MP3900 – HIGH EFFICIENCY BOOST CONTROLLER

MP3900 Rev. 1.0

www.MonolithicPower.com

8

12/17/2013

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For example, 2.2uF capacitance is sufficient to

achieve less then 1% input voltage ripple.

Meanwhile, it requires an adequate ripple

current rating. Use a capacitor with RMS

current rating greater than the inductor ripple

current (see Selecting the Inductor to determine

the inductor ripple current).

In addition, a smaller high quality ceramic

0.1μF~1µF capacitor may be placed to absorb

the high frequency noise. If using this technique,

it is recommended that the larger capacitor be a

tantalum or electrolytic type.

Selecting the Output Capacitor

Typically, a boost converter has significant

output voltage ripple because the current

through the output diode is discontinuous.

During the diode off state, all of the load current

is supplied by the output capacitor.

Low ESR capacitors are preferred to keep the

output voltage ripple to a minimum. The

characteristics of the output capacitor also

affect the stability of the regulation control

system. Ceramic, tantalum or low ESR

electrolytic capacitors are recommended. In the

case of ceramic capacitors, the impedance of

the capacitor at the switching frequency is

dominated by the capacitance, and so the

output voltage ripple is mostly independent of

the ESR. The output voltage ripple is estimated

to be:

SW

LOAD

OUT

IN

RIPPLE

f

2

C

I

V

V

-

1

V

×

×

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

≈

switching frequency and C2 is the output

capacitor.

In the case of tantalum or low-ESR electrolytic

capacitors, the ESR dominates the impedance

at the switching frequency. Therefore, the

output ripple is calculated as:

IN

OUT

ESR

LOAD

)

pk

_

pk

(

RIPPLE

V

V

R

I

V

×

×

≈

of the output capacitors.

For the application shown in page 1, use

ceramic

capacitor

as

an

example.

For

of

the

output

voltage,

the

ceramic

capacitance

could

dramatically

decrease as the voltage across the capacitor

increases. As a result, larger capacitance is

recommended. In this example, place four

4.7µF ceramic capacitors in parallel. The

voltage rating is also chosen as 50V.

In the meantime, the RMS current rating of the

output capacitor needs to be sufficient to handle

the large ripple current. The RMS current is

given by:

(

)

2

LOAD

OUT

IN

)

MAX

(

IN

LOAD

2

)

MAX

(

IN

)

RMS

(

RIPPLE

I

V

V

I

I

2

I

I

+

×

×

×

−

≈

INMAX

2

NMAX

I

)

RMS

(

RIPPLE

I

5

.

0

I

)

D

1

(

D

I

×

<

×

−

≈

For

and

the output capacitor can handle such an RMS

current.

In addition, a smaller high quality ceramic

0.1μF~1uF capacitor needs to be placed at the

output to absorb the high frequency noise

during the commutation between the power

MOSFET and the output diode. Basically, the

high frequency noise is caused by the parasitic

inductance of the trace and the parasitic

capacitors of devices. The ceramic capacitor

should be placed as close as possible to the

power MOSFET and output diode in order to

minimize the parasitic inductance and maximize

the absorption.

Selecting the Power MOSFET

The MP3900 is capable of driving a wide variety

of N-Channel power MOSFETS. The critical

parameters of selection of a MOSFET are: