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LM2595 Datasheet(PDF) 19 Page - National Semiconductor (TI) |
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LM2595 Datasheet(HTML) 19 Page - National Semiconductor (TI) |
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19 / 29 page  Application Information (Continued) INDUCTOR SELECTION All switching regulators have two basic modes of operation; continuous and discontinuous. The difference between the two types relates to the inductor current, whether it is flowing continuously, or if it drops to zero for a period of time in the normal switching cycle. Each mode has distinctively different operating characteristics, which can affect the regulators performance and requirements. Most switcher designs will operate in the discontinuous mode when the load current is low. The LM2595 (or any of the Simple Switcher family) can be used for both continuous or discontinuous modes of opera- tion. In many cases the preferred mode of operation is the con- tinuous mode. It offers greater output power, lower peak switch, inductor and diode currents, and can have lower out- put ripple voltage. But it does require larger inductor values to keep the inductor current flowing continuously, especially at low output load currents and/or high input voltages. To simplify the inductor selection process, an inductor selec- tion guide (nomograph) was designed (see Figure 4 through Figure 7). This guide assumes that the regulator is operating in the continuous mode, and selects an inductor that will al- low a peak-to-peak inductor ripple current to be a certain percentage of the maximum design load current. This peak-to-peak inductor ripple current percentage is not fixed, but is allowed to change as different design load currents are selected. (See Figure 16.) By allowing the percentage of inductor ripple current to in- crease for low load currents, the inductor value and size can be kept relatively low. When operating in the continuous mode, the inductor current waveform ranges from a triangular to a sawtooth type of waveform (depending on the input voltage), with the average value of this current waveform equal to the DC output load current. Inductors are available in different styles such as pot core, toroid, E-core, bobbin core, etc., as well as different core ma- terials, such as ferrites and powdered iron. The least expen- sive, the bobbin, rod or stick core, consists of wire wound on a ferrite bobbin. This type of construction makes for an inex- pensive inductor, but since the magnetic flux is not com- pletely contained within the core, it generates more Electro-Magnetic Interference (EMl). This magnetic flux can induce voltages into nearby printed circuit traces, thus caus- ing problems with both the switching regulator operation and nearby sensitive circuitry, and can give incorrect scope read- ings because of induced voltages in the scope probe. Also see section on Open Core Inductors. When multiple switching regulators are located on the same PC board, open core magnetics can cause interference be- tween two or more of the regulator circuits, especially at high currents. A toroid or E-core inductor (closed magnetic struc- ture) should be used in these situations. The inductors listed in the selection chart include ferrite E-core construction for Schott, ferrite bobbin core for Renco and Coilcraft, and powdered iron toroid for Pulse Engineer- ing. Exceeding an inductor’s maximum current rating may cause the 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 to look mainly resistive (the DC resistance of the winding). This can cause the switch current to rise very rapidly and force the switch into a cycle-by-cycle current limit, thus reducing the DC output load current. This can also result in overheat- ing of the inductor and/or the LM2595. Different inductor types have different saturation characteristics, and this should be kept in mind when selecting an inductor. The inductor manufacturer’s data sheets include current and energy limits to avoid inductor saturation. DS012565-30 FIGURE 15. Capacitor ESR Change vs Temperature DS012565-31 FIGURE 16. ( ∆I IND) Peak-to-Peak Inductor Ripple Current (as a Percentage of the Load Current) vs Load Current www.national.com 19 |
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