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ltc 1261 l 1 1261lfa typical application features description switched capacitor regulated voltage inverter the lt c ? 1261 l is a switched - capacitor voltage inverter designed to provide a regulated negative voltage from a single positive supply . the ltc 1261 l operates from a single 2.7 v to 5.25 v supply and provides an adjustable output voltage from C 1.23 v to C 5 v . the ltc 1261 l -4/ ltc 1261 l -4.5 needs a single 4.5 v to 5.25 v supply and provides a fixed output voltage of C4 v to C4.5 v respec - tively . three external capacitors are required : a 0.1 f flying capacitor and an input and output bypass capacitors . an optional compensation capacitor at adj ( comp ) can be used to reduce the output voltage ripple . each version of the ltc 1261 l will supply up to 20 ma output current with guaranteed output regulation of 4.5%. the ltc 1261 l includes an open - drain reg output that pulls low when the output is within 5% of the set value . quiescent current is typically 650 a when operating and 5 a in shutdown . the ltc 1261 l is available in 8- pin msop and so packages . applications n regulated negative voltage from a single positive supply n reg pin indicates output is in regulation n adjustable or fixed output voltages n output regulation : 4.5% n supply current : 650 a typ n shutdown mode drops supply current to 5 a n up to 20 ma output current n requires only three or four external capacitors n available in ms 8 and so -8 packages n gaas fet bias generators n negative supply generators n battery-powered systems n single supply applications l , lt , lt c , lt m , linear technology and the linear logo are registered trademarks of linear technology corporation . all other trademarks are the property of their respective owners . C4 v generator with power valid 1 2 3 4 8 7 6 5 ltc1261l-4 shdn reg out comp v cc c1 + c1 ? gnd c2 0.1f 1261l ta01 c4 3.3f v out = ? 4v at 10ma power valid 5v c1 1f 5v 10k c3* 1 00pf *optional + waveforms for C4 v generator with power valid 0v ?4v out shdn 0.1ms/div 1261l ta02 power valid 5v 5v 0v 0v
ltc 1261 l 2 1261lfa pin configuration absolute maximum ratings supply voltage (note 2) ........................................... 5.5v output voltage (note 3) ............................ 0.3v to C 5.3v total voltage, v cc to v out (note 2) ........................ 10.8v shdn pin ...................................... C0.3v to (v cc + 0.3v) reg pin ........................................................ C0.3v to 6v adj pin ............................. (v out C 0.3v) to (v cc + 0.3v) ( note 1) 1 2 3 4 v cc c1 + c1 ? gnd 8 7 6 5 shdn reg out adj (comp) top view ms8 package 8-lead plastic msop t jmax = 150 c , ja = 200 c / w 1 2 3 4 8 7 6 5 top view shdn reg out adj (comp) v cc c1 + c1 ? gnd s8 package 8-lead plastic so t jmax = 150 c , ja = 135 c / w order information lead free finish tape and reel part marking * package description temperature range ltc 1261 lcms 8# pbf ltc 1261 lcms 8# trpbf ltfm 8- lead plastic msop 0 c to 70 c ltc 1261 lims 8# pbf ltc 1261 lims 8# trpbf ltfm 8- lead plastic msop C40 c to 85 c ltc 1261 lcms 8-4# pbf ltc 1261 lcms 8-4# trpbf ltfn 8- lead plastic msop 0 c to 70 c ltc 1261 lims 8-4# pbf ltc 1261 lims 8-4# trpbf ltfn 8- lead plastic msop C40 c