MP3908 – HIGH EFFICIENCY BOOST CONTROLLER

MP3908 Rev.0.9

www.MonolithicPower.com

5

8/29/2008

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© 2008 MPS. All Rights Reserved.

APPLICATION INFORMATION

COMPONENT SELECTION

Setting the Output Voltage

Set the output voltage by selecting the resistive

voltage divider ratio. If we use 10kΩ for the low-

side resistor (R2) of the voltage divider, we can

determine the high-side resistor (R1) by the

equation:

REF

REF

OUT

V

)

V

V

(

2

R

1

R

−

×

=

R1=301kΩ.

ground programs an internal time delay circuit

that sets a delay from the turn off of the

synchronous MOSFET drive pin output to the

turn on of the main MOSFET drive pin output.

This delay is adjustable to program the required

time interval required by the circuitry to turn off

the synchronous MOSFET from the time that

the Synchronous output drive signal goes low.

This path may include active devices as well as

an isolating transformer used to electrically

isolate the secondary side output. Efficiency

losses can be quite significant if an overlap

occurs between the main and synchronous

switching MOSFETs if the delay is not carefully

has an internal resistance of approximately

50kOhms that should be added to any external

resistance used to approximate the delay

interval. Shorting the pin directly to ground will

result in approximately 30nSec delay as a result

of this internal resistance. A graph is provided

in the applications section of this data sheet to

illustrate the typical delay time generated

versus the resistor value selected

Selecting the Inductor and Current Sensing

Resistor

The inductor is required to transfer the energy

between the input source and the output

capacitors. A larger value inductor results in

less ripple current that results in lower peak

inductor current, and therefore reduces the

stress on the power MOSFET. However, the

larger value inductor has a larger physical size,

higher

series

resistance,

and/or

lower

saturation current.

A good rule of thumb is to allow the

peak-to-peak ripple current to be approximately

30-50% of the maximum input current. Make

sure that the peak inductor current is below

80% of the IC’s maximum current limit at the

operating duty cycle to prevent loss of

regulation. Make sure that the inductor does not

saturate under the worst-case load transient

and startup conditions. The required inductance

value can be calculated by :

I

f

V

)

V

-

(V

V

L

SW

OUT

IN(MIN)

OUT

)

MIN

(

IN

∆

×

×

×

=

η

×

×

=

)

MIN

(

IN

)

MAX

(

LOAD

OUT

)

MAX

(

IN

V

I

V

I

(

)

)

MAX

(

IN

I

%

50

%

30

I

−

=

∆

∆I is the peak-to-peak inductor ripple current

and η is the efficiency. For a typical design,

boost converter efficiency can reach 85%~95%.

ripple percentage being 30%,

η=95% and

8.8µH inductor (i.e. Sumida CDRH127/LDNP-

100MC).

The switch current is usually used for the peak

current mode control. In order to avoid hitting

the current limit, the voltage across the sensing

worst case current limit voltage, 200mV.

)

PEAK

(

L

SENSE

I

2

.

0

8

.

0

R

×

=

current.