LM2596 (961743), страница 7
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Substituting a closed core inductor such as a torroid or E-core will correct the problem, or re-arranging thePC layout may be necessary. Magnetic flux cutting the ICdevice ground trace, feedback trace, or the positive or negative traces of the output capacitor should be minimized.Sometimes, locating a trace directly beneath a bobbin inductor will provide good results, provided it is exactly in thecenter of the inductor (because the induced voltages cancelthemselves out), but if it is off center one direction or theother, then problems could arise. If flux problems arepresent, even the direction of the inductor winding can makea difference in some circuits.This discussion on open core inductors is not to frighten theuser, but to alert the user on what kind of problems to watchout for when using them.
Open core bobbin or “stick” inductors are an inexpensive, simple way of making a compactefficient inductor, and they are used by the millions in manydifferent applications.VIN = 12V, nominal, varying between 10V and 16V.The selection guide in Figure 5 shows that the vertical linefor a 2.5A load current, and the horizontal line for the 12Vinput voltage intersect approximately midway between theupper and lower borders of the 33 µH inductance region.A 33 µH inductor will allow a peak-to-peak inductor current(∆IIND) to flow that will be a percentage of the maximum loadcurrent. Referring to Figure 18, follow the 2.5A line approximately midway into the inductance region, and read thepeak-to-peak inductor ripple current (∆IIND) on the left handaxis (approximately 620 mA p-p).As the input voltage increases to 16V, it approaches theupper border of the inductance region, and the inductorripple current increases.
Referring to the curve in Figure 18,it can be seen that for a load current of 2.5A, thepeak-to-peak inductor ripple current (∆IIND) is 620 mA with12V in, and can range from 740 mA at the upper border (16Vin) to 500 mA at the lower border (10V in).Once the ∆IIND value is known, the following formulas can beused to calculate additional information about the switchingregulator circuit.23www.national.comLM2596Application Informationmaterial and the DC resistance, it could either act as a heatsink taking heat away from the board, or it could add heat tothe board.(Continued)THERMAL CONSIDERATIONSThe LM2596 is available in two packages, a 5-pin TO-220(T) and a 5-pin surface mount TO-263 (S).The TO-220 package needs a heat sink under most conditions.
The size of the heatsink depends on the input voltage,the output voltage, the load current and the ambient temperature. The curves in Figure 19 show the LM2596T junction temperature rises above ambient temperature for a 3Aload and different input and output voltages. The data forthese curves was taken with the LM2596T (TO-220 package) operating as a buck switching regulator in an ambienttemperature of 25˚C (still air).
These temperature rise numbers are all approximate and there are many factors that canaffect these temperatures. Higher ambient temperatures require more heat sinking.The TO-263 surface mount package tab is designed to besoldered to the copper on a printed circuit board. The copperand the board are the heat sink for this package and theother heat producing components, such as the catch diodeand inductor.
The PC board copper area that the package issoldered to should be at least 0.4 in2, and ideally shouldhave 2 or more square inches of 2 oz. (0.0028) in) copper.Additional copper area improves the thermal characteristics,but with copper areas greater than approximately 6 in2, onlysmall improvements in heat dissipation are realized. If further thermal improvements are needed, double sided, multilayer PC board with large copper areas and/or airflow arerecommended.The curves shown in Figure 20 show the LM2596S (TO-263package) junction temperature rise above ambient temperature with a 2A load for various input and output voltages.
Thisdata was taken with the circuit operating as a buck switchingregulator with all components mounted on a PC board tosimulate the junction temperature under actual operatingconditions. This curve can be used for a quick check for theapproximate junction temperature for various conditions, butbe aware that there are many factors that can affect thejunction temperature. When load currents higher than 2A areused, double sided or multilayer PC boards with large copper areas and/or airflow might be needed, especially for highambient temperatures and high output voltages.For the best thermal performance, wide copper traces andgenerous amounts of printed circuit board copper should beused in the board layout. (One exception to this is the output(switch) pin, which should not have large areas of copper.)Large areas of copper provide the best transfer of heat(lower thermal resistance) to the surrounding air, and movingair lowers the thermal resistance even further.Package thermal resistance and junction temperature risenumbers are all approximate, and there are many factorsthat will affect these numbers.
