LM2576D2T-3.3G 데이터 시트보기 (PDF) - ON Semiconductor

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LM2576D2T-3.3G

3.0 A, 15 V, Step−Down Switching Regulator

ON Semiconductor 

LM2576

Procedure (Adjustable Output Version: LM2576−ADJ) (continued)

Procedure

Example

4. Inductor Selection (L1)

A. Use the following formula to calculate the inductor Volt x

microsecond [V x ms] constant:

ǒ Ǔ E x T +

Vin – Vout

Vout

Vin

x

106

F[Hz]

[V

x ms]

B. Match the calculated E x T value with the corresponding

number on the vertical axis of the Inductor Value Selection

Guide shown in Figure 22. This E x T constant is a

measure of the energy handling capability of an inductor and

is dependent upon the type of core, the core area, the

number of turns, and the duty cycle.

C. Next step is to identify the inductance region intersected by

the E x T value and the maximum load current value on the

horizontal axis shown in Figure 25.

D. From the inductor code, identify the inductor value. Then

select an appropriate inductor from Table 2.

The inductor chosen must be rated for a switching

frequency of 52 kHz and for a current rating of 1.15 x ILoad.

The inductor current rating can also be determined by

calculating the inductor peak current:

Ip(max)

+

ILoad(max)

)

ǒVin

–

VoutǓ

2L

ton

where ton is the “on” time of the power switch and

ton

+

Vout

Vin

x

1.0

fosc

For additional information about the inductor, see the

inductor section in the “External Components” section of

this data sheet.

4. Inductor Selection (L1)

A. Calculate E x T [V x ms] constant:

E

xT

+ (25

– 8.0) x

8.0

25

x

1000

52

+

80 [V

x ms]

B. E x T = 80 [V x ms]

C. ILoad(max) = 2.5 A

Inductance Region = H150

D. Proper inductor value = 150 mH

Choose the inductor from Table 2.

5. Output Capacitor Selection (Cout)

A. Since the LM2576 is a forward−mode switching regulator

with voltage mode control, its open loop 2−pole−1−zero

frequency characteristic has the dominant pole−pair

determined by the output capacitor and inductor values.

For stable operation, the capacitor must satisfy the

following requirement:

Cout

w 13, 300

Vin(max)

Vout x L [μH]

[μF]

5. Output Capacitor Selection (Cout)

A.

Cout

w 13, 300 x

8

25

x 150

+

332.5

μF

To achieve an acceptable ripple voltage, select

Cout = 680 mF electrolytic capacitor.

B. Capacitor values between 10 mF and 2000 mF will satisfy

the loop requirements for stable operation. To achieve an

acceptable output ripple voltage and transient response, the

output capacitor may need to be several times larger than

the above formula yields.

C. Due to the fact that the higher voltage electrolytic capacitors

generally have lower ESR (Equivalent Series Resistance)

numbers, the output capacitor’s voltage rating should be at

least 1.5 times greater than the output voltage. For a 5.0 V

regulator, a rating of at least 8.0 V is appropriate, and a 10 V

or 16 V rating is recommended.

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