LM2596 (961743), страница 8
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A Schottky diode is recommended for low input voltages, (because of its lower voltagedrop) but for higher input voltages, a fast recovery diodecould be used.Without diode D3, when the input voltage is first applied, thecharging current of CIN can pull the output positive by several volts for a short period of time. Adding D3 prevents theoutput from going positive by more than a diode voltage.This delayed startup feature is useful in situations where theinput power source is limited in the amount of current it candeliver.
It allows the input voltage to rise to a higher voltagebefore the regulator starts operating. Buck regulators requireless input current at higher input voltages.UNDERVOLTAGE LOCKOUTSome applications require the regulator to remain off untilthe input voltage reaches a predetermined voltage. An undervoltage lockout feature applied to a buck regulator isshown in Figure 22, while Figure 23 and 24 applies the samefeature to an inverting circuit. The circuit in Figure 23 fea-25www.national.comLM2596Application InformationLM2596Application Information(Continued)01258339This circuit has hysteresisRegulator starts switching at VIN = 13VRegulator stops switching at VIN = 8VFIGURE 24.
Undervoltage Lockout with Hysteresis for Inverting Regulator01258340CIN— 68 µF/25V Tant. Sprague 595D470 µF/50V Elec. Panasonic HFQCOUT— 47 µF/20V Tant. Sprague 595D220 µF/25V Elec. Panasonic HFQFIGURE 25. Inverting −5V Regulator with Delayed Startupbe narrowed down to just a few values. Using the valuesshown in Figure 25 will provide good results in the majority ofinverting designs.This type of inverting regulator can require relatively largeamounts of input current when starting up, even with lightloads. Input currents as high as the LM2596 current limit(approx 4.5A) are needed for at least 2 ms or more, until theoutput reaches its nominal output voltage.
The actual timedepends on the output voltage and the size of the outputcapacitor. Input power sources that are current limited orsources that can not deliver these currents without gettingloaded down, may not work correctly. Because of the relatively high startup currents required by the inverting topology,the delayed startup feature (C1, R1 and R2) shown in Figure25 is recommended. By delaying the regulator startup, theinput capacitor is allowed to charge up to a higher voltagebefore the switcher begins operating. A portion of the highinput current needed for startup is now supplied by the inputcapacitor (CIN).
For severe start up conditions, the inputcapacitor can be made much larger than normal.01258341FIGURE 26. Inverting Regulator Typical Load CurrentBecause of differences in the operation of the invertingregulator, the standard design procedure is not used toselect the inductor value. In the majority of designs, a 33 µH,3.5A inductor is the best choice. Capacitor selection can alsowww.national.com26OFF. With the inverting configuration, some level shifting isrequired, because the ground pin of the regulator is nolonger at ground, but is now setting at the negative outputvoltage level. Two different shutdown methods for invertingregulators are shown in Figure 27 and 28.(Continued)INVERTING REGULATOR SHUTDOWN METHODSTo use the ON /OFF pin in a standard buck configuration issimple, pull it below 1.3V ( @25˚C, referenced to ground) toturn regulator ON, pull it above 1.3V to shut the regulator01258342FIGURE 27.
Inverting Regulator Ground Referenced Shutdown01258343FIGURE 28. Inverting Regulator Ground Referenced Shutdown using Opto Device27www.national.comLM2596Application InformationLM2596Application Information(Continued)TYPICAL THROUGH HOLE PC BOARD LAYOUT, FIXED OUTPUT (1X SIZE), DOUBLE SIDED01258344CIN — 470 µF, 50V, Aluminum Electrolytic Panasonic, “HFQ Series”COUT — 330 µF, 35V, Aluminum Electrolytic Panasonic, “HFQ Series”D1 — 5A, 40V Schottky Rectifier, 1N5825L1 — 47 µH, L39, Renco, Through HoleThermalloy Heat Sink #7020www.national.com28LM2596Application Information(Continued)TYPICAL THROUGH HOLE PC BOARD LAYOUT, ADJUSTABLE OUTPUT (1X SIZE), DOUBLE SIDED01258345CIN — 470 µF, 50V, Aluminum Electrolytic Panasonic, “HFQ Series”COUT — 220 µF, 35V Aluminum Electrolytic Panasonic, “HFQ Series”D1 — 5A, 40V Schottky Rectifier, 1N5825L1 — 47 µH, L39, Renco, Through HoleR1 — 1 kΩ, 1%R2 — Use formula in Design ProcedureCFF — See Figure 3.Thermalloy Heat Sink #7020FIGURE 29.
PC Board Layout29www.national.comLM2596Physical Dimensionsinches (millimeters)unless otherwise noted5-Lead TO-220 (T)Order Number LM2596T-3.3, LM2596T-5.0,LM2596T-12 or LM2596T-ADJNS Package Number T05Dwww.national.com30inches (millimeters) unless otherwise noted (Continued)5-Lead TO-263 Surface Mount Package (S)Order Number LM2596S-3.3, LM2596S-5.0,LM2596S-12 or LM2596S-ADJNS Package Number TS5BLIFE SUPPORT POLICYNATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORTDEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERALCOUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein:1. Life support devices or systems are devices orsystems which, (a) are intended for surgical implantinto the body, or (b) support or sustain life, andwhose failure to perform when properly used inaccordance with instructions for use provided in thelabeling, can be reasonably expected to result in asignificant injury to the user.National SemiconductorCorporationAmericasEmail: support@nsc.comwww.national.comNational SemiconductorEuropeFax: +49 (0) 180-530 85 86Email: europe.support@nsc.comDeutsch Tel: +49 (0) 69 9508 6208English Tel: +44 (0) 870 24 0 2171Français Tel: +33 (0) 1 41 91 87902.
A critical component is any component of a lifesupport device or system whose failure to performcan be reasonably expected to cause the failure ofthe life support device or system, or to affect itssafety or effectiveness.National SemiconductorAsia Pacific CustomerResponse GroupTel: 65-2544466Fax: 65-2504466Email: ap.support@nsc.comNational SemiconductorJapan Ltd.Tel: 81-3-5639-7560Fax: 81-3-5639-7507National 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.LM2596 SIMPLE SWITCHER Power Converter 150 kHz 3A Step-Down Voltage RegulatorPhysical Dimensions.
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