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1 LT1185 s f ea t u re low dropout regulator n low resistance pass transistor: 0.25 w n dropout voltage: 0.75v at 3a n 1% reference voltage n accurate programmable current limit n shutdown capability n internal reference available n standard 5-lead packages n full remote sense n low quiescent current: ? 2.5ma n good high frequency ripple rejection the LT1185 uses a saturation-limited npn transistor as the pass element. this device gives the linear dropout characteristics of an fet pass element with significantly less die area. high efficiency is maintained by using special anti-saturation circuitry that adjusts base drive to track load current. the on resistance is typically 0.25 w . accurate current limit is programmed with a single 1/8w external resistor, with a range of zero to three amperes. a second, fixed internal limit circuit prevents destructive currents if the programming current is accidentally over- ranged. shutdown of the regulator output is guaranteed when the program current is less than 1 m a, allowing external logic control of output voltage. the LT1185 has all the protection features of previous ltc regulators, including power limiting and thermal shut- down. the 4-lead to-3 package is specified for C55 c to 150 c operation and the 5-lead to-220 is specified over 0 c to 125 c. , ltc and lt are registered trademarks of linear technology corporation. u a o pp l ic at i ty p i ca l 5v, 3a regulator with 3.5a current limit dropout voltage + r1 2.37k r2 2.67k c1 2 m f tant ref gnd fb v out v in LT1185 c2 2 m f tant r lim * 4.3k + v in 6v to 16v v out 5v at 3a LT1185 ?ta01 *current limit = 15k/r lim = 3.5a + + load current (a) 0 v in ? out (v) 0.8 1.0 1.2 4 LT1185 ?ta02 0.6 0.4 0 1 2 3 0.2 1.6 1.4 t j = 125 c t j = 55 c t j = 25 c d u escriptio the lt ? 1185 is a 3a low dropout regulator with adjustable current limit and remote sense capability. it can be used as a positive output regulator with floating input or as a standard negative regulator with grounded input. the output voltage range is 2.5v to 25v, with 1% accuracy on the internal reference voltage.
2 LT1185 a u g w a w u w a r b s o lu t exi t i s 2 4 1 3 gnd fb ref v in (case) v out k package 4-lead to-3 metal can bottom view order part number LT1185mk order part number LT1185ct LT1185it q jc max = 2.5 c/ w, q ja = 50 c/w t package 5-lead plastic to-220 ref v out v in fb gnd front view tab is v in 5 4 3 2 1 q jc max = 2.5 c/ w, q ja = 35 c/w wu u package / o rder i for atio (note 1) input voltage .......................................................... 35v input-output differential ......................................... 30v fb voltage ................................................................ 7v ref voltage .............................................................. 7v output voltage ........................................................ 30v output reverse voltage ............................................ 2v operating ambient temperature range LT1185c ............................................... 0 c to 70 c LT1185m ......................................... C 55 c to 125 c operating junction temperature range* control section LT1185c ............................................. 0 c to 125 c LT1185i .......................................... C 40 c to 125 c LT1185m ........................................ C 55 c to 150 c power transistor section LT1185c ............................................. 0 c to 150 c LT1185i .......................................... C 40 c to 150 c LT1185m ........................................ C 55 c to 175 c storage temperature range ................ C 65 c to 150 c lead temperature (soldering, 10 sec)................ 300 c *see application section for details on calculating operation junction temperature parameter conditions min typ max units reference voltage (at fb pin) 2.37 v reference voltage tolerance (at fb pin) (note 2) v in C v out = 5v, v out = v ref 0.3 1% 1ma i out 3a l 1 2.5 % v in C v out = 1.2v to v in = 30v p 25w (note 6), v out = 5v t min t j t max (note 9) feedback pin bias current v out = v ref l 0.7 2 m a droput voltage (note 3) i out = 0.5a, v out = 5v 0.20 0.37 v i out = 3a, v out = 5v 0.67 1.00 v load regulation (note 7) i out = 5ma to 3a 0.05 0.3 % v in C v out = 1.5v to 10v, v out = 5v line regulation (note 7) v in C v out = 1v to 20v, v out = 5v 0.002 0.01 %/v minimum input voltage i out = 1a (note 4), v out = v ref 4.0 v i out = 3a 4.3 v internal current limit (see graph for 1.5v v in C v out 10v 3.3 3.6 4.2 a guaranteed curve) (note 12) l 3.1 4.4 a v in C v out = 15v l 2.0 3.0 4.2 a v in C v out = 20v l 1.0 1.7 2.6 a v in C v out = 30v l 0.2 0.4 1.0 a electrical characteristics the l denotes specifications which apply over the operating temperature range, otherwise specifications are at t a = 25 c. adjustable version, v in = 7.4v, v out = 5v, i out = 1ma, r lim = 4.02k, unless otherwise noted.
