MP8794

– 16V, 20A, SYNCHRONOUS STEP-DOWN CONVERTER

MP8794 Rev. 1.0

www.MonolithicPower.com

18

12/2/2019

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

© 2019 MPS. All Rights Reserved.

APPLICATION INFORMATION

Selecting the Input Capacitor

The step-down converter has a discontinuous

input current, and requires a capacitor to supply

AC current to the converter while maintaining

the DC input voltage. Use ceramic capacitors

for the best performance. During layout design,

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 and offer very low ESR.

The capacitors must have a ripple current rating

that exceeds the

converter’s maximum input

ripple current. Estimate the input ripple current

with Equation (8):

)

V

V

1

(

V

V

I

I

IN

OUT

IN

OUT

OUT

CIN









(8)

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

calculated with Equation (9):

2

I

I

OUT

CIN 

(9)

For simplification, choose an input capacitor

with an RMS current rating that exceeds half

the maximum load current. The input capacitor

value determines the converter input voltage

ripple. If there is an input voltage ripple

requirement in the system, select an input

capacitor that meets the specification.

Estimate the input voltage ripple with Equation

(10):

OUT

OUT

OUT

IN

SW

IN

IN

IN

I

V

V

V

(1

)

f

C

V

V







 



(10)

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

calculated with Equation (11):

OUT

IN

SW

IN

I

1

V

4

f

C



 



(11)

Selecting the Output Capacitor

The output capacitor maintains the DC output

voltage. Use POSCAP or ceramic capacitors.

Estimate the output voltage ripple with Equation

(12):

OUT

OUT

OUT

ESR

SW

IN

SW

OUT

VV

1

V

(1

) (R

)

f

L

V

8 F

C





 











(12)

When

using

ceramic

capacitors,

the

capacitance dominates the impedance at the

switching frequency and causes most of the

output

voltage

ripple.

For

simplification,

estimate the output voltage ripple with Equation

(13):

OUT

OUT

OUT

2

SW

OUT

IN

VV

V

(1

)

8 f

L C

V





 



 

(13)

For simplification, the output ripple can be

estimated with Equation (14):

OUT

OUT

OUT

ESR

SW

IN

VV

V

(1

) R

f

L

V





 





(14)

Selecting the 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. However, it also has a larger physical

size, a higher series resistance, and a lower

saturation current. It is usually recommended to

select an inductor value that sets the inductor

peak-to-peak ripple current between 30% and

40% of the maximum switch current limit.

Design for a peak inductor current that is below

the maximum switch current limit. Calculate the

inductance value with Equation (15):

OUT

OUT

SW

L

IN

VV

L

(1

)

f

I

V



 



(15)

Where

∆IL is the peak-to-peak inductor ripple

current.

Choose an inductor that does not saturate

under the maximum inductor peak current. The

peak inductor current can be calculated with

Equation (16):

OUT

OUT

LP

OUT

SW

IN

VV

I

I

(1

)

2 f

L

V





 



(16)