Datasheet

LM2597, LM2597HV
SNVS119C –MARCH 1998–REVISED APRIL 2013 www.ti.com
Figure 40. Peak-to-Peak Inductor
Ripple Current vs Load Current
When a switching regulator 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 and output voltage,
the peak-to-peak amplitude of this inductor current waveform remains 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 discontinuous mode of operation. Most switcher
designs (irregardless how large the inductor value is) will be forced to run discontinuous 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 (ΔI
IND
) can be
useful for determining 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 be calculated from the peak-to-peak ΔI
IND
. When the inductor nomographs shown in
Figure 26 through Figure 29 are used to select an inductor value, the peak-to-peak inductor ripple current can
immediately be determined. The curve shown in Figure 40 shows the range of (ΔI
IND
) that can be expected for
different load currents. The curve also shows how the peak-to-peak inductor ripple current (ΔI
IND
) changes as
you go from the lower border to the upper border (for a given load current) within an inductance region. The
upper border represents a higher input voltage, while the lower border represents a lower input voltage (see
INDUCTOR SELECTION).
These curves are only correct for continuous mode operation, and only if the inductor selection guides are used
to select the inductor value
Consider the following example:
V
OUT
= 5V, maximum load current of 300 mA
V
IN
= 15V, nominal, varying between 11V and 20V.
The selection guide in Figure 27 shows that the vertical line for a 0.3A load current, and the horizontal line for the
15V input voltage intersect approximately midway between the upper and lower borders of the 150 μH
inductance region. A 150 μH inductor will allow a peak-to-peak inductor current (ΔI
IND
) to flow that will be a
percentage of the maximum load current. Referring to Figure 40, follow the 0.3A line approximately midway into
the inductance region, and read the peak-to-peak inductor ripple current (ΔI
IND
) on the left hand axis
(approximately 150 mA p-p).
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 40, it can be seen that for a load current of 0.3A, the
peak-to-peak inductor ripple current (ΔI
IND
) is 150 mA with 15V in, and can range from 175 mA at the upper
border (20V in) to 120 mA at the lower border (11V in).
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