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lm1085 3a low dropout positive regulators general description the lm1085 is a series of low dropout positive voltage regulators with a maximum dropout of 1.5v at 3a of load current. it has the same pin-out as national semiconductor's industry standard lm317. the lm1085 is available in an adjustable version, which can set the output voltage with only two external resistors. it is also available in three fixed voltages: 3.3v, 5.0v and 12.0v. the fixed versions integrate the adjust resistors. the lm1085 circuit includes a zener trimmed bandgap ref- erence, current limiting and thermal shutdown. the lm1085 series is available in to-220 and to-263 pack- ages. refer to the lm1084 for the 5a version, and the lm1086 for the 1.5v version. features n available in 3.3v, 5.0v, 12v and adjustable versions n current limiting and thermal protection n output current 3a n line regulation 0.015% (typical) n load regulation 0.1% (typical) applications n high efficiency linear regulators n battery charger n post regulation for switching supplies n constant current regulator n microprocessor supply connection diagrams basic functional diagram, adjustable version to-220 ds100947-2 top view to-263 ds100947-4 top view ds100947-65 application circuit ds100947-52 1.2v to 15v adjustable regulator august 2001 lm1085 3a low dropout positive regulators ? 2001 national semiconductor corporation ds100947 www.national.com
ordering information package temperature range part number transport media nsc drawing 3-lead to-263 ?40c to +125c lm1085is-adj rails ts3b lm1085isx-adj tape and reel lm1085is-12 rails lm1085isx-12 tape and reel lm1085is-3.3 rails lm1085isx-3.3 tape and reel lm1085is-5.0 rails LM1085ISX-5.0 tape and reel 3-lead to-220 ?40c to + 125c lm1085it-adj rails t03b lm1085it-12 rails lm1085it-3.3 rails lm1085it-5.0 rails simplified schematic ds100947-34 lm1085 www.national.com 2
absolute maximum ratings (note 1) if military/aerospace specified devices are required, please contact the national semiconductor sales office/ distributors for availability and specifications. maximum input to output voltage differential lm1085-adj 29v lm1085-12 18v lm1085-3.3 27v lm1085-5.0 25v power dissipation (note 2) internally limited junction temperature (t j )(note 3) 150c storage temperature range -65c to 150c lead temperature 260c, to 10 sec esd tolerance (note 4) 2000v operating ratings (note 1) junction temperature range (t j ) (note 3) control section ?40c to 125c output section ?40c to 150c electrical characteristics typicals and limits appearing in normal type apply for t j = 25c. limits appearing in boldface type apply over the entire junc- tion temperature range for operation. symbol parameter conditions min (note 6) typ (note 5) max (note 6) units v ref reference voltage lm1085-adj i out = 10ma, v in ?v out =3v 10ma i out i full load ,1.5v (v in ?v out ) 15v (note 7) 1.238 1.225 1.250 1.250 1.262 1.270 v v v out output voltage (note 7) lm1085-3.3 i out = 0ma, v in =5v 0 i out i full load , 4.8v v in 15v 3.270 3.235 3.300 3.300 3.330 3.365 v v lm1085-5.0 i out = 0ma, v in =8v 0 i out i full load , 6.5v v in 20v 4.950 4.900 5.000 5.000 5.050 5.100 v v lm1085-12 i out = 0ma, v in = 15v 0 i out i full load , 13.5v v in 25v 11.880 11.760 12.000 12.000 12.120 12.240 v v d v out line regulation (note 8) lm1085-adj i out =10ma, 1.5v (v in -v out ) 15v 0.015 0.035 0.2 0.2 % % lm1085-3.3 i out = 0ma, 4.8v v in 15v 0.5 1.0 6 6 mv mv lm1085-5.0 i out = 0ma, 6.5v v in 20v 0.5 1.0 10 10 mv mv lm1085-12 i out =0ma, 13.5v v in 25v 1.0 2.0 25 25 mv mv d v out load regulation (note 8) lm1085-adj (v in -v out ) = 3v, 10ma i out i full load 0.1 0.2 0.3 0.4 % % lm1085-3.3 v in = 5v, 0 i out i full load 3 7 15 20 mv mv lm1085-5.0 v in = 8v, 0 i out i full load 5 10 20 35 mv mv lm1085-12 v in = 15v, 0 i out i full load 12 24 36 72 mv mv dropout voltage (note 9) lm1085-3.3/5/12/adj d v ref = 1%, i out =3a 1.3 1.5 v i limit current limit lm1085-adj v in ?v out =5v v in ?v out = 25v 3.2 0.2 5.5 0.5 a a lm1085-3.3 v in =8v 3.2 5.5 a lm1085-5.0 v in = 10v 3.2 5.5 a lm1085-12 v in = 17v 3.2 5.5 a lm1085 www.national.com 3
electrical characteristics (continued) typicals and limits appearing in normal type apply for t j = 25c. limits appearing in boldface type apply over the entire junc- tion temperature range for operation. symbol parameter conditions min (note 6) typ (note 5) max (note 6) units minimum load current (note 10) lm1085-adj v in ?v out = 25v 5.0 10.0 ma quiescent current lm1085-3.3 v in 18v 5.0 10.0 ma lm1085-5.0 v in 20v 5.0 10.0 ma lm1085-12 v in 25v 5.0 10.0 ma thermal regulation t a = 25c, 30ms pulse .004 0.02 %/w ripple rejection f ripple = 120hz, c out = 25f tantalum, i out = 3a 60 75 db lm1085-adj, c adj = 25f, (v in ?v o )=3v lm1085-3.3, v in = 6.3v 60 72 db lm1085-5.0, v in =8v 60 68 db lm1085-12 v in = 15v 54 60 db adjust pin current lm1085 55 120 a adjust pin current change 10ma i out i full load , 1.5v v in ?v out 25v 0.2 5 a temperature stability 0.5 % long term stability t a =125c, 1000hrs 0.3 1.0 % rms output noise (% of v out ) 10hz f 10khz 0.003 % thermal resistance junction-to-case 3-lead to-263: control section/output section 