IN

OUT

L

S

V

V

2 i

L

DT

-

D

=

t

L

i

'

OUT

I

S

T

S

DT

L

VOUT

L

- VOUT

VIN

OUT

D

IN

D

SW

V

V

D

V

V

V

+

=

+

-

OUT

IN

V

D

V

=

LM2831

www.ti.com

SNVS422D – AUGUST 2006 – REVISED SEPTEMBER 2015

Typical Applications (continued)

8.2.1.2 Detailed Design Procedure

Table 1. Bill of Materials

PART ID

PART VALUE

MANUFACTURER

PART NUMBER

U1

1.5-A Buck Regulator

TI

LM2831X

C1, Input Cap

22 µF, 6.3 V, X5R

TDK

C3216X5ROJ226M

C2, Output Cap

2x22 µF, 6.3 V, X5R

TDK

C3216X5ROJ226M

D1, Catch Diode

0.3 Vf Schottky 1.5 A, 30 VR

TOSHIBA

CRS08

L1

3.3 µH, 2.2 A

TDK

VLCF5020T-3R3N2R0-1

R2

15.0 k

Ω, 1%

Vishay

CRCW08051502F

R1

15.0 k

Ω, 1%

Vishay

CRCW08051502F

R3

100 k

Ω, 1%

Vishay

CRCW08051003F

8.2.1.2.1

Inductor Selection

The duty cycle (D) can be approximated quickly using the ratio of output voltage (VO) to input voltage (VIN):

(1)

The catch diode (D1) forward voltage drop and the voltage drop across the internal PMOS must be included to

calculate a more accurate duty cycle. Calculate D by using the following formula:

(2)

VSW can be approximated by:

VSW = IOUT × RDSON

(3)

The diode forward drop (VD) can range from 0.3 V to 0.7 V depending on the quality of the diode. The lower the

VD, the higher the operating efficiency of the converter. The inductor value determines the output ripple current.

Lower inductor values decrease the size of the inductor, but increase the output ripple current. An increase in the

inductor value will decrease the output ripple current.

One must ensure that the minimum current limit (1.8 A) is not exceeded, so the peak current in the inductor must

be calculated. The peak current (ILPK) in the inductor is calculated by:

ILPK = IOUT + ΔiL

(4)

Figure 20. Inductor Current

(5)

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