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| ltc 1261 1 1261fb for more information www.linear.com/ltc1261 typical application features description switched capacitor regulated voltage inverter the lt c ? 1261 is a switched-capacitor voltage inverter designed to provide a regulated negative voltage from a single positive supply. the ltc1261cs operates from a single 3v to 8v supply and provides an adjustable output voltage from C1.25v to C8v. an on-chip resistor string allows the ltc1261cs to be configured for output voltages of C3.5v, C4 v, C4.5v or C5v with no external components. the ltc1261cs8 is optimized for applica- tions which use a 5v or higher supply or which require low output voltages. it requires a single external 0.1f capacitor and provides adjustable and fixed output voltage options in 8-lead so packages. the ltc1261cs requires one or two external 0.1f capacitors, depending on input voltage. both versions require additional external 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 ltc1261 will supply up to 12ma output current with guaranteed output regulation of 5%. the ltc1261 includes an open-drain reg output which pulls low when the output is within 5% of the set value. output ripple is typically as low as 5mv. quiescent current is typically 600a when operating and 5a in shutdown. the ltc1261 is available in a 14-lead narrow body so package and an 8-lead so package. applications n regulated negative voltage from a single positive supply n can provide regulated C5v from a 3v supply n reg pin indicates output is in regulation n low output ripple: 5mv typ n supply current: 600a typ n shutdown mode drops supply current to 5a n up to 15ma output current n adjustable or fixed output voltages n requires only three or four external capacitors n available in 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. 1 2 3 4 8 7 6 5 ltc1261-4 shdn reg out comp v cc c1 + c1 ? gnd c2 0.1f c4 3.3f v out = ?4v at 10ma power valid 5v c1 1f 5v 10k c3* 100pf *optional + ltc1261 ? ta01 0v 0v 0v 0.2ms/div 5v ?4v power valid shdn out 5v ltc1261 ? ta02 C4v generator with power valid waveforms for C4v generator with power valid
ltc 1261 2 1261fb for more information www.linear.com/ltc1261 absolute maximum ratings supply voltage (note 2) ............................................. 9v output voltage (note 5) ............................... 0.3v to C 9v total voltage, v cc to v out (note 2) ........................... 12v input voltage shdn pin ..................................... C0.3v to v cc + 0.3v reg pin ................................................. C0.3v to 12v adj, r o, r1, r adj .............. v out C 0.3v to v cc + 0.3v (note 1) order information lead free finish tape and reel part marking* package description temperature range ltc1261cs8#pbf ltc1261cs8#trpbf 1261 8-lead plastic s0 0c to 70c ltc1261is8#pbf ltc1261is8#trpbf 1261 8-lead plastic s0 C40c to 85c ltc1261cs8-4#pbf ltc1261cs8-4#trpbf 12614 8-lead plastic s0 0c to 70c ltc1261cs8-4.5#pbf ltc1261cs8-4.5#trpbf 126145 8-lead plastic s0 0c to 70c ltc1261cs#pbf ltc1261cs#trpbf ltc1261cs 14-lead plastic s0 0c to 70c 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/ 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 *for fixed versions t jmax = 150c, ja = 150c/w top view s package 14-lead plastic so 1 2 3 4 5 6 7 14 13 12 11 10 9 8 nc c1 + c1 ? c2 + c2 ? gnd r0 v cc shdn reg out adj r adj r1 t jmax = 150c, ja = 110c/w pin configuration output short-circuit duration .......................... indefnite commercial temperature range (note 7) .... 0c to 70c industrial temperature range (note 7) ... C40c to 85c storage temperature range ................... C65c to 150c lead temperature (soldering, 10 sec) .................. 