to 85 c ltc 1261 lcms 8-4.5# pbf ltc 1261 lcms 8-4.5# trpbf ltfp 8- lead plastic msop 0 c to 70 c ltc 1261 lims 8-4.5# pbf ltc 1261 lims 8-4.5# trpbf ltfp 8- lead plastic msop C40 c to 85 c ltc 1261 lcs 8# pbf ltc 1261 lcs 8# trpbf 1261 l 8- lead plastic so 0 c to 70 c ltc 1261 lis 8# pbf ltc 1261 lis 8# trpbf 1261 l 8- lead plastic so C40 c to 85 c ltc 1261 lcs 8-4# pbf ltc 1261 lcs 8-4# trpbf 1261 l 4 8- lead plastic so 0 c to 70 c ltc 1261 lis 8-4# pbf ltc 1261 lis 8-4# trpbf 1261 l 4 8- lead plastic so C40 c to 85 c ltc 1261 lcs 8-4.5# pbf ltc 1261 lcs 8-4.5# trpbf 261 l 45 8- lead plastic so 0 c to 70 c ltc 1261 lis 8-4.5# pbf ltc 1261 lis 8-4.5# trpbf 261 l 45 8- lead plastic so C40 c to 85 c consult lt c marketing for parts specified with wider operating temperature ranges . * the temperature grade is identified by a label on the shipping container . consult lt c marketing for information on nonstandard lead based finish parts . for more information on lead free part marking , go to : http :// www . linear . com / leadfree / for more information on tape and reel specifications , go to : http :// www . linear . com / tapeandreel / output short-circuit duration ......................... indefnite commercial temperature range (note 4) .... 0c to 70c industrial temperature range (note 4) ... C40c to 85c storage temperature range ................... C65c to 150c lead temperature (soldering, 10 sec) .................. 300c
ltc 1261 l 3 1261lfa electrical characteristics the denotes the specifcations which apply over the full operating temperature range , otherwise specifcations are at t a = 25 c , c 1 = 0.1 f , c out = 3.3 f unless otherwise noted . ( notes 2, 4) symbol parameter conditions min typ max units v cc supply voltage ltc 1261 lcms 8/ ltc 1261 lcs 8 ltc 1261 lcms 8-4/ ltc 1261 lcs 8-4 ltc 1261 lcms 8-4.5/ ltc 1261 lcs 8-4.5 ( note 5) ( note 5) l l l 2.7 4.35 4.75 5.25 5.25 5.25 v v v v ref reference voltage 1.23 v i cc supply current v cc = 5.25 v , no load , shdn floating v cc = 5.25 v , no load , v shdn = v cc l l 650 5 1500 20 a a f osc internal oscillator frequency v cc = 5 v , v out = C4 v 650 khz v ol reg output low voltage i reg = 1 ma , v cc = 5 v , v out = C4 v l 0.1 0.8 v i reg reg sink current v reg = 0.8 v , v cc = 3.3 v v reg = 0.8 v , v cc = 5 v l l 4 5 8 12 ma ma i adj adjust pin current v adj = 1.23 v l 0.01 1 a v ih shdn input high voltage v cc = 5 v l 2 v v il shdn input low voltage v cc = 5 v l 0.8 v i in shdn input current v shdn = v cc l 2.5 25 a t on turn - on time v cc = 5 v , i out = 10 ma , C1.5 v v out C4 v ( ltc 1261 l ) v cc = 5 v , i out = 5 ma , v out = C4.5 v ( ltc 1261 l ) v cc = 5 v , i out = 10 ma , v out = C4 v ( ltc 1261 l -4) v cc = 5 v , i out = 5 ma , v out = C4.5 v ( ltc 1261 l -4.5) l l l l 250 250 250 250 1500 1500 1500 1500 s s s s v out output regulation ( ltc 1261 l ) 2.70 v v cc 5.25 v , 0 ma i out 10 ma 3.25 v v cc 5.25 v , 0 ma i out 20 ma l l C1.552 C1.552 C1.5 C1.5 C1.448 C1.448 v v 2.70 v v cc 5.25 v , 0 ma i out 5 ma 2.95 v v cc 5.25 v , 0 ma i out 10 ma 3.50 v v cc 5.25 v , 0 ma i out 20 ma l l l C2.070 C2.070 C2.070 C2.0 C2.0 C2.0 C1.930 C1.930 C1.91 v v v 2.95 v v cc 5.25 v , 0 ma i out 5 ma 3.30 v v cc 5.25 v , 0 ma i out 10 ma 3.85 v v cc 5.25 v , 0 ma i out 20 ma l l l C2.587 C2.587 C2.587 