3 LT1185 electrical characteristics the l denotes specifications which apply over the operating temperature range, otherwise specifications are at t a = 25 c. adjustable version, v in = 7.4v, v out = 5v, i out = 1ma, r lim = 4.02k, unless otherwise noted. parameter conditions min typ max units external current limit 5k r lim 15k, v out = 1v l 15k a? w programming constant (note 11) external current limit error 1a i lim 3a 0.02 i lim 0.06 i lim + 0.03 a r lim = 15k ? a/i lim l 0.04 i lim 0.09 i lim + 0.05 a quiescent supply current i out = 5ma, v out = v ref l 2.5 3.5 ma 4v v in 25v (note 5) supply current change with load v in C v out = v sat (note 10) l 25 40 ma/a v in C v out 3 2v l 10 25 ma/a ref pin shutoff current l 0.4 2 7 m a thermal regulation (see applications v in C v out = 10v 0.005 0.014 %/w information) i out = 5ma to 2a reference voltage temperature coefficient (note 8) 0.003 0.01 %/ c thermal resistance junction to case to-3 control area 1 c/w power transistor 3 c/w to-220 control area 1 c/w power transistor 3 c/w note 1: absolute maximum ratings are those values beyond which the life of a device may be impaired. note 2: reference voltage is guaranteed both at nominal conditions (no load, 25 c) and at worst-case conditions of load, line, power and temperature. an intermediate value can be calculated by adding the effects of these variables in the actual application. see the applications information section of this data sheet. note 3: dropout voltage is tested by reducing input voltage until the output drops 1% below its nominal value. tests are done at 0.5a and 3a. the power transistor looks basically like a pure resistance in this range so that minimum differential at any intermediate current can be calculated by interpolation; v dropout = 0.25v + 0.25 w ? i out . for load current less than 0.5a, see graph. note 4: minimum input voltage is limited by base emitter voltage drive of the power transistor section, not saturation as measured in note 3. for output voltages below 4v, minimum input voltage specification may limit dropout voltage before transistor saturation limitation. note 5: supply current is measured on the ground pin, and does not include load current, r lim , or output divider current. note 6: the 25w power level is guaranteed for an input-output voltage of 8.3v to 17v. at lower voltages the 3a limit applies, and at higher voltages the internal power limiting may restrict regulator power below 25w. see graphs. note 7: line and load regulation are measured on a pulse basis with a pulse width of ? 2ms, to minimize heating. dc regulation will be affected by thermal regulation and temperature coefficient of the reference. see applications information section for details. note 8: guaranteed by design and correlation to other tests, but not tested. note 9: t jmin = 0 c for the LT1185c, C 40 c for LT1185i, and C55 c for the LT1185m. power transistor area and control circuit area have different maximum junction temperatures. control area limits are t jmax = 125 c for the LT1185c and LT1185i and 150 c for the LT1185m. power area limits are 150 c for LT1185c and LT1185i and 175 c for LT1185m. note 10: v sat is the maximum specified dropout voltage; 0.25v + 0.25 ? i out . note 11: current limit is programmed with a resistor from ref pin to gnd pin. the value is 15k/i lim . note 12: for v in C v out = 1.5v; v in = 5v, v out = 3.5v. v out = 1v for all other current limit tests.
