MP8771

– 17V, 10A, SYNCHRONOUS, STEP-DOWN CONVERTER

MP8771 Rev. 1.0

www.MonolithicPower.com

18

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)

The output voltage ripple caused by the ESR is

very small. Therefore, an external ramp is

needed to stabilize the system. The external

and a capacitor (Cr).

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, 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 by 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 (the 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.