Application Information (Continued)
(see Figure 14 ). Often, capacitors with much higher voltage
ratings may be needed to provide the low ESR values re-
quired for low output ripple voltage.
The output capacitor for many different switcher designs of-
ten can be satisfied with only three or four different capacitor
values and several different voltage ratings. See the quick
design component selection tables in Figure 2 and Figure 3
for typical capacitor values, voltage ratings, and manufactur-
ers capacitor types.
Electrolytic capacitors are not recommended for tempera-
tures below −25˚C. The ESR rises dramatically at cold tem-
@
peratures and typically rises 3X
−25˚C and as much as
10X at −40˚C. See curve shown in Figure 15 .
Solid tantalum capacitors have a much better ESR spec for
cold temperatures and are recommended for temperatures
below −25˚C.
DS012439-30
FIGURE 15. 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 LM2594 (or any of the Simple Switcher family) can be
used for both continuous or discontinuous modes of opera-
tion.
In many cases the preferred mode of operation is the con-
tinuous mode. It offers greater output power, lower peak
switch, inductor and diode currents, and can have lower out-
put 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.
DS012439-29
FIGURE 14. Capacitor ESR vs Capacitor Voltage Rating
(Typical Low ESR Electrolytic Capacitor)
CATCH DIODE
To simplify the inductor selection process, an inductor selec-
tion guide (nomograph) was designed (see Figure 4 through
Figure 7 ). This guide assumes that the regulator is operating
in the continuous mode, and selects an inductor that will al-
low 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 16.)
Buck regulators require a diode to provide a return path for
the inductor current when the switch turns off. This must be
a fast diode and must be located close to the LM2594 using
short leads and short printed circuit traces.
Because of their very fast switching speed and low forward
voltage drop, Schottky diodes provide the best performance,
especially in low output voltage applications (5V and lower).
Ultra-fast recovery, or High-Efficiency rectifiers are also a
good choice, but some types with an abrupt turnoff charac-
teristic may cause instability or EMI problems. Ultra-fast re-
covery diodes typically have reverse recovery times of 50 ns
or less. Rectifiers such as the 1N4001 series are much too
slow and should not be used.
19
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