3-lead to-220: control section/output section 0.7/3.0 0.7/3.0 c/w c/w note 1: absolute maximum ratings indicate limits beyond which damage to the device may occur. operating ratings indicate conditions for which the device is intended to be functional, but specific performance is not guaranteed. for guaranteed specifications and the test conditions, see the electrical ch aracteristics. note 2: power dissipation is kept in a safe range by current limiting circuitry. refer to overload recovery in application notes. note 3: the maximum power dissipation is a function of t j(max) , q ja , and t a . the maximum allowable power dissipation at any ambient temperature is p d =(t j(max) t a )/ q ja . all numbers apply for packages soldered directly into a pc board. refer to thermal considerations in the application notes. note 4: for testing purposes, esd was applied using human body model, 1.5k w in series with 100pf. note 5: typical values represent the most likely parametric norm. note 6: all limits are guaranteed by testing or statistical analysis. note 7: i full load is defined in the current limit curves. the i full load curve defines the current limit as a function of input-to-output voltage. note that 30w power dissipation for the lm1085 is only achievable over a limited range of input-to-output voltage. note 8: load and line regulation are measured at constant junction temperature, and are guaranteed up to the maximum power dissipation of 30w. power dissipation is determined by the input/output differential and the output current. guaranteed maximum power dissipation will not be available over the full input/output range. note 9: dropout voltage is specified over the full output current range of the device. note 10: the minimum output current required to maintain regulation. lm1085 www.national.com 4
typical performance characteristics dropout voltage vs. output current ds100947-63 short-circuit current vs. input/output difference ds100947-68 percent change in output voltage vs. temperature ds100947-99 adjust pin current vs. temperature ds100947-98 maximum power dissipation vs. temperature ds100947-70 ripple rejection vs. frequency (lm1085-adj.) ds100947-43 lm1085 www.national.com 5
typical performance characteristics (continued) ripple rejection vs. output current (lm1085-adj.) ds100947-44 line transient response ds100947-72 load transient response ds100947-71 lm1085 www.national.com 6
application note general figure 1 shows a basic functional diagram for the lm1085-adj (excluding protection circuitry) . the topology is basically that of the lm317 except for the pass transistor. instead of a darlingtion npn with its two diode voltage drop, the lm1085 uses a single npn. this results in a lower dropout voltage. the structure of the pass transistor is also known as a quasi ldo. the advantage a quasi ldo over a pnp ldo is its inherently lower quiescent current. the lm1085 is guaranteed to provide a minimum dropout volt- age 1.5v over temperature, at full load. output voltage the lm1085 adjustable version develops at 1.25v reference voltage, (v ref ), between the output and the adjust terminal. as shown in figure 2, this voltage is applied across resistor r1 to generate a constant current i1. this constant current then flows through r2. the resulting voltage drop across r2 adds to the reference voltage to sets the desired output voltage. the current i adj from the adjustment terminal introduces an output error . but since it is small (120ua max), it becomes negligible when r1 is in the 100 w range. for fixed voltage devices, r1 and r2 are integrated inside the devices. stability consideration stability consideration primarily concern the phase response of the feedback loop. in order for stable operation, the loop must maintain negative feedback. the lm1085 requires a certain amount series resistance with capacitive loads. this series resistance introduces a zero within the loop to in- crease phase margin and thus increase stability. the equiva- lent series resistance (esr) of solid tantalum or aluminum electrolytic capacitors is used to provide the appropriate zero (approximately 500 khz). the aluminum electrolytic are less expensive than tantal- ums, but their esr varies exponentially at cold tempera- tures; therefore requiring close examination when choosing the desired transient response over temperature. tantalums are a convenient choice because their esr varies less than 2:1 over temperature. the recommended load/decoupling capacitance is a 10uf tantalum or a 50uf aluminum. these values will assure stability for the majority of applications. the adjustable versions allows an additional capacitor to be used at the adj pin to increase ripple rejection. if this is done the output capacitor should be increased to 22uf for tantal- ums or to 150uf for aluminum. capacitors other than tantalum or aluminum can be used at the adjust pin and the input pin. a 10uf capacitor is a reasonable value at the input. see ripple rejection section regarding the value for the adjust pin capacitor. it is desirable to have large output capacitance for applica- tions that entail large changes in load current (microproces- sors for example). the higher the capacitance, the