MPQ8626 – 16V, 6A, HIGH EFFICIENCY SYNC, STEP-DOWN CONVERTER W/ ADJUSTABLE CURRENT LIMIT

MPQ8626 Rev. 1.1

www.MonolithicPower.com

18

8/16/2022

MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.

© 2022 MPS. All Rights Reserved.

APPLICATION INFORMATION

Input Capacitor

The input current to the step-down converter is

discontinuous

and

therefore

requires

a

capacitor to supply AC current to the step-down

converter

while

maintaining

the

DC

input

voltage. Use ceramic capacitors for the best

performance. During layout, place the input

capacitors as close to VIN as possible.

The capacitance can vary significantly with the

temperature. Capacitors with X5R and X7R

ceramic dielectrics are recommended because

they are fairly stable over a wide temperature

range. The capacitors must have a ripple

current rating that exceeds the

converter’s

maximum input ripple current. Estimate the

input ripple current with Equation (9):

)

V

V

1

(

V

V

I

I

IN

OUT

IN

OUT

OUT

CIN

−





=

(9)

The worst-case condition occurs at VIN = 2VOUT,

shown in Equation (10):

2

I

I

OUT

CIN =

(10)

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 requirement.

Estimate the input voltage ripple with Equation

(11):

)

V

V

1

(

V

V

C

F

I

V

IN

OUT

IN

OUT

IN

SW

OUT

IN

−







=



(11)

The worst-case condition occurs at VIN = 2VOUT,

shown in Equation (12):

IN

SW

OUT

IN

C

F

I

4

1

V





=



(12)

Output Capacitor

The output capacitor maintains the DC output

voltage. Use ceramic capacitors or POSCAPs.

Estimate the output voltage ripple with Equation

(13):

)

8

1

(

)

1

(

OUT

SW

ESR

IN

OUT

SW

OUT

OUT

C

F

R

V

V

L

F

V

V





+



−





=



(13)

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

with Equation (14):

)

V

V

1

(

C

L

F

8

V

V

IN

OUT

OUT

2

SW

OUT

OUT

−









=



(14)

The ESR dominates the switching frequency

impedance for POSCAPs. For simplification,

the output ripple can be approximated with

Equation (15):

ESR

IN

OUT

SW

OUT

OUT

R

)

V

V

1

(

L

F

V

V



−





=



(15)

Inductor

The inductor supplies a 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 also has a larger physical size, a

higher series resistance, and a lower saturation

current. Generally, select an inductor value that

allows the inductor peak-to-peak ripple current

to be 30% to 40% of the maximum switch

current limit. Also design for a peak inductor

current that is below the maximum switch

current limit. Calculate the inductance value

with Equation (16):

)

V

V

1

(

I

F

V

L

IN

OUT

L

SW

OUT

−







=

(16)

Where

∆IL is the peak-to-peak inductor ripple

current.

Choose an inductor that will not saturate under

the maximum inductor peak current. The peak

inductor

current

can

be

calculated

with

Equation (17):

)

1

(

2

IN

OUT

SW

OUT

OUT

LP

V

V

L

F

V

I

I

−







+

=

(17)