4 LT1185 cc hara terist ics uw a t y p i ca lper f o r c e ripple rejection vs frequency ground pin current input-output differential (v) 0 0 output currnt (a) 1 2 3 5 10 15 20 LT1185 ?tpc01 25 4 5 30 typical test points guaranteed limit guaranteed limit internal current limit input voltage (v) 0 ground pin current (ma) 8 10 12 15 25 LT1185 ?tpc02 6 4 510 20 30 35 2 0 v out = 5v *does not include ref current or output divider current i load = 0 t j = 25 c quiescent ground pin current* junction temperature ( c) ?0 voltage (v) 2.37 2.38 2.39 150 LT1185 ?tpc03 2.36 2.35 2.33 0 50 100 2.34 2.41 2.40 ?5 25 75 125 feedback pin voltage temperature drift load current (a) 0 current (ma) 80 100 120 4 LT1185 ?tpc04 60 40 0 1 2 3 20 160 140 t j = 25 c regulator just at dropout point v in ?v out = 5v frequency (hz) ?0 ratio v out /v in (db) ?0 ?0 ?0 100 100 10k 100k 1m LT1185 ?tpc05 0 1k all output voltages with 0.05 m f across r2 v out = 5v v in ?v out = 1.5v load transient response time ( m s) 0 4 8 10 LT1185 ?tpc06 26 12 14 16 0.1a t r , f 100ns c out = 2.2 m f, esr = 1 w c out = 2.2 m f, esr = 2 w v out = 5v i out = 1a 100mv d i load output impedance frequency (hz) 0.01 impedance ( w ) 0.1 1 10 1m 10k 100k lt1183 ?tpc07 0.001 1k output impedance is set by output capacitor esr in this region v out = 5v i out = 1a c out = 2.2 m f
5 LT1185 block diagram a simplified block diagram of the LT1185 is shown in figure 1. a 2.37v bandgap reference is used to bias the input of the error amplifier a1, and the reference amplifier a2. a1 feeds a triple npn pass transistor stage which has the two driver collectors tied to ground so that the main pass transistor can completely saturate. this topology normally has a problem with unlimited current in q1 and q2 when the input voltage is less than the minimum required to create a regulated output. the standard fix for this problem is to insert a resistor in series with q1 and q2 collectors, but this resistor must be low enough in value to supply full base current for q3 under worst-case figure 1. block diagram applicatio s i for atio uu w u 300mv i1 2 m a r1 350 w 200mv d2 d4 d3 + a5 + a4 + a3 v in r2 0.055 w + a1 + a2 v ref 2.37v v out fb gnd r lim (external) ref d1 LT1185 ?bd q1 q2 q3 q4 conditions, resulting in very high supply current when the input voltage is low. to avoid this situation, the LT1185 uses an auxiliary emitter on q3 to create a drive limiting feedback loop which automatically adjusts the drive to q1 so that the base drive to q3 is just enough to saturate q3, but no more. under saturation conditions, the auxiliary emitter is acting like a collector to shunt away the output current of a1. when the input voltage is high enough to keep q3 out of saturation, the auxiliary emitter current drops to zero even when q3 is conducting full load current.
