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lm138/238 lm338 march 1993 adjustable voltage regulators three-terminal 5-a . guaranteed 7a peak output current . guaranteed 5a output current . adjustable output down to 1.2v . line regulation typically 0.005% /v . load regulation typically 0.1% . guaranteed thermal regulation . current limit constant with tem- perature . standard 3-lead transistor package description the lm138/lm238/lm338 are adjustable 3-terminal positive voltage regulators capable of supplying in ex- cess of 5a over a 1.2v to 32v output range. they are exceptionally easy to use and require only 2 resistors to set the output voltage. careful circuit design has re- sulted in outstanding load and line regulation com- parable to many commercial power supplies. the lm138 family is supplied in a standard 3-lead transis- tor package. a unique feature of the lm138 family is time-de- pendent current limiting. the current limit circuitry allows peak currents of up to 12a to be drawn from the regulator for short periods of time. this allows the lm138 to be used with heavy transient loads and speeds start-up under full-load conditions. under sus- tained loading conditions, the current limit decreases to a safe value protecting the regulator. also included on the chip are thermal overload protection and safe area protection for the power transistor. overload pro- tection remains functional even if the adjustment pin is accidentally disconnected. normally, no capacitors are needed unless the device is situated far from the input filter capacitors in which case an input bypass is needed. an optional output capacitor can be added to improve transient res- ponse. the adjustment terminal can be bypassed to achieve very high ripple rejection ratios which are dif- ficult to achieve with standard 3-terminal regulators. besides replacing fixed regulators or discrete designs, the lm238 is useful in a wide variety of other applica- tions. since the regulator is "floating" and sees only the input-to-output differential voltage, supplies of sev- eral hundred volts can be regulated as long as the maximum input to input differential is not exceeded. the lm138/lm238/lm338 are packaged in standard steel to-3 transistor packages. the lm138 is rated for operation from -55 o c to 150 o c, the lm238 from C 25 c to + 150 c and the lm338 from 0 c to + 125 c. to3 k suffix (steel can) order code part number temperature range package k lm138 lm238 lm338 -55 o c to + 150 o c -25 o c to + 150 o c 0 o c to + 125 o c example : lm138k pin connection (bottom view) case is output 1/12
absolute maximum rating symbol parameter value unit p tot power dissipation internally limited w v i - v o input-output voltage differential 35 v t oper operating junction temperature range lm138 lm238 lm338 -55 to 150 -25 to 150 0 to 125 o c t stg storage temperature range -65 to 150 o c t lead lead temperature (soldering, 10 seconds) 300 o c thermal characteristics symbol parameter value unit r th(j-c) typical junction-case thermal resistance 1.4 o c/w r th(j-a) max junction-ambient thermal resistance 35 o c/w schematic diagram lm138-lm238-lm338 2/12
electrical characteristics lm138: -55 t j 150 o c, v i - v o = 5v, i o = 2.5a lm238: -25 t j 150 o c, v i - v o = 5v, i o = 2.5a lm338: 0 t j 150 o c, v i - v o = 5v, i o = 2.5a although power dissipation is internally limited, these specifications apply to power dissipation up to 50w (unless otherwise specified). symbol parameter lm138-lm238 lm338 unit min. typ. max. min. typ. max. k vi line regulation - (note 1) t amb = 25 o c, 3 v (v i - v o ) 35 v 0.005 0.01 0.005 0.03 %/v k vo load regulation t amb = 25 o c, 10 ma i o 5 a v o 5v - (note 1) v o 3 5v - (note 1) 5 0.1 15 0.3 5 0.1 25 0.5 mv % thermal regulation (pulse = 20 ms) 0.002 0.01 0.002 0.02 %/w i adj adjustment pin current 45 100 45 100 m a d i adj adjustment pin current change 10 ma i l 5 a, 3 v (v i - v o ) 35 v 0.2 5 0.2 5 m a v (ref) reference voltage 3v (v i - v o ) 35 v, 10 ma i o 5a, p 50w 1.19 1.24 1.29 1.19 1.24 1.29 v k vi line regulation - (note 1) 3 v (v i - v o ) 35 v 0.02 0.04 0.02 0.06 %/v k vo load regulation 10 ma i o 5 a v o 5v - (note 1) v o 