300c ltc 1261 3 1261fb for more information www.linear.com/ltc1261 electrical characteristics the l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at t a = 25c. v cc = 3v to 6.5v unless otherwise specified. symbol parameter conditions 0c t a 70c C40c t a 85c (note 7) units min typ max min typ max v ref reference voltage l 1.20 1.24 1.28 1.20 1.24 1.28 v i s supply current no load, shdn floating, doubler mode no load, shdn floating, tripler mode no load, v shdn = v cc l l l 600 900 5 1000 1500 20 600 900 5 1500 2000 20 a a a f osc internal oscillator frequency 550 550 khz p eff power efficiency 65 65 % v ol reg output low voltage i reg = 1ma l 0.1 0.8 0.1 0.8 v i reg reg sink current v reg = 0.8v, v cc = 3.3v v reg = 0.8v, v cc = 5.0v l l 5 8 8 15 5 8 8 15 ma ma i adj adjust pin current v adj = 1.24v l 0.01 1 0.01 1 a v ih shdn input high voltage l 2 2 v v il shdn input low voltage l 0.8 0.8 v i in shdn input current v shdn = v cc l 5 20 5 25 a t on turn -on time i out = 15 ma 500 500 s doubler mode. v cc = 5v 10%, c1 = 0.1f, c2 = 0 (note 4), c out = 3.3f unless otherwise specified. ?v out output regulation (note 2) C1.24v v out C4v, 0 i out 8ma C1.24v v out C4v, 0 i out 7ma C4v v out C5v, 0 i out 8ma (note 6) l l 1 2 5 1 2 5 % % % i sc output short- circuit current v out = 0v l 60 125 60 125 ma v rip output ripple voltage i out = 5ma, v out = C4v 5 5 mv ltc1261cs only. tripler mode. v cc = 2.7v, c1 = c2 = 0.1f (note 4), c out = 3.3f unless otherwise specified. ?v out output regulation C1.24v v out C4v, 0 i out 5ma l 1 5 1 5 % i sc output short- circuit current v out = 0v l 60 125 60 125 ma v rip output ripple voltage i out = 5ma, v out = C4v 5 5 mv ltc1261cs only. tripler mode. v cc = 3.3v 10%, c1 = c2 = 0.1f (note 4), c out = 3.3f unless otherwise specified. ?v out output regulation (note 2) C1.24v v out C4.5v, 0 i out 6ma C4.5v v out C5v, 0 i out 3.5ma l l 1 2 5 5 1 2 5 % % i sc output short- circuit current v out = 0v l 35 75 35 75 ma v rip output ripple voltage i out = 5ma, v out = C4v 5 5 mv ltc1261cs only. tripler mode. v cc = 5v 10%, c1 = c2 = 0.1f (note 4), c out = 3.3f unless otherwise specified. ?v out output regulation C1.24v v out C4v, 0 i out 12ma C4v v out C5v, 0 i out 10ma l l 1 2 5 5 1 2 5 5 % % i sc output short- circuit current v out = 0v l 35 75 35 75 ma v rip output ripple voltage i out = 5ma, v out = C4v 5 5 mv 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: all typicals are given at t a = 25c. note 4: c1 = c2 = 0.1f means the specifications apply to tripler mode where v cc C v out = 3v cc (ltc1261cs only; the ltc1261cs8 cannot be connected in tripler mode) with c1 connected between c1 + and c1 C and c2 connected between c2 + and c2 C . c2 = 0 implies doubler mode where v cc C v out = 2v cc ; for the ltc1261cs this means c1 connects from c1 + to c2 C with c1 C and c2 + floating. for the ltc1261cs8 in doubler mode, c1 connects from c1 + to c1 C ; there are no c2 pins. note 5: setting output to ltc 1261 5 1261fb for more information www.linear.com/ltc1261 pin functions nc (pin 1/na): no internal connection. c1 + (pin 2/pin 2): c1 positive input. connect a 0.1f capacitor between c1 + and c1 C . with the ltc1261cs in doubler mode, connect a 0.1f capacitor from c1 + to c2 C . c1 C (pin 3/pin 3): c1 negative input. connect a 0.1f capacitor from c1 + to c1 C . with the ltc1261cs in doubler mode only, c1 C should float. c2 + (pin 4/na): c2 positive input. in tripler mode connect a 0.1f capacitor from c2 + to c2 C . this pin is used with the ltc1261cs in tripler mode only; in doubler mode this pin should float. c2 C (pin 5/na): c2 negative input. in tripler mode con- nect a 0.1f capacitor from c2 + to c2 C . in doubler mode connect a 0.1f capacitor from c1 + to c2 C . gnd (pin 6/pin 4): ground. connect to a low impedance ground. a ground plane will help to minimize regulation errors. r 0 ( pin 7/ na ): internal resistor string , 1 st tap . see table ?2 in the applications information section for information on internal resistor string pin connections vs output voltage. r1 (pin 8/na): internal resistor string, 2nd tap . r adj ( pin 9/na): internal resistor string output. connect this pin to adj to use the internal resistor divider . see table 2 in the applications information section for information on internal resistor string pin connections vs output voltage. adj (comp for fixed versions ) (pin 10/pin 5): output adjust/compensation pin. for adjustable parts this pin