LM2596 (961743), страница 5
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It must belocated near the regulator using short leads. This capacitorprevents large voltage transients from appearing at the input, and provides the instantaneous current needed eachtime the switch turns on.The important parameters for the Input capacitor are thevoltage rating and the RMS current rating. Because of the19www.national.comLM2596Application InformationRMS ripple current rating, voltage rating, and capacitancevalue. For the output capacitor, the ESR value is the mostimportant parameter.(Continued)A graph shown in Figure 13 shows the relationship betweenan electrolytic capacitor value, its voltage rating, and theRMS current it is rated for.
These curves were obtained fromthe Nichicon “PL” series of low ESR, high reliability electrolytic capacitors designed for switching regulator applications.Other capacitor manufacturers offer similar types of capacitors, but always check the capacitor data sheet.“Standard” electrolytic capacitors typically have much higherESR numbers, lower RMS current ratings and typically havea shorter operating lifetime.Because of their small size and excellent performance, surface mount solid tantalum capacitors are often used for inputbypassing, but several precautions must be observed. Asmall percentage of solid tantalum capacitors can short if theinrush current rating is exceeded.
This can happen at turn onwhen the input voltage is suddenly applied, and of course,higher input voltages produce higher inrush currents. Several capacitor manufacturers do a 100% surge current testing on their products to minimize this potential problem. Ifhigh turn on currents are expected, it may be necessary tolimit this current by adding either some resistance or inductance before the tantalum capacitor, or select a higher voltage capacitor. As with aluminum electrolytic capacitors, theRMS ripple current rating must be sized to the load current.The output capacitor requires an ESR value that has anupper and lower limit.
For low output ripple voltage, a lowESR value is needed. This value is determined by the maximum allowable output ripple voltage, typically 1% to 2% ofthe output voltage. But if the selected capacitor’s ESR isextremely low, there is a possibility of an unstable feedbackloop, resulting in an oscillation at the output.
Using thecapacitors listed in the tables, or similar types, will providedesign solutions under all conditions.If very low output ripple voltage (less than 15 mV) is required, refer to the section on Output Voltage Ripple andTransients for a post ripple filter.An aluminum electrolytic capacitor’s ESR value is related tothe capacitance value and its voltage rating.
In most cases,higher voltage electrolytic capacitors have lower ESR values(see Figure 14 ). Often, capacitors with much higher voltageratings may be needed to provide the low ESR values required for low output ripple voltage.The output capacitor for many different switcher designsoften can be satisfied with only three or four different capacitor values and several different voltage ratings. See thequick design component selection tables in Figure 2 and 4for typical capacitor values, voltage ratings, and manufacturers capacitor types.Electrolytic capacitors are not recommended for temperatures below −25˚C.
The ESR rises dramatically at cold temperatures and typically rises 3X @ −25˚C and as much as10X at −40˚C. See curve shown in Figure 15.Solid tantalum capacitors have a much better ESR spec forcold temperatures and are recommended for temperaturesbelow −25˚C.FEEDFORWARD CAPACITOR(Adjustable Output Voltage Version)CFF — A Feedforward Capacitor CFF, shown across R2 inFigure 1 is used when the ouput voltage is greater than 10Vor when COUT has a very low ESR.
This capacitor adds leadcompensation to the feedback loop and increases the phasemargin for better loop stability. For CFF selection, see thedesign procedure section.0125832901258328FIGURE 14. Capacitor ESR vs Capacitor Voltage Rating(Typical Low ESR Electrolytic Capacitor)FIGURE 13. RMS Current Ratings for Low ESRElectrolytic Capacitors (Typical)CATCH DIODEBuck regulators require a diode to provide a return path forthe inductor current when the switch turns off. This must bea fast diode and must be located close to the LM2596 usingshort leads and short printed circuit traces.Because of their very fast switching speed and low forwardvoltage drop, Schottky diodes provide the best performance,especially in low output voltage applications (5V and lower).Ultra-fast recovery, or High-Efficiency rectifiers are also aOUTPUT CAPACITORCOUT — An output capacitor is required to filter the outputand provide regulator loop stability.
Low impedance or lowESR Electrolytic or solid tantalum capacitors designed forswitching regulator applications must be used. When selecting an output capacitor, the important capacitor parametersare; the 100 kHz Equivalent Series Resistance (ESR), thewww.national.com20LM2596Application Information(Continued)good choice, but some types with an abrupt turnoff characteristic may cause instability or EMI problems. Ultra-fastrecovery diodes typically have reverse recovery times of 50ns or less. Rectifiers such as the 1N5400 series are muchtoo slow and should not be used.01258331FIGURE 16.
(∆IIND) Peak-to-Peak InductorRipple Current (as a Percentage of the Load Current)vs Load CurrentBy allowing the percentage of inductor ripple current toincrease for low load currents, the inductor value and sizecan be kept relatively low.When operating in the continuous mode, the inductor currentwaveform ranges from a triangular to a sawtooth type ofwaveform (depending on the input voltage), with the averagevalue of this current waveform equal to the DC output loadcurrent.Inductors are available in different styles such as pot core,toroid, E-core, bobbin core, etc., as well as different corematerials, such as ferrites and powdered iron.
The leastexpensive, the bobbin, rod or stick core, consists of wirewound on a ferrite bobbin. This type of construction makesfor an inexpensive inductor, but since the magnetic flux is notcompletely contained within the core, it generates moreElectro-Magnetic Interference (EMl). This magnetic flux caninduce voltages into nearby printed circuit traces, thus causing problems with both the switching regulator operation andnearby sensitive circuitry, and can give incorrect scope readings because of induced voltages in the scope probe.
Alsosee section on Open Core Inductors.When multiple switching regulators are located on the samePC board, open core magnetics can cause interferencebetween two or more of the regulator circuits, especially athigh currents. A torroid or E-core inductor (closed magneticstructure) should be used in these situations.The inductors listed in the selection chart include ferriteE-core construction for Schott, ferrite bobbin core for Rencoand Coilcraft, and powdered iron toroid for Pulse Engineering.Exceeding an inductor’s maximum current rating may causethe inductor to overheat because of the copper wire losses,or the core may saturate.
If the inductor begins to saturate,the inductance decreases rapidly and the inductor begins tolook mainly resistive (the DC resistance of the winding). Thiscan cause the switch current to rise very rapidly and forcethe switch into a cycle-by-cycle current limit, thus reducingthe DC output load current. This can also result in overheating of the inductor and/or the LM2596. Different inductortypes have different saturation characteristics, and thisshould be kept in mind when selecting an inductor.The inductor manufacturer’s data sheets include current andenergy limits to avoid inductor saturation.01258330FIGURE 15. Capacitor ESR Change vs TemperatureINDUCTOR SELECTIONAll switching regulators have two basic modes of operation;continuous and discontinuous.
The difference between thetwo types relates to the inductor current, whether it is flowingcontinuously, or if it drops to zero for a period of time in thenormal switching cycle. Each mode has distinctively differentoperating characteristics, which can affect the regulatorsperformance and requirements. Most switcher designs willoperate in the discontinuous mode when the load current islow.The LM2596 (or any of the Simple Switcher family) can beused for both continuous or discontinuous modes of operation.In many cases the preferred mode of operation is the continuous mode. It offers greater output power, lower peakswitch, inductor and diode currents, and can have loweroutput ripple voltage.
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