MP3908 – HIGH EFFICIENCY BOOST CONTROLLER

MP3908 Rev.0.9

www.MonolithicPower.com

6

8/29/2008

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MOSFET is used as the sensing resistor, be

calculated above, 30mΩ

Another factor to take into consideration is the

increases.. Device vendors will usually provide

temperature

coefficient

in

the

datasheet.

Generally, the MOSFET on resistance will

double from 25

°C to 125°C.

Selecting the Input Capacitor

An input capacitor (C1) is required to supply the

AC ripple current to the inductor, while limiting

noise at the input source. A low ESR capacitor

is required to keep the noise to the IC at a

minimum. Ceramic capacitors are preferred, but

tantalum or low-ESR electrolytic capacitors may

also suffice.

The capacitance can be calculated as:

SW

IN(RIPPLE)

1

f

V

8

I

C

×

∆

×

∆

≈

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

ripple. When using ceramic capacitors, take into

account the vendor specified voltage and

temperature coefficients for the particular

dielectric being used.

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