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LM2595 Datasheet(PDF) 20 Page - National Semiconductor (TI) |
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LM2595 Datasheet(HTML) 20 Page - National Semiconductor (TI) |
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20 / 29 page  Application Information (Continued) 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 (400 mA 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- mended. (See Figure 1.) The inductance required is typically between 1 µH and 5 µH, with low DC resistance, to maintain good load regulation. A low ESR output filter capacitor is also required to assure good dynamic load response and ripple reduction. The ESR of this capacitor may be as low as de- sired, because it is out of the regulator feedback loop. The photo shown in Figure 17 shows a typical output ripple volt- age, with and without a post ripple filter. When observing output ripple with a scope, it is essential that a short, low inductance scope probe ground connection be used. Most scope probe manufacturers provide a special probe terminator which is soldered onto the regulator board, preferable at the output capacitor. This provides a very short scope ground thus eliminating the problems associated with the 3 inch ground lead normally provided with the probe, and provides a much cleaner and more accurate picture of the ripple voltage waveform. The voltage spikes are caused by the fast switching action of the output switch and the diode, and the parasitic inductance of the output filter capacitor, and its associated wiring. To minimize these voltage spikes, the output capacitor should be designed for switching regulator applications, and the lead lengths must be kept very short. Wiring inductance, stray capacitance, as well as the scope probe used to evalu- ate these transients, all contribute to the amplitude of these spikes. When a switching regulator is operating in the continuous mode, the inductor current waveform ranges from a triangu- lar to a sawtooth type of waveform (depending on the input voltage). For a given input and output voltage, the peak-to-peak amplitude of this inductor current waveform re- mains constant. As the load current increases or decreases, the entire sawtooth current waveform also rises and falls. The average value (or the center) of this current waveform is equal to the DC load current. If the load current drops to a low enough level, the bottom of the sawtooth current waveform will reach zero, and the switcher will smoothly change from a continuous to a discon- tinuous mode of operation. Most switcher designs (irregard- less how large the inductor value is) will be forced to run dis- continuous if the output is lightly loaded. This is a perfectly acceptable mode of operation. In a switching regulator design, knowing the value of the peak-to-peak inductor ripple current ( ∆I IND) can be useful for determining a number of other circuit parameters. Param- DS012565-32 FIGURE 17. Post Ripple Filter Waveform DS012565-33 FIGURE 18. Peak-to-Peak Inductor Ripple Current vs Load Current www.national.com 20 |
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