained at full load . (1) power specification : item symbol min. typ. m ax. unit input voltage v in 10 14 v output voltage v out 4.75 5 5.25 v output current i out 0.2a 3a a output ripple voltage v ripple 100 mv
an 007 5 vin 1 duty 2 shdn 3 fb 4 gnd 5 dri 6 cs- 7 cs+ 8 q1 aic1578 l1 1 m h l2 47 m h c1 470 m f c2 470 m f c3 470 m f c4 470 m f c5 220 m f d1 1n5820 q2 4435 r1 47kf r2 15k c11 0.01 m f c6 300pf c9 0. 1 m f c10 0.1 m f vout1: 5v/2a vin :10v~14v vout 2 vin 3 adj 1 q3 aic1085 c8 10 m f c7 10 m f r4 750rf r3 1.2k f vout2: 3.3v/1a v ripple <50mv v ripple < 100mv lcd momitor power solution (1) switching + ldo fig4. aic1578 for lcd monitor power solution duty fig5. frequency & duty cycle vs v in v in ( v) duty (%) 4 6 8 10 12 14 16 18 20 55 60 65 70 75 80 85 90 ta = 27 c frequency frequency (khz) 0 50 100 150 200 250 300 350 r duty (m w ) fig6. duty cycle vs r duty duty cycle (%) 0 1 2 3 4 60 70 80 90 100 v in = 20v v in = 10v v in = 15v v in =5v r duty refer to typ. app. circuit.
an 007 6 ( ii ) design note and component selection : design note 1. dc - dc converter efficiency efficiency loss out out out out in in out out in out p v i v i v i v i p p + = = = 2. set feedback component ( r1,r2 ) following the datasheet equation : v out =1.22 (1+ r2 r1 ) t r1= 4 7kf , r2 = 15kf . (r1+r2) must be bigger than 50kr , for high efficiency request. c6 is noise filter depend on device?s switching frequency . 3. set duty range : ( if mosfet cem4435 : r ds - on =20 mr , 1n5820 : v f =0.475v) d min = 0.475 0.04 14 0.475 5 + - + = 37.9% d max = 0.475 0.04 8 0.475 5 + - + = 65% duty range is : 35.5% ~ 65% see fig 5 ,when v in = 10v ~ 14v , f sw range is 180khz ~ 230khz and duty range is 74% ~ 78% . so, duty pin can directly connect to vin pin . if you need larger duty cycle than typical applications , ca n reference fig6 add r duty to adjust it . 4. set output inductor l = di ) v (v 0 dc - = on on o dc 0.2i )t v (v - component selection : ( 1 ) sitching mosfet selection the power dissipation of mosfet is divide into two parts : conduc tion losses and switching losses . conduction losses : on - state losses are related to the load current and mosfet r ds ? on . p c = i 2 out r ds ? on d switching losses : these losses are encountered during the mosfet on and o ff states. they depend on the nature of the load as well ws the switching speed of the mosfet . p s = f s [ ts1 0 d ds dt i v + ts2 0 d ds dt i v ] ? 6 ts2)fs (ts1 i v d ds + f s : switching frequency ts1 : turn - on time ts2 : turn - off time v ds : supply voltage i d : drain current select mosfet key factors : 1. low r ds - on 2. low c iss 3. short reverse recovery time ( 2 ) schottky barrier rectifier selection : conduction losses : diode losses due to recovery time and conduction are strongly related to cir cuit topology and load impedance . p cr = v f i out (1 - d) v f : forward conduction voltage select schottky key factors : 1. low forward conduction voltage( v f ) 2. low esr 3. short reverse recovery time 4. large reverse breakdown voltage 5. i d - peak > i l - peak (3) pwm output capacitors selection
