LM2575

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9

Procedure (Fixed Output Voltage Version) In order to simplify the switching regulator design, a step−by−step

design procedure and example is provided.

Procedure

Example

Given Parameters:

Given Parameters:

1. Controller IC Selection

According to the required input voltage, output voltage and

current, select the appropriate type of the controller IC output

voltage version.

1. Controller IC Selection

According to the required input voltage, output voltage,

current polarity and current value, use the LM2575−5

controller IC

To prevent large voltage transients from appearing at the input

and for stable operation of the converter, an aluminium or

tantalum electrolytic bypass capacitor is needed between the

located close to the IC using short leads. This capacitor should

have a low ESR (Equivalent Series Resistance) value.

A 47

mF, 25 V aluminium electrolytic capacitor located near

to the input and ground pins provides sufficient bypassing.

3. Catch Diode Selection (D1)

A. Since the diode maximum peak current exceeds the

regulator maximum load current the catch diode current

rating must be at least 1.2 times greater than the maximum

load current. For a robust design the diode should have a

current rating equal to the maximum current limit of the

LM2575 to be able to withstand a continuous output short

B. The reverse voltage rating of the diode should be at least

1.25 times the maximum input voltage.

3. Catch Diode Selection (D1)

A. For this example the current rating of the diode is 1.0 A.

B. Use a 30 V 1N5818 Schottky diode, or any of the

suggested fast recovery diodes shown in the Table 4.

4. Inductor Selection (L1)

A. According to the required working conditions, select the

correct inductor value using the selection guide from

Figures 17 to 21

.

B. From the appropriate inductor selection guide, identify the

inductance region intersected by the Maximum Input

Voltage line and the Maximum Load Current line. Each

region is identified by an inductance value and an inductor

code.

C. Select an appropriate inductor from the several different

manufacturers part numbers listed in Table 1 or Table 2.

When using Table 2 for selecting the right inductor the

designer must realize that the inductor current rating must

be higher than the maximum peak current flowing through

the inductor. This maximum peak current can be calculated

as follows

:

For additional information about the inductor, see the

inductor section in the “External Components” section of

this data sheet.

I

p(max)

Load(max) )

V

in

–Vout ton

2L

V

out

V

in

x

1

fosc

4. Inductor Selection (L1)

A. Use the inductor selection guide shown in Figures 17

to 21.

B. From the selection guide, the inductance area intersected

by the 20 V line and 0.8 A line is L330.

C. Inductor value required is 330

mH. From the Table 1 or

Table 2

, choose an inductor from any of the listed

manufacturers.