MP8758–18V, HIGH CURRENT SYNCHRONOUS BUCK CONVERTER

MP8758 Rev. 1.0

www.MonolithicPower.com

17

1/13/2015

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Usually, R9 is set to 0Ω, and it can also be set

following equation (16) for a better noise

immunity. It should also set to be 5 times smaller

than R1//R2 to minimize its influence on Vramp.

9

4SW

1

R

2C

2F

=

π×

×

(16)

Using equation (14) to calculate the R1 can be

complicated. To simplify the calculation, a DC-

blocking capacitor Cdc can be added to filter the

DC influence from R4 and R9. Figure 12 shows

a

simplified

circuit

with

external

ramp

compensation and a DC-blocking capacitor. With

this capacitor, R1 can easily be obtained by

using the simplified equation for PWM mode

operation:

−−

=

+

OUT

REF

RAMP

12

REF

RAMP

1

(V

V

V

)

2

RR

1

VV

2

(17)

Cdc is suggested to be at least 10 times larger

than C4 for better DC blocking performance, and

should also not larger than 0.47uF considering

start up performance. In case one wants to use

larger Cdc for a better FB noise immunity,

combined with reduced R1 and R2 to limit the

Cdc in a reasonable value without affecting the

system start up. Be noted that even when the

Cdc is applied, the load and line regulation are

still Vramp related.

R1

R2

Ceramic

SW

FB

Vo

L

Cdc

R4

C4

Figure12—Simplified Circuit of Ceramic Capacitor

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.

Ceramic capacitors are recommended for best

performance and should be placed as close to

X7R ceramic dielectrics are recommended

because they are fairly stable with temperature

fluctuations.

The capacitors must also have a ripple current

rating greater than the maximum input ripple

current of the converter. The input ripple current

can be estimated as follows:

OUT

OUT

CIN

OUT

IN

IN

VV

II

(1

)

VV

=×

× −

(18)

where:

OUT

CIN

I

I

2

=

(19)

For simplification, choose the input capacitor with

an RMS current rating greater than half of the

maximum load current.

The input capacitance value determines the input

voltage ripple of the converter. If there is an input

voltage ripple requirement in the system, choose

the input capacitor that meets the specification.

The input voltage ripple can be estimated as

follows:

OUT

OUT

OUT

IN

SW

IN

IN

IN

IV

V

V(1

)

FC

V

V

Δ=

×

× −

×

(20)

OUT

IN

SW

IN

I

1

V

4F

C

Δ= ×

×

(21)

Output Capacitor

The output capacitor is required to maintain the

DC output voltage. Ceramic or POSCAP

capacitors are recommended. The output voltage

ripple can be estimated as:

OUT

OUT

OUT

ESR

SW

IN

SW

OUT

VV

1

V(1

) (R

)

FL

V

8 F

C

Δ=

× −

×

+

××

×

(22)

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 as: