LM2575EP datasheet(14/26 Pages) NSC

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Inductor Value Selection Guides (For Continuous Mode Operation) (Continued)

PROCEDURE (Adjustable Output Voltage Versions)

EXAMPLE (Adjustable Output Voltage Versions)

Given: V

OUT = Regulated Output Voltage VIN(Max) =

Maximum Input Voltage I

LOAD(Max) = Maximum Load

Current F = Switching Frequency (Fixed at 52 kHz)

Given: V

OUT = 10V VIN(Max) = 25V ILOAD(Max) = 1A F =

52 kHz

1. Programming Output Voltage (Selecting R1 and R2, as

shown in Figure 2 ) Use the following formula to select the

appropriate resistor values.

1 can be between 1k and 5k. (For best temperature coef-

ficient and stability with time, use 1% metal film resistors)

1.Programming Output Voltage (Selecting R1 and R2)

R2 = 1k (8.13 − 1) = 7.13k, closest 1% value is 7.15k

2. Inductor Selection (L1) A. Calculate the inductor Volt

•

microsecond constant, E

• T(V • µs), from the following

formula:

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 hori-

zontal axis, select the maximum load current. D. Identify the

inductance region intersected by the E

• T value and the

maximum load current value, and note the inductor code for

that region. E. Identify the inductor value from the inductor

code, and select an appropriate inductor from the table shown

in Figure 9. Part numbers are listed for three inductor manu-

facturers. The inductor chosen must be rated for operation at

the LM2575EP switching frequency (52 kHz) and for a current

rating of 1.15 x I

LOAD. For additional inductor information, see

the inductor section in the application hints section of this data

sheet.

2. Inductor Selection (L1) A. Calculate E

• T(V • µs)

B. E

• T = 115 V • µs C. I

LOAD(Max) = 1A D. Inductance

Region = H470 E. Inductor Value = 470 µH Choose from AIE

part #430-0634, Pulse Engineering part #PE-53118, or

Renco part #RL-1961.

3. Output Capacitor Selection (C

OUT)A. The value of the

output capacitor together with the inductor defines the

dominate pole-pair of the switching regulator loop. For

stable operation, the capacitor must satisfy the following

requirement:

The above formula yields capacitor values between 10 µF

and 2000 µF that will satisfy the loop requirements for stable

operation. But to achieve an acceptable output ripple voltage,

(approximately 1% of the output voltage) and transient re-

sponse, the output capacitor may need to be several times

larger than the above formula yields. B. The capacitor’s volt-

age rating should be at last 1.5 times greater than the output

voltage. For a 10V regulator, a rating of at least 15V or more

is recommended. Higher voltage electrolytic capacitors gen-

erally have lower ESR numbers, and for this reason it may be

necessary to select a capacitor rate for a higher voltage than

would normally be needed.

3. Output Capacitor Selection (C

OUT)A.

However, for acceptable output ripple voltage select C

OUT

≥

220 µF C

OUT = 220 µF electrolytic capacitor

4. Catch Diode Selection (D1) A. The catch-diode current

rating must be at least 1.2 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

LM2575EP. The most stressful condition for this diode is an

overload or shorted output. See diode selection guide in

Figure 8. B. The reverse voltage rating of the diode should

be at least 1.25 times the maximum input voltage.

4. Catch Diode Selection (D1) A. For this example, a 3A

current rating is adequate. B. Use a 40V MBR340 or

31DQ04 Schottky diode, or any of the suggested

fast-recovery diodes in Figure 8.

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