MP9459 – SYNCHRONOUS, STEP-DOWN CONVERTER W/ INTERNAL MOSFETS

MP9459 Rev. 1.0

www.MonolithicPower.com

13

1/13/2020

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The input voltage ripple can be estimated with

Equation (7):

OUT

OUT

OUT

IN

SW

IN

IN

IN

I

V

V

V

(1

)

f

C

V

V







 



(7)

The worst-case condition occurs at VIN = 2 x

VOUT, calculated with Equation (8):

OUT

IN

SW

IN

I

1

V

4

f

C



 



(8)

Selecting the Output Capacitor

An output capacitor is required to maintain the

DC

output

voltage.

Ceramic

or

POSCAP

capacitors

are

recommended.

The

output

voltage ripple can be estimated with Equation

(9):

OUT

OUT

OUT

ESR

SW

IN

SW

OUT

VV

1

V

(1

) (R

)

f

L

V

8 f

C





 











(9)

In

the

case

of

ceramic

capacitors,

the

capacitance dominates the impedance at the

switching frequency and causes most of the

output voltage ripple. For simplification, the

output voltage ripple can be calculated with

Equation (10):

OUT

OUT

OUT

2

SW

OUT

IN

VV

V

(1

)

8 f

L C

V





 



 

(10)

The output voltage ripple caused by the ESR is

very small.

In the case of POSCAP capacitors, the ESR

dominates the impedance at the switching

frequency. For simplification, the output ripple

can be estimated with Equation (11):

OUT

OUT

OUT

ESR

SW

IN

VV

V

(1

) R

f

L

V





 





(11)

A larger output capacitor can improve load

transient response, but the maximum output

capacitor limitation should also be considered in

the design application. If the output capacitance

is too high, the output voltage cannot reach the

design value during the soft-start time, and the

device fails to regulate.

The maximum output capacitor value (CO_MAX)

can be estimated with Equation (12):

O _ MAX

LIM _ AVG

OUT

ss

OUT

C

(I

I

) t / V







(12)

Where ILIM_AVG is the average start-up current

during the soft-start period, and tss is the soft-

start time.

Design Example

Table 2 shows a design example with ceramic

capacitors.

Table 2: Design Example

VIN

12V

VOUT

3.3V

IOUT

4A

For detailed application schematics, see the

Typical Applications Circuits section on page 15.

For typical performance and waveforms, see

the Typical Performance Characteristics section

on page 5. For more device applications, refer

to the related evaluation board datasheet.