LM2575 datasheet(11/28 Pages) MOTOROLA

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LM2575

MOTOROLA ANALOG IC DEVICE DATA

Procedure (Adjustable Output Version: LM2575–Adj) (continued)

Procedure

Example

4. Inductor Selection (L1)

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

microsecond [V x

µs] constant:

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 21. 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 21.

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

select an appropriate inductor from the Table 1 or Table 2.

The inductor chosen must be rated for a switching

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

The inductor current rating can also be determined by

calculating the inductor peak current

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

For additional information about the inductor, see the

inductor section in the “External Components” section of

this data sheet.

ExT

+ V

–Vout

Vout

Von

106

F[Hz]

[V x

ms]

p(max)

+ I

Load(max) )

–Vout ton

ton +

Vout

fosc

4. Inductor Selection (L1)

A. Calculate E x T [V x

µs] constant:

B. E x T = 51 [V x

µs]

C. ILoad(max) = 1.0 A

Inductance Region = L220

D. Proper inductor value = 220

µH

Choose the inductor from the Table 1 or Table 2.

ExT

+ (12 – 8.0) x 8.0

1000

+ 51 [V x ms]

5. Output Capacitor Selection (Cout)

A. Since the LM2575 is a forward–mode switching regulator

with voltage mode control, its open loop 2–pole–2–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:

B. Capacitor values between 10

µF and 2000 µF 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 8V is appropriate, and a 10 V

or 16 V rating is recommended.

out w

7.785

in(max)

Vout xL [µH]

[

µF]

5. Output Capacitor Selection (Cout)

To achieve an acceptable ripple voltage, select

Cout = 100 µF electrolytic capacitor.

out w

7.785

8.220

+ 53 µF

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