C2.5 C2.5 C2.5 C2.413 C2.413 C2.41 v v v 3.40 v v cc 5.25 v , 0 ma i out 5 ma 3.70 v v cc 5.25 v , 0 ma i out 10 ma 4.25 v v cc 5.25 v , 0 ma i out 20 ma l l l C3.105 C3.105 C3.105 C3.0 C3.0 C3.0 C2.895 C2.895 C2.885 v v v 3.85 v v cc 5.25 v , 0 ma i out 5 ma 4.10 v v cc 5.25 v , 0 ma i out 10 ma 4.60 v v cc 5.25 v , 0 ma i out 20 ma l l l C3.622 C3.622 C3.622 C3.5 C3.5 C3.5 C3.378 C3.378 C3.365 v v v output regulation ( ltc 1261 l / ltc 1261 l -4) 4.35 v v cc 5.25 v , 0 ma i out 5 ma 4.60 v v cc 5.25 v , 0 ma i out 10 ma 5.10 v v cc 5.25 v , 0 ma i out 20 ma l l l C4.140 C4.140 C4.140 C4.0 C4.0 C4.0 C3.860 C3.860 C3.83 v v v output regulation ( ltc 1261 l / ltc 1261 l -4.5) 4.75 v v cc 5.25 v , 0 ma i out 5 ma 5.05 v v cc 5.25 v , 0 ma i out 10 ma l l C4.657 C4.657 C4.5 C4.5 C4.343 C4.343 v v i sc output short - circuit current v out = 0 v , v cc = 5.25 v l 100 220 ma note 1: stresses beyond those listed under absolute maximum ratings may cause permanent damage to the device . exposure to any absolute maximum rating condition for extended periods may affect device reliability and lifetime . note 2: all currents into device pins are positive ; all currents out of device pins are negative . all voltages are referenced to ground unless otherwise specified . note 3: the output should never be set to exceed v cc C 10.8 v . note 4: the ltc 1261 lc is guaranteed to meet specifications from 0 c to 70 c and is designed , characterized and expected to meet industrial temperature limits , but is not tested at C40 c and 85 c . the ltc 1261 li is guaranteed to meet specifications from C40 c and 85 c . note 5: the ltc 1261 l -4 and ltc 1261 l -4.5 will operate with less than the minimum v cc specified in the electrical characteristics table , but they are not guaranteed to meet the 4.5% v out specification .
ltc 1261 l 4 1261lfa typical performance characteristics output voltage vs supply voltage minimum required v cc vs v out and i out maximum output current vs supply voltage output voltage vs output current output voltage vs output current output voltage vs supply voltage ( see test circuits ) output current (ma) 0 ? 2.25 output voltage (v) ? 2.00 ? 1.50 ? 1.25 ? 1.00 20 0 1261l g01 ? 1.75 10 5 25 15 30 ? 0.75 ? 0.50 ? 0.25 t a = 25c v out = ? 2v v cc = 2.7v v cc = 3v output current (ma) 0 output voltage (v) ? 3.0 ? 3.1 ? 3.2 ? 3.3 ? 3.4 ? 3.5 ? 3.6 ? 3.7 ? 3.8 ? 3.9 ? 4.0 ? 4.1 ? 4.2 5 10 15 20 1261l g02 25 30 t a = 25c v out = ? 4v v cc = 4.5v v cc = 5v supply voltage (v) 2.5 3.0 3.5 4.0 4.5 5.0 output voltage (v) ? 2.00 ?1.95 1261l g03 ? 2.05 ? 2.10 ?1.90 t a = ? 40c v out = ? 2v t a = 85c t a = 25c supply voltage (v) 4.5 ? 4.15 output voltage (v) ? 4.10 ? 4.05 ? 4.00 ? 3.95 4.7 4.9 5.1 5.3 1261l g04 ? 3.90 ? 3.85 4.6 4.8 5.0 5.2 v out = ? 4v t a = ? 40c t a = 85c t a = 25c output voltage (v) ? 5 2.0 positive supply voltage (v) 2.4 3.2 3.6 4.0 ? 2 ? 1 5.6 1261l g05 2.8 ? 4 ? 3 0 4.4 4.8 5.2 i out = 5ma i out = 10ma i out = 20ma supply voltage (v) 2.7 maximum output current (ma) 40 50 60 5.1 1261l g06 30 20 0 3.3 3.9 4.5 3.0 5.4 3.6 4.2 4.8 10 80 70 t a = 25c v out = ? 2v v out = ? 3v v out = ? 4v supply current vs supply voltage reference voltage vs temperature supply current vs supply voltage supply voltage (v) 0 supply current (ma) 1.0 2.0 3.0 0.5 1.5 2.5 3.0 3.5 4.0 4.5 1261l g07 5.0 2.5 t a = ? 40c t a = 85c t a = 25c v out = ? 2v i out = 0 supply voltage (v) 4.5 0 supply current (ma) 0.5 1.5 2.0 2.5 5.0 3.5 4.7 4.9 5.0 1261l g08 1.0 4.0 4.5 3.0 4.6 4.8 5.1 5.2 5.3 t a = 85c v out = ? 4.5v i out = 0 t a = 25c t a = ? 40c temperature (c) ? 55 1.21 reference voltage (v) 1.22 1.23 1.24 1.25 ? 35 ? 15 5 25 1261l g09 45 65 85 105 125 v cc = 5v adj = 0v
