LM2675

SNVS129E – MAY 2004 – REVISED JUNE 2005

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PROCEDURE (Fixed Output Voltage Version)

EXAMPLE (Fixed Output Voltage Version)

3. Catch Diode Selection (D1)

3. 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. In this example, a 1A, 20V

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

Schottky diode will provide the best performance. If the circuit must

duty cycle, which is approximately the output voltage divided by the

withstand a continuous shorted output, a higher current Schottky

input voltage). The largest value of the catch diode average current

diode is recommended.

occurs at the maximum load current and 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 equal

to 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.

This 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 12V, 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

higher capacitor voltage rating is 16V.

current. The capacitor manufacturer data sheet must be checked to

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

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

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

Figure 28 show typical RMS current ratings for several different

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

aluminum electrolytic capacitor values. A parallel connection of two

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

or more capacitors may be required to increase the total minimum

an appropriate input capacitor. From the curves, locate the 16V line

RMS current rating to suit the application requirements.

and note which capacitor values have RMS current ratings greater

For an aluminum electrolytic capacitor, the voltage rating should be

than 500 mA.

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

For a through hole design, a 330

μF/16V electrolytic capacitor

exercised if solid tantalum capacitors are used. The tantalum

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

capacitor voltage rating should be twice the maximum input voltage.

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

The tables in Table 3 show the recommended application voltage for

capacitors can be used provided the RMS ripple current ratings are

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

adequate. Additionally, for a complete surface mount design,

recommended that they be surge current tested by the manufacturer. electrolytic capacitors such as the Sanyo CV-C or CV-BS and the

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

Nichicon WF or UR and the NIC Components NACZ series could be

from Sprague are all surge current tested. Another approach to

considered.

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

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

a small inductor in series with the input supply line.

but caution must be exercised with regard to the capacitor surge

Use caution when using ceramic capacitors for input bypassing,

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

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

μF,

25V 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.

12

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