LM2598S-ADJ National Semiconductor, LM2598S-ADJ Datasheet - Page 24

• Manufacturer Part Number: LM2598S-ADJ
• Description: SIMPLE SWITCHER Power Converter 150 kHz 1A Step-Down Voltage Regulator/ with Features
• Manufacturer: National Semiconductor

www.national.com
Application Information
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 19 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, 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-
FIGURE 20. Peak-to-Peak Inductor
Ripple Current vs Load Current
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(Continued)
) can be useful for
DS012593-37
24
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 4 through Figure 7 are used to
select an inductor value, the peak-to-peak inductor ripple
current can immediately be determined. The curve shown in
Figure 20 shows the range of ( I
for different load currents. The curve also shows how the
peak-to-peak inductor ripple current ( I
go from the lower border to the upper border (for a given load
current) within an inductance region. The upper border rep-
resents a higher input voltage, while the lower border repre-
sents a lower input voltage (see Inductor Selection Guides).
These curves are only correct for continuous mode opera-
tion, and only if the inductor selection guides are used to se-
lect the inductor value
Consider the following example:
The selection guide in Figure 5 shows that the vertical line
for a 0.8A load current, and the horizontal line for the 12V in-
put voltage intersect approximately midway between the up-
per and lower borders of the 68 µH inductance region. A 68
µH inductor will allow a peak-to-peak inductor current ( I
to flow that will be a percentage of the maximum load cur-
rent. Referring to Figure 20 , follow the 0.8A line approxi-
mately midway into the inductance region, and read the
peak-to-peak inductor ripple current ( I
axis (approximately 300 mA p-p).
As the input voltage increases to 14V, it approaches the up-
per border of the inductance region, and the inductor ripple
current increases. Referring to the curve in Figure 20 , it can
be seen that for a load current of 0.8A, the peak-to-peak in-
ductor ripple current ( I
range from 340 mA at the upper border (14V in) to 225 mA at
the lower border (10V in).
Once the I
used to calculate additional information about the switching
regulator circuit.
1. Peak Inductor or peak switch current
2. Minimum load current before the circuit becomes dis-
3. Output Ripple Voltage = ( I
4. ESR of C
OPEN CORE INDUCTORS
Another possible source of increased output ripple voltage or
unstable operation is from an open core inductor. Ferrite
bobbin or stick inductors have magnetic lines of flux flowing
through the air from one end of the bobbin to the other end.
V
V
OUT
IN
continuous
= 0.3A x 0.16 = 48 mV p-p
= 12V, nominal, varying between 10V and 14V.
= 5V, maximum load current of 800 mA
IND
OUT
value is known, the following formulas can be
IND
) is 300 mA with 12V in, and can
IND
IND
) x (ESR of C
IND
) that can be expected
IND
IND
. When the inductor
) changes as you
) on the left hand
OUT
)
IND
)