LM2576
Procedure (Fixed Output Voltage Version)
In order to simplify the switching regulator design, a step−by−step
design procedure and some examples are provided.
Procedure
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
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.
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.
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
LM2576 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.
4. Inductor Selection (L1)
A.
According to the required working conditions, select the
correct inductor value using the selection guide from
Figures 18 to 22.
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:
I
p(max)
+
I
Example
Given Parameters:
V
out
= 5.0 V
V
in(max)
= 15 V
I
Load(max)
= 3.0 A
1. Controller IC Selection
According to the required input voltage, output voltage,
current polarity and current value, use the LM2576−5
controller IC
2. Input Capacitor Selection (C
in
)
A 100
mF,
25 V aluminium electrolytic capacitor located near
to the input and ground pins provides sufficient bypassing.
3. Catch Diode Selection (D1)
A.
For this example the current rating of the diode is 3.0 A.
B.
Use a 20 V 1N5820 Schottky diode, or any of the
suggested fast recovery diodes shown in Table 1.
4. Inductor Selection (L1)
A.
Use the inductor selection guide shown in Figures 19.
B.
From the selection guide, the inductance area intersected
by the 15 V line and 3.0 A line is L100.
C.
Inductor value required is 100
mH.
From Table 2, choose
an inductor from any of the listed manufacturers.
Load(max)
)
V –V out t on
in
2L
where t
on
is the “on” time of the power switch and
V
ton
+
out x 1.0
fosc
V
in
For additional information about the inductor, see the
inductor section in the “Application Hints” section of
this data sheet.
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