LM2574, LM2574HV
SNVS104C –JUNE 1999–REVISED APRIL 2013
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To simplify the inductor selection process, an inductor selection guide (nomograph) was designed (see Figure 25
through Figure 29). This guide assumes continuous mode operation, and selects an inductor that will allow a
peak-to-peak inductor ripple current (ΔI
IND
) to be a certain percentage of the maximum design load current. In the
LM2574 SIMPLE SWITCHER, the peak-to-peak inductor ripple current percentage (of load current) is allowed to
change as different design load currents are selected. By allowing the percentage of inductor ripple current to
increase for lower current applications, the inductor size and value can be kept relatively low.
INDUCTOR RIPPLE CURRENT
When the switcher is operating in the continuous mode, the inductor current waveform ranges from a triangular
to a sawtooth type of waveform (depending on the input voltage). For a given input voltage and output voltage,
the peak-to-peak amplitude of this inductor current waveform remains constant. As the load current rises or falls,
the entire sawtooth current waveform also rises or falls. The average DC value of this waveform is equal to the
DC load current (in the buck regulator configuration).
If the load current drops to a low enough level, the bottom of the sawtooth current waveform will reach zero, and
the switcher will change to a discontinuous mode of operation. This is a perfectly acceptable mode of operation.
Any buck switching regulator (no matter how large the inductor value is) will be forced to run discontinuous if the
load current is light enough.
The curve shown in Figure 30 illustrates how the peak-to-peak inductor ripple current (ΔI
IND
) is allowed to change
as different maximum load currents are selected, and also how it changes as the operating point varies from the
upper border to the lower border within an inductance region (see INDUCTOR SELECTION).
Figure 30. Inductor Ripple Current (ΔI
IND
) Range
Based on Selection Guides from Figure 25 through Figure 29.
Consider the following example:
V
OUT
= 5V @ 0.4A
V
IN
= 10V minimum up to 20V maximum
The selection guide in Figure 26 shows that for a 0.4A load current, and an input voltage range between 10V and
20V, the inductance region selected by the guide is 330 μH. This value of inductance will allow a peak-to-peak
inductor ripple current (ΔI
IND
) to flow that will be a percentage of the maximum load current. For this inductor
value, the ΔI
IND
will also vary depending on the input voltage. As the input voltage increases to 20V, it
approaches the upper border of the inductance region, and the inductor ripple current increases. Referring to the
curve in Figure 30, it can be seen that at the 0.4A load current level, and operating near the upper border of the
330 μH inductance region, the ΔI
IND
will be 53% of 0.4A, or 212 mA p-p.
This ΔI
IND
is important because from this number the peak inductor current rating can be determined, the
minimum load current required before the circuit goes to discontinuous operation, and also, knowing the ESR of
the output capacitor, the output ripple voltage can be calculated, or conversely, measuring the output ripple
voltage and knowing the ΔI
IND
, the ESR can be calculated.
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