LM138,LM338 LM138/LM338 5-Amp Adjustable Regulators Literature Number: SNVS771A LM138/LM338 5-Amp Adjustable Regulators General Description The LM138 series of adjustable 3-terminal positive voltage regulators is capable of supplying in excess 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 resulted in outstanding load and line regulation — comparable to many commercial power supplies. The LM138 family is supplied in a standard 3-lead transistor package. A unique feature of the LM138 family is time-dependent 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 sustained 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 protection remains functional even if the adjustment pin is accidentally disconnected. Normally, no capacitors are needed unless the device is situated more than 6 inches from the input filter capacitors in which case an input bypass is needed. An output capacitor can be added to improve transient response, while bypassing the adjustment pin will increase the regulator’s ripple rejection. Besides replacing fixed regulators or discrete designs, the LM138 is useful in a wide variety of other applications. Since the regulator is “floating” and sees only the input-to-output differential voltage, supplies of several hundred volts can be regulated as long as the maximum input to output differential is not exceeded, i.e., do not short-circuit output to ground. The part numbers in the LM138 series which have a K suffix are packaged in a standard Steel TO-3 package, while those with a T suffix are packaged in a TO-220 plastic package. The LM138 is rated for −55˚C ≤ TJ ≤ +150˚C, and the LM338 is rated for 0˚C ≤ TJ ≤ +125˚C. Features n n n n n n n Guaranteed 7A peak output current Guaranteed 5A output current Adjustable output down to 1.2V Guaranteed thermal regulation Current limit constant with temperature P+ Product Enhancement tested Output is short-circuit protected Applications n Adjustable power supplies n Constant current regulators n Battery chargers Connection Diagrams (See Physical Dimension section for further information) Connection Diagrams (TO-3 STEEL) Metal Can Package (TO-220) Plastic Package 00906031 Front View Order Number LM338T See NS Package Number T03B 00906030 Bottom View Order Number LM138K STEEL or LM338K STEEL See NS Package Number K02A © 2004 National Semiconductor Corporation DS009060 www.national.com LM138/LM338 5-Amp Adjustable Regulators May 1998 LM138/LM338 Absolute Maximum Ratings (Note 1) Storage Temperature If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. Lead Temperature Metal Package (Soldering, 10 seconds) Plastic Package (Soldering, 4 seconds) (Note 4) Power Dissipation Internally limited Input/Output Voltage Differential −65˚C to +150˚C 300˚C 260˚C ESD Tolerance +40V, −0.3V TBD Operating Temperature Range −55˚C ≤ TJ ≤ +150˚C LM138 0˚C ≤ TJ ≤ +125˚C LM338 Electrical Characteristics Specifications with standard type face are for TJ = 25˚C, and those with boldface type apply over full Operating Temperature Range. Unless otherwise specified, VIN − VOUT = 5V; and IOUT = 10 mA. (Note 2) Symbol Parameter Conditions VREF Reference Voltage 3V ≤ (VIN − VOUT) ≤ 35V, VRLINE Line Regulation 3V ≤ (VIN − VOUT) ≤ 35V (Note 3) LM138 Units Min Typ Max 1.19 1.24 1.29 V 0.005 0.01 %/V 0.02 0.04 %/V 0.1 0.3 % 10 mA ≤ IOUT ≤ 5A, P ≤ 50W VRLOAD Load Regulation 10 mA ≤ IOUT ≤ 5A (Note 3) Thermal Regulation 20 ms Pulse IADJ Adjustment Pin Current ∆IADJ Adjustment Pin Current Change 10 mA ≤ IOUT ≤ 5A, 0.3 0.6 % 0.002 0.01 %/W 45 100 µA 0.2 5 µA 5 mA 3V ≤ (VIN − VOUT) ≤ 35V ∆VR/T Temperature Stability TMIN ≤ TJ ≤ TMAX 1 ILOAD(Min) Minimum Load Current VIN − VOUT = 35V 3.5 ICL Current Limit VIN − VOUT ≤ 10V VN RMS Output Noise, % of VOUT Ripple Rejection Ratio DC 5 8 A 0.5 ms Peak 7 12 A VIN − VOUT = 30V 1 10 Hz ≤ f ≤ 10 kHz 0.003 % 60 dB 75 dB VOUT = 10V, f = 120 Hz, CADJ = 0 µF VOUT = 10V, f = 120 Hz, CADJ = 10 µF θJC % Long-Term Stability TJ = 125˚C, 1000 Hrs Thermal Resistance, K Package 60 0.3 1 A 1 % 1 ˚C/W Junction to Case θJA Thermal Resistance, Junction to K Package 35 ˚C/W Ambient (No Heat Sink) Electrical Characteristics Symbol Parameter Conditions VREF Reference Voltage 3V ≤ (VIN − VOUT) ≤ 35V, VRLINE Line Regulation 3V ≤ (VIN − VOUT) ≤ 35V (Note 3) LM338 Units Min Typ Max 1.19 1.24 1.29 V 0.005 0.03 %/V 0.02 0.06 %/V 0.1 0.5 % 0.3 1 % 10 mA ≤ IOUT ≤ 5A, P ≤ 50W VRLOAD www.national.com Load Regulation 10 mA ≤ IOUT ≤ 5A (Note 3) 2 Symbol (Continued) Parameter Conditions LM338 Min Thermal Regulation IADJ Adjustment Pin Current ∆IADJ Adjustment Pin Current Change 20 ms Pulse 10 mA ≤ IOUT ≤ 5A, 3V ≤ (VIN − VOUT) ≤ 35V Max 0.002 0.02 %/W 45 100 µA 0.2 5 µA 