ltc 1261 l 5 1261lfa typical performance characteristics oscillator frequency vs temperature start - up time vs supply voltage output short - circuit current vs temperature temperature (c) ? 40 oscillator frequency (khz) 625 650 675 80 1261l g10 600 575 525 ?10 20 50 ? 25 95 5 35 65 550 725 700 t a = 25c v out = ? 4v v cc = 5v supply voltage (v) 2.5 0 start-up time (ms) 0.1 0.3 0.4 0.5 0.7 1261l g11 0.2 0.6 3.0 3.5 4.0 4.5 5.0 v out = ? 2v v out = ? 4v t a = 25c i out = 10ma temperature (c) ? 40 short-circuit current (ma) 80 100 120 80 1261l g12 60 40 0 ? 10 20 50 ? 25 95 5 35 65 20 160 140 v cc = 5.25v v cc = 5v v cc = 3v v cc = 2.7v pin functions v cc ( pin 1): power supply . this requires an input voltage between 2.7 v and 5.25 v . v cc must be bypassed to ground with at least a 1 f capacitor placed in close proximity to the chip . see the applications information section for details . c 1 + ( pin 2): c 1 positive input . connect a 0.1 f capacitor between c 1 + and c 1 C . c 1 C ( pin 3): c 1 negative input . connect a 0.1 f capacitor from c 1 + to c 1 C . gnd ( pin 4): ground . connect to a low impedance ground . a ground plane will help to minimize regulation errors . adj ( comp for fixed versions ) ( pin 5): output adjust / compensation pin . for adjustable parts this pin is used to set the output voltage . the output voltage is divided down with an external resistor divider and fed back to this pin to set the regulated output voltage . typically the resistor string should draw 10 a from the output to minimize errors due to the bias current at the adjust pin . fixed output voltage parts have the internal resistor string connected to this pin inside the package . the pin can be used to trim the output voltage if desired . it can also be used as an optional feedback compensation pin to reduce output ripple on both the adjustable and fixed output voltage parts . see the applications information section for more information on compensation and output ripple . out ( pin 6): negative voltage output . this pin must be bypassed to ground with a 1 f or larger capacitor . the value of the output capacitor and its esr have a strong effect on output ripple . see the applications information section for more details . reg ( pin 7): this is an open - drain output that pulls low when the output voltage is within 5% of the set value . it will sink 5 ma to ground with a 5 v supply . the external circuitry must provide a pull - up or reg will not swing high . the voltage at reg may exceed v cc and can be pulled up to 6 v above ground without damage . shdn ( pin 8): shutdown . when this pin is at ground the ltc 1261 l operates normally . an internal 5 a pull - down keeps shdn low if it is left floating . when shdn is pulled high , the ltc 1261 l enters shutdown mode . in shutdown , the charge pump is disabled , the output collapses to 0 v and the quiescent current drops to 5 a typically .
