LM2675 datasheet(17/26 Pages) NSC

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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:

OUT = Regulated Output Voltage

OUT = 20V

IN(max) = Maximum Input Voltage

IN(max) = 28V

LOAD(max) = Maximum Load Current

LOAD(max) = 1A

F = Switching Frequency (Fixed at a nominal 260 kHz).

1. Programming Output Voltage (Selecting R

1 and R2,as

shown in Figure 3)

1. Programming Output Voltage (Selecting R

1 and R2,as

shown in Figure 3)

Use the following formula to select the appropriate resistor

values.

Select R

1 to be 1 k

Ω, 1%. Solve for R

where V

REF = 1.21V

Select a value for R

1 between 240

Ω and 1.5 kΩ. 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.)

2 = 1k (16.53 − 1) = 15.53 k

Ω, closest 1% value is 15.4 kΩ.

2 = 15.4 k

Ω.

AVX TPS

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Voltage

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+85˚C Rating

3.3

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510

Sprague 594D

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FIGURE 15.

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