10 mA ∆VR/T Temperature Stability TMIN ≤ TJ ≤ TMAX 1 ILOAD(Min) Minimum Load Current VIN − VOUT = 35V 3.5 ICL Current Limit VIN − VOUT ≤ 10V DC 5 8 0.5 ms Peak 7 12 VIN − VOUT = 30V VN θJA % A A 1 A RMS Output Noise, % of VOUT 10 Hz ≤ f ≤ 10 kHz Ripple Rejection Ratio VOUT = 10V, f = 120 Hz, CADJ = 0 µF Long-Term Stability TJ = 125˚C, 1000 hrs Thermal Resistance Junction to Case Thermal Resistance, Junction to K Package 35 ˚C/W Ambient (No Heat Sink) T Package 50 ˚C/W VOUT = 10V, f = 120 Hz, CADJ = 10 µF θJC Units Typ 60 0.003 % 60 dB 75 0.3 dB 1 % K Package 1 ˚C/W T Package 4 ˚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 do not guarantee specific performance limits. For guaranteed specifications and test conditions, see the Electrical Characteristics. Note 2: These specifications are applicable for power dissipations up to 50W for the TO-3 (K) package and 25W for the TO-220 (T) package. Power dissipation is guaranteed at these values up to 15V input-output differential. Above 15V differential, power dissipation will be limited by internal protection circuitry. All limits (i.e., the numbers in the Min. and Max. columns) are guaranteed to National’s AOQL (Average Outgoing Quality Level). Note 3: Regulation is measured at a constant junction temperature, using pulse testing with a low duty cycle. Changes in output voltage due to heating effects are covered under the specifications for thermal regulation. Note 4: Refer to RETS138K drawing for military specifications of LM138K. Typical Performance Characteristics Current Limit Current Limit 00906032 00906033 3 www.national.com LM138/LM338 Electrical Characteristics LM138/LM338 Typical Performance Characteristics (Continued) Current Limit Load Regulation 00906035 00906034 Adjustment Current Dropout Voltage 00906037 00906036 Temperature Stability Output Impedance 00906038 www.national.com 00906039 4 LM138/LM338 Typical Performance Characteristics (Continued) Minimum Operating Current Ripple Rejection 00906041 00906040 Ripple Rejection Ripple Rejection 00906042 00906043 Line Transient Response Load Transient Response 00906044 00906045 Application Hints In operation, the LM138 develops a nominal 1.25V reference voltage, VREF, between the output and adjustment terminal. The reference voltage is impressed across program resistor R1 and, since the voltage is constant, a constant current I1 then flows through the output set resistor R2, giving an output voltage of 5 www.national.com LM138/LM338 Application Hints mum performance. The current set resistor connected between the adjustment terminal and the output terminal (usually 240Ω) should be tied directly to the output of the regulator (case) 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 0.05Ω resistance between the regulator and load will have a load regulation due to line resistance of 0.05Ω x IL. If the set resistor is connected near the load the effective line resistance will be 0.05Ω (1 + R2/R1) or in this case, 11.5 times worse. Figure 2 shows the effect of resistance between the regulator and 240Ω set resistor. (Continued) 00906006 FIGURE 1. Since the 50 µA current from the adjustment terminal represents an error term, the LM138 was designed to minimize IADJ and make it very constant with line and load changes. To do this, all quiescent operating current is returned to the output establishing a minimum load current requirement. If there is insufficient load on the output, the output will rise. 00906007 FIGURE 2. Regulator with Line Resistance in Output Lead EXTERNAL CAPACITORS An input bypass capacitor is recommended. A 0.1 µF disc or 1 µF solid tantalum on the input is suitable input bypassing for almost all applications. The device is more sensitive to the absence of input bypassiing when adjustment or output capacitors are used but the above values will eliminate the possiblity of problems. The adjustment terminal can be bypassed to ground on the LM138 to improve ripple rejection. This bypass capacitor prevents ripple from being amplified as the output voltage is increased. With a 10 µF bypass capacitor 75 dB ripple rejection is obtainable at any output level. Increases over 20 µF do not appreciably improve the ripple rejection at frequencies above 120 Hz. If the bypass capacitor is used, it is sometimes necessary to include protection diodes to prevent the capacitor from discharging 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 µF in aluminum electrolytic to equal 1 µF 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 0.5 MHz. For this reason, 0.01 µF disc may seem to work better than a 0.1 µF disc as a bypass. Although the LM138 is stable with no output capacitors, like