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LM2595 Datasheet(PDF) 21 Page - National Semiconductor (TI) |
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LM2595 Datasheet(HTML) 21 Page - National Semiconductor (TI) |
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21 / 29 page  Application Information (Continued) eters such as, peak inductor or peak switch current, mini- mum load current before the circuit becomes discontinuous, output ripple voltage and output capacitor ESR can all be calculated from the peak-to-peak ∆I IND. When the inductor nomographs shown in Figure 4 through Figure 7 are used to select an inductor value, the peak-to-peak inductor ripple current can immediately be determined. The curve shown in Figure 18 shows the range of ( ∆I IND) that can be expected for different load currents. The curve also shows how the peak-to-peak inductor ripple current ( ∆I IND) changes as you go from the lower border to the upper border (for a given load current) within an inductance region. The upper border rep- resents a higher input voltage, while the lower border repre- sents a lower input voltage (see Inductor Selection Guides). These curves are only correct for continuous mode opera- tion, and only if the inductor selection guides are used to se- lect the inductor value Consider the following example: V OUT = 5V, maximum load current of 800 mA V IN = 12V, nominal, varying between 10V and 14V. The selection guide in Figure 5 shows that the vertical line for a 0.8A load current, and the horizontal line for the 12V in- put voltage intersect approximately midway between the up- per and lower borders of the 68 µH inductance region. A 68 µH inductor will allow a peak-to-peak inductor current ( ∆I IND) to flow that will be a percentage of the maximum load cur- rent. Referring to Figure 18, follow the 0.8A line approxi- mately midway into the inductance region, and read the peak-to-peak inductor ripple current ( ∆I IND) on the left hand axis (approximately 300 mA p-p). As the input voltage increases to 14V, it approaches the up- per border of the inductance region, and the inductor ripple current increases. Referring to the curve in Figure 18,itcan be seen that for a load current of 0.8A, the peak-to-peak in- ductor ripple current ( ∆I IND) is 300 mA with 12V in, and can range from 340 mA at the upper border (14V in) to 225 mA at the lower border (10V in). Once the ∆I IND value is known, the following formulas can be used to calculate additional information about the switching regulator circuit. 1. Peak Inductor or peak switch current 2. Minimum load current before the circuit becomes dis- continuous 3. Output Ripple Voltage = ( ∆I IND)x(ESR of COUT) = 0.30Ax0.16 Ω=48 mV p-p 4. ESR of C OUT OPEN CORE INDUCTORS Another possible source of increased output ripple voltage or unstable operation is from an open core inductor. Ferrite bobbin or stick inductors have magnetic lines of flux flowing through the air from one end of the bobbin to the other end. These magnetic lines of flux will induce a voltage into any wire or PC board copper trace that comes within the induc- tor’s magnetic field. The strength of the magnetic field, the orientation and location of the PC copper trace to the mag- netic field, and the distance between the copper trace and the inductor, determine the amount of voltage generated in the copper trace. Another way of looking at this inductive coupling is to consider the PC board copper trace as one turn of a transformer (secondary) with the inductor winding as the primary. Many millivolts can be generated in a copper trace located near an open core inductor which can cause stability problems or high output ripple voltage problems. If unstable operation is seen, and an open core inductor is used, it’s possible that the location of the inductor with re- spect to other PC traces may be the problem. To determine if this is the problem, temporarily raise the inductor away from the board by several inches and then check circuit op- eration. If the circuit now operates correctly, then the mag- netic flux from the open core inductor is causing the problem. Substituting a closed core inductor such as a torroid or E-core will correct the problem, or re-arranging the PC layout may be necessary. Magnetic flux cutting the IC device 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 induc- tor will provide good results, provided it is exactly in the cen- ter of the inductor (because the induced voltages cancel themselves out), but if it is off center one direction or the other, then problems could arise. If flux problems are present, even the direction of the inductor winding can make a difference in some circuits. This discussion on open core inductors is not to frighten the user, but to alert the user on what kind of problems to watch out for when using them. Open core bobbin or “stick” induc- tors are an inexpensive, simple way of making a compact ef- ficient inductor, and they are used by the millions in many dif- ferent applications. THERMAL CONSIDERATIONS The LM2595 is available in two packages, a 5-pin TO-220 (T) and a 5-pin surface mount TO-263 (S). The TO-220 package can be used without a heat sink for ambient temperatures up to approximately 50˚C (depending on the output voltage and load current). The curves in Figure 19 show the LM2595T junction temperature rises above am- bient temperature for different input and output voltages. The data tor these curves was taken with the LM2595T (TO-220 package) operating as a switching regutator in an ambient temperature of 25˚C (still air). These temperature rise num- bers are all approximate and there are many factors that can affect these temperatures. Higher ambient temperatures re- quire some heat sinking, either to the PC board or a small external heat sink. The TO-263 surface mount package tab is designed to be soldered to the copper on a printed circuit board. The copper and the board are the heat sink for this package and the other heat producing components, such as the catch diode and inductor. The PC board copper area that the package is soldered to should be at least 0.4 in 2, and ideally should have 2 or more square inches of 2 oz. (0.0028 in) copper. Additional copper area improves the thermal characteristics, www.national.com 21 |
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