
LM2574
10
MOTOROLA ANALOG IC DEVICE DATA
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:
Vout = Regulated Output Voltage (3.3 V, 5.0 V, 12 V or 15 V)
Vin(max) = Maximum Input Voltage
ILoad(max) = Maximum Load Current
Given Parameters:
Vout = 5.0 V
Vin(max) = 15 V
ILoad(max) = 0.4 A
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 LM2574–5 controller IC.
2. Input Capacitor Selection (Cin)
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 
input pin +Vin and ground pin Gnd. This capacitor should be 
located close to the IC using short leads. This capacitor should
have a low ESR (Equivalent Series Resistance) value.
2. Input Capacitor Selection (Cin)
A 22 
μ
F, 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 
LM2574 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 20 V 1N5817 Schottky diode, or any of the 
suggested fast recovery diodes shown in Table 1.
4. Inductor Selection (L1)
A.
According to the required working conditions, select the 
correct inductor value using the selection guide from 
Figures 19 to 23.
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 2. 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:
where ton is the “on” time of the power switch and
V
out
V
in
For additional information about the inductor, see the inductor 
section in the “EXTERNAL COMPONENTS” section of this 
data sheet.
I
p
(
max
)
I
Load
(
max
)
V
in
V
out
t
on
2L
t
on
xf
osc
4. Inductor Selection (L1)
A.
Use the inductor selection guide shown in Figure 20.
B.
From the selection guide, the inductance area intersected 
by the 15 V line and 0.4 A line is 330.
C.
Inductor value required is 330 
μ
H. From Table 2, choose an
inductor from any of the listed manufacturers.