MP8792 – 16V, 12A, SYNCHRONOUS STEP-DOWN CONVERTER

MP8792 Rev. 1.0

www.MonolithicPower.com

17

12/2/2019

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APPLICATION INFORMATION

Selecting the Input Capacitor

The step-down converter has a discontinuous

input current, and requires a capacitor to supply

the

AC

current

to

the

converter

while

maintaining the DC input voltage. Use ceramic

capacitors

for

optimal

performance.

While

designing the PCB layout, place the input

capacitors as close to IN as possible.

The capacitance can vary significantly with

temperature. Use capacitors with X5R and X7R

ceramic dielectrics because they are fairly

stable over a wide temperature range, and they

offer low ESR.

The capacitors must have a ripple current rating

that exceeds the converter’s maximum input

ripple current. Estimate the input ripple current

with Equation (8):

)

1

(

IN

OUT

IN

OUT

OUT

CIN

V

V

V

V

I

I









(8)

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

calculated with Equation (9):

2

I

I

OUT

CIN 

(9)

For simplification, choose an input capacitor

with an RMS current rating that exceeds half

the maximum load current. The input capacitor

value determines the converter input voltage

ripple. If there is an input voltage ripple

requirement in the system, select an input

capacitor that meets the specification.

Estimate the input voltage ripple with Equation

(10):

)

1

(

IN

OUT

IN

OUT

IN

SW

OUT

IN

V

V

V

V

C

f

I

V













(10)

The worst-case condition occurs when VIN =

2VOUT, calculated with Equation (11):

IN

SW

OUT

IN

C

f

I

V









4

1

(11)

Selecting the Output Capacitor

The output capacitor maintains the DC output

voltage. Use POSCAP or ceramic capacitors.

Estimate the output voltage ripple with Equation

(12):

)

8

1

(

)

1

(

OUT

SW

ESR

IN

OUT

SW

OUT

OUT

C

f

R

V

V

L

f

V

V



















(12)

When

using

ceramic

capacitors,

the

capacitance dominates the impedance at the

switching frequency and causes most of the

output

voltage

ripple.

For

simplification,

estimate the output voltage ripple with Equation

(13):

)

1

(

8

2

IN

OUT

OUT

SW

OUT

OUT

V

V

C

L

f

V

V















(13)

The ESR dominates the switching frequency

impedance for the POSCAP capacitors. For

simplification,

the

output

ripple

can

be

calculated with Equation (14):

ESR

IN

OUT

SW

OUT

OUT

R

V

V

L

f

V

V













)

1

(

(14)

Selecting the Inductor

The inductor supplies constant current to the

output load while being driven by the switching

input voltage. A larger-value inductor results in

less ripple current and lower output ripple

voltage. However, it has a larger physical size,

a

higher

series

resistance,

and

a

lower

saturation current. It is usually recommended to

select an inductor value that allows the inductor

peak-to-peak ripple current to be 30% to 40% of

the maximum switch current limit. Design for a

peak

inductor

current

that

is

below

the

maximum switch current limit.

Calculate the

inductance value using Equation (15):

)

1

(

IN

OUT

L

SW

OUT

V

V

I

f

V

L











(15)

Where

∆IL is the peak-to-peak inductor ripple

current.

Choose an inductor that will not saturate under

the maximum inductor peak current. The peak

inductor

current

can

be

calculated

with

Equation (16):