LM2575 /LM2575HV Series Buck Regulator Design Procedure (Adjustable Output)

PROCEDURE (Adjustable Output Voltage Version)

EXAMPLE (Adjustable Output Voltage Version)

Given:

V

OUT = Regulated Output Voltage

V

IN(max) = Maximum Input Voltage

I

LOAD(max) = Maximum Load Current

F = Switching Frequency (Fixed at a nominal 52 kHz).

Given:

V

OUT = 20V

V

IN(max) = 28V

I

LOAD(max) = 2A

F = Switching Frequency (Fixed at a nominal 52 kHz).

1. Programming Output Voltage (Selecting R

1 and R2,as

shown in

Figure 1 )

Use the following formula to select the appropriate resistor

values.

Select a value for R

1 between 240Ω and 1.5 kΩ. The lower

resistor values minimize noise pickup in the sensitive feed-

back pin. (For the lowest temperature coefficient and the best

stability with time, use 1% metal film resistors.)

1. Programming Output Voltage (Selecting R

1 and R2,as

shown in

Figure 1 )

Select R

1 to be 1 kΩ, 1%. Solve for R2.

R

2 = 1k (16.26 − 1) = 15.26k, closest 1% value is 15.4 kΩ.

R

2 = 15.4 kΩ.

2. Inductor Selection (L1)

A. Calculate the inductor Volt

• microsecond constant E • T(V

• µs), from the following formula:

where V

SAT = internal switch saturation voltage = 1.16V

and V

D = diode forward voltage drop = 0.5V

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 horizontal axis, select the maximum load current.

D. Identify the inductance region intersected by the E

• T value

and the Maximum Load Current value. Each region is identi-

fied by an inductance value and an inductor code (LXX).

E. Select an appropriate inductor from the four manufacturer’s

part numbers listed in

Figure 8.

2. Inductor Selection (L1)

A. Calculate the inductor Volt

• microsecond constant

(E

• T),

B. E

• T = 34.2 (V • µs)

C. I

LOAD(max) = 2A

D. From the inductor value selection guide shown in Figure 7,

the inductance region intersected by the 34 (V

• µs) horizontal

L39.

E. From the table in

Figure 8, locate line L39, and select an

bers.

10

www.umw-ic.com

友台半导体有限公司

2A Step-Down Voltage Regulator

UMW

R

UMW LM2575/LM2575HV

UMW LM2575/LM2575HV