Some of these factors includeboard size, shape, thickness, position, location, and evenboard temperature. Other factors are, trace width, totalprinted circuit copper area, copper thickness, single- ordouble-sided, multilayer board and the amount of solder onthe board. The effectiveness of the PC board to dissipateheat also depends on the size, quantity and spacing of othercomponents on the board, as well as whether the surrounding air is still or moving.
Furthermore, some of these components such as the catch diode will add heat to the PCboard and the heat can vary as the input voltage changes.For the inductor, depending on the physical size, type of corewww.national.com01258334Circuit Data for Temperature Rise CurveTO-220 Package (T)CapacitorsThrough hole electrolyticInductorThrough hole, RencoDiodeThrough hole, 5A 40V, SchottkyPC board3 square inches single sided 2 oz.
copper(0.0028")FIGURE 19. Junction Temperature Rise, TO-22001258335Circuit Data for Temperature Rise CurveTO-263 Package (S)CapacitorsSurface mount tantalum, molded “D” sizeInductorSurface mount, Pulse Engineering, 68 µHDiodeSurface mount, 5A 40V, SchottkyPC board9 square inches single sided 2 oz. copper(0.0028")FIGURE 20. Junction Temperature Rise, TO-26324tures a constant threshold voltage for turn on and turn off(zener voltage plus approximately one volt). If hysteresis isneeded, the circuit in Figure 24 has a turn ON voltage whichis different than the turn OFF voltage. The amount of hysteresis is approximately equal to the value of the output voltage. If zener voltages greater than 25V are used, an additional 47 kΩ resistor is needed from the ON /OFF pin to theground pin to stay within the 25V maximum limit of the ON/OFF pin.(Continued)INVERTING REGULATORThe circuit in Figure 25 converts a positive input voltage to anegative output voltage with a common ground.
The circuitoperates by bootstrapping the regulator’s ground pin to thenegative output voltage, then grounding the feedback pin,the regulator senses the inverted output voltage and regulates it.01258336FIGURE 21. Delayed Startup0125833701258338This circuit has an ON/OFF threshold of approximately 13V.FIGURE 22.
Undervoltage Lockoutfor Buck RegulatorFIGURE 23. Undervoltage Lockoutfor Inverting RegulatorDELAYED STARTUPThe circuit in Figure 21 uses the the ON /OFF pin to providea time delay between the time the input voltage is appliedand the time the output voltage comes up (only the circuitrypertaining to the delayed start up is shown). As the inputvoltage rises, the charging of capacitor C1 pulls the ON /OFFpin high, keeping the regulator off.
Once the input voltagereaches its final value and the capacitor stops charging, andresistor R2 pulls the ON /OFF pin low, thus allowing thecircuit to start switching. Resistor R1 is included to limit themaximum voltage applied to the ON /OFF pin (maximum of25V), reduces power supply noise sensitivity, and also limitsthe capacitor, C1, discharge current.
When high input ripplevoltage exists, avoid long delay time, because this ripple canbe coupled into the ON /OFF pin and cause problems.This example uses the LM2596-5.0 to generate a −5V output, but other output voltages are possible by selecting otheroutput voltage versions, including the adjustable version.Since this regulator topology can produce an output voltagethat is either greater than or less than the input voltage, themaximum output current greatly depends on both the inputand output voltage. The curve shown in Figure 26 provides aguide as to the amount of output load current possible for thedifferent input and output voltage conditions.The maximum voltage appearing across the regulator is theabsolute sum of the input and output voltage, and this mustbe limited to a maximum of 40V.
For example, when converting +20V to −12V, the regulator would see 32V between theinput pin and ground pin. The LM2596 has a maximum inputvoltage spec of 40V.Additional diodes are required in this regulator configuration.Diode D1 is used to isolate input voltage ripple or noise fromcoupling through the CIN capacitor to the output, under lightor no load conditions. Also, this diode isolation changes thetopology to closley resemble a buck configuration thus providing good closed loop stability.
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