6 LT1185 applicatio s i for atio uu w u amplifier a2 is used to generate an internal current through q4 when an external resistor is connected from the ref pin to ground. this current is equal to 2.37v divided by r lim . it generates a current limit sense voltage across r1. the regulator will current limit via a4 when the voltage across r2 is equal to the voltage across r1. these two resistors essentially form a current amplifier with a gain of 350/0.055 = 6,360. good temperature drift is inherent because r1 and r2 are made from the same diffusions. their ratio, not absolute value, determines current limit. initial accuracy is enhanced by trimming r1 slightly at wafer level. current limit is equal to 15k w /r lim . d1 and i 1 are used to guarantee regulator shutdown when ref pin current drops below 2 m a. a current less than 2 m a through q4 causes the +input of a5 to go low and shut down the regulator via d2. a3 is an internal current limit amplifier which can override the external current limit. it provides goof proof protec- tion for the pass transistor. although not shown, a3 has a nonlinear foldback characteristic at input-output volt- ages above 12v to guarantee safe area protection for q3. see the graph, internal current limit in the typical perfor- mance characteristics of this data sheet. setting output voltage the LT1185 output voltage is set by two external resistors (see figure 2). internal reference voltage is trimmed to 2.37v so that a standard 1% 2.37k resistor (r1) can be used to set divider current at 1ma. r2 is then selected from: for r1 = 2.37k and v ref = 2.37v, this reduces to: r2 = v out C 2.37k suggested values of 1% resistors are shown. v out r2 when r1 = 2.37k 5v 2.67k 5.2v 2.87k 6v 3.65k 12v 9.76k 15v 12.7k output capacitor the LT1185 has a collector output npn pass transistor, which makes the open-loop output impedance much higher than an emitter follower. open-loop gain is a direct function of load impedance, and causes a main-loop pole to be created by the output capacitor, in addition to an internal pole in the error amplifier. to ensure loop stability, the output capacitor must have an esr (effective series resistance) which has an upper limit of 2 w , and a lower limit of 0.2 divided by the capacitance in m f. a 2 m f output capacitor, for instance, should have a maximum esr of 2 w , and a minimum of 0.2/2 = 0.1 w . these values are easily encompassed by standard solid tantalum capacitors, but occasionally a solid tantalum unit will have abnormally high esr, especially at very low tempera- tures. the suggested 2 m f value shown in the circuit applications should be increased to 4.7 m f for C 40 c and C55 c designs if the 2 m f units cannot be guaranteed to stay below 2 w at these temperatures. although solid tantalum capacitors are suggested, other types can be used if they meet the esr requirements. standard aluminum electrolytic capacitors need to be upward of 25 m f in general to hold 2 w maximum esr, especially at low temperatures. ceramic, plastic film, and monolithic capacitors have a problem with esr being too low . these types should have a 1 w carbon resistor in series to guarantee loop stability. the output capacitor should be located close to the regu- lator ( 3") to avoid excessive impedance due to lead inductance. a six inch lead length (2 ? 3") will generate an extra 0.8 w inductive reactance at 1mhz, and unity-gain frequency can be up to that value. for remote sense applications, the capacitor should still be located close to the regulator. additional capacitance can be added at the remote sense point, but the remote capacitor must be at least 2 m f solid tantalum. it cannot be a low esr type like ceramic or mylar unless a 0.5 w to 1 w carbon resistor is added in series with the capacitor. logic boards with multiple low esr bypass capacitors should have a solid tantalum unit added in parallel whose value is approximately five times the combined value of low esr capacitors. r2 = (v out ?2.37) r1 v ref
7 LT1185 applicatio s i for atio uu w u large output capacitors (electrolytic or solid tantalum) will not cause the LT1185 to oscillate, but they will cause a damped ringing at light load currents where the esr of the capacitor is several orders of magnitude lower than the load resistance. this ringing only occurs as a result of transient load or line conditions and normally causes no problems because of its low amplitude ( 25mv). heat sinking the LT1185 will normally be used with a heat sink. the size of the heat sink is determined by load current, input and output voltage, ambient temperature, and the thermal resistance of the regulator, junction-to-case ( q jc ). the LT1185 has two separate values for q jc : one for the power transistor section, and a second, lower value for the control section. the reason for two values is that the power transistor is capable of operating at higher continu- ous temperature than the control circuitry. at low power levels, the two areas are at nearly the same temperature, and maximum temperature is limited by the control area. at high power levels, the power transistor will be at a significantly higher temperature than the control area and its maximum operating temperature will be the limiting factor. to calculate heat sink requirements, you must solve a thermal resistance formula twice, one for the power transistor and one for the control area. the lowest value obtained for heat sink thermal resistance must be used. in these equations, two values for maximum junction tem- perature and junction-to-case thermal resistance are used, as given in electrical specifications. example: a commercial