ltc 1261 l 6 1261lfa test circuits fixed output adjustable output 1 2 3 4 8 7 6 5 shdn reg out comp v cc c1 + c1 ? gnd 0.1f 1261l tco1 3.3f v out = ? 4v (ltc1261l-4) v out = ? 4.5v (ltc1261l-4.5) 5v 10f + + ltc1261l-x 1 2 3 4 8 7 6 5 shdn reg out adj v cc c1 + c1 ? gnd v cc 0.1f 1261l tco2 3.3f v out + ltc1261l applications information the ltc 1261 l uses an inverting charge pump to generate a regulated negative output voltage that is either equal to or less than the supply voltage . the ltc 1261 l needs only three external capacitors and is available in the msop and so -8 packages theory of operation a block diagram of the ltc 1261 l is shown in figure 1. the heart of the ltc 1261 l is the charge pump core shown in the dashed box . it generates a negative output voltage by first charging the flying capacitor ( c 1) between v cc and ground . it then connects the top of the flying capaci - tor to ground , forcing the bottom of the flying capacitor to a negative voltage . the charge on the flying capacitor is transferred to the output bypass capacitor , leaving it charged to the negative output voltage . this process is driven by the internal 650 khz clock . figure 1 shows the charge pump configuration . with the clock low , c 1 is charged to v cc by s 1 and s 3. at the next rising clock edge , s 1 and s 3 are open and s 2 and s 4 closed . s 2 connects c 1 + to ground , c 1 C is connected to the output by s 4. the charge in c 1 is transferred to c out , setting it to a negative voltage . the output voltage is monitored by comp 1 which com - pares a divided replica of the output at adj ( comp for fixed output voltage parts ) to the internal reference . at the beginning of a cycle the clock is low , forcing the output of the and gate low and charging the flying capacitor . the next rising clock edge sets the rs latch , setting the charge pump to transfer charge from the flying capacitor to the output capacitor . as long as the output is below the set point , comp 1 stays low , the latch stays set and the charge pump runs at the full 50% duty cycle of the clock gated through the and gate . as the output approaches the set voltage , comp 1 will trip whenever the divided signal exceeds the internal 1.23 v reference relative to out . this resets the rs latch and truncates the clock pulses , reducing the amount of charge transferred to the output capacitor and regulating the output voltage . if the output exceeds the set point , comp 1 stays high , inhibiting the rs latch and disabling the charge pump . comp 2 also monitors the divided signal at adj but it is connected to a 1.17 v reference , 5% below the main refer - ence voltage . when the divided output exceeds this lower reference voltage indicating that the output is within 5% of the set value , comp 2 goes high turning on the reg output transistor . this is an open - drain n - channel device capable of sinking 4 ma with a 3.3 v v cc and 5 ma with a 5 v v cc . when in the off state ( divided output is more than 5% below v ref ) the drain can be pulled above v cc without damage up to a maximum of 6 v above ground . note that the reg output only indicates if the magnitude of the output is below the magnitude of the set point by 5% ( i . e ., v out > C4.75 v for a C5 v set point ). if the magnitude of the output is forced higher than the magnitude of the set point ( i . e ., to C5.25 v when the output is set for C5 v ) the reg output will stay low .
ltc 1261 l 7 1261lfa applications information ? + ? + clk 650khz s r q s2 s3 s1 v cc v out 1261l f01 60mv 1.17v v ref = 1.23v c out c1 ? c1 + c1 s4 r1 internally connected for fixed output voltage parts r2 comp1 comp2 adj (comp) reg out + figure 1. block diagram output ripple output ripple in the ltc 1261 l is present from two sources ; voltage droop at the output capacitor between clocks and frequency response of the regulation loop . voltage droop is easy to calculate . with a typical clock frequency of 650 khz , the charge on the output capacitor is refreshed once every 1.54 s . with a 15 ma load and a 3.3 f output capacitor , the output will droop by : i l o a d ? t c o u t ? ? ? ? ? ? = 15 m a 1.54 s 3.3 f ? ? ? ? ? ? = 7 m v this can be a significant ripple component when the output is heavily loaded , especially if the output capacitor is small . if absolute minimum output ripple is required , a 10 f or greater output capacitor should be used . regulation loop frequency response is the other major contributor to output ripple . the ltc 1261 l regulates the output voltage by limiting the amount of charge trans - ferred to the output capacitor on a cycle - by - cycle basis . the output voltage is sensed at the adj pin ( comp for fixed output voltage versions ) through an internal or external resistor divider from the out pin to ground . as the flying capacitor is first connected to the output , the output voltage begins to change quite rapidly . as soon as it exceeds the set point comp 1 trips , switching the state of the charge pump and stopping the charge transfer . because the rc time constant of the capacitors and the switches is quite short , the adj pin must have a wide ac bandwidth to be able to respond to the output in time . external parasitic capacitance at the adj pin can reduce the bandwidth to the point where the comparator cannot respond by the time the clock pulse finishes . when this happens the comparator will allow a few complete pulses through , then overcorrect and disable the charge pump until the output drops below the set point . under these conditions the output will remain in regulation but the output ripple will increase as the comparator hunts for the correct value . to prevent this from happening , an external capacitor can be connected from adj ( or comp for fixed output voltage parts ) to ground to compensate for external parasitics and
