MP1660

– SYNCHRONOUS, STEP-DOWN CONVERTER WITH INTERNAL MOSFETS

MP1660 Rev. 1.0

www.MonolithicPower.com

13

6/9/2020

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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 (5):

OUT

OUT

CIN

OUT

IN

IN

VV

I

I

(1

)

VV





 

(5)

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

calculated with Equation (6):

OUT

CIN

I

I

2



(6)

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 specifications.

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 = 2VOUT,

estimated 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)

With ceramic capacitors, the impedance at the

switching frequency the output voltage ripple.

For simplification, the output voltage ripple can

be estimated with Equation (10):

OUT

OUT

OUT

2

SW

OUT

IN

VV

V

(1

)

8 f

L C

V





 



 

(10)

With POSCAP capacitors, the ESR dominates

the impedance at the switching frequency. For

simplification,

the

output

ripple

can

be

calculated with Equation (11):

OUT

OUT

OUT

ESR

SW

IN

VV

V

(1

) R

f

L

V





 





(11)

A larger output capacitor can achieve a better

load transient response, but

consider the

maximum output capacitor limitation in the

design application. If the output capacitor value

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

3

lists

recommended

values

when

ceramic capacitors are applied.

Table 3: Design Example

VIN

12V

VOUT

3.3V

IOUT

3A

Figure 7 on page 15 shows the detailed

application circuit. For typical performance

values

and

waveforms,

see

the

Typical

Performance Characteristics section on page 5.

For more device applications, refer to the

related evaluation board datasheet.