is used to set the output voltage. the output voltage should be divided down with a resistor divider and fed back to this pin to set the regulated output voltage. the resistor divider can be external or the internal divider string can be used if it can provide the required output voltage. typically (cs/cs8) the resistor string should draw 10a from the output to minimize errors due to the bias current at the adjust pin. fixed output parts have the internal resistor string con- nected 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 adjustable and fixed output voltage parts. see applications information section for more information on compensation and output ripple. out (pin 11/pin 6): negative voltage output. this pin must be bypassed to ground with a 1f or larger capaci- tor; it must be at least 3.3f to provide specified output ripple. the size of the output capacitor has a strong effect on output ripple. see the applications information section for more details. reg (pin 12/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 8ma to ground with a 5v 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 12v above ground without damage. shdn ( pin 13/pin 8): shutdown. when this pin is at ground the ltc 1261 operates normally . an internal 5 a pull - down keeps shdn low if it is left floating. when shdn is pulled high, the ltc1261 enters shutdown mode. in shutdown the charge pump stops, the output collapses to 0v and the quiescent current drops to 5a typically. v cc (pin 14/pin 1): power supply. this requires an input voltage between 3v and 6.5v. certain combinations of output voltage and operating mode may place additional restrictions on the input voltage. v cc must be bypassed to ground with at least a 0.1f capacitor placed in close proximity to the chip. see the applications information section for details. ltc 1261 6 1261fb for more information www.linear.com/ltc1261 applications information modes of operation the ltc1261 uses a charge pump to generate a nega- tive output voltage that can be regulated to a value either higher or lower than the original input voltage. it has two modes of operation: a doubler inverting mode, which can provide a negative output equal to or less than the positive power supply and a tripler inverting mode, which can provide negative output voltages either larger or smaller in magnitude than the original positive supply. the tripler offers greater versatility and wider input range but requires four external capacitors and a 14-lead pack- age. the doubler offers the so-8 package and requires only three external capacitors. doubler mode doubler mode allows the ltc1261 to generate negative output voltage magnitudes up to that of the supply volt- age, creating a voltage between v cc and out of up to two times v s . in doubler mode the lt1261 uses a single flying capacitor to invert the input supply voltage, and the output voltage is stored on the output bypass capacitor between switch cycles. the ltc1261cs8 is always configured in doubler mode and has only one pair of flying capacitor pins (figure 1a). the ltc1261cs can be configured in doubler mode by connecting a single flying capacitor between the c1 + and c2 C pins. c1 C and c2 + should be left floating (figure 1b). tripler mode the ltc1261cs can be used in a tripler mode which can generate negative output voltages up to twice the supply voltage. the total voltage between the v cc and out pins can be up to three times v s . for example, tripler mode can be used to generate C 5v from a single positive 3.3v supply . tripler mode requires two external flying capacitors . the first connects between c1 + and c1 C and the second between c2 + and c2 C (figure 1c). because of the relatively high voltages that can be generated in this mode , care must be taken to ensure that the total input-to-output voltage never exceeds 12v or the ltc1261 may be damaged. in most applications the output voltage will be kept in check by the regulation loop. damage is possible however, with supply voltages above 4v in tripler mode and above 6v in doubler mode. as the input supply voltage rises the allowable output voltage drops, finally reaching C 4v with an 8.5 v supply. to avoid this problem use doubler mode whenever possible with high input supply voltages. 