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LM2672 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 1.0 is available 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)

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 =1k

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

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) = 1A

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 1A vertical line is 68 µH, and the inductor code is

L30.

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 L30, and select an

inductor part number from the list of manufacturers’ part

numbers.

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.

3. Output Capacitor SeIection (C

OUT)

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 68 µH is C20.

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