LM2596 (961743), страница 3
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Rectifiers such as the 1N5400series are much too slow and should not be used.3. Catch Diode Selection (D1)4. Input Capacitor (CIN)A low ESR aluminum or tantalum bypass capacitor is neededbetween the input pin and ground pin to prevent large voltagetransients from appearing at the input. This capacitor shouldbe located close to the IC using short leads. In addition, theRMS current rating of the input capacitor should be selected tobe at least 1⁄2 the DC load current. The capacitor manufacturers data sheet must be checked to assure that this currentrating is not exceeded. The curve shown in Figure 13 showstypical RMS current ratings for several different aluminumelectrolytic capacitor values.For an aluminum electrolytic, the capacitor voltage ratingshould be approximately 1.5 times the maximum input voltage.
Caution must be exercised if solid tantalum capacitorsare used (see Application Information on input capacitor). Thetantalum capacitor voltage rating should be 2 times the maximum input voltage and it is recommended that they be surgecurrent tested by the manufacturer.Use caution when using ceramic capacitors for input bypassing, because it may cause severe ringing at the VIN pin.For additional information, see section on input capacitors in Application Information section.4. Input Capacitor (CIN)The important parameters for the Input capacitor are the inputvoltage rating and the RMS current rating. With a nominalinput voltage of 12V, an aluminum electrolytic capacitor with avoltage rating greater than 18V (1.5 x VIN) would be needed.The next higher capacitor voltage rating is 25V.The RMS current rating requirement for the input capacitor ina 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 Figure13 can be used to select an appropriate input capacitor. Fromthe curves, locate the 35V line and note which capacitorvalues have RMS current ratings greater than 1.5A. A 680 µF/35V capacitor could be used.For a through hole design, a 680 µF/35V 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 rating (see Application Information on 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. In this example, a 5A,20V, 1N5823 Schottky diode will provide the best performance, and will not be overstressed even for a shorted output.11www.national.comLM2596LM2596 Series Buck Regulator Design Procedure (Fixed Output)LM2596LM2596 Series Buck Regulator Design Procedure (Fixed Output)ConditionsInductor(Continued)Output CapacitorThrough Hole ElectrolyticSurface Mount TantalumOutputLoadMax InputInductanceInductorPanasonicNichiconAVX TPSSpragueVoltageCurrentVoltage(µH)(#)HFQ SeriesPL SeriesSeries595D Series(V)(A)(V)3.3325321232522 L41(µF/V)(µF/V)(µF/V)(µF/V)470/25560/16330/6.3390/6.3722 L41560/35560/35330/6.3390/6.31022 L41680/35680/35330/6.3390/6.34033 L40560/35470/35330/6.3390/6.3622 L33470/25470/35330/6.3390/6.31033 L32330/35330/35330/6.3390/6.34047 L39330/35270/50220/10330/10822 L41470/25560/16220/10330/101022 L41560/25560/25220/10330/101533 L40330/35330/35220/10330/104047 L39330/35270/35220/10330/10922 L33470/25560/16220/10330/102068 L38180/35180/35100/10270/104068 L38180/35180/35100/10270/101522 L41470/25470/25100/16180/161833 L40330/25330/25100/16180/163068 L44180/25180/25100/16120/204068 L44180/35180/35100/16120/201533 L32330/25330/25100/16180/162068 L38180/25180/25100/16120/2040150 L4282/2582/2568/2068/25FIGURE 2.
LM2596 Fixed Voltage Quick Design Component Selection Tablewww.national.com12PROCEDURE (Adjustable Output Voltage Version)EXAMPLE (Adjustable Output Voltage Version)Given:VOUT = Regulated Output VoltageGiven:VIN(max) = Maximum Input VoltageILOAD(max) = Maximum Load CurrentF = Switching Frequency (Fixed at a nominal 150 kHz).VIN(max) = 28VILOAD(max) = 3AF = Switching Frequency (Fixed at a nominal 150 kHz).1. Programming Output Voltage (Selecting R1 and R2, asshown in Figure 1 )1. Programming Output Voltage (Selecting R1 and R2, asshown in Figure 1 )Use the following formula to select the appropriate resistorvalues.Select R1 to be 1 kΩ, 1%. Solve for R2.VOUT = 20VR2 = 1k (16.26 − 1) = 15.26k, closest 1% value is 15.4 kΩ.R2 = 15.4 kΩ.Select a value for R1 between 240Ω and 1.5 kΩ.
