Datasheet

LM2599
SNVS123C –APRIL 1998–REVISED APRIL 2013 www.ti.com
Figure 37. 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 25 through 7 are used to select an inductor value, the peak-to-peak inductor ripple current can
immediately be determined. The curve shown in Figure 37 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 Guides).
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 2.5A
V
IN
= 12V, nominal, varying between 10V and 16V.
The selection guide in Figure 26 shows that the vertical line for a 2.5A load current, and the horizontal line for the
12V input voltage intersect approximately midway between the upper and lower borders of the 33 μH inductance
region. A 33 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 37, follow the 2.5A line approximately midway into the inductance
region, and read the peak-to-peak inductor ripple current (ΔI
IND
) on the left hand axis (approximately 620 mA p-
p).
As the input voltage increases to 16V, it approaches the upper border of the inductance region, and the inductor
ripple current increases. Referring to the curve in Figure 37, it can be seen that for a load current of 2.5A, the
peak-to-peak inductor ripple current (ΔI
IND
) is 620 mA with 12V in, and can range from 740 mA at the upper
border (16V in) to 500 mA at the lower border (10V in).
Once the ΔI
IND
value is known, the following formulas can be used to calculate additional information about the
switching regulator circuit.
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