ltc 1261 l 8 1261lfa applications information increase the regulation loop bandwidth ( figure 2). this sounds counter intuitive until we remember that the internal reference is generated with respect to out , not ground . the feedback loop actually sees ground as its output , thus the compensation capacitor should be connected across the top of the resistor divider , from adj ( or comp ) to ground . by the same token , avoid adding capacitance between adj ( or comp ) and v out . this will slow down the feedback loop and increase output ripple . a 100 pf capacitor from adj or comp to ground will compensate the loop properly under most conditions for fixed voltage versions of the ltc 1261 l . for the adjustable ltc 1261 l , the capacitor value will be dependent upon the values of the external resistors in the divider network . capacitor selection capacitor sizing the performance of the ltc 1261 l is affected by the capacitors to which it connects . the ltc 1261 l requires bypass capacitors to ground for both the v cc and out pins . the input capacitor provides most of ltc 1261 l s supply current while it is charging the flying capacitors . this capacitor should be mounted as close to the package as possible and its value should be at least ten times larger than the flying capacitor . ceramic capacitors generally provide adequate performance . avoid using a tantalum capacitor as the input bypass unless there is at least a 0.1 f ceramic capacitor in parallel with it . the charge pump capacitor is somewhat less critical since its peak current is limited by the switches inside the ltc 1261 l . most ap - plications should use a 0.1 f as the flying capacitor value . conveniently , ceramic capacitors are the most common type of 0.1 f capacitor and they work well here . usually the easiest solution is to use the same capacitor type for both the input bypass capacitor and the flying capacitor . in applications where the maximum load current is well - defined and output ripple is critical or input peak currents need to be minimized , the flying capacitor value can be comp1 1.23v r2 v out adj/comp resistors are internal for fixed output voltage parts 1261l f02 r1 c c 100pf to charge pump ref + ? figure 2. regulator loop compensation output filtering if extremely low output ripple (<5 mv ) is required , additional output filtering is required . because the ltc 1261 l uses a high 650 khz switching frequency , fairly low value rc or lc networks can be used at the output to effectively filter the output ripple . a 10 series output resistor and a 3.3 f capacitor will cut output ripple to below 3 mv ( figure ?3). further reductions can be obtained with larger filter capacitors or by using an lc output filter . ltc1261l-4 v cc 5v c1 + c1 ? 4 6 5 2 3 out 0.1f 100pf 3.3f 10 comp 1261l f03 gnd v out = ? 4v 1f + 3.3f + figure 3. output filter cuts ripple below 3 mv
ltc 1261 l 9 1261lfa applications information tailored to the application . reducing the value of the flying capacitor reduces the amount of charge transferred with each clock cycle . this limits maximum output current , but also cuts the size of the voltage step at the output with each clock cycle . the smaller capacitor draws smaller pulses of current out of v cc as well , limiting peak currents and reducing the demands on the input supply . table 1 shows recommended values of flying capacitor vs maximum load capacity . table 1. typical max load ( ma ) vs flying capacitor value at t a = 25 c , v out = C4 v flying capacitor value ( f ) max load ( ma ) v cc = 5 v 0.1 20 0.047 15 0.033 10 0.022 5 0.01 1 the output capacitor performs two functions : it provides output current to the load during half of the charge pump cycle and its value helps to set the output ripple voltage . for applications that are insensitive to output ripple , the output bypass capacitor can be as small as 1 f . larger output capacitors will reduce output ripple further at the expense of turn - on time . capacitor esr output capacitor equivalent series resistance ( esr ) is another factor to consider . excessive esr in the output capacitor can fool the regulation loop into keeping the output artificially low by prematurely terminating the charging cycle . as the charge pump switches to recharge the output a brief surge of current flows from the flying capacitors to the output capacitor . this current surge can be as high as 100 ma under full load conditions . a typical 3.3 f tantalum capacitor has 1 or 2 of esr ; 100 ma 2 = 200 mv . if the output is within 200 mv of the set point this additional 200 mv surge will trip the feedback comparator and terminate the charging cycle . the pulse dissipates quickly and the comparator returns to the correct state , but the rs latch will not allow the charge pump to respond until the next clock edge . this prevents the charge pump from going into very high frequency oscillation under such conditions but it also creates an output error as the feedback loop regulates based on the top of the spike , not the average value of the output ( figure 4). the resulting output voltage behaves as if a resistor of value c esr ( i pk / i ave ) was placed in series with the output . to avoid this nasty sequence of events , connect a 0.1 f ceramic capacitor in parallel with the larger output capacitor . the ceramic capacitor will eat the high frequency spike , preventing it from fooling the feedback loop , while the larger but slower tantalum or aluminum output capacitor supplies output current to the load between charge cycles . low esr output cap clock v out average v set comp1 output v out high esr output cap v out average v set comp1 output v out 1261l f04 figure 4. output ripple with low and high esr capacitors note that esr in the flying capacitor will not cause the same condition ; in fact , it may actually improve the situation by cutting the peak current and lowering the amplitude of the spike . however , more flying capacitor esr is not neces - sarily better . as soon as the rc time constant approaches half of a clock period ( the time the capacitors have to share charge at full duty cycle ) the output current capability of the ltc 1261 l starts to diminish . for a 0.1 f flying capacitor , this gives a maximum total series resistance of : 1 2 t c l k c f l y ? ? ? ? ? ? = 1 2 1 650 k h z ? ? ? ? ? ? / 0.1 f = 7.7 most of this resistance is already provided by the internal switches in the ltc 1261 l . more than 1 or 2 of esr on the flying capacitors will start to affect the regulation at maximum load .