larger the available charge per demand. it is also desirable to provide low esr to reduce the change in output voltage: d v= d i x esr it is common practice to use several tantalum and ceramic capacitors in parallel to reduce this change in the output voltage by reducing the overall esr. output capacitance can be increased indefinitely to improve transient response and stability. ripple rejection ripple rejection is a function of the open loop gain within the feed-back loop (refer to figure 1 and figure 2 ). the lm1085 exhibits 75db of ripple rejection (typ.). when adjusted for voltages higher than v ref , the ripple rejection decreases as function of adjustment gain: (1+r1/r2) or v o /v ref . there- fore a 5v adjustment decreases ripple rejection by a factor of four (?12db); output ripple increases as adjustment voltage increases. however, the adjustable version allows this degradation of ripple rejection to be compensated. the adjust terminal can be bypassed to ground with a capacitor (c adj ). the imped- ance of the c adj should be equal to or less than r1 at the desired ripple frequency. this bypass capacitor prevents ripple from being amplified as the output voltage is in- creased. 1/(2 p * f ripple * c adj ) r 1 load regulation the lm1085 regulates the voltage that appears between its output and ground pins, or between its output and adjust pins. in some cases, line resistances can introduce errors to the voltage across the load. to obtain the best load regula- tion, a few precautions are needed. figure 3 shows a typical application using a fixed output regulator. rt1 and rt2 are the line resistances. v load is less than the v out by the sum of the voltage drops along the line resistances. in this case, the load regulation seen at the r load would be degraded from the data sheet specification. ds100947-65 figure 1. basic functional diagram for the lm1085, excluding protection circuitry ds100947-17 figure 2. basic adjustable regulator lm1085 www.national.com 7
application note (continued) to improve this, the load should be tied directly to the output terminal on the positive side and directly tied to the ground terminal on the negative side. when the adjustable regulator is used ( figure 4 ), the best performance is obtained with the positive side of the resistor r1 tied directly to the output terminal of the regulator rather than near the load. this eliminates line drops from appearing effectively in series with the reference and degrading regu- lation. for example, a 5v regulator with 0.05 w resistance between the regulator and load will have a load regulation due to line resistance of 0.05 w xi l .ifr1(=125 w )is connected near the load the effective line resistance will be 0.05 w (1 + r2/r1) or in this case, it is 4 times worse. in addition, the ground side of the resistor r2 can be returned near the ground of the load to provide remote ground sens- ing and improve load regulation. 3.0 protection diodes under normal operation, the lm1085 regulator does not need any protection diode. with the adjustable device, the internal resistance between the adjustment and output ter- minals limits the current. no diode is needed to divert the current around the regulator even with a capacitor on the adjustment terminal. the adjust pin can take a transient signal of 25v with respect to the output voltage without damaging the device. when an output capacitor is connected to a regulator and the input is shorted, the output capacitor will discharge into the output of the regulator. the discharge current depends on the value of the capacitor, the output voltage of the regulator, and rate of decrease of v in . in the lm1085 regu- lator, the internal diode between the output and input pins can withstand microsecond surge currents of 10a to 20a. with an extremely large output capacitor ( 3 1000 f), and with input instantaneously shorted to ground, the regulator could be damaged. in this case, an external diode is recom- mended between the output and input pins to protect the regulator, shown in figure 5 . overload recovery overload recovery refers to regulator's ability to recover from a short circuited output. a key factor in the recovery process is the current limiting used to protect the output from drawing too much power. the current limiting circuit reduces the output current as the input to output differential increases. refer to short circuit curve in the curve section. during normal start-up, the input to output differential is small since the output follows the input. but, if the output is shorted, then the recovery involves a large input to output differential. sometimes during this condition the current lim- iting circuit is slow in recovering. if the limited current is too low to develop a voltage at the output, the voltage will stabilize at a lower level. under these conditions it may be necessary to recycle the power of the regulator in order to get the smaller differential voltage and thus adequate start up conditions. refer to curve section for the short circuit current vs. input differential voltage. thermal considerations ics heats up when in operation, and power consumption is one factor in how hot it gets. the other factor is how well