MP8719 – 26V, 12A, HIGH-CURRENT, SYNC BUCK CONVERTER WITH ±1A LDO

MP8719 Rev1.01

www.MonolithicPower.com

17

8/3/2017

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shown in Equation (7):

OUT

CIN

I

I

2



(7)

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 an input capacitor that meets

the specification.

The input voltage ripple can be estimated with

Equation (8):

OUT

OUT

OUT

IN

SW

IN

IN

IN

IV

V

V(1

)

FC

V

V





 



(8)

shown in Equation (9):

OUT

IN

SW

IN

I

1

V

4F

C

 



(9)

Selecting the 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 using Equation

(10):

OUT

OUT

OUT

ESR

SW

IN

SW

OUT

VV

1

V(1

) (R

)

FL

V

8 F

C



 









(10)

When using ceramic capacitors, the impedance

at the switching frequency is dominated by the

capacitance, which mainly causes the output

voltage ripple. For simplification, the output

voltage ripple can be estimated with Equation

(11):

OUT

OUT

OUT

2

SW

OUT

IN

VV

V(1

)

8F

L C

V



 

 

(11)

The output voltage ripple caused by ESR is

very small. Therefore, an external ramp is

needed to stabilize the system. The external

ramp can be generated through resistor R4 and

capacitor C4.

When using POSCAP capacitors, the ESR

dominates the impedance at the switching

frequency. The ramp voltage generated from

the ESR dominates the output ripple. The

output ripple can be approximated with

Equation (12):

OUT

OUT

OUT

ESR

SW

IN

VV

V(1

) R

FL

V



 





(12)

The maximum output capacitor limitation should

be considered in the design application. The

MP8719 has a soft-start time period of around

1.6ms. If the output capacitor value is too high,

the output voltage cannot reach the design

value during the soft-start time, causing it to fail

to regulate. The maximum output capacitor

Equation (13):

O_ MAX

LIM _ AVG

OUT

ss

OUT

C(I

I

) T / V





(13)

during the soft-start period (which can be

soft-start time.

Selecting the Inductor

An inductor is necessary for supplying constant

current to the output load while being driven by

the switched input voltage. A larger-value

inductor results in less ripple current, resulting

in lower output ripple voltage, but also has a

larger physical footprint, a higher series

resistance, and a lower saturation current. A

good rule for determining the inductance value

is to design the peak-to-peak ripple current in

the inductor to be in the range of 30% to 50% of

the maximum output current, and the peak

inductor current below the maximum switch

current limit. The inductance value can be

calculated with Equation (14):

OUT

OUT

SW

L

IN

VV

L(1

)

FI

V

 



(14)

current.

The inductor should not saturate under the

maximum inductor peak current (including a

be >13A.