LM2671 datasheet(17/26 Pages) NSC

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LM2671 Series Buck Regulator Design Procedure (Fixed Output) (Continued)

LM2671 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 is available on (version 6.0)

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) = 500 mA

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

1. Programming Output Voltage (Selecting R

1 and R2,as

shown in Figure 3)

Use the following formula to select the appropriate resistor

values.

1. Programming Output Voltage (Selecting R

1 and R2,as

shown in Figure 3)

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 =1 k

Ω (16.53 − 1) = 15.53 kΩ, closest 1% value is 15.4 kΩ.

2 = 15.4 k

Ω.

2. Inductor Selection (L1)

A. Calculate the inductor Volt

• microsecond constant E • T

• µs), from the following formula:

2. Inductor Selection (L1)

A. Calculate the inductor Volt

• microsecond constant (E • T),

where V

SAT=internal switch saturation voltage=0.25V and

D = diode forward voltage drop = 0.5V

AVX TPS

Recommended

Application Voltage

Voltage

Rating

+85˚C Rating

3.3

6.3

510

Sprague 594D

Recommended

Application Voltage

Voltage

Rating

+85˚C Rating

2.5

3.3

6.3

510

816

FIGURE 15. Recommended Application Voltage for AVX TPS and

Sprague 594D Tantalum Chip Capacitors Derated for 85˚C.

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