LM2671 datasheet(18/26 Pages) NSC

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LM2671 Series Buck Regulator Design Procedure (Adjustable Output)

(Continued)

PROCEDURE (Adjustable Output Voltage Version)

EXAMPLE (Adjustable Output Voltage Version)

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 500 mA 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 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 100 µH is

C20.

B. Select an appropriate capacitor value and voltage rating,

using the capacitor code, from the output capacitor selection

table in Figure 17. There are two solid tantalum (surface mount)

capacitor manufacturers and four electrolytic (through hole)

capacitor manufacturers to choose from. It is recommended that

both the manufacturers and the manufacturer’s series that are

listed in the table be used. A table listing the manufacturers’

phone numbers is located in Figure 11.

B. From the output capacitor selection table in Figure 17,

choose a capacitor value (and voltage rating) that intersects the

capacitor code(s) selected in section A, C20.

The capacitance and voltage rating values corresponding to the

capacitor code C20 are the:

Surface Mount:

33 µF/25V

Sprague 594D Series.

33 µF/25V

AVX TPS Series.

Through Hole:

33 µF/25V

Sanyo OS-CON SC Series.

120 µF/35V

Sanyo MV-GX Series.

120 µF/35V

Nichicon PL Series.

120 µF/35V

Panasonic HFQ Series.

Other manufacturers or other types of capacitors may also be

used, provided the capacitor specifications (especially the 100

kHz ESR) closely match the characteristics of the capacitors

listed in the output capacitor table. Refer to the capacitor

manufacturers’ data sheet for this information.

4. Catch Diode Selection (D1)

A. In normal operation, the average current of the catch diode is

the load current times the catch diode duty cycle, 1-D (D is the

switch duty cycle, which is approximately V

OUT/VIN). The largest

value of the catch diode average current occurs at the maximum

input voltage (minimum D). For normal operation, the catch

diode current rating must be at least 1.3 times greater than its

maximum average current. However, if the power supply design

must withstand a continuous output short, the diode should have

a current rating greater than the maximum current limit of the

LM2671. The most stressful condition for this diode is a shorted

output condition.

4. Catch Diode Selection (D1)

A. Refer to the table shown in Figure 12. Schottky diodes

provide the best performance, and in this example a 1A, 40V

Schottky diode would be a good choice. If the circuit must

withstand a continuous shorted output, a higher current (at least

1.2A) Schottky diode is recommended.

B. The reverse voltage rating of the diode should be at least

1.25 times the maximum input voltage.

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