LM2675 Series Buck Regulator Design Procedure (Fixed Output) (Continued)
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Sprague 594D
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FIGURE 15.
LM2675 Series Buck Regulator Design Procedure (Adjustable Output)
PROCEDURE (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:
EXAMPLE (Adjustable Output Voltage Version)
Given:
VOUT = 20V
VOUT = Regulated Output Voltage
V
IN(max) = Maximum Input Voltage
VIN(max) = 28V
I
LOAD(max) = Maximum Load Current
ILOAD(max) = 1A
F = Switching Frequency (Fixed at a nominal 260 kHz).
1. Programming Output Voltage (Selecting R1 and R2, as
shown in Figure 3)
F = Switching Frequency (Fixed at a nominal 260 kHz).
1. Programming Output Voltage (Selecting R1 and R2, as
shown in Figure 3)
Use the following formula to select the appropriate resistor
values.
Select R1 to be 1 kΩ, 1%. Solve for R2.
where VREF = 1.21V
Select a value for R1 between 240Ω and 1.5 kΩ. The lower
R2 = 1k (16.53 − 1) = 15.53 kΩ, closest 1% value is 15.4 kΩ.
resistor values minimize noise pickup in the sensitive feedback R2 = 15.4 kΩ.
pin. (For the lowest temperature coefficient and the best stability
with time, use 1% metal film resistors.)
17
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