LM2574 データシートの表示（PDF） - Motorola => Freescale

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LM2574

EASY SWITCHER™ 0.5 A STEP–DOWN VOLTAGE REGULATOR

Motorola => Freescale 

LM2574

Procedure (Fixed Output Voltage Version) (continued) In order to simplify the switching regulator design, a step–by–step

design procedure and example is provided.

Procedure

Example

5. Output Capacitor Selection (Cout)

A. Since the LM2574 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 and an acceptable ripple voltage,

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

100 µF and 470 µF is recommended.

B. 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 at least 8.0 V is appropriate, and a 10 V

or 16 V rating is recommended.

5. Output Capacitor Selection (Cout)

A. Cout = 100 µF to 470 µF standard aluminium electrolytic.

B. Capacitor voltage rating = 20 V.

Procedure (Adjustable Output Version: LM2574–ADJ)

Procedure

Given Parameters:

Vout = Regulated Output Voltage

Vin(max) = Maximum DC Input Voltage

ILoad(max) = Maximum Load Current

1. Programming Output Voltage

To select the right programming resistor R1 and R2 value (see

+ ǒ ) Ǔ Figure 2) use the following formula:

Vout

Vref 1.0

R2

R1

where Vref = 1.23 V

Resistor R1 can be between 1.0 kΩ and 5.0 kΩ. (For best

+ ǒ * Ǔ temperature coefficient and stability with time, use 1% metal

film resistors).

R2

R1

Vout

Vref

1.0

Example

Given Parameters:

Vout = 24 V

Vin(max) = 40 V

ILoad(max) = 0.4 A

1. Programming Output Voltage (selecting R1 and R2)

Select R1 and R2 :

ǒ ) Ǔ Vout = 1.23 1.0

R2

R1

Select R1 = 1.0 kΩ

+ ǒ * Ǔ + ǒ * Ǔ R2

R1

Vout

Vref

1.0

1.0

k

10 V

1.23 V

1.0

R2 = 18.51 kΩ, choose a 18.7 kΩ metal film resistor.

2. Input Capacitor Selection (Cin)

To prevent large voltage transients from appearing at the input

and for stable operation of the converter, an aluminium or

tantalum electrolytic bypass capacitor is needed between the

input pin +Vin and ground pin Gnd. This capacitor should be

located close to the IC using short leads. This capacitor should

have a low ESR (Equivalent Series Resistance) value.

For additional information see input capacitor section in the

“EXTERNAL COMPONENTS” section of this data sheet.

3. Catch Diode Selection (D1)

A. Since the diode maximum peak current exceeds the

regulator maximum load current the catch diode current

rating must be at least 1.2 times greater than the maximum

load current. For a robust design, the diode should have a

current rating equal to the maximum current limit of the

LM2574 to be able to withstand a continuous output short.

B. The reverse voltage rating of the diode should be at least

1.25 times the maximum input voltage.

2. Input Capacitor Selection (Cin)

A 22 µF aluminium electrolytic capacitor located near the input

and ground pin provides sufficient bypassing.

3. Catch Diode Selection (D1)

A. For this example, a 1.0 A current rating is adequate.

B. Use a 50 V MBR150 Schottky diode or any suggested fast

recovery diodes in Table 1.

MOTOROLA ANALOG IC DEVICE DATA

11

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