LM2594 datasheet(19/28 Pages) NSC

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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.

CATCH DIODE

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.

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.

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.)

DS012439-29

FIGURE 14. Capacitor ESR vs Capacitor Voltage Rating

(Typical Low ESR Electrolytic Capacitor)

DS012439-30

FIGURE 15. Capacitor ESR Change vs Temperature

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