Datasheet

LM2597, LM2597HV
www.ti.com SNVS119C –MARCH 1998–REVISED APRIL 2013
Figure 37. Capacitor ESR Change vs Temperature
INDUCTOR SELECTION
All switching regulators have two basic modes of operation; continuous and discontinuous. The difference
between the two types relates to the inductor current, whether it is flowing continuously, or if it drops to zero for a
period of time in the normal switching cycle. Each mode has distinctively different operating characteristics,
which can affect the regulators performance and requirements. Most switcher designs will operate in the
discontinuous mode when the load current is low.
The LM2597 (or any of the Simple Switcher family) can be used for both continuous or discontinuous modes of
operation.
In many cases the preferred mode of operation is the continuous mode. It offers greater output power, lower
peak switch, inductor and diode currents, and can have lower output ripple voltage. But it does require larger
inductor values to keep the inductor current flowing continuously, especially at low output load currents and/or
high input voltages.
To simplify the inductor selection process, an inductor selection guide (nomograph) was designed (see Figure 26
through Figure 29). This guide assumes that the regulator is operating in the continuous mode, and selects an
inductor that will allow a peak-to-peak inductor ripple current to be a certain percentage of the maximum design
load current. This peak-to-peak inductor ripple current percentage is not fixed, but is allowed to change as
different design load currents are selected. (See Figure 38.)
Figure 38. (ΔI
IND
) Peak-to-Peak Inductor
Ripple Current (as a Percentage
of the Load Current) vs Load Current
By allowing the percentage of inductor ripple current to increase for low load currents, the inductor value and size
can be kept relatively low.
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