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LM2596

Simple switcher Power Converter 150kHz 3A Step-Down Voltage Regulator

First Components International 

LM2596 Simple switcher Power Converter 150kHz

3A Step-Down Voltage Regulator

PROCEDURE (Adjustable Output Voltage Version)

EXAMPLE (Adjustable Output Voltage Version)

ILOAD(max) = Maximum Load Current

F=Switching Frequency (Fixed at a nominal 150 kHz).

ILOAD(max) = 3A

F=Switching Frequency (Fixed at a nominal 150 kHz).

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

as shown in Figure 1)

R2, as shown in Figure 1)

Use the following formula to select the appropriate resistor Select R1 to be 1 kΩ, 1%. Solve for R2.

values.

VOUT = VREF (1 + R 2 )

R1

where VREF = 1.23 V

Select a value for R1 between 240Ω and 1.5kΩ. The lower

resistor values minimize noise pickup in the sensitive

feedback pin. (For the lowest temperature coefficient and

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

R 2 = R 1( VOUT − 1) = 1k( 20 V − 1)

VREF

1.23 V

R2=1k (16.26-1)=15.26k, closest 1% value is 15.4kΩ

R2 = 15.4 kΩ.

R 2 = R 1( VOUT − 1)

VREF

2. Inductor Selection (L1)

2. Inductor Selection (L1)

A. Calculate the inductor Volt • microsecond constant E•T A. Calculate the inductor Volt • microsecond constant

(V•Us), from the following formula:

(E•T),

E•T=(VIN− VOUT− VSAT) • VOUT+ VD • 1000 (V• µs) E•T = (28−20−1.16)• 20+0.5 •1000(V•µs)

VIN− VSAT+ VD 150kHz

28−1.16+0.5 150

where VSAT = internal switch saturation voltage = 1.16V

and VD = diode forward voltage drop = 0.5V

E •T = ( 6.84)• 20.5 •6.67(V • µs) = 34.2(V • µs)

27.34

B. Use the E•T value from the previous formula and match B. E•T=34.2 (V•Us)

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.

C. ILOAD (max) = 3A

D. Identify the inductance region intersected by the E•T D. From the inductor value selection guide shown in

value and the Maximum Load Current value. Each region is Figure 7, the inductance region intersected by the 34

identified by an inductance value and an inductor code (V•Us) horizontal line and the 3A vertical line is 47 UH,

(LXX).

and the inductor code is L39.

E. Select an appropriate inductor from the four

manufacturer’s part numbers listed in Figure 8.

E. From the table in Figure 8, locate line L39, and

select an inductor part number from the list of

manufacturers part numbers.

3. Output Capacitor Selection (COUT)

A. In the majority of applications, low ESR electrolytic or solid

tantalum capacitors between 82 UF and 820 UF provide the

best results. This capacitor should be located close to the IC

using short capacitor leads and short copper traces. Do not

use capacitors larger than 820 UF.

B. To simplify the capacitor selection procedure, refer to

the quick design table shown in Figure 3. This table

contains different output voltages, and lists various output

capacitors that will provide the best design solutions.

3. Output Capacitor SeIection (COUT)

B. From the quick design table shown in Figure 3, locate

the output voltage column. From that column, locate the

output voltage closest to the output voltage in your

application. In this example, select the 24V line. Under the

output capacitor section, select a capacitor from the list

of through hole electrolytic or surface mount tantalum

types from four different capacitor manufacturers. It is

recommended that both the manufacturers and the

manufacturers series that are listed in the table be used.

In this example, through hole aluminum electrolytic

capacitors from several different manufacturers are

available.

220 UF/35V Panasonic HFQ Series

150 UF/35V Nichicon PL Series

C. The capacitor voltage rating should be at least 1.5

times greater than the output voltage, and often much

higher voltage ratings are needed to satisfy the low ESR

requirements needed for low output ripple voltage.

C. For a 20V output, a capacitor rating of at least 30V

or more is needed. In this example, either a 35V or

50V capacitor would work. A 35V rating was chosen,

although a 50V rating could also be used if a lower

output ripple voltage is needed.

Other manufacturers or other types of capacitors may

also be used, provided the capacitor specifications

10

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