LM2596 (961743), страница 2
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See Application Information in this data sheet and the thermalmodel in Switchers Made Simple™ version 4.3 software.Typical Performance CharacteristicsNormalizedOutput Voltage(Circuit of Figure 1)Line Regulation01258304Efficiency01258305501258306www.national.comLM2596Typical Performance CharacteristicsSwitch SaturationVoltage(Circuit of Figure 1) (Continued)Switch Current Limit01258307OperatingQuiescent Current01258310Minimum OperatingSupply Voltage01258311ON /OFF PinCurrent (Sinking)01258313www.national.com0125830901258308ShutdownQuiescent CurrentON /OFF ThresholdVoltageDropout VoltageSwitching Frequency0125831460125831201258315LM2596Typical Performance Characteristics(Circuit of Figure 1) (Continued)Feedback PinBias Current012583167www.national.comLM2596Typical Performance CharacteristicsDiscontinuous Mode Switching WaveformsVIN = 20V, VOUT = 5V, ILOAD = 500 mAL = 10 µH, COUT = 330 µF, COUT ESR = 45 mΩContinuous Mode Switching WaveformsVIN = 20V, VOUT = 5V, ILOAD = 2AL = 32 µH, COUT = 220 µF, COUT ESR = 50 mΩ0125831701258318Horizontal Time Base: 2 µs/div.Horizontal Time Base: 2 µs/div.A: Output Pin Voltage, 10V/div.A: Output Pin Voltage, 10V/div.B: Inductor Current 1A/div.B: Inductor Current 0.5A/div.C: Output Ripple Voltage, 50 mV/div.C: Output Ripple Voltage, 100 mV/div.Load Transient Response for Discontinuous ModeVIN = 20V, VOUT = 5V, ILOAD = 500 mA to 2AL = 10 µH, COUT = 330 µF, COUT ESR = 45 mΩLoad Transient Response for Continuous ModeVIN = 20V, VOUT = 5V, ILOAD = 500 mA to 2AL = 32 µH, COUT = 220 µF, COUT ESR = 50 mΩ01258320Horizontal Time Base: 200 µs/div.01258319A: Output Voltage, 100 mV/div.
(AC)Horizontal Time Base: 100 µs/div.B: 500 mA to 2A Load PulseA: Output Voltage, 100 mV/div. (AC)B: 500 mA to 2A Load PulseTest Circuit and Layout GuidelinesFixed Output Voltage Versions01258322CIN— 470 µF, 50V, Aluminum Electrolytic Nichicon “PL Series”COUT— 220 µF, 25V Aluminum Electrolytic, Nichicon “PL Series”D1— 5A, 40V Schottky Rectifier, 1N5825L1— 68 µH, L38www.national.com8LM2596Test Circuit and Layout Guidelines(Continued)Adjustable Output Voltage Versions01258323where VREF = 1.23VSelect R1 to be approximately 1 kΩ, use a 1% resistor for best stability.CIN — 470 µF, 50V, Aluminum Electrolytic Nichicon “PL Series”COUT— 220 µF, 35V Aluminum Electrolytic, Nichicon “PL Series”D1— 5A, 40V Schottky Rectifier, 1N5825L1— 68 µH, L38R1— 1 kΩ, 1%CFF— See Application Information SectionFIGURE 1.
Standard Test Circuits and Layout GuidesAs in any switching regulator, layout is very important. Rapidly switching currents associated with wiring inductance cangenerate voltage transients which can cause problems. Forminimal inductance and ground loops, the wires indicated byheavy lines should be wide printed circuit traces andshould be kept as short as possible. For best results,external components should be located as close to theswitcher lC as possible using ground plane construction orsingle point grounding.If open core inductors are used, special care must betaken as to the location and positioning of this type of inductor.