an 007 7 the bulk filter capacitor values are generally determined by the esr(effective series resistance) and esl (effective series inductance) parameters rather than actual capacitance . high frequency decoupling capacitors should be placed as close to the power pins of the load as physically possible . be careful not to add inductance in the circuit board wiring that could cancel the usefulness of these low inductance component , use only specialized low - esr capacitors intended for switching reg ulator applications for the bulk capacitors . the bulk capacitor?s esr determines the output ripple voltage and the initial voltage drop after a high slew - rate transient . an aluminum electrolytic capacitor?s esr value is related to the case size with lower esr available in larger case sizes . (4) pwm output inductor selection the output inductor is selected to meet the output voltage ripple requirements and sets the converter?s response time to a load transient . the inductor value determines the converter?s r ipple current and the ripple voltage is a function of the ripple current . the ripple voltage and current are approximate by the following equation : g i= o s out in l f v v - ?? in out v v , g v out = g i ?? esr increasing the value of inductance re duces the ripple current and converter?s response time to a load transient . 1. efficiency test : input voltage input current output voltage output current output load efficiency 10v 290 ma 5.06v 503ma 500ma 87.8 % 10v 570 ma 5.06v 1003 ma 1a 89.0 % 10 v 1149 ma 5.05v 2001 ma 2a 87.9 % 10v 1754 ma 5.05v 3001 ma 3a 86.4 % 12v 252 ma 5.06v 503ma 500ma 84.2 % 12v 489 ma 5.06v 1003 ma 1a 86.5 % 12v 979 ma 5.05v 2001 ma 2a 86.1 % 12v 1491 ma 5.06v 3001 ma 3a 84.9 % 14v 217 ma 5.09v 503ma 500ma 84.3 % 1 4v 419 ma 5.09v 1003 ma 1a 87.0 % 14v 836 ma 5.08v 2001 ma 2a 86.9% 14v 1271 ma 5.07v 3001 ma 3a 85.5 %
an 007 8 2.temperature test load = 1a load = 2a load = 3a load v in 1578 mos l1 l2 1578 mos l1 l2 1578 mos l1 l2 8v 34.1 35.5 32.8 35.9 38.1 42.2 35.1 44.1 40.2 51.3 37.3 61.1 10v 36.7 37.9 34.5 36.4 41.5 48.8 35.6 49.9 45.5 53.3 38.9 66.5 12v 39.3 38.3 34.5 36.6 43.7 51.2 36.1 50.7 49.7 65.8 37.4 70.7 14v 40.8 40.2 34.8 37.5 44.3 56.6 38.7 62.1 50.3 66.4 39.2 74.6 15v 42.4 40.6 35.1 39.3 45.5 58.9 3 9.5 69.4 53.9 67.2 44.3 80.1 unit: c 3.test waveform : fig 1: switching signal ch1: vg - gnd (5v / div) ch2: vs - gnd (5v / div) status: v in = 10v dc v out = 5.06v dc output load = 1a fig 2: switching signal ch1: vg - gnd (5v / div) ch2: vs - gnd (5v / d iv) status: vin= 12v dc v out = 5.05v dc output load = 2a
an 007 9 fig3: switching signal ch1: vg - gnd (5v / div) ch2: vs - gnd (5v / div) status: v in = 10vdc v out = 5.05vdc output load = 3a fig4: switching signal ch1: vg - gnd (5v / div) ch2: vs - gnd (5v / div) status: v in = 10vdc v out = 5.05vdc output load = 2a fig5: switching signal ch1: vg - gnd (5v / div) ch2: vs - gnd (5v / div) status: v in = 12v dc v out = 5.05vdc output load = 2a fig 6: switching signal ch1: vg - gnd (5v / div) ch2: vs - gnd (5v / div) status: v in = 12v dc v out = 5.06vdc output load = 3a
an 007 10 fig7: 5v output ripple voltage ch1: 5v output (ripple voltage) status: input voltage: 10v output load: 1a fig 8: switching signal ch1: 5v output (ripple voltage) status: input voltage: 10v output load: 3a fig 9: 5v output ripple ch1: 5v output (ripple voltage) status: input voltage: 12v output load: 1a fig 10: 5v output ripple ch1: 5v output (ripple voltage) status: input voltage: 12v output load: 3a
an 007 11 4. lcd monitor bom l ist reference part number qty pkg manufacturer remark q1 aic1578cs 1 so - 8 aic q2 cem4435 1 so - 8 cet n - mosfet q3 aic1085cm 1 to - 263 aic l1 1 m h / 2a 1 smd h&d / cailcraft l2 47 m h / 3a 1 smd h&d / cailcraft d1 1n5820 1 dip schottky c1,c2,c3,c4 470 m f / 16v 4 dip c5 220 m f / 16v 1 dip c6 330 pf 1 smd c7,c8 10 m f / 16v 2 dip c9,10 0.1 m f 2 smd c 11 0.01 m f 1 smd r1 47k w / 1% 1 smd r2 15k w / 1% 1 smd r3 12k w / 1% 1 smd r4 750 w / 1% 1 smd

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