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LM2599 Datasheet(PDF) 22 Page - National Semiconductor (TI) |
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LM2599 Datasheet(HTML) 22 Page - National Semiconductor (TI) |
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22 / 31 page  Application Information (Continued) 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 torroid 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 LM2599. 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. DISCONTINUOUS MODE OPERATION The selection guide chooses inductor values suitable for continuous mode operation, but for low current applications and/or high input voltages, a discontinuous mode design may be a better choice. It would use an inductor that would be physically smaller, and would need only one half to one third the inductance value needed for a continuous mode de- sign. The peak switch and inductor currents will be higher in a discontinuous design, but at these low load currents (1A and below), the maximum switch current will still be less than the switch current limit. Discontinuous operation can have voltage waveforms that are considerable different than a continuous design. The out- put pin (switch) waveform can have some damped sinusoi- dal ringing present. (See Typical Performance Characteris- tics photo titled Discontinuous Mode Switching Waveforms) This ringing is normal for discontinuous operation, and is not caused by feedback loop instabilities. In discontinuous op- eration, there is a period of time where neither the switch or the diode are conducting, and the inductor current has dropped to zero. During this time, a small amount of energy can circulate between the inductor and the switch/diode parasitic capacitance causing this characteristic ringing. Nor- mally this ringing is not a problem, unless the amplitude be- comes great enough to exceed the input voltage, and even then, there is very little energy present to cause damage. Different inductor types and/or core materials produce differ- ent amounts of this characteristic ringing. Ferrite core induc- tors have very little core loss and therefore produce the most ringing. The higher core loss of powdered iron inductors pro- duce less ringing. If desired, a series RC could be placed in parallel with the inductor to dampen the ringing. The com- puter aided design software Switchers Made Simple (ver- sion 4.3) will provide all component values for continuous and discontinuous modes of operation. OUTPUT VOLTAGE RIPPLE AND TRANSIENTS The output voltage of a switching power supply operating in the continuous mode will contain a sawtooth ripple voltage at the switcher frequency, and may also contain short voltage spikes at the peaks of the sawtooth waveform. The output ripple voltage is a function of the inductor saw- tooth ripple current and the ESR of the output capacitor. A typical output ripple voltage can range from approximately 0.5% to 3% of the output voltage. To obtain low ripple volt- age, the ESR of the output capacitor must be low, however, caution must be exercised when using extremely low ESR capacitors because they can affect the loop stability, result- ing in oscillation problems. If very low output ripple voltage is needed (less than 20 mV), a post ripple filter is recom- DS012582-36 FIGURE 18. ( ∆I IND) Peak-to-Peak Inductor Ripple Current (as a Percentage of the Load Current) vs Load Current DS012582-37 FIGURE 19. Post Ripple Filter Waveform www.national.com 22 |
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