LM2575 datasheet(16/28 Pages) MOTOROLA

16 / 28 page

LM2575

MOTOROLA ANALOG IC DEVICE DATA

conditions, the circuit will be forced to the discontinuous

mode when inductor current falls to zero for certain period of

time (see Figure 22 and Figure 23). Each mode has

distinctively different operating characteristics, which can

affect the regulator performance and requirements. In many

cases the preferred mode of operation is the continuous

mode. It offers greater output power, lower peak currents in

the switch, inductor and diode, and can have a lower output

ripple voltage. On the other hand 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

selection guide for the LM2575 regulator was added to this

data sheet (Figures 17 through 21). 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 percentage is allowed to change as

different design load currents are selected. For light loads

(less than approximately 200 mA) it may be desirable to

operate the regulator in the discontinuous mode, because

the inductor value and size can be kept relatively low.

Consequently, the percentage of inductor peak–to–peak

current increases. This discontinuous mode of operation is

perfectly acceptable for this type of switching converter. Any

buck regulator will be forced to enter discontinuous mode if

the load current is light enough.

Figure 22. Continuous Mode Switching

Current Waveforms

1.0

HORTIZONTAL TIME BASE: 5.0

µs/DIV

1.0

Selecting the Right Inductor Style

Some important considerations when selecting a core

type are core material, cost, the output power of the power

supply, the physical volume the inductor must fit within, and

the amount of EMI (Electro–Magnetic Interference) shielding

that the core must provide. The inductor selection guide

covers different styles of inductors, such as pot core, E–core,

toroid and bobbin core, as well as different core materials

such as ferrites and powdered iron from different

manufacturers.

For high quality design regulators the toroid core seems to

be the best choice. Since the magnetic flux is completely

contained within the core, it generates less EMI, reducing

noise problems in sensitive circuits. The least expensive is

the bobbin core type, which consists of wire wound on a

ferrite rod core. This type of inductor generates more EMI

due to the fact that its core is open, and the magnetic flux is

not completely contained within the core.

When multiple switching regulators are located on the

same printed circuit board, open core magnetics can cause

interference between two or more of the regulator circuits,

especially at high currents due to mutual coupling. A toroid,

pot core or E–core (closed magnetic structure) should be

used in such applications.

Do Not Operate an Inductor Beyond its

Maximum Rated Current

Exceeding an inductor’s maximum current rating may

cause the inductor to overheat because of the copper wire

losses, or the core may saturate. Core saturation occurs

when the flux density is too high and consequently the cross

sectional area of the core can no longer support additional

lines of magnetic flux.

This causes the permeability of the core to drop, the

inductance value decreases rapidly and the inductor begins

to look mainly resistive. It has only the dc resistance of the

winding. This can cause the switch current to rise very rapidly

and force the LM2575 internal switch into cycle–by–cycle

current limit, thus reducing the dc output load current. This

can also result in overheating of the inductor and/or the

LM2575. Different inductor types have different saturation

characteristics, and this should be kept in mind when

selecting an inductor.

Figure 23. Discontinuous Mode Switching

Current Waveforms

0.1

HORTIZONTAL TIME BASE: 5.0

µs/DIV

LM2575 Scheda tecnica(PDF) 16 Page - Motorola, Inc

LM2575 Scheda tecnica(HTML) 16 Page - Motorola, Inc

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

Scheda tecnica Scarica

Link URL