3 5v - (note 1) 20 0.3 30 0.6 20 0.3 50 1 mv % k vt temperature stability (t min t j t max )11% i o(min) minimum load current (v i - v o 35 v) 3.5 5 3.5 10 ma i o(max) current limit (v i - v o 10 v) dc 0.5 ms peak v i - v o = 30 v 5 7 8 12 1 5 7 8 12 1 a rms output noise, % of v o (t amb = 25 o c, 10 hz f 10 khz) 0.003 0.003 % r vf ripple rejection ratio v o = 10 v, f = 120 hz c adi = 10 m f60 60 75 60 60 75 db k vh long term stability (t amb = 125 o c) 0.3 1 0.3 1 % note 1 : regulation is measured at constant junction temperature. changes in output voltage due to heating effects are taken into account separately by thermal rejection. lm138-lm238-lm338 3/12
lm138-lm238-lm338 4/12
lm138-lm238-lm338 5/12
typical applications + 1.2v to + 25v adjustable regulator application hints in operation, the lm338 develops a nominal 1.25v reference voltage, v (ref) , between the output and adjustment terminal. the reference voltage is im- pressed across program resistor r1 and, since the voltage is constant, a constant current i 1 then flows through the output set resistor r2, giving an output voltage of v o = v (ref) ( 1+ r 2 r 1 ) + i adj r 2 figure 1. since the 50 m a current from the adjustment termi- nal represents an error term, the lm338 was de- signed to minimize i adj and make it very constant with line and load changes. to do this, all quiescent operating current is returned to the output establi- shing a minimum load current requirement. if there is insufficient load on the output, the output will rise. external capacitors an input bypass capacitor is recommended. a 0.1 m f disc or 1 m f solid tantalum on the input is suit- able input by passing for almost all applications. the device is more sensitive to the absence of input bypassing when adjustment or output capacitors are used byt the above values will eliminate the possibility of problems. the adjustment terminal can be bypassed to ground on the lm338 to improve ripple rejection. this bypass capacitor prevents ripple form being amplified as the output voltage is increased. with a 10 m f bypass capacitor 75db ripple rejection is obtainable at any output level. increases over 20 m f do not appreciably improve the ripple rejection at frequencies above 120hz. if the bypass capacitor is used, it is sometimes necessary to include pro- tection diodes to prevent the capacitor from dis- charging through internal low current paths and damaging the device. in general, the best type of capacitors to use are solid tantalum. solid tantalum capacitors have low impedance even at high frequencies. depending upon capacitor construction, it takes about 25 m f in aluminum electrolytic to equal 1 m f solid tantalum at high frequencies. ceramic capacitors are also good at high frequencies, but some types have a large decrease in capacitance at frequencies around 0.5mhz. for this reason, 0.01 m f disc may seem to work better than a 0.1 m f disc as a bypass. although the lm338 is stable with no output capa- citors, like any feedback circuit, certain values of external capacitance can cause excessive ringing. this occurs with values between 500pf and 5000pf. a 1 m f solid tantalum (or 25 m f aluminium electrolytic) on the output swamps this effect and insures stability. load regulation the lm338 is capable of providing extremely good load regulation but a few precautions are needed to obtain maximum performance. the current set resistor connected between the adjustment termi- nal and the output terminal (usually 240 w ) should be tied directly to the output of the regulator rather than near the load. this eliminates line drops from appearing effectively in series with the reference and degrading regulation. for example, a 15v regulator with 0.05 w resistance between the regu- lator and load will have a load regulation due to line resistance of 0.05 w x i l . if the set resistor is con- nected near the load the effective line resistance d needed if device is far from filter capacitors. * optional-improves transient response. output capacitors in the range of 1 m f to 100 m f of aluminium or tantalum electrolytic are commonly used to provide improved output impedance and rejec- tion of tran-sients. r2 * * v o = 1.25v (1 + ) r1 * * * r1 = 240 w for lm138 and lm238 lm138-lm238-lm338 6/12
