MP1542 – 700KHz/1.3MHz BOOST CONVERTER WITH A 2A SWITCH

MP1542 Rev. 1.3

www.MonolithicPower.com

7

9/7/2006

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Selecting the Output Capacitor

The output capacitor is required to maintain the

DC output voltage. Low ESR capacitors are

preferred to keep the output voltage ripple to a

minimum. The characteristic of the output

capacitor also affects 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 capacitance

of the output capacitor.

In the case of tantalum or low-ESR electrolytic

capacitors, the ESR dominates the impedance

at the switching frequency, and so the output

ripple is calculated as:

IN

OUT

ESR

LOAD

SW

LOAD

OUT

IN

RIPPLE

V

V

R

I

f

2

C

I

)

V

V

1

(

V

×

×

+

×

×

−

≈

of the output capacitors.

Choose an output capacitor to satisfy the output

ripple and load transient requirements of the

design. A 4.7µF-22µF ceramic capacitor is

suitable for most applications.

Selecting the Inductor

The inductor is required to force the higher

output voltage while being driven by the input

voltage. A larger value inductor results in less

ripple current that results in lower peak inductor

current,

reducing

stress

on

the

internal

inductor has a larger physical size, higher

series resistance, and/or lower saturation

current.

A 4.7µH inductor is recommended for most

1.3MHz applications and a 10µH inductor is

recommended for most 700KHz applications.

However, a more exact inductance value can

be calculated. 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 75% of the current limit at the

operating duty cycle to prevent loss of

regulation due to the current limit. Also make

sure that the inductor does not saturate under

the worst-case load transient and startup

conditions. Calculate the required inductance

value by the equation:

I

f

V

)

V

-

(V

V

L

SW

OUT

IN

OUT

IN

∆

×

×

=

×

η

×

×

=

IN

)

MAX

(

LOAD

OUT

)

MAX

(

IN

V

I

V

I

(

)

)

MAX

(

IN

I

%

50

%

30

I

−

=

∆

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

is efficiency.

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

recovery time, use a Schottky diode with the

MP1542. The diode should be rated for a

reverse voltage equal to or greater than the

output voltage used. The average current rating

must be greater than the maximum load current

expected, and the peak current rating must be

greater than the peak inductor current.