
LM2675 Series Buck Regulator Design Procedure (Fixed Output) (Continued)
LM2675 Series Buck Regulator Design Procedure (Adjustable Output)
PROCEDURE (Adjustable Output Voltage Version)
EXAMPLE (Adjustable Output Voltage Version)
To simplify the buck regulator design procedure, National
Semiconductor is making available computer design software to
be used with the SIMPLE SWITCHER line of switching
regulators. LM267X Made Simple version 6.0 is available for
use on Windows 3.1, NT, or 95 operating systems.
Given:
V
OUT = Regulated Output Voltage
V
OUT = 20V
V
IN(max) = Maximum Input Voltage
V
IN(max) = 28V
I
LOAD(max) = Maximum Load Current
I
LOAD(max) = 1A
F = Switching Frequency
(Fixed at a nominal 260 kHz).
F = Switching Frequency
(Fixed at a nominal 260 kHz).
1. Programming Output Voltage (Selecting R
1 and R2,as
shown in
1. Programming Output Voltage (Selecting R
1 and R2,as
shown in
Use the following formula to select the appropriate resistor
values.
Select R
1 to be 1 k, 1%. Solve for R2.
where V
REF = 1.21V
Select a value for R
1 between 240 and 1.5 k. The lower
resistor values minimize noise pickup in the sensitive feedback
pin. (For the lowest temperature coefficient and the best stability
with time, use 1% metal film resistors.)
R
2 = 1k (16.53 1) = 15.53 k, closest 1% value is 15.4 k.
R
2 = 15.4 k.
2. Inductor Selection (L1)
AVX TPS
Recommended
Voltage
Application Voltage
Rating
+85C Rating
3.3
6.3
510
10
20
12
25
15
35
Sprague 594D
Recommended
Voltage
Application Voltage
Rating
+85C Rating
2.5
4
3.3
6.3
510
816
12
20
18
25
24
35
29
50
FIGURE 15.
LM2675
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