ltc 1261 l 10 1261lfa applications information typical applications resistor selection resistor selection is easy with the fixed output voltage versions of the ltc 1261 l no resistors are needed ! se - lecting the right resistors for the adjustable parts is only a little more difficult . a resistor divider should be used to divide the signal at the output to give 1.23 v at the adj pin with respect to v out ( figure 5). the ltc 1261 l uses a positive reference with respect to v out , not a negative reference with respect to ground ( figure 1 shows the reference connection ). be sure to keep this in mind when connecting the resistors ! if the initial output is not what you expected , try swapping the two resistors . ltc1261l gnd r1 4 5 6 1261l f05 v out = ?1.23v r2 r1 + r2 r2 adj out ( ) figure 5. external resistor connections 1 mv ripple , 5 v input , C 4 v output gaas fet bias generator 5 v input , C 4 v output gaas fet bias generator 1 2 3 4 8 7 6 5 shdn reg out comp v cc c1 + c2 ? gnd 100pf 1261 ta03 3.3f ? 4v bias 10k p-channel power switch 0.1f shutdown 5v v bat ltc1261l-4 gaas transmitter 1f + 1 2 3 4 8 7 6 5 shdn reg out comp v cc c1 + c2 ? gnd 100pf 1261 ta04 10f ? 4v bias 100h 10k p-channel power switch 0.1f shutdown 5v v bat 10f ltc1261l-4 gaas transmitter 1f + + the ltc 1261 l can be internally configured for other fixed output voltages . contact the linear technology marketing department for details .
ltc 1261 l 11 1261lfa typical applications 5 v input , C 0.5 v output gaas fet bias generator 1 2 3 4 8 7 6 5 shdn reg out adj v cc c1 + c2 ? gnd 100pf 1261 ta05 ? 0.5v bias 10k p-channel power switch 0.1f shutdown 5v v bat 3.3f ltc1261l gaas transmitter 43.2k 1% 12.4k 1% 1f + package description please refer to http :// www . linear . com / designtools / packaging / for the most recent package drawings . msop (ms8) 0307 rev f 0.53 0.152 (.021 .006) seating plane note: 1. dimensions in millimeter/(inch) 2. drawing not to scale 3. dimension does not include mold flash, protrusions or gate burrs. mold flash, protrusions or gate burrs shall not exceed 0.152mm (.006") per side 4. dimension does not include interlead flash or protrusions. interlead flash or protrusions shall not exceed 0.152mm (.006") per side 5. lead coplanarity (bottom of leads after forming) shall be 0.102mm (.004") max 0.18 (.007) 0.254 (.010) 1.10 (.043) max 0.22 ? 0.38 (.009 ? .015) typ 0.1016 0.0508 (.004 .002) 0.86 (.034) ref 0.65 (.0256) bsc 0 ? 6 typ detail ?a? detail ?a? gauge plane 1 2 3 4 4.90 0.152 (.193 .006) 8 7 6 5 3.00 0.102 (.118 .004) (note 3) 3.00 0.102 (.118 .004) (note 4) 0.52 (.0205) ref 5.23 (.206) min 3.20 ? 3.45 (.126 ? .136) 0.889 0.127 (.035 .005) recommended solder pad layout 0.42 0.038 (.0165 .0015) typ 0.65 (.0256) bsc ms8 package 8-lead plastic msop (reference ltc dwg # 05-08-1660 rev f)
ltc 1261 l 12 1261lfa package description please refer to http :// www . linear . com / designtools / packaging / for the most recent package drawings . .016 ? .050 (0.406 ? 1.270) .010 ? .020 (0.254 ? 0.508) 45 0? 8 typ .008 ? .010 (0.203 ? 0.254) so8 rev g 0212 .053 ? .069 (1.346 ? 1.752) .014 ? .019 (0.355 ? 0.483) typ .004 ? .010 (0.101 ? 0.254) .050 (1.270) bsc 1 2 3 4 .150 ? .157 (3.810 ? 3.988) note 3 8 7 6 5 .189 ? .197 (4.801 ? 5.004) note 3 .228 ? .244 (5.791 ? 6.197) .245 min .160 .005 recommended solder pad layout .045 .005 .050 bsc .030 .005 typ inches (millimeters) note: 1. dimensions in 2. drawing not to scale 3. these dimensions do not include mold flash or protrusions. mold flash or protrusions shall not exceed .006" (0.15mm) 4. pin 1 can be bevel edge or a dimple s8 package 8-lead plastic small outline (narrow .150 inch) (reference ltc dwg # 05-08-1610 rev g)
