MP8763D ― 12A, 18V, SYNCHRONOUS STEP-DOWN CONVERTER

MP8763D Rev. 1.1

www.MonolithicPower.com

20

1/15/2015

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2

R

V

2

1

V

V

2

1

V

V

1

R

RAMP

REF

RAMP

REF

OUT

×

×

+

×

−

−

=

(19)

C4 for better DC blocking performance, but

smaller

than

0.47µF

account for

start-up

noise immunity, reduce R1 and R2 to limit effects

on system start-up. Note that even with Cdc, the

R1

R2

Ceramic

SW

FB

L

R4

C4

Figure12—Simplified Ceramic Capacitor

Circuit with DC Blocking Capacitor

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. Use ceramic capacitors for best

performance. During layout, Place the input

capacitors as close to the IN pin 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.

The capacitors must also have a ripple current

rating that exceeds the converter’s maximum

input ripple current. Estimate the input ripple

current as follows:

)

V

V

1

(

V

V

I

I

IN

OUT

IN

OUT

OUT

CIN

−

×

×

=

(20)

where:

2

I

I

OUT

(21)

For simplification, choose an input capacitor with

an RMS current rating that exceeds half the

maximum load current.

The input capacitance value determines the

converter input voltage ripple. Select a capacitor

value that meets any input voltage ripple

requirements.

Estimate the input voltage ripple as follows:

)

V

V

1

(

V

V

C

F

I

V

IN

OUT

IN

OUT

IN

SW

OUT

IN

−

×

×

×

=

Δ

(22)

where:

IN

SW

OUT

IN

C

F

I

4

1

V

×

×

=

Δ

(23)

Output Capacitor

The output capacitor maintains the DC output

voltage. Use ceramic capacitors or POSCAPs.

Estimate the output voltage ripple as:

)

C

F

8

1

R

(

)

V

V

1

(

L

F

V

V

OUT

SW

ESR

IN

OUT

SW

OUT

OUT

×

×

+

×

−

×

×

=

Δ

(24)

When using ceramic capacitors, the capacitance

dominates the impedance at the switching

frequency. The capacitance also dominates the

output voltage ripple. For simplification, estimate

the output voltage ripple as:

)

V

V

1

(

C

L

F

8

V

V

IN

OUT

OUT

2

SW

OUT

OUT

−

×

×

×

×

=

Δ

(25)

The ESR only contributes minimally to the output

voltage ripple, thus requiring an external ramp to

stabilize the system. Design the external ramp

with R4 and C4 as per equation 5, 8 and 9.