version of the LT1185 in the to-220 package is to be used with a maximum ambient temperature of 60 c. output voltage is 5v at 2a. input voltage can vary from 6v to 10v. assume an interface resistance of 1 c/w. first solve for control area, where the maximum junction temperature is 125 c for the to-220 package, and q jc = 1 c/w: next, solve for power transistor limitation, with t jmax = 150 c, q jc = 3 c/w: the lowest number must be used, so heat sink resistance must be less than 4.2 c/w. some heat sink data sheets show graphs of heat sink temperature rise vs power dissipation instead of listing a value for thermal resistance. the formula for q hs can be rearranged to solve for maximum heat sink temperature rise: d t hs = t jmax C t amax C p( q jc + q chs ) using numbers from the previous example: d t hs = 125 c C 60 C 10.5(1 + 1) = 44 c control section d t hs = 150 c C 60 C 10.5(3 + 1) = 48 c power transistor the smallest rise must be used, so heat sink temperature rise must be less than 44 c at a power level of 10.5w. for board level applications, where heat sink size may be critical, one is often tempted to use a heat sink which barely meets the requirements. this is permissible if correct assumptions were made concerning maximum ambient temperature and power levels. one complicating p = (10v ?5v) (2a) + 2a 40 (10v) = 10.5w q hs = 125? ?60? 10.5w ?1?/w ?1?/w = 4.2?/w q hs = 150 ?60 10.5 ?3 ?1 = 4.6?/w q hs = (t jmax ?t amax ) p ? q jc ? q chs . q hs = maximum heat sink thermal resistance q jc = LT1185 junction-to-case thermal resistance q chs = case-to-heat sink (interface) thermal resistance, including any insulating washers t jmax = LT1185 maximum operating junction temperature t amax = maximum ambient temperature in customers application p = device dissipaton = (v in ?v out ) (i out ) + i out 40 (v in )
8 LT1185 applicatio s i for atio uu w u factor is that local ambient temperature may be somewhat higher because of the point source of heat. the conse- quences of excess junction temperature include poor reliability, especially for plastic packages, and the possi- bility of thermal shutdown or degraded electrical charac- teristics. the final design should be checked in situ with a thermocouple attached to the regulator case under worst- case conditions of high ambient, high input voltage and full load. what about overloads? ic regulators with thermal shutdown, like the LT1185, allow heat sink designs which concentrate on worst-case normal conditions and ignore fault conditions. an output overload or short may force the regulator to exceed its maximum junction temperature rating, but thermal shutdown is designed to prevent regulator failure under these conditions. a word of caution however; thermal shutdown temperatures are typically 175 c in the control portion of the die and 180 c to 225 c in the power transistor section. extended operation at these tempera- tures can cause permanent degradation of plastic encap- sulation. designs which may be subjected to extended periods of overload should either use the hermetic to-3 package or increase heat sink size. foldback current limiting can be implemented to minimize power levels under fault conditions. external current limit the LT1185 requires a resistor to set current limit. the value of this resistor is 15k divided by the desired current limit (in amps). the resistor for 2a current limit would be 15k/2a = 7.5k. tolerance over temperature is 10%, so current limit is normally set 15% above maximum load current. foldback limiting can be employed if short-circuit current must be lower than full load current (see typical applications). the LT1185 has internal current limiting which will over- ride external current limit if power in the pass transistor is excessive. the internal limit is ? 3.6a with a foldback characteristic which is dependent on input-output voltage, not output voltage per se (see typical performace charac- teristics). ground pin current ground pin current for the LT1185 is approximately 2ma plus i out /40. at i out = 3a, ground pin current is typically 2ma + 3/40 = 77ma. worst case guarantees on the ratio of i out to ground pin current are contained in the electrical specifications. ground pin current can be important for two reasons. it adds to power dissipation in the regulator and it can affect load/line regulation if a long line is run from the ground pin to load ground. the additional power dissipation is found by multiplying ground pin current by input voltage. in a typical example, with v in = 8v, v out = 5v and i out = 2a, the LT1185 will dissipate (8v C 5v)(2a) = 6w in the pass transistor and (2a/40)(8v) = 0.4w in the internal drive circuitry. this is only a 1.5% efficiency loss, and a 6.7% increase in regulator power dissipation, but these values will increase at higher output voltages. ground pin current can affect regulation as shown in figure 2. parasitic resistance in the ground pin lead will create a voltage drop which increases output voltage as load current is increased. similarly, output voltage can decrease as input voltage increases because the i out /40 component of ground pin current drops significantly at higher input-output differentials. these effects are small enough to be ignored for local regulation applications, but for remote sense applications, they may need to be consid- ered. ground lead resistance of 0.4 w would cause an output voltage error of up to (3a/40)(0.4 w ) = 30mv, or 0.6% at v out = 5v. note that if the sense leads are connected as shown in figure 2, with r a ? 0 w , this error is a fixed number of millivolts, and does not increase as a function of dc output voltage.
