LM2672 datasheet(12/25 Pages) NSC

12 / 25 page

LM2672 Series Buck Regulator Design Procedure (Fixed Output) (Continued)

PROCEDURE (Fixed Output Voltage Version)

EXAMPLE (Fixed Output Voltage Version)

3. 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 the output voltage

divided by the input voltage). The largest value of the catch

diode average current 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 LM2672. The most stressful condition for this diode is a

shorted output condition.

3. Catch Diode Selection (D1)

A. Refer to the table shown in Figure 12. In this example, a 1A,

20V Schottky diode will provide the best performance. If the

circuit must withstand a continuous shorted output, a higher

current 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. This Schottky diode must be located close to the

LM2672 using short leads and short printed circuit traces.

4. 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.

4. 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 12V, an aluminum electrolytic capacitor with a

voltage rating greater than 15V (1.25 x V

IN) would be needed.

The next higher capacitor voltage rating is 16V.

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 16V line and note which capacitor values

have RMS current ratings greater than 500 mA.

For a through hole design, a 330 µF/16V 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, 25V capacitor is adequate.

5. 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.

5. Boost Capacitor (C

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

ceramic capacitor.

6. Soft-Start Capacitor (C

SS - optional)

This capacitor controls the rate at which the device starts up.

The formula for the soft-start capacitor C

SS is:

6. Soft-Start Capacitor (C

SS - optional)

For this application, selecting a start-up time of 10 ms and using

the formula for C

SS results in a value of:

LM2672 датащи(PDF) 12 Page - National Semiconductor (TI)

LM2672 датащи(HTML) 12 Page - National Semiconductor (TI)

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

датащи скачать

ссылки URL