MP8758H–22V, HIGH-CURRENT SYNCHRONOUS BUCK CONVERTER

MP8758H Rev.1.0

www.MonolithicPower.com

18

10/13/2015

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R8 is set to 0Ω and can be set using Equation

(16) for better noise immunity. Also, it should be

five times smaller than R1//R2 to minimize its

Equation (16):

8

4SW

1

R

2C

2F

=

π×

×

(16)

Using Equation (14) on page 17 to calculate R1

can be complicated. To simplify the calculation,

a DC-blocking capacitor (CDC) can be added to

filter the DC influence from R4 and R8. 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

shown in Equation (17):

−−

=

+

OUT

REF

RAMP

12

REF

RAMP

1

(V

V

V

)

2

RR

1

VV

2

(17)

R1

R2

Ceramic

SW

FB

Vo

L

Cdc

R4

C4

Figure 12: Simplified Circuit of Ceramic

Capacitor with DC Blocking Capacitor

CDC is suggested to be at least 10 times larger

than C4 for better DC-blocking performance,

and should not be larger than 0.47µF,

considering start-up performance. If a larger

CDC is needed for better FB noise immunity,

combine it with reduced R1 and R2 to limit

CDC reasonably without affecting the system

start-up. Note that even when the CDC is

applied, the load and line regulations are still

Selecting the 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. For best performance, use a ceramic

capacitor placed as close to VIN as possible.

Ceramic

capacitors

with

X5R

and

X7R

dielectrics are recommended because of their

low ESR and small temperature coefficients.

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 with Equation (18):

OUT

OUT

CIN

OUT

IN

IN

VV

II

(1

)

VV

=×

× −

(18)

shown in Equation (19):

OUT

CIN

I

I

2

=

(19)

For simplification, choose an 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 with

Equation (20):

OUT

OUT

OUT

IN

SW

IN

IN

IN

IV

V

V(1

)

FC

V

V

Δ=

×

× −

×

(20)

shown in Equation (21):

OUT

IN

SW

IN

I

1

V

4F

C

Δ= ×

×

(21)

Selecting the Output Capacitor

The output capacitor maintains the DC output

voltage. Ceramic or POSCAP capacitors are

recommended. The output voltage ripple can be

estimated with Equation (22):

OUT

OUT

OUT

ESR

SW

IN

SW

OUT

VV

1

V(1

) (R

)

FL

V

8 F

C

Δ=

× −

×

+

××

×

(22)

For

ceramic

capacitors,

the

capacitance

dominates the impedance at the switching

frequency, and the capacitance causes the

majority of the output voltage ripple. For

simplification, the output voltage ripple can be

estimated with Equation (23):