ltc 1261 l 13 1261lfa information furnished by linear technology corporation is believed to be accurate and reliable . however , no responsibility is assumed for its use . linear technology corporation makes no representa - tion that the interconnection of its circuits as described herein will not infringe on existing patent rights . revision history rev date description page number a 9/12 added i - grade option modified output regulation specifications modified package / order information tables modified notes 4 and 5 throughout 1, 3 2 4
ltc 1261 l 14 1261lfa linear technology corporation 1630 mccarthy blvd., milpitas, ca 95035-7417 (408) 432-1900 fax : (408) 434-0507 www.linear.com linear technology corporation 1999 lt 0912 rev a ? printed in usa related parts typical applications minimum parts count C 4.5 v generator low output voltage generator 2 3 5 6 ltc1261l adj out c1 + c1 ? 1261l ta06 3.3f = v cc ? 9.92a (r s + 124k) = ? 0.5v (r s = 432k) = ? 1v (r s = 487k) 1n5817 1 5v 4 0.1f 1f 100pf r s v out v cc gnd 124k + 1 2 3 4 8 7 6 5 shdn reg out comp v cc c1 + c1 ? gnd 0.1f 1261l ta07 3.3f v out = ? 4.5v at 5ma 5v 1f + ltc1261l-4.5 part number description comments lt 1121 micropower low dropout regulator with shutdown 0.4 v dropout voltage at 150 ma , low noise , switched capacitor regulated voltage inverter ltc 1429 clock synchronized switched capacitor regulated voltage inverter gaas fet bias ltc 1503 -1.8/ ltc 1503 -2 high efficiency inductorless step - down dc / dc converter fixed 1.8 v or 2 v output from 2.4 v to 6 v input , i out = 100 ma ltc 1514 / ltc 1515 step - up / step - down switched capacitors dc / dc converters v in : 2 v to 10 v , v out is fixed or adjustable , i out up to 50 ma ltc 1516 micropower , regulated 5 v charge pump dc / dc converter i out = 20 ma ( v in 2 v ), i out = 50 ma ( v in 3 v ) ltc 1517 -5 micropower , regulated 5 v charge pump dc / dc converter ltc 1522 without shutdown and packaged in sot -23 ltc 1522 micropower , regulated 5 v charge pump dc / dc converter i out = 10 ma ( v in 2.7 v ), i out = 20 ma ( v in 3 v ) ltc 1550 l / ltc 1551 l low noise switched capacitor regulated voltage inverter gaas fet bias with linear regulator , <1 mv ripple , msop ltc 1555 / ltc 1556 sim power supply and level translator step - up / step - down sim power supply and level translators lt 1611 1.4 mhz inverting mode switching regulator C5 v at 150 ma from a 5 v input , 5- lead sot -23 lt 1614 inverting 600 khz switching regulator with low battery detector C5 v at 200 ma from 5 v input in msop lt 1617 / lt 1617 -1 micropower inverting dc / dc converters C15 v at 12 ma from 2.5 v input , 5- lead sot -23 ltc 1682 / ltc 1682 -3.3/ ltc 1682 -5 low noise doubler charge pumps output noise = 60 v rms , 2.5 v to 5.5 v output ltc 1754 -5 micropower , regulated 5 v charge pump with shutdown in sot -23 i cc = 13 a , i out = 50 ma ( v in 3 v ), i out = 25 ma ( v in 2.7 v )

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