NB679, 28 V VIN, FIXED 5V-8A BUCK CONVERTER WITH LDO

NB679 Rev. 1.03

www.MonolithicPower.com

15

7/27/2017

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APPLICATION INFORMATION

Input Capacitor

The input current to the step-down converter is

discontinuous, and therefore requires a capacitor

to supply the AC current to the step-down

converter while maintaining the DC input voltage.

Ceramic capacitors are recommended for best

performance and should be placed as close to

the VIN pin as possible. Capacitors with X5R and

X7R

ceramic

dielectrics

are

recommended

because they are fairly stable with temperature

fluctuations.

The capacitors must have a ripple-current rating

greater than the maximum input ripple current of

the converter. The input ripple current can be

estimated with Equation (3) and Equation (4):

OUT

OUT

CIN

OUT

IN

IN

VV

I

I

(1

)

VV





 

(3)

The worst-case condition occurs at VIN = 2VOUT,

where:

OUT

CIN

I

I

2



(4)

For simplification, choose the input capacitor with

an RMS current rating greater than half of the

maximum load current.

The input capacitor value determines the input

voltage ripple of the converter. If there is an input

voltage ripple requirement in the system, choose

an input capacitor that meets the specification.

The input voltage ripple can be estimated using

Equation (5) and Equation (6):

OUT

OUT

OUT

IN

SW

IN

IN

IN

I

V

V

V

(1

)

F

C

V

V







 



(5)

The worst-case condition occurs at VIN = 2VOUT,

where:

OUT

IN

SW

IN

I

1

V

4

F

C



 



(6)

Output Capacitor

An output capacitor is required to maintain the

DC

output

voltage.

Ceramic

or

POSCAP

capacitors are recommended. The output voltage

ripple can be estimated using Equation (7):

OUT

OUT

OUT

ESR

SW

IN

SW

OUT

VV

1

V

(1

) (R

)

F

L

V

8 F

C





 











(7)

When using ceramic capacitors, the impedance

at the switching frequency is dominated by the

capacitance. The output voltage ripple is caused

mainly by the capacitance. For simplification, the

output voltage ripple can be estimated using

Equation (8):

OUT

OUT

OUT

2

SW

OUT

IN

VV

V

(1

)

8 F

L C

V





 



 

(8)

When using POSCAP capacitors, the ESR

dominates

the

impedance

at

the

switching

frequency.

The

output

ripple

can

be

approximated with Equation (9):

OUT

OUT

OUT

ESR

SW

IN

VV

V

(1

) R

F

L

V





 





(9)

The maximum output capacitor limitation should

be considered in design application. For a small

soft-start time period (if the output capacitor

value is too high), the output voltage cannot

reach the design value during the soft-start time,

causing it to fail to regulate. The maximum output

capacitor

value

(Co_max)

can

be

limited

approximately using Equation (10):

O _ MAX

LIM _ AVG

OUT

ss

OUT

C

(I

I

) T / V







(10)

Where, ILIM_AVG is the average start-up current

during a soft-start period (it can be equivalent to

the current limit value), and Tss is the soft-start

time.

Inductor

The inductor is necessary to supply constant

current to the output load while being driven by

the switched input voltage. A larger value

inductor results in less ripple current, resulting in

a lower output ripple voltage. However, a larger

value inductor has a larger physical footprint, a

higher

series

resistance,

and/or

a

lower

saturation current. A good rule for determining

the inductance value is to design the peak-to-

peak ripple current in the inductor to be in the

range of 30 percent to 50 percent of the

maximum output current, with the peak inductor

current below the maximum switch current limit.