LM2599 datasheet(13/31 Pages) NSC

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LM2599 Series Buck Regulator Design Procedure (Adjustable Output)

(Continued)

PROCEDURE (Adjustable Output Voltage Version)

EXAMPLE (Adjustable Output Voltage Version)

2. Inductor Selection (L1)

A. Calculate the inductor Volt

• microsecond constant E • T

• µs), from the following formula:

where V

SAT = internal switch saturation voltage = 1.16V and

D = diode forward voltage drop = 0.5V

B. Use the E

• T value from the previous formula and match

it with the E

• T number on the vertical axis of the Inductor

Value Selection Guide shown in

Figure 7.

C. on the horizontal axis, select the maximum load current.

D. Identify the inductance region intersected by the E

• T

value and the Maximum Load Current value. Each region is

identified by an inductance value and an inductor code

(LXX).

E. Select an appropriate inductor from the four manufactur-

er’s part numbers listed in

Figure 8.

2. Inductor Selection (L1)

A. Calculate the inductor Volt

• microsecond constant (E •

T),

B. E

• T = 34.2 (V • µs)

C. I

LOAD(max) = 3A

D. From the inductor value selection guide shown in

Figure 7,

the inductance region intersected by the 34 (V

• µs) horizon-

tal line and the 3A vertical line is 47 µH, and the inductor

code is L39.

E. From the table in

Figure 8, locate line L39, and select an

inductor part number from the list of manufacturers part num-

bers.

3. Output Capacitor Selection (C

OUT)

A. In the majority of applications, low ESR electrolytic or solid

tantalum capacitors between 82 µF and 820 µF provide the

best results. This capacitor should be located close to the IC

using short capacitor leads and short copper traces. Do not

use capacitors larger than 820 µF. For additional informa-

tion, see section on output capacitors in application in-

formation section.

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

quick design table shown in

Figure 3. This table contains dif-

ferent output voltages, and lists various output capacitors

that will provide the best design solutions.

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.

3. Output Capacitor SeIection (C

OUT)

A. See section on C

OUT in Application Information section.

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.

220/35 Panasonic HFQ Series

150/35 Nichicon PL Series

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 50V rating was chosen because it has a

lower ESR which provides a lower output ripple voltage.

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. Re-

fer to the capacitor manufacturers data sheet for this informa-

tion.

4. Feedforward Capacitor (C

FF) (See Figure 1)

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

tional capacitor is required. The compensation capacitor is

typically between 100 pF and 33 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, a 560 pF

capacitor is needed.

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