LM2596 (961743), страница 4
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This diode must be fast (short reverse recovery time) andmust be located close to the LM2596 using short leads andshort printed circuit traces. Because of their fast switchingspeed and low forward voltage drop, Schottky diodes providethe best performance and efficiency, and should be the firstchoice, especially in low output voltage applications. Ultra-fastrecovery, or High-Efficiency rectifiers are also a good choice,but some types with an abrupt turn-off characteristic maycause instability or EMl problems. Ultra-fast recovery diodestypically have reverse recovery times of 50 ns or less.
Rectifiers such as the 1N4001 series are much too slow and shouldnot be used.5. Catch Diode Selection (D1)6. Input Capacitor (CIN)A low ESR aluminum or tantalum bypass capacitor is neededbetween the input pin and ground to prevent large voltagetransients from appearing at the input. In addition, the RMScurrent rating of the input capacitor should be selected to be atleast 1⁄2 the DC load current.
The capacitor manufacturersdata sheet must be checked to assure that this current ratingis not exceeded. The curve shown in Figure 13 shows typicalRMS current ratings for several different aluminum electrolyticcapacitor values.This capacitor should be located close to the IC using shortleads and the voltage rating should be approximately 1.5times the maximum input voltage.If solid tantalum input capacitors are used, it is recomendedthat they be surge current tested by the manufacturer.Use caution when using a high dielectric constant ceramiccapacitor for input bypassing, because it may cause severeringing at the VIN pin.For additional information, see section on input capacitors in application information section.6.
Input Capacitor (CIN)The important parameters for the Input capacitor are the inputvoltage rating and the RMS current rating. With a nominalinput voltage of 28V, an aluminum electrolytic aluminum electrolytic capacitor with a voltage rating greater than 42V (1.5 xVIN) would be needed. Since the the next higher capacitorvoltage rating is 50V, a 50V capacitor should be used. Thecapacitor voltage rating of (1.5 x VIN) is a conservative guideline, and can be modified somewhat if desired.The RMS current rating requirement for the input capacitor ofa buck regulator is approximately 1⁄2 the DC load current. Inthis example, with a 3A load, a capacitor with a RMS currentrating of at least 1.5A is needed.The curves shown in Figure 13 can be used to select anappropriate input capacitor.
From the curves, locate the 50Vline and note which capacitor values have RMS current ratingsgreater than 1.5A. Either a 470 µF or 680 µF, 50V capacitorcould be used.For a through hole design, a 680 µF/50V electrolytic capacitor(Panasonic HFQ series or Nichicon PL series or equivalent)would be adequate. Other types or other manufacturers capacitors can be used provided the RMS ripple current ratingsare adequate.For surface mount designs, solid tantalum capacitors can beused, but caution must be exercised with regard to the capacitor surge current rting (see Application Information or inputcapacitors in this data sheet).
The TPS series available fromAVX, and the 593D series from Sprague are both surge current tested.A. Refer to the table shown in Figure 11. Schottky diodesprovide the best performance, and in this example a 5A, 40V,1N5825 Schottky diode would be a good choice. The 5A dioderating is more than adequate and will not be overstressedeven for a shorted output.To further simplify the buck regulator design procedure, National Semiconductor is making available computer designsoftware to be used with the Simple Switcher line ot switchingregulators. Switchers Made Simple (version 4.3 or later) isavailable on a 31⁄2" diskette for IBM compatible computers.15www.national.comLM2596LM2596 Series Buck Regulator Design Procedure (Adjustable Output)LM2596LM2596 Series Buck Regulator Design Procedure (Adjustable Output)OutputVoltage(V)Through Hole Output CapacitorSurface Mount Output CapacitorPanasonicNichicon PLFeedforwardAVX TPSSpragueFeedforwardHFQ SeriesSeriesCapacitorSeries595D SeriesCapacitor(µF/V)(µF/V)(µF/V)(µF/V)2820/35820/3533 nF330/6.3470/433 nF4560/35470/3510 nF330/6.3390/6.310 nF6470/25470/253.3 nF220/10330/103.3 nF1.5 nF9330/25330/251.5 nF100/16180/1612330/25330/251 nF100/16180/161 nF15220/35220/35680 pF68/20120/20680 pF24220/35150/35560 pF33/2533/25220 pF28100/50100/50390 pF10/3515/50220 pFFIGURE 3.