1 2 3 4 5 6 7 14 13 12 11 10 9 8 c1 c2 c1 + c1 ? c2 + ltc1261 ltc1261 ? f01 c2 ? 1 2 3 4 5 6 7 14 13 12 11 10 9 8 c1 c1 + c1 ? c2 + ltc1261 c2 ? 1 2 3 4 8 7 6 5 c1 + c1 ? c1 ltc1261 a.) ltc1261cs8 doubler mode b.) ltc1261cs doubler mode c.) ltc1261cs tripler mode figure 1. flying capacitor connections theory of operation a block diagram of the ltc1261 is shown in figure 2. the heart of the ltc1261 is the charge pump core shown in the dashed box. it generates a negative output voltage by first charging the flying capacitors between v cc and ground. it then stacks the flying capacitors on top of each other and connects the top of the stack to ground forcing the bottom of the stack to a negative voltage. the charge on the flying capacitors is transferred to the output bypass capacitor , leaving it charged to the negative output voltage . this process is driven by the internal clock. figure 2 shows the charge pump configured in tripler mode . with the clock low, c1 and c2 are charged to v cc by s1, s3, s5 and s7. at the next rising clock edge, s1, s3, s5 and s7 open and s2, s4 and s6 close, stacking c1 and c2 on top of each other. s2 connects c1 + to ground, s4 connects c1 C to c2 + and c2 C is connected to the output by s6. the charge in c1 and c2 is transferred to c out , setting it to a negative voltage. doubler mode works the same way except that the single flying capacitor (c1) is connected between c1 + and c2 C . s3, s4 and s5 dont do anything useful in doubler mode. c1 is charged initially by s1 and s7 and connected to the output by s2 and s6. ltc 1261 7 1261fb for more information www.linear.com/ltc1261 applications information figure 2. block diagram ? + ? + clk 550khz s r q s2 s3 s7 s1 s5 v cc out v out ltc1261 ? f02 60mv 1.18v v ref = 1.24v r adj * r1* r0* *ltc1261cs14 only c out c1 ? c1 + c1 s4 s6 c2 ? 50k 100k 226k internally connected for fixed output voltage parts 124k c2 + c2 comp 1 comp 2 adj/comp reg + the output voltage is monitored by comp 1 which compares a divided replica of the output at adj (comp for fixed output 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 capacitors. the next rising clock edge sets the rs latch, setting the charge pump to transfer charge from the flying capacitors to the output capacitor. as long as the output is below the set point, comp1 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, comp1 will trip whenever the divided signal exceeds the internal 1.24v 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, comp1 stays high, inhibiting the rs latch and disabling the charge pump. comp2 also monitors the divided signal at adj but it is connected to a 1.18v 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, comp2 goes high turning on the reg output transistor. this is an open drain n-channel device capable of sinking 5ma with a 3.3v v cc and 8ma with a 5v v cc . when in the off state (divided output more than 5% below v ref ) the drain can be pulled above v cc without damage up to a maximum of 12v 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 C5v set point). if the magnitude of the output is forced higher than the magnitude of the set point ( i.e., to C 6v when the output is set for C5v ) the reg output will stay low. ltc 1261 8 1261fb for more information www.linear.com/ltc1261 applications information output ripple output ripple in the ltc1261 comes 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 550khz, the charge on the output capacitor is refreshed once every 1.8s. with a 15ma load and a 3.3f output capacitor, the output will droop by: i load ? ? t c out ? ? ? ? ? ? = 15ma ? 