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LM2672

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SNVS136K – SEPTEMBER 1998 – REVISED APRIL 2013

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, Texas Instruments

is making available computer design software to be used with the

SIMPLE SWITCHERline of switching regulators.LM267X Made

Simple version 6.0 is available onWindows3.1, 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) = 1A

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.

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

C. ILOAD(max) = 1A

D. Identify the inductance region intersected by the E • T value and

D. From the inductor value selection guide shown in Figure 27, the

the Maximum Load Current value. Each region is identified by an

inductance region intersected by the 21.6 (V •

μs) horizontal line and

inductance value and an inductor code (LXX).

the 1A vertical line is 68

μH, and the inductor code is L30.

E. Select an appropriate inductor from the four manufacturer's part

E. From the table in Table 1, locate line L30, and select an inductor

numbers listed in Table 1. For information on the different types of

part number from the list of manufacturers' part numbers.

inductors, see the inductor selection in the fixed output voltage

design procedure.

3. Output Capacitor SeIection (COUT)

A. Select an output capacitor from the capacitor code selection guide A. Use the appropriate row of the capacitor code selection guide, in

in Table 9. Using the inductance value found in the inductor

Table 9. For this example, use the 15–20V row. The capacitor code

selection guide, step 1, locate the appropriate capacitor code

corresponding to an inductance of 68

μH is C20.

corresponding to the desired output voltage.

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