LM2574DW View Datasheet(PDF) - Motorola => Freescale
Part Name
Description
Manufacturer
LM2574DW Datasheet PDF : 24 Pages
| |||
LM2574
Procedure (Adjustable Output Version: LM2574–ADJ)
Procedure
Example
4. Inductor Selection (L1)
A. Use the following formula to calculate the inductor Volt x
microsecond [V x µs] constant:
+ * ƪ ƫ E x T
(Vin
Vout)
Vout
Vin
x
106
F[Hz]
V x ms
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 23. 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 27.
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
+ ) ǒ * Ǔ peak current:
Ip(max) ILoad(max)
Vin Vout ton
2L
ƪ ƫ 4. Inductor Selection (L1)
A. Calculate E x T V x ms constant :
+ * + ƪ ƫ E x T
(40
24)
x
24
40
x
1000
52
105 V x ms
+ ƪ ƫ B. E x T 185 V x ms
C. ILoad(max) = 0.4 A
Inductance Region = 1000
D. Proper inductor value = 1000 µH
Choose the inductor from Table 2.
where ton is the “on” time of the power switch and
+ ton
Vout
Vin
x
1.0
fosc
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 LM2574 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:
w ƪ ƫ ƪ ƫ Cout
13, 300 Vin(max)
Vout x L mH
mF
5. Output Capacitor Selection (Cout)
w + A.
Cout
13, 300
x
24
40
x 1000
22.2 mF
To achieve an acceptable ripple voltage, select
Cout = 100 µF electrolytic capacitor.
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 16V rating is recommended.
12
MOTOROLA ANALOG IC DEVICE DATA
Share Link: 