LM2671

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SNVS008K – SEPTEMBER 1998 – REVISED APRIL 2013

PROCEDURE (Adjustable Output Voltage Version)

EXAMPLE (Adjustable Output Voltage Version)

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

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 500 mA vertical line is 100

μH, and the inductor code is L20.

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

E. From the table in Table 1, locate line L20, 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.

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 100

μH is C20.

corresponding to the desired output voltage.

B. Select an appropriate capacitor value and voltage rating, using

B. From the output capacitor selection table in Table 10, choose a

the capacitor code, from the output capacitor selection table in

capacitor value (and voltage rating) that intersects the capacitor

Table 10. There are two solid tantalum (surface mount) capacitor

code(s) selected in section A, C20.

manufacturers and four electrolytic (through hole) capacitor

The capacitance and voltage rating values corresponding to the

manufacturers to choose from. It is recommended that both the

capacitor code C20 are the:

manufacturers and the manufacturer's series that are listed in the

Surface Mount:

table be used. A table listing the manufacturers' phone numbers is

33

μF/25V

Sprague 594D Series.

located in Table 4.

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)

4. Catch Diode Selection (D1)

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

A. Refer to the table shown in Table 5. Schottky diodes provide the

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

best performance, and in this example a 1A, 40V Schottky diode

would be a good choice. If the circuit must withstand a continuous

the catch diode average current occurs at the maximum input

shorted output, a higher current (at least 1.2A) Schottky diode is

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

recommended.

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.

B. The reverse voltage rating of the diode should be at least 1.25

times the maximum input voltage.

C. Because of their fast switching speed and low forward voltage

drop, Schottky diodes provide the best performance and efficiency.

The Schottky diode must be located close to the LM2671 using short

leads and short printed circuit traces.

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