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LM2675 Series Buck Regulator Design Procedure (Adjustable Output)
(Continued)
PROCEDURE (Adjustable Output Voltage Version)
EXAMPLE (Adjustable Output Voltage Version)
A. Calculate the inductor Volt
microsecond constant E T(V
s), from the following formula:
A. Calculate the inductor Volt
microsecond constant (E T),
where V
SAT = internal switch saturation voltage = 0.25V and VD
= diode forward voltage drop = 0.5V
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.
B. E
T = 21.6 (V s)
C. On the horizontal axis, select the maximum load current.
C. I
LOAD(max) = 1A
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).
D. From the inductor value selection guide shown in
Figure 7,
the inductance region intersected by the 21.6 (V
s) horizontal
line and the 1A vertical line is 68 H, and the inductor code is
L30.
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.
E. From the table in
Figure 8, locate line L30, and select an
inductor part number from the list of manufacturers part
numbers.
3. Output Capacitor Selection (C
OUT)
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.
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 68 H is C20.
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.
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 F/25V
Sprague 594D Series.
33 F/25V
AVX TPS Series.
Through Hole:
33 F/25V
Sanyo OS-CON SC Series.
120 F/35V
Sanyo MV-GX Series.
120 F/35V
Nichicon PL Series.
120 F/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.
LM2675
www.national.com
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