LM2675

SNVS129E – MAY 2004 – REVISED JUNE 2005

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PROCEDURE (Adjustable Output Voltage Version)

EXAMPLE (Adjustable Output Voltage Version)

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 2.2A) 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 LM2675. 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 LM2675 using short

leads and short printed circuit traces.

A low ESR aluminum or tantalum bypass capacitor is needed

The important parameters for the input capacitor are the input

between the input pin and ground to prevent large voltage transients

voltage rating and the RMS current rating. With a maximum input

from appearing at the input. This capacitor should be located close

voltage of 28V, an aluminum electrolytic capacitor with a voltage

to the IC using short leads. In addition, the RMS current rating of the

input capacitor should be selected to be at least ½ the DC load

The RMS current rating requirement for the input capacitor in a buck

current. The capacitor manufacturer data sheet must be checked to

regulator is approximately ½ the DC load current. In this example,

assure that this current rating is not exceeded. The curves shown in

with a 1A load, a capacitor with a RMS current rating of at least 500

Figure 28 show typical RMS current ratings for several different

mA is needed. The curves shown in Figure 28 can be used to select

aluminum electrolytic capacitor values. A parallel connection of two

an appropriate input capacitor. From the curves, locate the 35V line

or more capacitors may be required to increase the total minimum

and note which capacitor values have RMS current ratings greater

RMS current rating to suit the application requirements.

than 500 mA.

For an aluminum electrolytic capacitor, the voltage rating should be

For a through hole design, a 330

μF/35V electrolytic capacitor

at least 1.25 times the maximum input voltage. Caution must be

(Panasonic HFQ series, Nichicon PL, Sanyo MV-GX series or

exercised if solid tantalum capacitors are used. The tantalum

equivalent) would be adequate. Other types or other manufacturers'

capacitor voltage rating should be twice the maximum input voltage.

capacitors can be used provided the RMS ripple current ratings are

Table 7 and Table 8 show the recommended application voltage for

adequate. Additionally, for a complete surface mount design,

AVX TPS and Sprague 594D tantalum capacitors. It is also

electrolytic capacitors such as the Sanyo CV-C or CV-BS, and the

recommended that they be surge current tested by the manufacturer. Nichicon WF or UR and the NIC Components NACZ series could be

The TPS series available from AVX, and the 593D and 594D series

considered.

from Sprague are all surge current tested. Another approach to

For surface mount designs, solid tantalum capacitors can be used,

minimize the surge current stresses on the input capacitor is to add

but caution must be exercised with regard to the capacitor surge

a small inductor in series with the input supply line.

current rating and voltage rating. In this example, checking Table 8,

Use caution when using ceramic capacitors for input bypassing,

and the Sprague 594D series datasheet, a Sprague 594D 15

μF,

50V capacitor is adequate.

This capacitor develops the necessary voltage to turn the switch

For this application, and all applications, use a 0.01

μF, 50V ceramic

gate on fully. All applications should use a 0.01

μF, 50V ceramic

capacitor.

capacitor.

18

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