LM2598 datasheet(14/33 Pages) NSC

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LM2598 Series Buck Regulator Design Procedure (Adjustable Output)

(Continued)

PROCEDURE (Adjustable Output Voltage Version)

EXAMPLE (Adjustable Output Voltage Version)

B. To simplify the capacitor selection procedure, refer to the

quick design table shown in

Figure 3. This table contains

different output voltages, and lists various output capacitors

that will provide the best design solutions.

B. From the quick design table shown in

Figure 3, locate the

output voltage column. From that column, locate the output

voltage closest to the output voltage in your application. In

this example, select the 24V line. Under the output capacitor

section, select a capacitor from the list of through hole elec-

trolytic or surface mount tantalum types from four different

capacitor manufacturers. It is recommended that both the

manufacturers and the manufacturers series that are listed in

the table be used.

In this example, through hole aluminum electrolytic capaci-

tors from several different manufacturers are available.

82 µF

35V

Panasonic HFQ Series

82 µF

35V

Nichicon PL Series

C. The capacitor voltage rating should be at least 1.5 times

greater than the output voltage, and often much higher volt-

age ratings are needed to satisfy the low ESR requirements

needed for low output ripple voltage.

C. For a 20V output, a capacitor rating of at least 30V or

more is needed. In this example, either a 35V or 50V capaci-

tor would work. A 35V rating was chosen although a 50V

rating could also be used if a lower output ripple voltage is

needed.

Other manufacturers or other types of capacitors may also

be used, provided the capacitor specifications (especially the

100 kHz ESR) closely match the types listed in the table.

Refer to the capacitor manufacturers data sheet for this

information.

4. Feedforward Capacitor (C

FF) (See Figure 1)

For output voltages greater than approximately 10V, an ad-

ditional capacitor is required. The compensation capacitor is

typically between 50 pF and 10 nF, and is wired in parallel

with the output voltage setting resistor, R

2. It provides addi-

tional stability for high output voltages, low input-output volt-

ages, and/or very low ESR output capacitors, such as solid

tantalum capacitors.

This capacitor type can be ceramic, plastic, silver mica, etc.

(Because of the unstable characteristics of ceramic capaci-

tors made with Z5U material, they are not recommended.)

4. Feedforward Capacitor (C

FF)

The table shown in

Figure 3 contains feed forward capacitor

values for various output voltages. In this example,a1nF

capacitor is needed.

5. Catch Diode Selection (D1)

A. The catch diode current rating must be at least 1.3 times

greater than the maximum load current. Also, 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 LM2598. The most stressful condition for

this diode is an overload or shorted output condition.

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

1.25 times the maximum input voltage.

C. This diode must be fast (short reverse recovery time) and

must be located close to the LM2598 using short leads and

short printed circuit traces. Because of their fast switching

speed and low forward voltage drop, Schottky diodes provide

the best performance and efficiency, and should be the first

choice, especially in low output voltage applications.

Ultra-fast recovery, or High-Efficiency rectifiers are also a

good choice, but some types with an abrupt turn-off charac-

teristic may cause instability or EMl problems. Ultra-fast re-

covery diodes typically have reverse recovery times of 50 ns

or less. Rectifiers such as the 1N4001 series are much too

slow and should not be used.

5. Catch Diode Selection (D1)

A. Refer to the table shown in

Figure 11. Schottky diodes

provide the best performance, and in this example a 3A, 40V,

1N5822 Schottky diode would be a good choice. The 3A

diode rating is more than adequate and will not be over-

stressed even for a shorted output.

LM2598 Datasheet(PDF) 14 Page - National Semiconductor (TI)

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