MP3209 – 1.4MHZ, 350mA BOOST CONVERTER

MP3209 Rev.0.941

www.MonolithicPower.com

7

11/30/2011

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APPLICATIONS INFORMATION

COMPONENT SELECTION

Setting the Output Voltage

Set the output voltage by selecting the resistive

voltage divider ratio. Use 510kΩ for the high-

side resistor R1 of the voltage divider.

Determine the low-side resistor R2 by the

equation:

FB

OUT

FB

V

V

V

R1

R2

−

×

=

(It is recommended to add feed-forward

resistor R3 and C3 to improve the transient

performance.) (Figure 2)

Selecting the Input Capacitor

An input capacitor is required to supply the AC

ripple current to the inductor, while limiting noise

at the input source. This capacitor must have low

ESR, so ceramic is the best choice.

Use an input capacitor value of 4.7μF or greater.

This capacitor must be placed physically close

to the IN pin. Since it reduces the voltage ripple

seen at IN, it also reduces the amount of EMI

passed back along that line to the other circuitry.

Selecting the Output Capacitor

A single 4.7

μF to 10μF ceramic capacitor

usually provides sufficient output capacitance

for most applications. If larger amount of

capacitance is desired for improved line support

and transient response, tantalum capacitors

can be used in parallel with the ceramic. The

impedance of the ceramic capacitor at the

switching frequency is dominated by the

capacitance, and so the output voltage ripple is

mostly independent of the ESR. The output

(

)

SW

UT

O

IN

UT

O

LOAD

RIPPLE

f

2

C

V

V

V

I

V

×

×

−

=

current, C2 is the capacitance of the output

frequency.

Selecting the Inductor

The inductor is required to force the output

voltage higher while being driven by the lower

input voltage. Choose an inductor that does not

saturate at the SW current limit. A good rule for

determining the inductance 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 75% of

the typical current limit at the duty cycle used to

prevent loss of regulation due to the current

limit variation.

Calculate the required inductance value L using

the equations:

I

f

V

)

V

-

(V

V

L

SW

OUT

IN

OUT

IN

Δ

×

×

=

η

×

×

=

IN

)

MAX

(

LOAD

OUT

)

MAX

(

IN

V

I

V

I

(

)

)

MAX

(

IN

I

%

50

%

30

I

−

=

Δ

is the peak-to-peak inductor ripple current, and η

is efficiency. For the MP3209, 10µH is

recommended for input voltages less than 3.3V

and 22µH for inputs greater than 3.3V.

Selecting the Diode

The output rectifier diode supplies current to the

inductor when the internal MOSFET is off. To

reduce losses due to diode forward voltage and

reverse recovery, use a Schottky diode. Choose a

diode whose maximum reverse voltage rating is

greater than the maximum output voltage. For

output voltage less than 20V, it is recommended

to choose the CMH5H-3 for most applications.

This diode is used for load currents less than

300mA. For ultra-low-profile applications, choose

BAT54-02 Schottky diode. Use two BAT54-02 in

parallel for high load current.