LM2574, NCV2574
http://onsemi.com
10
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:
V
out
= Regulated Output Voltage (3.3 V, 5.0 V, 12 V or 15 V)
V
in(max)
= Maximum Input Voltage
I
Load(max)
= Maximum Load Current
Given Parameters:
V
out
= 5.0 V
V
in(max)
= 15 V
I
Load(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 (C
in
)
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 +V
in
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 (C
in
)
A 22 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
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 t
on
is the “on” time of the power switch and
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
)
)
ǒ
V
in
* V
out
Ǔ
t
on
2L
t
on
+
V
out
V
in
x
1.0
f
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 mH. From Table 2, choose
an inductor from any of the listed manufacturers.
