MP3908 – HIGH EFFICIENCY BOOST CONTROLLER

MP3908 Rev.0.9

www.MonolithicPower.com

7

8/29/2008

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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 MP3908 is capable of driving a wide variety

of N-Channel power MOSFETS. The critical

parameters of selection of a MOSFET are:

Ideally, the off-state voltage across the

MOSFET is equal to the output voltage.

Considering the voltage spike when it turns off,

output voltage.

The maximum current through the power

MOSFET happens when the input voltage is

minimum and the output power is maximum.

The maximum RMS current through the

MOSFET is given by

MAX

IN(MAX)

)

MAX

(

RMS

D

I

I

×

=

Where:

OUT

)

MIN

(

IN

OUT

MAX

V

V

V

D

−

≈

The current rating of the MOSFET should be

The on resistance of the MOSFET determines

the conduction loss, which is given by:

k

R

I

P

(on)

DS

2

RMS

cond

×

×

=

Where k is the temperature coefficient of the

as the current sensing resistor, make sure the

voltage drop across the device does not exceed

the current limit value of 190mV.

the charge between the threshold voltage and

the plateau voltage when a driver charges the

charge during the plateau voltage. These two

parameters are needed to estimate the turn on

and turn off loss.

SW

IN

DS

PLT

DR

G

GD

SW

IN

DS

TH

DR

G

GS1

SW

f

I

V

V

V

R

Q

f

I

V

V

V

R

Q

P

×

×

×

−

×

+

×

×

×

−

×

=

is the drain-source voltage. Please note that the

switching loss is the most difficult part in the

loss estimation. The formula above provides a

simple physical expression. If more accurate

estimation is required, the expressions will be

much more complex.

For extended knowledge of the power loss

estimation, readers should refer to the book

“Power MOSFET Theory and Applications”

written by Duncan A. Grant and John Gowar.

the gate drive loss. The expression is

SW

DR

G

DR

f

V

Q

P

×

×

=

For the application in page 1, a FDS6630 or

equivalent MOSFET is chosen. Read from the