introduction some applications requires high performances power supply characteristics in terms of fast recover to properly work. in particular it happen in the sequential system, where if an unexpected and high current absorption happens the output voltage go dowun due to the slow power supply response. if in the sys- tem there are microprocessor or counters, functionality problems could happen. some companies sug- gest integrated solutions, but the performances are not optimum due to recovery times around 40 m s, with output current medium or low and high dropout voltage ( as it is shown in figure 1, lt1585 perform- ances ). using the lpr30 it is easy to realize power supply system with improved performances thanks to the high flexibility given by this device . in fact using an external power mosfet , the ctm can use compo- nents with ver y low r load . in this way it is possible to supply current more than 10a, with lower dropout voltage. another important feature of the lpr30 is the un compensated internal error amplifier; which al- lows for realization of an optimized external compensated network in terms of faster response. december 1997 AN964 application note very fast load transient response with the new lpr30 application lt1585 load variation from 0.2a to 4a time= 50us/div, vertical = 50mv/div lpr30 load variation from 0ma to 4a time= 50us/div, vertical = 50mv/div figure 1. lt1585 and lpr30 load variations (r2 = 50 w , cout = 220 m f). 1/5
circuit description the electric schematic is shown in figure 2. table 1. components description resistor value capacitor value r1 1k w c1 470 m f r2 1k w c2 100nf r3 27 w c3 6800mf r4 33k w c4 50pf r5 1k w c5 22nf r6 480 w c6 47mf r7 20m w r8 150 w r9 2.7k w r10 33 w in the circuit there are two different compesation ; - the first one implements the current limit amplifier, - the second one implents the error amplifier. the first compensation is given by r3, r8, r9, c2, d1and tr1 . the transistor tr1 is utilized to decrease the r2 resistor value; consequently it follows that there is a reduction of the total esponse time. the second compensation is given by r10, c4 and c3. in particular c3 in combination with r load constitutes the domination pole. it is important that the ocon- stant timeo associated with c3 ? r load is t 1.8ms. so if the ctm needs a v out 3.3v and i outmax = gate gnd 5 lim- 3 lim+ 2 err- 7 vref err+ 8 46 lpr30 6800 m f r7 20m w 1k 22nf vcc2 = 12v vcc1 = 5v 27k w 1k w 480 w bc337 1n4148 r2 r3 c5 r6 r5 c3 50pf c4 c1 1 470 m f r1 r4 p50n06 vcc r10 33 w 1k 33k c2 100nf r9 2.7k w r8 150 w 47uf c3 tr1 figure 2. electric schematic. AN964 application note 2/5
15a it follows : r loadmin = v out i outmax then 3.3 15 = 0.22 w and c3 >( 1.8 ? 10 - 3 ) r loadmin 8.200mf r10 and c4 decrease the amplifier error gain at the high frequencies, while c5 improves the fast re- sponse. here below in figure n. 3 is shown the typical application demoboard. hints.. . in order to allow the complete power mosfet driving , v2 must be greater than v out +v gsat ? r1 and r4 value are calculated in according to i outmax and i outc.c performances. looking to the power dissi- pated at point a is: p da =(v in -v out -r sense ? imax) ? imax. while the power dissipated at point b is p db =(v in -r sense ? i sc ) ? i sc @ v in ? i sc . in the operating mode it is good choice to impose p db p da to avoid a larger heatsink dimension- ing in short circuit condition.if we let p da =p db , we obtain: i sc = ( v in - v out - r sense ? i max ) ? i max v in figure 3 . typical application demoboard. AN964 application note 3/5
figure 4 reports the folback limitation characteristic. in some applications it is not possible to impose p db p da because a high ratio i max /i sc implies an higher drop on the r sense . in fact : r sense ? i sc = 50 ? 10 - 3 ? r1 + r4 r4 > 50mv r sense > 50mv i sc and then r sense ? i max = v sense > 50mv ? ? ? ? i max i sc ? ? ? so the starting point to determinate all the compo- nents to be used is to impose the dissipation in short circuit condition : p d-b =(v in -r sense ? i sc ) ? i sc @ v in ? i sc >v in ? 50mv ? i max. /v sense and then 1) v sense > 50mv ? ? ? ? ( v in ? i max ) p d - b ? ? ? 2) r sense = v sense i max 3) r onmax ? ? ? ( v drop - v sense ) i max ? ? ? and so the powermosfet. 4) now we can calculate the r1 and r4 resistors. ( r1 + r4 ) = v out + v sense v out + 50mv - 1 and so imposing the r1 we can determinate the r4 5) i sc = ? ? ? 50mv r sense ? ? ? ? ? ? ? ( r1 + r4 ) r4 ? ? ? performances figure 5 shows the load transient response of the power supply when the load increases from 0a to 10a . it is possible to see response times extremely short (around 1 m s), even if we start from no-load condi- tion. consequently the output voltage variations are extremely reduced. it is possible to reduce this load variation even further by increasing the out- put capacitor (c3) value. vout iout icc icc imax a b figure 4. load variation from 0a to 10a time= 2us/div, vertical = 50mv/div, 2a/div vout = 3.3v iout = 0a 1 m s vin = 5v, vout = 3.3v figure 5. AN964 application note 4/5
information furnished is believed to be accurate and reliable. however, sgs-thomson micr oelectronics assumes no responsibility for the consequences of use of such information nor for any infringement of patents or other rights of third parties which may result from its use. no license is granted by implication or otherwise under any patent or patent rights of sgs-thomson microelectronics. specification men- tioned in this publication are subject to change without notice. this publication supersedes and replaces all information previously supplied. sgs-thomson microelectronics products are not authorized for use as critical components in life support devices or systems without ex- press written approval of sgs-thomson microelectronics. ? 1997 sgs-thomson microelectronics printed in italy all rights reserved sgs-thomson microelectronics group of companies australia - brazil - canada - china - france - germany - italy - japan - korea - malaysia - malta - morocco - the netherlands - singapore - spain - sweden - switzerland - taiwan - thailand - united kingdom - u.s.a. AN964 application note 5/5
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