any feedback circuit, certain values of external capacitance can cause excessive ringing. This occurs with values between 500 pF and 5000 pF. A 1 µF solid tantalum (or 25 µF aluminum electrolytic) on the output swamps this effect and insures stability. With the TO-3 package, it is easy to minimize the resistance 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 regulation. PROTECTION DIODES When external capacitors are used with any IC regulator it is sometimes necessary to add protection diodes to prevent the capacitors from discharging through low current points into the regulator. Most 20 µF capacitors have low enough internal series resistance to deliver 20A spikes when shorted. Although the surge is short, there is enough energy to damage parts of the IC. 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 the rate of decrease of VIN. In the LM138 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 µF 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. Discharge occurs when either the input or output is shorted. Internal to the LM138 is a 50Ω resistor which limits the peak discharge current. No protection is needed for output voltages of 25V or less and 10 µF capacitance. Figure 3 shows an LM138 with protection diodes included for use with outputs greater than 25V and high values of output capacitance. LOAD REGULATION The LM138 is capable of providing extremely good load regulation but a few precautions are needed to obtain maxi- www.national.com 6 LM138/LM338 Application Hints (Continued) 00906008 D1 protects against C1 D2 protects against C2 FIGURE 3. Regulator with Protection Diodes 7 www.national.com LM138/LM338 Typical Applications Regulator and Voltage Reference 1.2V–25V Adjustable Regulator 00906001 00906003 Temperature Controller 00906010 Full output current not available at high input-output voltages †Optional — improves transient response. Output capacitors in the range of 1 µF to 1000 µF of aluminum or tantalum electrolytic are commonly used to provide improved output impedance and rejection of transients. *Needed if device is more than 6 inches from filter capacitors. **R1 = 240Ω for LM138. R1, R2 as an assembly can be ordered from Bourns: MIL part no. 7105A-AT2-502 COMM part no. 7105A-AT7-502 www.national.com 8 LM138/LM338 Schematic Diagram 00906009 Typical Applications Precision Power Regulator with Low Temperature Coefficient 00906012 * Adjust for 3.75 across R1 9 www.national.com LM138/LM338 Typical Applications (Continued) Slow Turn-On 15V Regulator Adjustable Regulator with Improved Ripple Rejection 00906014 †Solid tantalum 00906013 *Discharges C1 if output is shorted to ground **R1 = 240Ω for LM138 High Stability 10V Regulator Digitally Selected Outputs 00906015 00906016 *Sets maximum VOUT **R1 = 240Ω for LM138 www.national.com 10 LM138/LM338 Typical Applications (Continued) 15A Regulator 00906017 * Minimum load — 100 mA 5V Logic Regulator with Electronic Shutdown** Light Controller 00906011 00906018 ** Minimum output ≈ 1.2V 11 www.national.com LM138/LM338 Typical Applications (Continued) 0 to 22V Regulator 00906019 * R1 = 240Ω, R2 = 5k for LM138 Full output current not available at high input-output voltages 12V Battery Charger 00906020 www.national.com 12 LM138/LM338 Typical Applications (Continued) Adjustable Current Regulator Precision Current Limiter 00906022 00906021 5A Current Regulator Tracking Preregulator 00906023 00906024 Adjusting Multiple On-Card Regulators with Single Control* 00906025 † Minimum load — 10 mA * All outputs within ± 100 mV 13 www.national.com LM138/LM338 Typical Applications (Continued) Power Amplifier 00906027 AV = 1, RF = 10k, CF = 100 pF AV = 10, RF = 100k, CF = 10 pF Bandwidth ≥ 100 kHz Distortion ≤ 0.1% Simple 12V Battery Charger 00906028 Use of RS allows low charging rates with fully charged battery. **The 1000 µF is recommended to filter out input transients www.national.com 14 LM138/LM338 Typical Applications (Continued) Adjustable 15A Regulator 00906026 Current Limited 6V Charger 00906029 * Set max charge current to 3A ** THE 1000 µF is recommended to filter out input transients. 15 www.national.com LM138/LM338 Typical Applications (Continued) 10A Regulator 00906002 * Minimum load — 100 mA www.national.com 16 LM138/LM338 Physical Dimensions inches (millimeters) unless otherwise noted 2 Lead TO-3 Metal Can Package (K) Order Number LM138K or LM338K STEEL NS Package Number K02A 3 Lead Molded TO-220 (T) Order Number LM338T NS Package Number T03B 17 www.national.com LM138/LM338 5-Amp Adjustable Regulators Notes National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications. 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