LM2675 datasheet(19/26 Pages) NSC

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LM2675 Series Buck Regulator Design Procedure (Adjustable Output)

(Continued)

PROCEDURE (Adjustable Output Voltage Version)

EXAMPLE (Adjustable Output Voltage Version)

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

LM2675. 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

2.2A) Schottky diode is recommended.

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.

5. Input Capacitor (C

IN)

A low ESR aluminum or tantalum bypass capacitor is needed

between the input pin and ground to prevent large voltage

transients from appearing at the input. This capacitor should be

located close to the IC using short leads. In addition, the RMS

current rating of the input capacitor should be selected to be at

least 1⁄2 the DC load current. The capacitor manufacturer data

sheet must be checked to assure that this current rating is not

exceeded. The curves shown in Figure 14 show typical RMS

current ratings for several different aluminum electrolytic

capacitor values. A parallel connection of two or more

capacitors may be required to increase the total minimum RMS

current rating to suit the application requirements.

For an aluminum electrolytic capacitor, the voltage rating should

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

be exercised if solid tantalum capacitors are used. The tantalum

capacitor voltage rating should be twice the maximum input

voltage. The tables in Figure 15 show the recommended

application voltage for AVX TPS and Sprague 594D tantalum

capacitors. It is also recommended that they be surge current

tested by the manufacturer. The TPS series available from AVX,

and the 593D and 594D series from Sprague are all surge

current tested. Another approach to minimize the surge current

stresses on the input capacitor is to add a small inductor in

series with the input supply line.

Use caution when using ceramic capacitors for input bypassing,

because it may cause severe ringing at the V

IN pin.

5. Input Capacitor (C

IN)

The important parameters for the input capacitor are the input

voltage rating and the RMS current rating. With a maximum

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

voltage rating of at least 35V (1.25 x V

IN) would be needed.

The RMS current rating requirement for the input capacitor in a

buck regulator is approximately 1⁄2 the DC load current. In this

example, with a 1A load, a capacitor with a RMS current rating

of at least 500 mA is needed. The curves shown in Figure 14

can be used to select an appropriate input capacitor. From the

curves, locate the 35V line and note which capacitor values

have RMS current ratings greater than 500 mA.

For a through hole design, a 330 µF/35V electrolytic capacitor

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

equivalent) would be adequate. Other types or other

manufacturers’ capacitors can be used provided the RMS ripple

current ratings are adequate. Additionally, for a complete

surface mount design, electrolytic capacitors such as the Sanyo

CV-C or CV-BS, and the Nichicon WF or UR and the NIC

Components NACZ series could be considered.

For surface mount designs, solid tantalum capacitors can be

used, but caution must be exercised with regard to the capacitor

surge current rating and voltage rating. In this example,

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

Sprague 594D 15 µF, 50V capacitor is adequate.

6. Boost Capacitor (C

This capacitor develops the necessary voltage to turn the switch

gate on fully. All applications should use a 0.01 µF, 50V ceramic

capacitor.

For this application, and all applications, use a 0.01 µF, 50V

ceramic capacitor.

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