the heat is dissipated. heat dissipation is predictable by knowing the thermal resistance between the ic and ambient ( q ja ). thermal resistance has units of temperature per power (c/w). the higher the thermal resistance, the hotter the ic. the lm1085 specifies the thermal resistance for each pack- age as junction to case ( q jc ). in order to get the total resistance to ambient ( q ja ), two other thermal resistance must be added, one for case to heat-sink ( q ch ) and one for heatsink to ambient ( q ha ). the junction temperature can be predicted as follows: t j =t a +p d ( q jc + q ch + q ha )=t a +p d q ja t j is junction temperature, t a is ambient temperature, and p d is the power consumption of the device. device power consumption is calculated as follows: i in =i l +i g p d =(v in ?v out )i l +v in i g ds100947-18 figure 3. typical application using fixed output regulator ds100947-19 figure 4. best load regulation using adjustable output regulator ds100947-15 figure 5. regulator with protection diode lm1085 www.national.com 8
application note (continued) figure 6 shows the voltages and currents which are present in the circuit. once the devices power is determined, the maximum allow- able ( q ja(max) ) is calculated as: q ja(max) =t r(max) /p d =t j(max ?t a(max) )/p d the lm1085 has different temperature specifications for two different sections of the ic: the control section and the output section. the electrical characteristics table shows the junc- tion to case thermal resistances for each of these sections, while the maximum junction temperatures (t j(max) ) for each section is listed in the absolute maximum section of the datasheet. t j(max) is 125c for the control section, while t j(max) is 150c for the output section. q ja(max) should be calculated separately for each section as follows: q ja (max, control section) = (125c - t a(max) )/p d q ja (max, output section) = (150c - t a(max) )/p d the required heat sink is determined by calculating its re- quired thermal resistance ( q ha(max) ). q ha(max) = q ja(max) ?( q jc + q ch ) q ha(max) should also be calculated twice as follows: q ha(max) = q ja (max, control section) - ( q jc (con- trol section) + q ch ) q ha(max) = q ja (max, output section) - ( q jc (output section) + q ch ) if thermal compound is used, q ch can be estimated at 0.2 c/w. if the case is soldered to the heat sink, then a q ch can be estimated as 0 c/w. after, q ha(max) is calculated for each section, choose the lower of the two q ha(max) values to determine the appropriate heat sink. if pc board copper is going to be used as a heat sink, then figure 7 can be used to determine the appropriate area (size) of copper foil required. ds100947-16 figure 6. power dissipation diagram ds100947-64 figure 7. heat sink thermal resistance vs area lm1085 www.national.com 9
typical applications ds100947-67 5v to 3.3v, 1.5a regulator ds100947-50 adjustable @ 5v ds100947-52 1.2v to 15v adjustable regulator ds100947-53 5v regulator with shutdown ds100947-54 battery charger ds100947-55 adjustable fixed regulator ds100947-56 regulator with reference ds100947-57 high current lamp driver protection lm1085 www.national.com 10
typical applications (continued) ds100947-59 battery backup regulated supply ds100947-60 ripple rejection enhancement ds100947-61 automatic light control ds100947-62 generating negative supply voltage ds100947-58 remote sensing lm1085 www.national.com 11
physical dimensions inches (millimeters) unless otherwise noted 3-lead to-263 ns package number ts3b lm1085 www.national.com 12
physical dimensions inches (millimeters) unless otherwise noted (continued) life support policy national's products are not authorized for use as critical components in life support devices or systems without the express written approval of the president and general counsel of national semiconductor corporation. as used herein: 1. life support devices or systems are devices or systems which, (a) are intended for surgical implant into the body, or (b) support or sustain life, and whose failure to perform when properly used in accordance with instructions for use provided in the labeling, can be reasonably expected to result in a significant injury to the user. 2. a critical component is any component of a life support device or system whose failure to perform can be reasonably expected to cause the failure of the life support device or system, or to affect its safety or effectiveness. national semiconductor corporation americas email: support@nsc.com national semiconductor europe fax: +49 (0) 180-530 85 86 email: europe.support@nsc.com deutsch tel: +49 (0) 69 9508 6208 english tel: +44 (0) 870 24 0 2171 fran?ais tel: +33 (0) 1 41 91 8790 national semiconductor asia pacific customer response group tel: 65-2544466 fax: 65-2504466 email: ap.support@nsc.com national semiconductor japan ltd. tel: 81-3-5639-7560 fax: 81-3-5639-7507 www.national.com 3-lead to-220 ns package number t03b lm1085 3a low dropout positive regulators national does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and national reserves the righ t at any time without notice to change said circuitry and specifications.

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