1.8s 3.3f ? ? ? ? ? ? = 8.2mv 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 10f or greater output capacitor should be used. regulation loop frequency response is the other major contributor to output ripple. the ltc1261 regulates the output voltage by limiting the amount of charge transferred to the output capacitor on a cycle-by-cycle basis. the output voltage is sensed at the adj pin (comp for fixed output versions) through an internal or external resistor divider from the out pin to ground. as the flying capaci- tors are first connected to the output, the output voltage begins to change quite rapidly. as soon as it exceeds the set point comp1 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 parts) to ground to compensate for external parasitics and increase the regulation loop bandwidth (figure 3). this sounds coutnterintuitive until we remember that the internal reference is generated with respect to out, not ground. comp 1 1.24v r2 v out adj/comp resistors are internal for fixed output parts ltc1261 ? f03 r1 c c 100pf to charge pump ref + ? figure 3. regulator loop compensation 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 100pf capacitor from adj or comp to ground will compensate the loop properly under most conditions. output filtering if extremely low output ripple (< 5mv) is required, addi- tional output filtering is required. because the ltc1261 uses a high 550khz 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 3.3f capacitor will cut output ripple to below 3mv (figure ?4). further reductions can be obtained with larger filter capacitors or by using an lc output filter. ltc 1261 9 1261fb for more information www.linear.com/ltc1261 applications information figure 4. output filter cuts ripple below 3mv ltc1261cs8-4 v cc 5v c1 + c1 ? 4 6 5 2 3 out 0.1f 100pf 3.3f 10 comp ltc1261 ? f04 gnd v out = ?4v 1f + + 3.3f capacitor selection capacitor sizing the performance of the ltc1261 can be affected by the capacitors it is connected to . the ltc 1261 requires bypass capacitors to ground for both the v cc and out pins. the input capacitor provides most of ltc 1261 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 five times larger than the flying capacitor. ceramic capacitors generally provide adequate performance but avoid using a tantalum capaci- tor as the input bypass unless there is at least a 0.1f ceramic capacitor in parallel with it. the charge pump capacitors are somewhat less critical since their peak currents are limited by the switches inside the ltc1261. most applications should use 0.1f as the flying capaci- tor value. conveniently, ceramic capacitors are the most common type of 0.1f capacitor and they work well here. usually the easiest solution is to use the same capacitor type for both the input bypass and the flying capacitors. 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 values can be tailored to the application . reducing the value of the flying capacitors 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 capacitors draw smaller pulses of current out of v cc as well, limiting peak cur - rents 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 = 25c, v out = C 4v flying capacitor value (f) max load (ma) v cc = 5v doubler mode max load (ma) v cc = 3.3v tripler mode 0.1 22 20 0.047 16 15 0.033 8 11 0.022 4 5 0.01 1 3 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 1f. to achieve specified output ripple with 0.1f flying capacitors, the output capacitor should be at least 3.3f. 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 100ma under full load conditions. a typical 3.3f tantalum capacitor has 1 or 2 of esr; 100ma ? 2 = 200 mv. if the output is within 200mv of the set point this additional 200mv 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 ltc 1261 10 1261fb for more information www.linear.com/ltc1261 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 5). 