MP8718 – SYNCHRONOUS STEP-DOWN CONVERTER WITH INTERNAL MOSFETS

MP8718 Rev. 0.9

www.MonolithicPower.com

14

2/9/2010

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Selecting the Output Capacitor

The output capacitor (C2) is required to

maintain the DC output voltage. Ceramic,

tantalum, or low ESR electrolytic capacitors are

recommended. Low ESR capacitors are

preferred to keep the output voltage ripple low.

The output voltage ripple can be estimated by:

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

×

×

+

×

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

×

×

=

∆

2

C

f

8

1

R

V

V

1

L

f

V

V

S

ESR

IN

OUT

S

OUT

OUT

equivalent series resistance (ESR) value of the

output capacitor.

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 by:

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

×

×

×

×

=

IN

OUT

2

S

OUT

OUT

V

V

1

2

C

L

f

8

V

∆V

In the case of tantalum or electrolytic capacitors,

the ESR dominates the impedance at the

switching frequency. For simplification, the

output ripple can be approximated to:

ESR

IN

OUT

S

OUT

OUT

R

V

V

1

L

f

V

∆V

×

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

×

×

=

The characteristics of the output capacitor also

affect the stability of the regulation system. The

MP8718 can be optimized for a wide range of

capacitance and ESR values.

PCB Layout

PCB layout is very important to achieve stable

operation. Please follow these guidelines and

take Figure 4 for references.

1) Keep the connection of input ground and

GND pin as short and wide as possible.

2) Keep the connection of input capacitor and

IN pin as short and wide as possible.

3) Ensure all feedback connections are short

and direct. Place the feedback resistors

and compensation components as close to

the chip as possible.

4) Route SW away from sensitive analog

areas such as FB.

5) Connect IN, SW, and especially GND

respectively to a large copper area to cool

the chip to improve thermal performance

and long-term reliability.

6) Adding RC snubber circuit from IN pin to

SW pin can reduce SW spikes.

Top Layer

Bottom Layer

Figure 4—PCB Layout