MP8774

– 18V, 12A, SYNCHRONOUS, STEP-DOWN CONVERTER

MP8774 Rev. 1.0

www.MonolithicPower.com

19

1/15/2018

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© 2018 MPS. All Rights Reserved.

The input voltage ripple can be estimated with

Equation (8):

OUT

OUT

OUT

IN

SW

IN

IN

IN

I

V

V

V

(1

)

F

C

V

V







 



(8)

shown in Equation (9):

OUT

IN

SW

IN

I

1

V

4 F

C



 



(9)

Selecting the 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 with Equation

(10):

OUT

OUT

OUT

ESR

SW

IN

SW

OUT

VV

1

V

(1

) (R

)

F

L

V

8 F

C





 











(10)

In

the

case

of

ceramic

capacitors,

the

impedance

at

the

switching

frequency

is

dominated by the capacitance. The output

voltage

ripple

is

mainly

caused

by

the

capacitance.

For

simplification,

the

output

voltage ripple can be estimated with Equation

(11):

OUT

OUT

OUT

2

SW

OUT

IN

VV

V

(1

)

8 F

L C

V





 



 

(11)

In the case of POSCAP capacitors, the ESR

dominates the impedance at the switching

frequency. For simplification, the output ripple

can be approximated with Equation (12):

OUT

OUT

OUT

ESR

SW

IN

VV

V

(1

) R

F

L

V





 





(12)

Besides considering the output ripple, choosing

a larger output capacitor can also result in a

better load transient response. Be sure to

consider

the

maximum

output

capacitor

limitation in the design application. If the output

capacitor value is too high, the output voltage

cannot reach the design value during the soft-

start time and fails to regulate.

can be limited approximately with Equation (13):

O _ MAX

LIM _ AVG

OUT

ss

OUT

C

(I

I

) T / V







(13)

start time.

PCB Layout Guidelines

Efficient PCB layout of the switching power

supplies is critical for stable operation. A poor

layout design can result in poor line or load

regulation

and

stability

issues.

For

better

performance, it is recommended to use a four-

layer board (two middle layers are GND). For

best results, refer to Figure 8 and follow the

guidelines below.

1. Place the high-current paths (GND, VIN,

and SW) very close to the device with short,

direct, and wide traces.

2. Place the input capacitor as close to VIN

and GND as possible.

3. Place a VCC decoupling capacitor close to

the device.

4. Connect AGND and PGND at the point of

the VCC capacitor

’s ground connection.

5. Place the external feedback resistors next

to FB.

6. Keep the switching node (SW) short and

away from the feedback network.