LM2575

10

MOTOROLA ANALOG IC DEVICE DATA

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 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 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 8V 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 = 16 V.

Procedure (Adjustable Output Version: LM2575–Adj)

Procedure

Example

Given Parameters:

Vout = Regulated Output Voltage

Vin(max) = Maximum DC Input Voltage

ILoad(max) = Maximum Load Current

Given Parameters:

Vout = 8.0 V

Vin(max) = 12 V

ILoad(max) = 1.0 A

1. Programming Output Voltage

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

Figure 14) use the following formula:

Resistor R1 can be between 1.0 k and 5.0 k

Ω. (For best

temperature coefficient and stability with time, use 1% metal

film resistors).

Vout + Vref 1 )

R2

R1

R2

+ R1

Vout

V

ref

–1

where Vref = 1.23 V

1. Programming Output Voltage (selecting R1 and R2)

Select R1 and R2:

R2 = 9.91 k

Ω, choose a 9.88 k metal film resistor.

R2

+ R1

Vout

V

ref

* 1 + 1.8 k

8.0 V

1.23 V

* 1

Vout + 1.23 1 )

R2

R1

Select R1 = 1.8 k

Ω

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.

2. Input Capacitor Selection (Cin)

A 100

µF aluminium electrolytic capacitor located near the

input and ground pin provides sufficient bypassing.

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

LM2575 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.

3. Catch Diode Selection (D1)

A. For this example, a 3.0 A current rating is adequate.

B. Use a 20 V 1N5820 or MBR320 Schottky diode or any

suggested fast recovery diode in the Table 4.