9 LT1185 applicatio s i for atio uu w u figure 2. proper connection of positive sense lead shutdown techniques the LT1185 can be shut down by open-circuiting the ref pin. the current flowing into this pin must be less than 0.4 m a to guarantee shutdown. figure 3 details several ways to create the open condition, with various logic levels. for variations on these schemes, simply remember that the voltage on the ref pin is 2.4v negative with respect to the ground pin. output overshoot very high input voltage slew rate during start-up may cause the LT1185 output to overshoot. up to 20% over- shoot could occur with input voltage ramp-up rate exceed- ing 1v/ m s. this condition cannot occur with normal 50hz to 400hz rectified ac inputs because parasitic resistance and inductance will limit rate of rise even if the power switch is closed at the peak of the ac line voltage. this assumes that the switch is in the ac portion of the circuit. if instead, a switch is placed directly in the regulator input so that a large filter capacitor is precharged, fast input slew rates will occur on switch closure. the output of the regulator will slew at a rate set by current limit and output capacitor size; dvdt = i lim /c out . with i lim = 3.6a and c out = 2.2 m f, the output will slew at 1.6v/ m s and overshoot can occur. this overshoot can be reduced to a few hundred millivolts or less by increasing the output capacitor to 10 m f and/or reducing current limit so that output slew rate is held below 0.5v/ m s. a second possibility for creating output overshoot is recovery from an output short. again, the output slews at a rate set by current limit and output capacitance. to avoid overshoot, the ratio i lim /c out should be less than 0.5 10 6 . remember that load capacitance can be added to c out for this calculation. many loads will have multiple supply bypass capacitors that total more than c out . + r1* 2.37k r2 ref gnd fb v out v in LT1185 r lim + v in v out LT1185 ?f02 load parasitic lead resistances ? r b + i gnd r a *r1 should be connected directly to ground lead, not to the load, so that r a ? 0 w . this limits the output voltage error to (i gnd )(r b ). errors created by r a are multiplied by (1 + r2/r1). note that v out increases with increasing ground pin current. r2 should be connected directly to load for remote sensing
10 LT1185 applicatio s i for atio uu w u figure 3. shutdown techniques r lim ? 4k + v in r6 30k r7 2.4k ? r1 r2 v out + q1 2n3906 * LT1185 ?f3a *cmos logic ? for higher values of r lim , make r7 = (r lim )(0.6) 5v r5 300k + ref gnd fb v out v in LT1185 5v logic, positive regulated output 5v logic, negative regulated output r lim LT1185 ?f03b ref gnd fb v out LT1185 v in r4 33k 5v ?i?= output ?ff 3 ea 1n4148 v in q1 2n3906
11 LT1185 applicatio s i for atio uu w u thermal regulation ic regulators have a regulation term not found in discrete designs because the power transistor is thermally coupled to the reference. this creates a shift in the output voltage which is proportional to power dissipation in the regulator. d v out = p(k1 + k2 q ja ) = (i out )(v in C v out )(k1 + k2 q ja ) k1 and k2 are constants. k1 is a fast time constant effect caused by die temperature gradients which are estab- lished within 50ms of a power change. k1 is specified on the data sheet as thermal regulation, in percent per watt. k2 is a long time constant term caused by the temperature drift of the regulator reference voltage. it is also specified, but in percent per degree centigrade. it must be multiplied by overall thermal resistance, junction-to-ambient, q ja . as an example, assume a 5v regulator with an input voltage of 8v, load current of 2a, and a total thermal resistance of 4 c/w, including junction-to-case, (use control area specification), interface, and heat sink resis- tance. k1 and k2, respectively, from the data sheet are 0.014%/w and 0.01%/ c. d v out = (2a)(8v C 5v)(0.014 + 0.01 ? 4) = 0.32% this shift in output voltage could be in either direction because k1 and k2 can be either positive or negative. thermal regulation is already included in the worst case reference specification. output voltage reversal some ic regulators suffer from a latch-up state when their output is forced to a reverse voltage of as little as one diode drop. the latch-up state can be triggered without a fault condition when the load is connected to an opposite polarity supply instead of to ground. if the second supply is turned on first, it will pull the output of the first supply to a reverse voltage through the load. the first supply may then latch off when turned on. this problem is particularly annoying because the diode clamps which should always be used to protect against polarity reversal do not usually stop the latch-up problem. the LT1185 is designed to allow output reverse polarity of several volts without damage or latch-up, so that a simple diode clamp can be used.
