LM2594 datasheet(20/28 Pages) NSC

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Application Information (Continued)

By allowing the percentage of inductor ripple current to in-

crease 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 ma-

terials, such as ferrites and powdered iron. The least expen-

sive, the bobbin, rod or stick core, consists of wire wrapped

on a ferrite bobbin. This type of construction makes for a in-

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

pletely 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 LM2594. Different inductor

types have different saturation characteristics, and this

should be kept in mind when selecting an inductor.

The inductor manufacturers data sheets include current and

energy limits to avoid inductor saturation.

DISCONTINUOUS MODE OPERATION

The selection guide chooses inductor values suitable for

continuous mode operation, but for low current applications

and/or high input voltages, a discontinuous mode design

may be a better choice. It would use an inductor that would

be physically smaller, and would need only one half to one

third the inductance value needed for a continuous mode de-

sign. The peak switch and inductor currents will be higher in

a discontinuous design, but at these low load currents

(200 mA and below), the maximum switch current will still be

less than the switch current limit.

Discontinuous operation can have voltage waveforms that

are considerable different than a continuous design. The out-

put pin (switch) waveform can have some damped sinusoi-

dal ringing present. (See photo titled; Discontinuous Mode

Switching Waveforms) This ringing is normal for discontinu-

ous operation, and is not caused by feedback loop instabili-

ties. In discontinuous operation, there is a period of time

where neither the switch or the diode are conducting, and

the inductor current has dropped to zero. During this time, a

small amount of energy can circulate between the inductor

and the switch/diode parasitic capacitance causing this char-

acteristic ringing. Normally this ringing is not a problem, un-

less the amplitude becomes great enough to exceed the in-

put voltage, and even then, there is very little energy present

to cause damage.

Different inductor types and/or core materials produce differ-

ent amounts of this characteristic ringing. Ferrite core induc-

tors have very little core loss and therefore produce the most

ringing. The higher core loss of powdered iron inductors pro-

duce less ringing. If desired, a series RC could be placed in

parallel with the inductor to dampen the ringing. The com-

puter aided design software

Switchers Made Simple (ver-

sion 4.1) will provide all component values for continuous

and discontinuous modes of operation.

OUTPUT VOLTAGE RIPPLE AND TRANSIENTS

The output voltage of a switching power supply operating in

the continuous mode will contain a sawtooth ripple voltage at

the switcher frequency, and may also contain short voltage

spikes at the peaks of the sawtooth waveform.

The output ripple voltage is a function of the inductor saw-

tooth ripple current and the ESR of the output capacitor. A

typical output ripple voltage can range from approximately

0.5% to 3% of the output voltage. To obtain low ripple volt-

age, the ESR of the output capacitor must be low, however,

caution must be exercised when using extremely low ESR

capacitors because they can affect the loop stability, result-

ing in oscillation problems. If very low output ripple voltage is

needed (less than 15 mV), a post ripple filter is recom-

mended. (See

Figure 1.) The inductance required is typically

between 1 µH and 5 µH, with low DC resistance, to maintain

good load regulation. A low ESR output filter capacitor is also

required to assure good dynamic load response and ripple

DS012439-31

FIGURE 16. (

∆I

IND) Peak-to-Peak

Inductor Ripple Current

(as a Percentage of the Load Current) vs Load Current

DS012439-32

FIGURE 17. Post Ripple Filter Waveform

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