MP8763 — 12A, 18V, SYNCHRONOUS STEP-DOWN CONVERTER

MP8763 Rev. 1.3

www.MonolithicPower.com

18

6/21/2016

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R1

R2

Ceramic

SW

FB

L

R4

C4

Figure 12: 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

the

input

voltage

ripple

requirement

Estimate the input voltage ripple as follows:

)

V

V

(

V

V

C

f

I

V

IN

OUT

IN

OUT

IN

SW

OUT

IN

−

×

×

×

=

∆

1

(22)

where:

IN

SW

OUT

IN

C

f

I

V

×

×

=

∆

4

1

(23)

Output Capacitor

The output capacitor maintains the DC output

voltage. Use ceramic capacitors or POSCAPs

Estimate the output voltage ripple as:

)

C

f

R

(

)

V

V

(

L

f

V

V

OUT

SW

ESR

IN

OUT

SW

OUT

OUT

×

×

+

×

−

×

×

=

∆

8

1

1

(24)

When using ceramic capacitors, the capacitance

dominates the impendence at the switching

frequency. The capacitance also dominates the

output voltage ripple. For simplification, estimate

the output voltage ripple as:

)

V

V

(

C

L

f

V

V

IN

OUT

OUT

SW

OUT

OUT

−

×

×

×

×

=

∆

1

8

2

(25)

The ESR 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 equations 5, 8, and 9.

The ESR dominates the switching-frequency

impedance for POSCAPs. The ESR ramp

voltage is high enough to stabilize the system

thus eliminating the need for an external ramp.

Select a minimum ESR value of ~12m

to

ensure stable operation. For simplification, the

output ripple can be approximated as:

ESR

IN

OUT

SW

OUT

OUT

R

)

V

V

(

L

f

V

V

×

−

×

×

=

∆

1

(26)

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,

but is physically larger, has a higher series

resistance, and often a lower saturation current.

Generally, select an inductor value that allows

the inductor peak-to-peak ripple current that is

30% to 40% of the maximum switch current limit.

Also, design for a peak inductor current that is