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 0.1f 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. applications information 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 ltc1261 ? f05 figure 5. output ripple with low and high esr capacitors figure 6. external resistor connections note that esr in the flying capacitors will not cause the same condition; in fact, it may actually improve the situ- ation by cutting the peak current and lowering the ampli- tude of the spike. however, more flying capacitor esr is not necessarily better. as soon as the rc time constant approaches half of a clock period (the time the capaci- tors have to share charge at full duty cycle) the output current capability of the ltc1261 will begin to diminish. for 0.1f flying capacitors, this gives a maximum total series resistance of: 1 2 t clk c fly ? ? ? ? ? ? = 1 2 1 550khz ? ? ? ? ? ? / 0.1f = 9.1 ? most of this resistance is already provided by the internal switches in the ltc1261 (especially in tripler mode). more than 1 or 2 of esr on the flying capacitors will start to affect the regulation at maximum load. resistor selection resistor selection is easy with the fixed output versions of the ltc1261 no resistors are needed! selecting 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 1.24v at the adj pin with respect to v out (figure 6). the ltc1261 uses a positive reference with respect to v out , not a negative reference with respect to ground (figure 2 shows the reference con- nection). 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. ltc1261 gnd r1 6 (4*) 10 (5*) 11 (6*) *ltc1261cs8 ltc1261 ? f06 v out = ?1.24v r2 r1 + r2 r2 adj out ( ) the 14-lead adjustable parts include a built-in resistor string which can provide an assortment of output voltages by using different pin-strapping options at the r0, r1, and r adj pins ( table 2). the internal resistors are roughly 124k, 226k, 100k, and 50k (see figure 2) giving output options of C3.5v, C4v, C4.5v, and C5v. the resistors are carefully matched to provide accurate divider ratios, but the absolute values can vary substantially from part to part. it is not a good idea to create a divider using an external resistor and one of the internal resistors unless the output voltage accuracy is not critical. ltc 1261 11 1261fb for more information www.linear.com/ltc1261 typical applications applications information table 2. output voltages using the internal resistor divider pin connections output voltage adj to radj C5v adj to radj, r0 to gnd C4.5v adj to radj, r1 to r0 C4v adj to radj, r1 to gnd C3.5v adj to r1 C1.77v adj to r0 C1.38v adj to gnd C1.24v there are some oddball output voltages available by con- necting adj to r0 or r1 and shorting out some of the internal resistors. if one of these combinations gives you the output voltage you want, by all means use it! the internal resistor values are the same for the fixed output versions of the ltc1261 as they are for the adjustable. the output voltage can be trimmed, if desired, by connecting external resistance from the comp pin to out or ground to alter the divider ratio. as in the adjustable parts, the absolute value of the internal resistors may vary significantly from unit to unit. as a result, the further the trim shifts the output voltage the less accurate the output voltage will be. if a precise output voltage other than one of the available fixed voltages is required, it is better to use an adjustable ltc1261 and use precision external resistors. the internal reference is trimmed at the factory to within 3.5% of 1.24v; with 1% external resistors the output will be within 5.5% of the nominal value, even under worst case conditions. the ltc1261 can be internally configured with nonstan- dard fixed output voltages. contact the linear technology marketing department for details. 3.3v input, C4.5v output gaas fet bias generator 2 3 4 5 8 13 12 11 10 9 shdn reg out adj r adj c1 + c1 ? c2 + c2 ? r1 0.1f 100pf 1f nc ltc1261 ? ta03 3.3f ?4.5v bias 10k 7 6 14 0.1f shutdown 3.3v v bat ltc1261 v cc r0 gnd gaas transmitter p-channel power switch + ltc 1261 12 1261fb for more information www.linear.com/ltc1261 typical applications 5v input, C 4v output gaas fet bias generator 7 cells to C 1.24v output gaas fet bias generator 1mv ripple, 5v input, C 4v output gaas fet bias generator 1 2 3 4 8 7 6 5 shdn reg out comp v cc c1 + c2 ? gnd 100pf ltc1261 ? ta04 3.3f ? 4v bias 10k p-channel power switch 0.1f shutdown 5v v bat ltc1261-4 gaas transmitter 1f + 1 2 3 4 8 7 6 5 shdn reg out adj v cc c1 + c2 ? gnd ltc1261 ? ta05 3.3f ?1.24v bias 10k p-channel power switch 0.1f shutdown v bat = 8.4v (7 nicd cells) ltc1261 gaas transmitter 1f + 1 2 3 4 8 7 6 5 shdn reg out comp v cc c1 + c2 ? gnd 100pf ltc1261 ? ta06 10f ? 