Allowing the inductor flux to intersect sensitive feedback,lC groundpath and COUT wiring can cause problems.When using the adjustable version, special care must betaken as to the location of the feedback resistors and theassociated wiring. Physically locate both resistors near theIC, and route the wiring away from the inductor, especially anopen core type of inductor. (See application section for moreinformation.)9www.national.comLM2596LM2596 Series Buck Regulator Design Procedure (Fixed Output)PROCEDURE (Fixed Output Voltage Version)EXAMPLE (Fixed Output Voltage Version)Given:VOUT = Regulated Output Voltage (3.3V, 5V or 12V)Given:VOUT = 5VVIN(max) = Maximum DC Input VoltageILOAD(max) = Maximum Load CurrentVIN(max) = 12VILOAD(max) = 3A1.
Inductor Selection (L1)A. Select the correct inductor value selection guide from Figures Figure 4, Figure 5, or Figure 6. (Output voltages of 3.3V,5V, or 12V respectively.) For all other voltages, see the designprocedure for the adjustable version.B. From the inductor value selection guide, identify the inductance region intersected by the Maximum Input Voltage lineand the Maximum Load Current line. Each region is identifiedby an inductance value and an inductor code (LXX).C. Select an appropriate inductor from the four manufacturer’spart numbers listed in Figure 8.1.
Inductor Selection (L1)A. Use the inductor selection guide for the 5V version shownin Figure 5.B. From the inductor value selection guide shown in Figure 5,the inductance region intersected by the 12V horizontal lineand the 3A vertical line is 33 µH, and the inductor code is L40.C. The inductance value required is 33 µH. From the table inFigure 8, go to the L40 line and choose an inductor partnumber from any of the four manufacturers shown. (In mostinstance, both through hole and surface mount inductors areavailable.)2.
Output Capacitor Selection (COUT)A. In the majority of applications, low ESR (Equivalent SeriesResistance) electrolytic capacitors between 82 µF and 820 µFand low ESR solid tantalum capacitors between 10 µF and470 µF provide the best results. This capacitor should belocated close to the IC using short capacitor leads and shortcopper traces. Do not use 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 component selection table shown in Figure 2.This table contains different input voltages, output voltages,and load currents, and lists various inductors and output capacitors that will provide the best design solutions.C. The capacitor voltage rating for electrolytic capacitorsshould be at least 1.5 times greater than the output voltage,and often much higher voltage ratings are needed to satisfythe low ESR requirements for low output ripple voltage.D.
For computer aided design software, see Switchers MadeSimple™ version 4.3 or later.2. Output Capacitor Selection (COUT)A. See section on output capacitors in application information section.B. From the quick design component selection table shown inFigure 2, locate the 5V output voltage section. In the loadcurrent column, choose the load current line that is closest tothe current needed in your application, for this example, usethe 3A line.
In the maximum input voltage column, select theline that covers the input voltage needed in your application, inthis example, use the 15V line. Continuing on this line arerecommended inductors and capacitors that will provide thebest overall performance.The capacitor list contains both through hole electrolytic andsurface mount tantalum capacitors from four different capacitor manufacturers. It is recommended that both the manufacturers and the manufacturer’s series that are listed in the tablebe used.In this example aluminum electrolytic capacitors from severaldifferent manufacturers are available with the range of ESRnumbers needed.330 µF 35V Panasonic HFQ Series330 µF 35V Nichicon PL SeriesC. For a 5V output, a capacitor voltage rating at least 7.5V ormore is needed. But even a low ESR, switching grade, 220 µF10V aluminum electrolytic capacitor would exhibit approximately 225 mΩ of ESR (see the curve in Figure 14 for the ESRvs voltage rating).
This amount of ESR would result in relatively high output ripple voltage. To reduce the ripple to 1% ofthe output voltage, or less, a capacitor with a higher value orwith a higher voltage rating (lower ESR) should be selected. A16V or 25V capacitor will reduce the ripple voltage by approximately half.www.national.com10PROCEDURE (Fixed Output Voltage Version)(Continued)EXAMPLE (Fixed Output Voltage Version)3.
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. 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 also provide good results. Ultra-fast recovery diodes typically have reverse recovery times of 50 ns or less.
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