LM2596 (961743), страница 6
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But it does require larger inductorvalues 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 selection guide (nomograph) was designed (see Figure 4 through8). This guide assumes that the regulator is operating in thecontinuous mode, and selects an inductor that will allow apeak-to-peak inductor ripple current to be a certain percentage of the maximum design load current.
This peak-to-peakinductor ripple current percentage is not fixed, but is allowedto change as different design load currents are selected.(See Figure 16.)21www.national.comLM2596Application Informationvoltage, the ESR of the output capacitor must be low, however, caution must be exercised when using extremely lowESR capacitors because they can affect the loop stability,resulting in oscillation problems.
If very low output ripplevoltage is needed (less than 20 mV), a post ripple filter isrecommended. (See Figure 1.) The inductance required istypically between 1 µH and 5 µH, with low DC resistance, tomaintain 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 maybe as low as desired, because it is out of the regulatorfeedback loop. The photo shown in Figure 17 shows atypical output ripple voltage, with and without a post ripplefilter.When observing output ripple with a scope, it is essentialthat a short, low inductance scope probe ground connectionbe used. Most scope probe manufacturers provide a specialprobe terminator which is soldered onto the regulator board,preferable at the output capacitor.
This provides a very shortscope ground thus eliminating the problems associated withthe 3 inch ground lead normally provided with the probe, andprovides a much cleaner and more accurate picture of theripple voltage waveform.The voltage spikes are caused by the fast switching action ofthe output switch and the diode, and the parasitic inductanceof the output filter capacitor, and its associated wiring. Tominimize these voltage spikes, the output capacitor shouldbe designed for switching regulator applications, and thelead lengths must be kept very short. Wiring inductance,stray capacitance, as well as the scope probe used to evaluate these transients, all contribute to the amplitude of thesespikes.When a switching regulator is operating in the continuousmode, the inductor current waveform ranges from a triangular to a sawtooth type of waveform (depending on the inputvoltage).
For a given input and output voltage, thepeak-to-peak amplitude of this inductor current waveformremains constant. As the load current increases or decreases, the entire sawtooth current waveform also risesand falls. The average value (or the center) of this currentwaveform is equal to the DC load current.If the load current drops to a low enough level, the bottom ofthe sawtooth current waveform will reach zero, and theswitcher will smoothly change from a continuous to a discontinuous mode of operation. Most switcher designs (irregardless how large the inductor value is) will be forced to rundiscontinuous if the output is lightly loaded. This is a perfectly acceptable mode of operation.(Continued)DISCONTINUOUS MODE OPERATIONThe selection guide chooses inductor values suitable forcontinuous mode operation, but for low current applicationsand/or high input voltages, a discontinuous mode designmay be a better choice.
It would use an inductor that wouldbe physically smaller, and would need only one half to onethird the inductance value needed for a continuous modedesign. The peak switch and inductor currents will be higherin a discontinuous design, but at these low load currents (1Aand below), the maximum switch current will still be less thanthe switch current limit.Discontinuous operation can have voltage waveforms thatare considerable different than a continuous design. Theoutput pin (switch) waveform can have some damped sinusoidal ringing present. (See Typical Performance Characteristics photo titled Discontinuous Mode Switching Waveforms) This ringing is normal for discontinuous operation,and is not caused by feedback loop instabilities.
In discontinuous operation, there is a period of time where neither theswitch or the diode are conducting, and the inductor currenthas dropped to zero. During this time, a small amount ofenergy can circulate between the inductor and the switch/diode parasitic capacitance causing this characteristic ringing. Normally this ringing is not a problem, unless the amplitude becomes great enough to exceed the input voltage, andeven then, there is very little energy present to cause damage.Different inductor types and/or core materials produce different amounts of this characteristic ringing.
Ferrite core inductors have very little core loss and therefore produce the mostringing. The higher core loss of powdered iron inductorsproduce less ringing. If desired, a series RC could be placedin parallel with the inductor to dampen the ringing. Thecomputer aided design software Switchers Made Simple(version 4.3) will provide all component values for continuous and discontinuous modes of operation.01258332FIGURE 17. Post Ripple Filter WaveformOUTPUT VOLTAGE RIPPLE AND TRANSIENTSThe output voltage of a switching power supply operating inthe continuous mode will contain a sawtooth ripple voltage atthe switcher frequency, and may also contain short voltagespikes at the peaks of the sawtooth waveform.The output ripple voltage is a function of the inductor sawtooth ripple current and the ESR of the output capacitor. Atypical output ripple voltage can range from approximately0.5% to 3% of the output voltage.
To obtain low ripplewww.national.com22(Continued)LM2596Application Information1.Peak Inductor or peak switch current2.Minimum load current before the circuit becomes discontinuous3.Output Ripple Voltage = (∆IIND)x(ESR of COUT)= 0.62Ax0.1Ω=62 mV p-p4.01258333FIGURE 18. Peak-to-Peak InductorRipple Current vs Load CurrentIn a switching regulator design, knowing the value of thepeak-to-peak inductor ripple current (∆IIND) can be useful fordetermining a number of other circuit parameters. Parameters such as, peak inductor or peak switch current, minimum load current before the circuit becomes discontinuous,output ripple voltage and output capacitor ESR can all becalculated from the peak-to-peak ∆IIND.
When the inductornomographs shown in Figure 4 through 8 are used to selectan inductor value, the peak-to-peak inductor ripple currentcan immediately be determined. The curve shown in Figure18 shows the range of (∆IIND) that can be expected fordifferent load currents. The curve also shows how thepeak-to-peak inductor ripple current (∆IIND) changes as yougo from the lower border to the upper border (for a given loadcurrent) within an inductance region.
The upper border represents a higher input voltage, while the lower border represents a lower input voltage (see Inductor Selection Guides).These curves are only correct for continuous mode operation, and only if the inductor selection guides are used toselect the inductor valueConsider the following example:VOUT = 5V, maximum load current of 2.5AOPEN CORE INDUCTORSAnother possible source of increased output ripple voltage orunstable operation is from an open core inductor. Ferritebobbin or stick inductors have magnetic lines of flux flowingthrough the air from one end of the bobbin to the other end.These magnetic lines of flux will induce a voltage into anywire or PC board copper trace that comes within the inductor’s magnetic field.
The strength of the magnetic field, theorientation and location of the PC copper trace to the magnetic field, and the distance between the copper trace andthe inductor, determine the amount of voltage generated inthe copper trace. Another way of looking at this inductivecoupling is to consider the PC board copper trace as oneturn of a transformer (secondary) with the inductor windingas the primary. Many millivolts can be generated in a coppertrace located near an open core inductor which can causestability problems or high output ripple voltage problems.If unstable operation is seen, and an open core inductor isused, it’s possible that the location of the inductor withrespect to other PC traces may be the problem.
To determine if this is the problem, temporarily raise the inductoraway from the board by several inches and then checkcircuit operation. If the circuit now operates correctly, thenthe magnetic flux from the open core inductor is causing theproblem.
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