LM2574-12 Motorola Inc, LM2574-12 Datasheet - Page 10

• Manufacturer Part Number: LM2574-12
• Description: EASY SWITCHERE 0.5 A STEP-DOWN VOLTAGE REGULATOR
• Manufacturer: Motorola Inc

Procedure (Fixed Output Voltage Version) In order to simplify the switching regulator design, a step–by–step
design procedure and example is provided.
10
4. Inductor Selection (L1)
Given Parameters:
1. Controller IC Selection
2. Input Capacitor Selection (C in )
3. Catch Diode Selection (D1)
A. According to the required working conditions, select the
B. From the appropriate inductor selection guide, identify the
C. Select an appropriate inductor from the several different
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
According to the required input voltage, output voltage and
current, select the appropriate type of the controller IC output
voltage version.
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.
A. Since the diode maximum peak current exceeds the
B. The reverse voltage rating of the diode should be at least
correct inductor value using the selection guide from
Figures 19 to 23.
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.
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.
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.
1.25 times the maximum input voltage.
I
p
(
max
)
+
I
t
Load
on
+
Procedure
(
max
V
V
out
in
)
)
x 1.0
V
f
in
osc
*
2L
V
out
t
on
LM2574
Given Parameters:
1. Controller IC Selection
2. Input Capacitor Selection (C in )
3. Catch Diode Selection (D1)
4. Inductor Selection (L1)
V out = 5.0 V
V in(max) = 15 V
I Load(max) = 0.4 A
According to the required input voltage, output voltage, current
polarity and current value, use the LM2574–5 controller IC.
A 22 F, 25 V aluminium electrolytic capacitor located near to
the input and ground pins provides sufficient bypassing.
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
A. Use the inductor selection guide shown in Figure 20.
B. From the selection guide, the inductance area intersected
C. Inductor value required is 330 H. From Table 2, choose an
suggested fast recovery diodes shown in Table 1.
by the 15 V line and 0.4 A line is 330.
inductor from any of the listed manufacturers.
MOTOROLA ANALOG IC DEVICE DATA
Example