LM2597 datasheet(24/34 Pages) NSC

24 / 34 page

Application Information (Continued)

quick design component selection tables in

Figure 1 and

Figure 2 for typical capacitor values, voltage ratings, and

manufacturers 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 18.

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

recovery 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 LM2597 (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

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 volt-

ages.

To simplify the inductor selection process, an inductor selec-

tion guide (nomograph) was designed (see

Figure 3 through

Figure 6). 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 19.)

By allowing the percentage of inductor ripple current to

increase for low load currents, the inductor value and size

can be kept relatively low.

When operating in the continuous mode, the inductor current

waveform ranges from a triangular to a sawtooth type of

waveform (depending on the input voltage), with the average

value of this current waveform equal to the DC output load

current.

Inductors are available in different styles such as pot core,

toroid, E-core, bobbin core, etc., as well as different core

materials, such as ferrites and powdered iron. The least

expensive, the bobbin, rod or stick core, consists of wire

wrapped on a ferrite bobbin. This type of construction makes

for a inexpensive inductor, but since the magnetic flux is not

completely contained within the core, it generates more

Electro-Magnetic Interference (EMl). This magnetic flux can

induce voltages into nearby printed circuit traces, thus caus-

ing problems with both the switching regulator operation and

nearby sensitive circuitry, and can give incorrect scope read-

ings because of induced voltages in the scope probe. Also

see section on Open Core Inductors.

The inductors listed in the selection chart include ferrite

E-core construction for Schott, ferrite bobbin core for Renco

and Coilcraft, and powdered iron toroid for Pulse Engineer-

ing.

Exceeding an inductor’s maximum current rating may cause

the inductor to overheat because of the copper wire losses,

or the core may saturate. If the inductor begins to saturate,

the inductance decreases rapidly and the inductor begins to

look mainly resistive (the DC resistance of the winding). This

can cause the switch current to rise very rapidly and force

the switch into a cycle-by-cycle current limit, thus reducing

the DC output load current. This can also result in overheat-

ing of the inductor and/or the LM2597. Different inductor

types have different saturation characteristics, and this

should be kept in mind when selecting an inductor.

DS012440-37

FIGURE 18. Capacitor ESR Change vs Temperature

DS012440-31

FIGURE 19. (

∆I

IND) Peak-to-Peak Inductor

Ripple Current (as a Percentage

of the Load Current) vs Load Current

LM2597 Datasheet(PDF) 24 Page - National Semiconductor (TI)

LM2597 Datasheet(HTML) 24 Page - National Semiconductor (TI)

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34

Datasheet Download

Link URL