12 LT1185 typical applicatio s u foldback current limiting LT1185 ?ta03a v in r4 5.36k r3 15k q1 2n3906 r1 2.37k + 2 m f tant ref gnd fb v out LT1185 v in + v out 2 m f tant r2 2.61k + + auxiliary + 12v low dropout regulator for switching supply i out v out (normalized) 0.6 0.8 1.0 LT1185 ?ta03b 0.4 0.2 0 1.2 1.4 1.6 i short-circuit = 15k r3 + i full load = 15k r3 10.8k r4 LT1185 ?ta04 r lim r1 2.37k r2 9.76k 12v regulated auxiliary 5v main output * * 5v control primary *diode connection indicates a flyback switching topology, but forward converters may also be used + + + ref gnd fb v out v in LT1185
13 LT1185 typical applicatio s u time delayed start-up low input voltage monitor tracks dropout characteristics r6** 1k + 4k r1 2.37k r2 2.6k r7 27k c1 2.2 m f tant v out + optional hystersis ? 2m lt1006 ? v v + 3 2 4 7 ?ow?for low input output swings from v in + to v in r3 360k r4** 1k r5* 0.01 w c2 2.2 m f tant ref gnd fb v out LT1185 v in + v in *3" #26 wire **r4 determines trip point at i out = 0 r6 determines increase of trip point as i out increases trip point for v in = v out () 1 + r4 ?r7 r3 ?r6 + i out r5 ?r7 r6 for values shown, trip point for v in is: v out + 0.37v at i out = 0 and v out = 1.18v at i out = 3a ? do not substitute. op amp must have common mode range equal to negative supply LT1185 ?ta05 + + + ref gnd fb v out LT1185 v in r lim *** d1 d2 r3** 15k c3* d3 ? c2 2.2 m f v in r2 r1 2.37k c1 2.2 m f tant + v out LT1185 ?ta06 all diodes 1n4148 *see chart for delay time versus (c3)(r3//r lim ) product **for long delay times, replace d2 with 2n3906 transistor and use r3 only for calculating delay time. r3 can increase to 100k ***i lim is ? 11k/r lim , instead of 15k, because of voltage drop in d1. temperature coefficient of i lim will be ? 0.11%/ c, so adequate margin must be allowed for cold operation ? d3 provides fast reset of timing. input must drop to a low value to reset timing q1** + + delay time input voltage (v) 0 time constants (t)* 1.5 2.0 2.5 15 25 LT1185 ?ta07 1.0 0.5 0 510 20 3.0 3.5 4.0 30 *t = (r3//r lim )(c3) = () r3 ?r lim r3 + r lim (c3)
14 LT1185 sche atic diagra ww r1 5.5k c1 10pf r2 3k r3 3k r4 520 w r5 600 w r7 500 w r6 750 w 500 w r9 2.7k q48 r54 4k r8 6.5k q9 q15 q16 r11 220 w r53 10k r52 10k c2 r55 30k r49 700 w r56 600 w r50 160 w 10k r46 8k q36 r47 4k q51 q35 r48 2k r45 1.3k r40 1k r42 50k r43 50k r44 5k c3 30pf r39 1k d1 r12 2k r13 2k r14 3.2k r17 6k r15 4k r16 1k r18 2k q17 q18 q19 q28 r35 20k c4 10pf r37 1k r38 400 w c5 10pf r34 300 w r36 20k r38 20k r28 0.055 w r26 1k r24 6k r23 80 w r19 20k d4 v in v out fb gnd ref q52 r31 200 w LT1185 ?sd q20 q53 q21 q22 q23 q24 q25 q26 q27 q29 q30 q31 q14 q13 q8 q12 q11 q6 q5 q2 q4 q1 q3 q49 q47 q43 q44 q41 q46 q40 q34 q39 q37 q50 q33 q32 q7 q45 q42