The lowerresistor values minimize noise pickup in the sensitive feedback pin. (For the lowest temperature coefficient and the beststability with time, use 1% metal film resistors.)2. Inductor Selection (L1)A. Calculate the inductor Volt • microsecond constant E • T (V• µs), from the following formula:2.
Inductor Selection (L1)A. Calculate the inductor Volt • microsecond constant(E • T),where VSAT = internal switch saturation voltage = 1.16Vand VD = diode forward voltage drop = 0.5VB. E • T = 34.2 (V • µs)C. ILOAD(max) = 3AB. Use the E • T value from the previous formula and match itwith the E • T number on the vertical axis of the Inductor ValueSelection Guide shown in Figure 7.C. on the horizontal axis, select the maximum load current.D.
Identify the inductance region intersected by the E • T valueand the Maximum Load Current value. Each region is identified by an inductance value and an inductor code (LXX).E. Select an appropriate inductor from the four manufacturer’spart numbers listed in Figure 8.D. From the inductor value selection guide shown in Figure 7,the inductance region intersected by the 34 (V • µs) horizontalline and the 3A vertical line is 47 µH, and the inductor code isL39.E. From the table in Figure 8, locate line L39, and select aninductor part number from the list of manufacturers part numbers.13www.national.comLM2596LM2596 Series Buck Regulator Design Procedure (Adjustable Output)LM2596LM2596 Series Buck Regulator Design Procedure (Adjustable Output)(Continued)PROCEDURE (Adjustable Output Voltage Version)EXAMPLE (Adjustable Output Voltage Version)3.
Output Capacitor Selection (COUT)3. Output Capacitor SeIection (COUT)A. In the majority of applications, low ESR electrolytic or solidtantalum capacitors between 82 µF and 820 µF provide thebest results. This capacitor should be located close to the ICusing short capacitor leads and short copper traces. Do notuse capacitors larger than 820 µF. For additional information, see section on output capacitors in application information section.B. To simplify the capacitor selection procedure, refer to thequick design table shown in Figure 3.
This table containsdifferent output voltages, and lists various output capacitorsthat will provide the best design solutions.C. The capacitor voltage rating should be at least 1.5 timesgreater than the output voltage, and often much higher voltageratings are needed to satisfy the low ESR requirementsneeded for low output ripple voltage.A.
See section on COUT in Application Information section.B. From the quick design table shown in Figure 3, locate theoutput voltage column. From that column, locate the outputvoltage closest to the output voltage in your application. In thisexample, select the 24V line. Under the output capacitor section, select a capacitor from the list of through hole electrolyticor surface mount tantalum types from four different capacitormanufacturers.
It is recommended that both the manufacturers and the manufacturers series that are listed in the table beused.In this example, through hole aluminum electrolytic capacitorsfrom several different manufacturers are available.220 µF/35V Panasonic HFQ Series150 µF/35V Nichicon PL SeriesC. For a 20V output, a capacitor rating of at least 30V or moreis needed.
In this example, either a 35V or 50V capacitorwould work. A 35V rating was chosen, although a 50V ratingcould also be used if a lower output ripple voltage is needed.Other manufacturers or other types of capacitors may also beused, provided the capacitor specifications (especially the 100kHz ESR) closely match the types listed in the table. Refer tothe capacitor manufacturers data sheet for this information.4. Feedforward Capacitor (CFF) (See Figure 1)For output voltages greater than approximately 10V, an additional capacitor is required. The compensation capacitor istypically between 100 pF and 33 nF, and is wired in parallelwith the output voltage setting resistor, R2.
It provides additional stability for high output voltages, low input-output voltages, and/or very low ESR output capacitors, such as solidtantalum capacitors.4. Feedforward Capacitor (CFF)The table shown in Figure 3 contains feed forward capacitorvalues for various output voltages. In this example, a 560 pFcapacitor is needed.This capacitor type can be ceramic, plastic, silver mica, etc.(Because of the unstable characteristics of ceramic capacitorsmade with Z5U material, they are not recommended.)www.national.com14(Continued)PROCEDURE (Adjustable Output Voltage Version)EXAMPLE (Adjustable Output Voltage Version)5.
Catch Diode Selection (D1)A. The catch diode current rating must be at least 1.3 timesgreater than the maximum load current. Also, if the powersupply design must withstand a continuous output short, thediode should have a current rating equal to the maximumcurrent limit of the LM2596. The most stressful condition forthis diode is an overload or shorted output condition.B. The reverse voltage rating of the diode should be at least1.25 times the maximum input voltage.C.
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