LM2574 datasheet(17/24 Pages) NSC

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Application Hints (Continued)

INDUCTOR RIPPLE CURRENT

When the switcher is operating in the continuous mode, the

inductor current waveform ranges from a triangular to a saw-

tooth type of waveform (depending on the input voltage). For

a given input voltage and output voltage, the peak-to-peak

amplitude of this inductor current waveform remains con-

stant. As the load current rises or falls, the entire sawtooth

current waveform also rises or falls. The average DC value

of this waveform is equal to the DC load current (in the buck

regulator configuration).

If the load current drops to a low enough level, the bottom of

the sawtooth current waveform will reach zero, and the

switcher will change to a discontinuous mode of operation.

This is a perfectly acceptable mode of operation. Any buck

switching regulator (no matter how large the inductor value

is) will be forced to run discontinuous if the load current is

light enough.

The curve shown in

Figure 10 illustrates how the peak-to-

peak inductor ripple current (

∆I

IND) is allowed to change as

different maximum load currents are selected, and also how

it changes as the operating point varies from the upper bor-

der to the lower border within an inductance region (see In-

ductor Selection guides).

Consider the following example:

OUT = 5V @ 0.4A

IN = 10V minimum up to 20V maximum

The selection guide in

Figure 5 shows that for a 0.4A load

current, and an input voltage range between 10V and 20V,

the inductance region selected by the guide is 330 µH. This

value of inductance will allow a peak-to-peak inductor ripple

current (

∆I

IND) to flow that will be a percentage of the maxi-

mum load current. For this inductor value, the

∆I

IND will also

vary depending on the input voltage. As the input voltage in-

creases to 20V, it approaches the upper border of the induc-

tance region, and the inductor ripple current increases. Re-

ferring to the curve in

Figure 10, it can be seen that at the

0.4A load current level, and operating near the upper border

of the 330 µH inductance region, the

∆I

IND will be 53% of

0.4A, or 212 mA p-p.

This

∆I

IND is important because from this number the peak

inductor current rating can be determined, the minimum load

current required before the circuit goes to discontinuous op-

eration, and also, knowing the ESR of the output capacitor,

the output ripple voltage can be calculated, or conversely,

measuring the output ripple voltage and knowing the

∆I

IND,

the ESR can be calculated.

From the previous example, the Peak-to-peak Inductor

Ripple Current (

∆I

IND) = 212 mA p-p. Once the ∆IND value is

known, the following three formulas can be used to calculate

additional information about the switching regulator circuit:

Peak Inductor or peak switch current

Minimum load current before the circuit becomes dis-

continuous

Output Ripple Voltage = (

∆I

IND) x (ESR of COUT)

The selection guide chooses inductor values suitable for

continuous mode operation, but if the inductor value chosen

is prohibitively high, the designer should investigate the pos-

sibility of discontinuous operation. The computer design soft-

ware

Switchers Made Simple will provide all component

values for discontinuous (as well as continuous) mode of op-

eration.

Inductors are available in different styles such as pot core,

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

materials, such as ferrites and powdered iron. The least ex-

pensive, the bobbin core type, consists of wire wrapped on a

ferrite rod core. This type of construction makes for an inex-

pensive inductor, but since the magnetic flux is not com-

pletely contained within the core, it generates more electro-

magnetic interference (EMI). This EMl can cause problems

in sensitive circuits, or can give incorrect scope readings be-

cause of induced voltages in the scope probe.

The inductors listed in the selection chart include powdered

iron toroid for Pulse Engineering, and ferrite bobbin core for

Renco.

An inductor should not be operated beyond its maximum

rated current because it may saturate. When an inductor be-

gins to saturate, the inductance decreases rapidly and the

inductor begins to look mainly resistive (the DC resistance of

the winding). This can cause the inductor current to rise very

rapidly and will affect the energy storage capabilities of the

inductor and could cause inductor overheating. Different in-

ductor types have different saturation characteristics, and

this should be kept in mind when selecting an inductor. The

inductor manufacturers’ data sheets include current and en-

ergy limits to avoid inductor saturation.

OUTPUT CAPACITOR

An output capacitor is required to filter the output voltage and

is needed for loop stability. The capacitor should be located

near the LM2574 using short pc board traces. Standard alu-

minum electrolytics are usually adequate, but low ESR types

are recommended for low output ripple voltage and good

stability. The ESR of a capacitor depends on many factors,

some which are: the value, the voltage rating, physical size

and the type of construction. In general, low value or low

voltage (less than 12V) electrolytic capacitors usually have

higher ESR numbers.

The amount of output ripple voltage is primarily a function of

the ESR (Equivalent Series Resistance) of the output ca-

DS011394-18

FIGURE 10. Inductor Ripple Current (

∆I

IND) Range

Based on Selection Guides from

Figure 4 through

Figure 8.

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