4v bias 100h 10k p-channel power switch 0.1f shutdown 5v v bat 10f ltc1261-4 gaas transmitter 1f + + ltc 1261 13 1261fb for more information www.linear.com/ltc1261 typical applications high supply voltage, C 5v output gaas fet bias generator low output voltage generator C 5v supply generator minimum parts count C 4v generator 2 3 4 5 8 13 12 11 10 9 shdn reg out adj r adj c1 + c1 ? c2 + c2 ? r1 0.1f 100pf 1f 1n4733a 5.1v nc nc ltc1261 ? ta07 3.3f ?5v bias 10k 7 6 14 0.1f shutdown 8v v bat 12v ltc1261 v cc r0 gnd gaas transmitter p-channel power switch + 2 3 5 6 ltc1261 adj out c1 + c1 ? ltc1261 ? ta10 3.3f = v cc C 10a (r s + 124k) = ? 0.5v (r s = 426k) = ? 1v (r s = 476k) 1n5817 1 5v 4 0.1f 1f 100pf r s v out v cc gnd 124k + 2 3 4 5 10 9 8 7 11 adj r adj r1 r0 out c1 + c1 ? c2 + c2 ? 0.1f ltc1261 ? ta09 nc nc 6 14 0.1f 100pf 1f ltc1261 3v v cc 7v v cc gnd 3.3f ?5v 5% at 10ma + 1 2 3 4 8 7 6 5 ltc1261-4 shdn reg out comp v cc c1 + c1 ? gnd 0.1f ltc1261 ? ta12 3.3f v out = ?4v at 10ma 5v 1f + ltc 1261 14 1261fb for more information www.linear.com/ltc1261 typical applications this circuit uses the ltc1261cs8 to generate a C1.24v output at 20ma. attached to this output is a 312 resistor to make the current / voltage conversion . 4 ma through 312 generates 1.24v, giving a net 0v output. 20ma through 312 gives 6.24v across the resistor, giving a net 5v output. if the 4ma to 20ma source requires an operat- ing voltage greater than 8v, it should be powered from a separate supply; the ltc1261 can then be powered from any convenient supply, 3v v s 8 v. the schottky diode prevents the external voltage from damaging the ltc1261 in shutdown or under fault conditions. the ltc1261s reference is trimmed to 3.5% and the resistor adds 1% uncertainty, giving 4.5% total output error. 6 5 2 3 ltc1261 out adj c1 + c1 ? ltc1261 ? ta11 1 8v 3.3f 312 1% 0.1f 1n5817 4ma to 20ma sensor optional input protection diodes ?1.24v 4 1f 0v to 5v 5% v cc gnd + ? + C 1.24v generator for 4ma-20ma to 0v-5v conversion ltc 1261 15 1261fb for more information www.linear.com/ltc1261 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 16 1261fb for more information www.linear.com/ltc1261 package description please refer to http://www .linear.com/designtools/packaging/ for the most recent package drawings. 1 n 2 3 4 .150 ? .157 (3.810 ? 3.988) note 3 14 13 .337 ? .344 (8.560 ? 8.738) note 3 .228 ? .244 (5.791 ? 6.197) 12 11 10 9 5 6 7 n/2 8 .016 ? .050 (0.406 ? 1.270) .010 ? .020 (0.254 ? 0.508) 45 0 ? 8 typ .008 ? .010 (0.203 ? 0.254) s14 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 .245 min n 1 2 3 n/2 .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 s package 14-lead plastic small outline (narrow .150 inch) (reference ltc dwg # 05-08-1610 rev g) ltc 1261 17 1261fb for more information www.linear.com/ltc1261 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 b 02/13 updated part numbers for lead free added i-grade option 2 2, 3 (revision history begins at rev b) ltc 1261 18 1261fb for more information www.linear.com/ltc1261 ? linear technology corporation 2013 lt 0313 rev b ? printed in usa linear technology corporation 1630 mccarthy blvd., milpitas, ca 95035-7417 (408) 432-1900 fax : (408) 434-0507 www.linear.com/ltc1261 related parts typical application 5v input, C 0.5v output gaas fet bias generator 1 2 3 4 8 7 6 5 shdn reg out adj v cc c1 + c2 ? gnd 100pf ltc1261 ? ta08 ?0.5v bias 5.5% 10k p-channel power switch 0.1f shutdown 5v v bat 3.3f ltc1261 gaas transmitter 42.2k 12.4k 1f + part number description comments ltc1550/ltc1551 low noise switched capacitor regulated voltage inverter gaas fet bias with linear regulator 1mv ripple ltc1429 clock synchronized switched capacitor regulated voltage inverter gaas fet bias lt1121 micropower low dropout regulators with shutdown 0.4v dropout voltage at 150ma, low noise, switched capacitor regulated voltage inverter |
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