Output Capacitor and Feedforward Capacitor Selection TableLM2596 Series Buck Regulator Design ProcedureINDUCTOR VALUE SELECTION GUIDES (For Continuous Mode Operation)0125832601258324FIGURE 6. LM2596-12FIGURE 4. LM2596-3.30125832701258325FIGURE 7. LM2596-ADJFIGURE 5. LM2596-5.0www.national.com16Inductance Current(µH)(A)SchottLM2596LM2596 Series Buck Regulator Design Procedure(Continued)RencoPulse EngineeringThroughSurfaceThroughSurfaceHoleMountHoleMountThroughHoleCoilcraftSurfaceSurfaceMountMountL15220.9967148350 67148460 RL-1284-22-43 RL1500-22PE-53815PE-53815-S DO3308-223L21680.9967144070 67144450 RL-5471-5RL1500-68PE-53821PE-53821-S DO3316-683L22471.1767144080 67144460 RL-5471-6—PE-53822PE-53822-S DO3316-473L23331.4067144090 67144470 RL-5471-7—PE-53823PE-53823-S DO3316-333L24221.7067148370 67148480 RL-1283-22-43—PE-53824PE-53825-S DO3316-223L25152.1067148380 67148490 RL-1283-15-43—PE-53825PE-53824-S DO3316-153L263300.8067144100 67144480 RL-5471-1—PE-53826PE-53826-SDO5022P-334L272201.0067144110 67144490 RL-5471-2—PE-53827PE-53827-SDO5022P-224L281501.2067144120 67144500 RL-5471-3—PE-53828PE-53828-SDO5022P-154L291001.4767144130 67144510 RL-5471-4—PE-53829PE-53829-SDO5022P-104L30681.7867144140 67144520 RL-5471-5—PE-53830PE-53830-SDO5022P-683L31472.2067144150 67144530 RL-5471-6—PE-53831PE-53831-S DO5022P-473L32332.5067144160 67144540 RL-5471-7—PE-53932PE-53932-S DO5022P-333L33223.1067148390 67148500 RL-1283-22-43—PE-53933PE-53933-S DO5022P-223L34153.4067148400 67148790 RL-1283-15-43—PE-53934PE-53934-S DO5022P-153L352201.7067144170—RL-5473-1—PE-53935PE-53935-S—L361502.1067144180—RL-5473-4—PE-54036PE-54036-S—L371002.5067144190—RL-5472-1—PE-54037PE-54037-S—L38683.1067144200—RL-5472-2—PE-54038PE-54038-S—L39473.5067144210—RL-5472-3—PE-54039PE-54039-S—L40333.5067144220 67148290 RL-5472-4—PE-54040PE-54040-S—L41223.5067144230 67148300 RL-5472-5—PE-54041PE-54041-S—L421502.7067148410—RL-5473-4—PE-54042PE-54042-S—L431003.4067144240—RL-5473-2—PE-54043—L44683.4067144250—RL-5473-3—PE-54044—FIGURE 8.
