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LM2674 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) = 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)

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.

2. Inductor Selection (L1)

A. Calculate the inductor Volt

• microsecond constant E • T(V •

µs), from the following formula:

A. Calculate the inductor Volt

• microsecond constant (E • T),

where V

SAT=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 the E

• T number on the vertical axis of the Inductor Value

Selection Guide shown in Figure 7.

B. E

• T = 21.6 (V • µs)

C. On the horizontal axis, select the maximum load current.

C. I

LOAD(max) = 500 mA

D. 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).

D. From the inductor value selection guide shown in Figure 7,

the inductance region intersected by the 21.6 (V

• µs) horizontal

line and the 500mA vertical line is 100 µH, and the inductor

code is L20.

E. Select an appropriate inductor from the four manufacturer’s

part numbers listed in Figure 8. For information on the different

types of inductors, see the inductor selection in the fixed output

voltage design procedure.

E. From the table in Figure 8, locate line L20, and select an

inductor part number from the list of manufacturers part

numbers.

3. Output Capacitor Selection (C

OUT)

3. Output Capacitor SeIection (C

OUT)

A. Select an output capacitor from the capacitor code selection

guide in Figure 16. Using the inductance value found in the

inductor selection guide, step 1, locate the appropriate capacitor

code corresponding to the desired output voltage.

A. Use the appropriate row of the capacitor code selection

guide, in Figure 16. For this example, use the 15–20V row. The

capacitor code corresponding to an inductance of 100 µH is

C20.

LM2674 Datasheet(PDF) 18 Page - National Semiconductor (TI)

LM2674 Datasheet(HTML) 18 Page - National Semiconductor (TI)

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