LM2671 datasheet(21/36 Pages) TI1

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LM2671

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SNVS008L – SEPTEMBER 1998 – REVISED JUNE 2016

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9.2.2.1 Design Requirements

Table 7 lists the design parameters for this example.

Table 7. Design Parameters

PARAMETER

VALUE

Regulated output voltage, VOUT

20 V

Maximum input voltage, VIN(max)

28 V

Maximum load current, ILOAD(max)

500 mA

Switching frequency, F

Fixed at a nominal 260 kHz

9.2.2.2 Detailed Design Procedure

9.2.2.2.1

Programming Output Voltage

Select R1 and R2, as shown in Figure 21.

Use the following formula to select the appropriate resistor values.

where

•

VREF = 1.21 V

(3)

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

(4)

Select a value for R1 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.

(5)

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

R2 = 15.4 kΩ.

9.2.2.2.2

Inductor Selection (L1)

1. Calculate the inductor Volt • microsecond constant E • T (V • μs) from Equation 6.

where

•

VSAT = internal switch saturation voltage = 0.25 V

•

VD = diode forward voltage drop = 0.5 V

(6)

Calculate the inductor Volt • microsecond constant (E • T) with Equation 7.

(7)

2. Use the E • T value from the previous formula and match it with the E • T number on the vertical axis of the

inductor value selection guide shown in Figure 20.

E • T = 21.6 (V • μs)

(8)

3. On the horizontal axis, select the maximum load current in Equation 9.

ILOAD(max) = 500 mA

(9)

4. Identify the inductance region intersected by the E • T value and the maximum load current value. Each

region is identified by an inductance value and an inductor code (LXX). From the inductor value selection

guide shown in Figure 20, the inductance region intersected by the 21.6 (V • μs) horizontal line and the 500-

mA vertical line is 100 μH, and the inductor code is L20.

5. Select an appropriate inductor from the four manufacturer's part numbers listed in Table 2. For information

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