Inductor Manufacturers Part NumbersCoilcraft Inc.Coilcraft Inc., EuropePulse Engineering Inc.Phone(800) 322-2645FAX(708) 639-1469Phone+11 1236 730 595FAX+44 1236 730 627Phone(619) 674-8100FAX(619) 674-8262Pulse Engineering Inc., Phone+353 93 24 107EuropeFAX+353 93 24 459Renco Electronics Inc.Phone(800) 645-5828FAX(516) 586-5562Phone(612) 475-1173FAX(612) 475-1786Schott Corp.FIGURE 9. Inductor Manufacturers Phone Numbers17www.national.comLM2596LM2596 Series Buck Regulator Design ProcedureNichicon Corp.PanasonicAVX Corp.Sprague/Vishay(Continued)Phone(708) 843-7500FAX(708) 843-2798Phone(714) 373-7857FAX(714) 373-7102Phone(803) 448-9411FAX(803) 448-1943Phone(207) 324-4140FAX(207) 324-7223FIGURE 10. Capacitor Manufacturers Phone NumbersVR3A DiodesSurface MountSchottkyUltra Fast4A–6A DiodesThrough HoleSchottkyRecovery20VSK3230V 30WQ03SK33All ofthesediodesarerated toat least50V.40V SK3450V SK35orMBRS360More 30WQ05SR302MBR3201N5821MBR33031DQ031N5822Ultra FastSchottkyRecoveryAll ofthesediodesarerated toat least50V.All ofthesediodesarerated toat least50V.50WQ0350WQ0430WF1031DQ04Through HoleSchottkyUltra FastRecoverySR5021N5823SB520SR5031N5824SB530All ofthesediodesarerated toat least50V.SR504MBR340MURS3201N5825MUR320MURS620SR305SB54050WF10MBR35050WQ0531DQ0518MUR620HER601SB55050SQ080FIGURE 11.
Diode Selection Tablewww.national.comUltra FastRecoverySR304MBRS34030WQ041N5820Surface MountLM2596Block Diagram01258321FIGURE 12.relatively high RMS currents flowing in a buck regulator’sinput capacitor, this capacitor should be chosen for its RMScurrent rating rather than its capacitance or voltage ratings,although the capacitance value and voltage rating are directly related to the RMS current rating.The RMS current rating of a capacitor could be viewed as acapacitor’s power rating. The RMS current flowing throughthe capacitors internal ESR produces power which causesthe internal temperature of the capacitor to rise.
The RMScurrent rating of a capacitor is determined by the amount ofcurrent required to raise the internal temperature approximately 10˚C above an ambient temperature of 105˚C. Theability of the capacitor to dissipate this heat to the surrounding air will determine the amount of current the capacitor cansafely sustain. Capacitors that are physically large and havea large surface area will typically have higher RMS currentratings. For a given capacitor value, a higher voltage electrolytic capacitor will be physically larger than a lower voltagecapacitor, and thus be able to dissipate more heat to thesurrounding air, and therefore will have a higher RMS current rating.The consequences of operating an electrolytic capacitorabove the RMS current rating is a shortened operating life.The higher temperature speeds up the evaporation of thecapacitor’s electrolyte, resulting in eventual failure.Selecting an input capacitor requires consulting the manufacturers data sheet for maximum allowable RMS ripplecurrent.
For a maximum ambient temperature of 40˚C, ageneral guideline would be to select a capacitor with a ripplecurrent rating of approximately 50% of the DC load current.For ambient temperatures up to 70˚C, a current rating of75% of the DC load current would be a good choice for aconservative design. The capacitor voltage rating must be atleast 1.25 times greater than the maximum input voltage,and often a much higher voltage capacitor is needed tosatisfy the RMS current requirements.Application InformationPIN FUNCTIONS+VIN — This is the positive input supply for the IC switchingregulator.
A suitable input bypass capacitor must be presentat this pin to minimize voltage transients and to supply theswitching currents needed by the regulator.Ground — Circuit ground.Output — Internal switch. The voltage at this pin switchesbetween (+VIN − VSAT) and approximately −0.5V, with a dutycycle of approximately VOUT/VIN.
To minimize coupling tosensitive circuitry, the PC board copper area connected tothis pin should be kept to a minimum.Feedback — Senses the regulated output voltage to complete the feedback loop.ON /OFF — Allows the switching regulator circuit to be shutdown using logic level signals thus dropping the total inputsupply current to approximately 80 µA. Pulling this pin belowa threshold voltage of approximately 1.3V turns the regulatoron, and pulling this pin above 1.3V (up to a maximum of 25V)shuts the regulator down. If this shutdown feature is notneeded, the ON /OFF pin can be wired to the ground pin orit can be left open, in either case the regulator will be in theON condition.EXTERNAL COMPONENTSINPUT CAPACITORCIN — A low ESR aluminum or tantalum bypass capacitor isneeded between the input pin and ground pin.
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