LM2597HVM-12 National Semiconductor, LM2597HVM-12 Datasheet - Page 24

• Manufacturer Part Number: LM2597HVM-12
• Description: SIMPLE SWITCHER Power Converter 150 kHz 0.5A Step-Down Voltage Regulator/ with Features
• Manufacturer: National Semiconductor

www.national.com
Application Information
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 (200
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 photo titled; Discontinuous Mode
Switching Waveforms) This ringing is normal for discontinu-
ous operation, and is not caused by feedback loop instabili-
ties. In discontinuous operation, 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 char-
acteristic ringing. Normally this ringing is not a problem, un-
less the amplitude becomes great enough to exceed the in-
put 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 4.1) 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 15 mV), a post ripple filter is recom-
mended. (See Figure 12 .) The inductance required is typi-
cally between 1 µH and 5 µH, with low DC resistance, to
maintain good load regulation. A low ESR output filter ca-
pacitor is also required to assure good dynamic load re-
FIGURE 19. Post Ripple Filter Waveform
(Continued)
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24
sponse and ripple reduction. The ESR of this capacitor may
be as low as desired, because it is out of the regulator feed-
back loop. The photo shown in Figure 19 shows a typical
output ripple voltage, 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, the diode, and the parasitic inductance of
the output filter capacitor, and its associated wiring. To mini-
mize 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 ca-
pacitance, as well as the scope probe used to evaluate 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
determining a number of other circuit parameters. Param-
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
nomographs shown in Figure 3 through Figure 6 are used to
select an inductor value, the peak-to-peak inductor ripple
current can immediately be determined. The curve shown in
FIGURE 20. Peak-to-Peak Inductor
Ripple Current vs Load Current
IND
IND
. When the inductor
) can be useful for
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