15 LT1185 package descriptio u dimensions in inches (millimeters) unless otherwise noted. k package 4-lead to-3 metal can (ltc dwg # 05-08-1311) t package 5-lead plastic to-220 (standard) (ltc dwg # 05-08-1421) 0.038 ?0.043 (0.965 ?1.09) 0.060 ?0.135 (1.524 ?3.429) 0.320 ?0.350 (8.13 ?8.89) 0.420 ?0.480 (10.67 ?12.19) 0.760 ?0.775 (19.30 ?19.69) k4(to-3) 0695 72 18 0.495 ?0.525 (12.57 ?13.34) r 0.470 tp p.c.d. 0.167 ?0.177 (4.24 ?4.49) r 0.151 ?0.161 (3.84 ?4.09) dia 2 plc 0.655 ?0.675 (16.64 ?19.05) 1.177 ?1.197 (29.90 ?30.40) t5 (to-220) 0398 0.028 ?0.038 (0.711 ?0.965) 0.057 ?0.077 (1.448 ?1.956) 0.135 ?0.165 (3.429 ?4.191) 0.700 ?0.728 (17.78 ?18.491) 0.045 ?0.055 (1.143 ?1.397) 0.095 ?0.115 (2.413 ?2.921) 0.013 ?0.023 (0.330 ?0.584) 0.620 (15.75) typ 0.155 ?0.195 (3.937 ?4.953) 0.152 ?0.202 (3.861 ?5.131) 0.260 ?0.320 (6.60 ?8.13) 0.165 ?0.180 (4.191 ?4.572) 0.147 ?0.155 (3.734 ?3.937) dia 0.390 ?0.415 (9.906 ?10.541) 0.330 ?0.370 (8.382 ?9.398) 0.460 ?0.500 (11.684 ?12.700) 0.570 ?0.620 (14.478 ?15.748) 0.230 ?0.270 (5.842 ?6.858) 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 represen- tation that the interconnection of circuits as described herein will not infringe on existing patent rights.
16 LT1185 ? linear technology corporation 1994 1185fd lt/gp 0499 2k rev d ? printed in usa typical applicatio s u logic controlled 3a low-side switch with fault protection LT1185 ?ta08 ref gnd v out LT1185 fb v in 5v load 1n4001 add for inductive loads r lim 4k improved high frequency ripple rejection + v out + v in LT1185 ?ta09 r lim c2 2.2 m f tant r1 2.37k r2 c3 0.05 m f c1 4.7 m f tant note: c3 impoves high frequency ripple rejection by 6db at v out = 5v, and by 14db at v out = 12v. c1 is increased to 4.7 m f to ensure good stabiltity when c3 is used ref gnd fb v out LT1185 v in + part number description comments lt1085 7.5a low dropout regulator 1v dropout voltage lt1117 800ma low dropout regulator with shutdown reverse voltage and reverse current protection lt1120a micropower regulator with comparator and shutdown 20 m a supply current, 2.5v reference output lt1129 200ma micropower low dropout regulator 400mv dropout voltage, 50 m a supply current lt1175 500ma negative low dropout micropower regulator 45 m a supply current, adjustable current limit lt1585 4.6a low dropout fast transient response regulator for high performance microprocessors related parts linear technology corporation 1630 mccarthy blvd., milpitas, ca 95035-7417 (408) 432-1900 l fax: (408) 434-0507 l www.linear-tech.com

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