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| 型號(hào)： | LM2673 |
| 文件頁數(shù)： | 18/25頁 |
| 文件大?。?/td> | 495K |
| 代理商： | LM2673 |
LM2671 Series Buck Regulator Design Procedure (Adjustable Output)
(Continued)
PROCEDURE (Adjustable Output Voltage Version)
B.
Use the E
T value from the previous formula and match it
with the E
T number on the vertical axis of the Inductor Value
Selection Guide shown in Figure 7
C.
On the horizontal axis, select the maximum load current.
D.
Identify the inductance region intersected by the E
T value
and the Maximum Load Current value. Each region is identified
by an inductance value and an inductor code (LXX).
EXAMPLE (Adjustable Output Voltage Version)
B.
E
T = 21.6 (V
μs)
C.
I
LOAD
(max) = 500 mA
D.
From the inductor value selection guide shown in Figure 7
the inductance region intersected by the 21.6 (V
μs) horizontal
line and the 500 mA vertical line is 100 μH, and the inductor
code is L20.
E.
From the table in Figure 8 locate line L20, and select an
inductor part number from the list of manufacturers’ part
numbers.
E.
Select an appropriate inductor from the four manufacturer’s
part numbers listed in Figure 8 For information on the different
types of inductors, see the inductor selection in the fixed output
voltage design procedure.
3. Output Capacitor SeIection (C
OUT
)
A.
Select an output capacitor from the capacitor code selection
guide in Figure 16 Using the inductance value found in the
inductor selection guide, step 1, locate the appropriate capacitor
code corresponding to the desired output voltage.
B.
Select an appropriate capacitor value and voltage rating,
using the capacitor code, from the output capacitor selection
table in Figure 17 There are two solid tantalum (surface mount)
capacitor manufacturers and four electrolytic (through hole)
capacitor manufacturers to choose from. It is recommended that
both the manufacturers and the manufacturer’s series that are
listed in the table be used. A table listing the manufacturers’
phone numbers is located in Figure 11
3. Output Capacitor SeIection (C
OUT
)
A.
Use the appropriate row of the capacitor code selection
guide, in Figure 16 For this example, use the 15–20V row. The
capacitor code corresponding to an inductance of 100 μH is
C20.
B.
From the output capacitor selection table in Figure 17
choose a capacitor value (and voltage rating) that intersects the
capacitor code(s) selected in section A, C20.
The capacitance and voltage rating values corresponding to the
capacitor code C20 are the:
Surface Mount:
33 μF/25V
Sprague 594D Series.
33 μF/25V
AVX TPS Series.
Through Hole:
33 μF/25V
Sanyo OS-CON SC Series.
120 μF/35V
Sanyo MV-GX Series.
120 μF/35V
Nichicon PL Series.
120 μF/35V
Panasonic HFQ Series.
Other manufacturers or other types of capacitors may also be
used, provided the capacitor specifications (especially the 100
kHz ESR) closely match the characteristics of the capacitors
listed in the output capacitor table. Refer to the capacitor
manufacturers’ data sheet for this information.
4. Catch Diode Selection (D1)
A.
Refer to the table shown in Figure 12 Schottky diodes
provide the best performance, and in this example a 1A, 40V
Schottky diode would be a good choice. If the circuit must
withstand a continuous shorted output, a higher current (at least
1.2A) Schottky diode is recommended.
4. Catch Diode Selection (D1)
A.
In normal operation, the average current of the catch diode is
the load current times the catch diode duty cycle, 1-D (D is the
switch duty cycle, which is approximately V
OUT
/V
IN
). The largest
value of the catch diode average current occurs at the maximum
input voltage (minimum D). For normal operation, the catch
diode current rating must be at least 1.3 times greater than its
maximum average current. However, if the power supply design
must withstand a continuous output short, the diode should have
a current rating greater than the maximum current limit of the
LM2671. The most stressful condition for this diode is a shorted
output condition.
B.
The reverse voltage rating of the diode should be at least
1.25 times the maximum input voltage.
L
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