will be 0.05 w (1 + r 2 /r 1 ) or in this case, 11.5 times worse. figure 2 shows the effect of resistance between the regulator and 140 w set resistor. with the to-3 package, it is easy to minimize the re- sistance from the case to the set resistor, by using 2 separate leads to the case. the ground of r2 can be returned near the ground of the load to provide remote ground sensing and improve load regula- tion. protection diodes when external capacitors are used with any ic regu- lator it is sometimes necessary to add protection diodes to prevent the capacitors from discharging through low current points into the regulator. most figure 2 : regulator with line resistance in out- put lead. 20 m f capacitors have low enough internal series re- sistance to deliver 20a spikes when shorted. al- though the surge is short, there is enough energy to damage parts of the ic. when an output capacitor is connected to a regula- tor and the input is shorted, the output capacitor will discharge into the output of the regulator. the dis- charge current depends on the value of the capaci- tor, the output voltage of the regulator, and the rate of decrease of v i . in the lm338 this discharge path is through a large junction that is able to sustain 25a surge with no problem. this is not true of other types of positive regulators. for output capacitors of 100 m f or less at output of 15v or less, there is no need to use diodes. the bypass capacitor on the adjustment terminal can discharge through a low current junction. dis- charge occurs when either the input or output is shorted. internal to the lm338 is a 50 w resistor which limits the peak discharge current. no protec- tion is needed for output voltages of 25v or less and 10 m f capacitance. figure 3 shows an lm338 with protection diodes included for use with outputs greater than 25v and high values of output capacit- ance. figure 3 : regulator with protection diodes. lm138-lm238-lm338 7/12
10a regulator * minimum load C 100ma v i 3 10v v 0 3 3v v i C v 0 3 3.5v 5a current regulator * minimum load C 100ma v i 3 10v v 0 3 3v v i C v 0 3 3.5v lm138-lm238-lm338 8/12
15a regulator * minimum load C 100ma v i 3 + 10v v 0 3 + 3v v i C v 0 3 + 4v 5v logic regulator with electronic shutdown** * r1 = 240 w for lm138 or lm238 * r2 = 720 w for lm138 or lm238 ** minimum output ? + 1.2v lm138-lm238-lm338 9/12
* r1 = 240 w for lm138 or lm238 * r2 = 720 w for lm138 or lm238 ** minimum output ? + 1.2v slow turn-on 15v regulator * r1 = 240 w } for lm138 and lm238 * r2 = 2.7k w tracking preregulator lm138-lm238-lm338 10/12
dim. mm inch min. typ. max. min. typ. max. a 11.00 13.10 0.433 0.516 b 0.97 1.15 0.038 0.045 c 1.50 1.65 0.059 0.065 d 8.32 8.92 0.327 0.351 e 19.00 20.00 0.748 0.787 g 10.70 11.10 0.421 0.437 n 16.50 17.20 0.649 0.677 p 25.00 26.00 0.984 1.023 r 4.00 4.09 0.157 0.161 u 38.50 39.30 1.515 1.547 v 30.00 30.30 1.187 1.193 c d n b v u r a p e g o p003f to-3 mechanical data lm138-lm238-lm338 11/12
information furnished is believed to be accurate and reliable. however, sgs-thomson microelectronics assumes no responsability for the consequences of use of such information nor for any infringement of patents or other rights of third parties which may results from its use. no license is granted by implication or otherwise under any patent or patent rights of sgs-thomson microelectronics. specification s mentioned in this publication are subject to change without notice. this publication supersedes and replaces all information previously s upplied. sgs-thomson microelectronics products are not authorized for use as critical components in life support devices or systems with out express written approval of sgs-thomson microelectonics. ? 1994 sgs-thomson microelectronics - all rights reserved sgs-thomson microelectronics group of companies australia - brazil - france - germany - hong kong - italy - japan - korea - malaysia - malta - morocco - the netherlands - singapore - spain - sweden - switzerland - taiwan - thailand - united kingdom - u.s.a lm138-lm238-lm338 12/12

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