LM2576
Procedure (Adjustable Output Version: LM2576–ADJ) (continued)
Procedure
4. Inductor Selection (L1)
A.
Use the following formula to calculate the inductor Volt x
microsecond [V x
µs]
constant:
V out
6
E x T
V – V out
x 10
[V x
m
s]
in
V
F[Hz]
in
B.
Match the calculated E x T value with the corresponding
number on the vertical axis of the Inductor Value Selection
Guide shown in Figure 22. This E x T constant is a
measure of the energy handling capability of an inductor and
is dependent upon the type of core, the core area, the
number of turns, and the duty cycle.
C.
Next step is to identify the inductance region intersected by
the E x T value and the maximum load current value on the
horizontal axis shown in Figure 25.
D.
From the inductor code, identify the inductor value. Then
select an appropriate inductor from Table 2.
The inductor chosen must be rated for a switching
frequency of 52 kHz and for a current rating of 1.15 x ILoad.
The inductor current rating can also be determined by
calculating the inductor peak current:
Example
4. Inductor Selection (L1)
A.
Calculate E x T [V x
µs]
constant:
+
E x T
+
(25 – 8.0) x 8.0
25
x 1000
52
+
80 [V x
m
s]
B.
E x T = 80 [V x
µs]
C.
ILoad(max) = 2.5 A
Inductance Region = H150
D.
Proper inductor value = 150
µH
Choose the inductor from Table 2.
I
+
I
)
p(max) Load(max)
t on
V
in
– V out ton
2L
where ton is the “on” time of the power switch and
osc
in
For additional information about the inductor, see the
inductor section in the “External Components” section of
this data sheet.
5. Output Capacitor Selection (Cout)
A.
Since the LM2576 is a forward–mode switching regulator
with voltage mode control, its open loop 2–pole–1–zero
frequency characteristic has the dominant pole–pair
determined by the output capacitor and inductor values.
For stable operation, the capacitor must satisfy the
following requirement:
V
in(max)
Cout
13, 300
[µF]
V out x L [µH]
5. Output Capacitor Selection (Cout)
A.
25
Cout
13, 300 x
8 x 150
+
VVout x f1.0
w
+
332.5
µF
To achieve an acceptable ripple voltage, select
Cout = 680
µF
electrolytic capacitor.
w
B.
Capacitor values between 10
µF
and 2000
µF
will satisfy
the loop requirements for stable operation. To achieve an
acceptable output ripple voltage and transient response, the
output capacitor may need to be several times larger than
the above formula yields.
C.
Due to the fact that the higher voltage electrolytic capacitors
generally have lower ESR (Equivalent Series Resistance)
numbers, the output capacitor’s voltage rating should be at
least 1.5 times greater than the output voltage. For a 5.0 V
regulator, a rating of at least 8.0 V is appropriate, and a 10 V
or 16 V rating is recommended.
12
MOTOROLA ANALOG IC DEVICE DATA
MOTOROLA [ MOTOROLA, INC ]
