LM2671 datasheet(18/31 Pages) TI

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LM2671

SNVS008K – SEPTEMBER 1998 – REVISED APRIL 2013

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Table 7. AVX TPS (continued)

Recommended

Voltage

Application Voltage

Rating

+85°C Rating

Table 8. Sprague 594D

Recommended

Voltage

Application Voltage

Rating

+85°C Rating

2.5

3.3

6.3

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, Texas Instrumnets

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:

VOUT = Regulated Output Voltage

VOUT = 20V

VIN(max) = Maximum Input Voltage

VIN(max) = 28V

ILOAD(max) = Maximum Load Current

ILOAD(max) = 500 mA

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

1. Programming Output Voltage (Selecting R1 and R2, as shown in 1. Programming Output Voltage (Selecting R1 and R2, as shown in

Figure 23)

Use the following formula to select the appropriate resistor values.

Select R1 to be 1 kΩ, 1%. Solve for R2.

where VREF = 1.21V

(3)

(4)

Select a value for R1 between 240Ω and 1.5 kΩ. The lower resistor

R2 = 1 kΩ (16.53 − 1) = 15.53 kΩ, closest 1% value is 15.4 kΩ.

values minimize noise pickup in the sensitive feedback pin. (For the

R2 = 15.4 kΩ.

lowest temperature coefficient and the best stability with time, use

1% metal film resistors.)

(5)

2. Inductor Selection (L1)

A. Calculate the inductor Volt • microsecond constant E • T (V •

μs),

A. Calculate the inductor Volt • microsecond constant (E • T),

from the following formula:

(6)

(7)

where VSAT=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

B. E • T = 21.6 (V •

μs)

the E • T number on the vertical axis of the Inductor Value Selection

Guide shown in Figure 27.

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