LM1575, LM2575-N, LM2575HV

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SNVS106E – MAY 1999 – REVISED APRIL 2013

LM2575 Series Buck Regulator Design Procedure

PROCEDURE (Fixed Output Voltage Versions)

EXAMPLE (Fixed Output Voltage Versions)

Given:

Given:

1. Inductor Selection (L1)

1. Inductor Selection (L1)

A. Select the correct Inductor value selection guide from Figure 27, A. Use the selection guide shown in Figure 28.

Figure 28, Figure 29 and Figure 30 (Output voltages of 3.3V, 5V, B. From the selection guide, the inductance area intersected by the

12V or 15V respectively). For other output voltages, see the design 20V line and 0.8A line is L330.

procedure of Figure 26 .

C. Inductor value required is 330

μH. From the table in Table 2,

B. From the inductor value selection guide, identify the inductance choose AIE 415-0926, Pulse Engineering PE-52627, or RL1952.

inductor code for that region.

C. Identify the inductor value from the inductor code, and select an

appropriate inductor from the table shown in Table 2. Part numbers

are listed for three inductor manufacturers. The inductor chosen

must be rated for operation at the LM2575 switching frequency (52

information, see INDUCTOR SELECTION.

defines the dominate pole-pair of the switching regulator loop. For B. Capacitor voltage rating = 20V.

stable

operation

and

an

acceptable

output

ripple

voltage,

(approximately 1% of the output voltage) a value between 100

μF

and 470

μF is recommended.

B. The capacitor's voltage rating should be at least 1.5 times greater

than the output voltage. For a 5V regulator, a rating of at least 8V is

appropriate, and a 10V or 15V rating is recommended.

Higher voltage electrolytic capacitors generally have lower ESR

numbers, and for this reason it may be necessary to select a

capacitor rated for a higher voltage than would normally be needed.

3. Catch Diode Selection (D1)

3. Catch Diode Selection (D1)

A. The catch-diode current rating must be at least 1.2 times greater A. For this example, a 1A current rating is adequate.

than the maximum load current. Also, if the power supply design B. Use a 30V 1N5818 or SR103 Schottky diode, or any of the

must withstand a continuous output short, the diode should have a suggested fast-recovery diodes shown in Table 1.

current rating equal to the maximum current limit of the LM2575. 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.

An aluminum or tantalum electrolytic bypass capacitor located close A 47

μF, 25V aluminum electrolytic capacitor located near the input

to the regulator is needed for stable operation.

and ground pins provides sufficient bypassing.

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Product Folder Links: LM1575 LM2575-N LM2575HV