MP1543 — 1.5A, 500KHZ SYNCHRONOUS RECTIFIED STEP-UP CONVERTER

MP1543 Rev. 1.0

www.MonolithicPower.com

6

1/11/2006

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© 2006 MPS. All Rights Reserved.

TM

APPLICATION INFORMATION

COMPONENT SELECTION

Setting the Output Voltage

Set the output voltage by selecting the resistive

voltage divider ratio. The voltage divider drops

the output voltage to the 1.2V feedback voltage.

Use a 100kΩ resistor for R2 of the voltage

divider. Determine the high-side resistor R1 by

the equation:

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

=

2

R

V

V

V

1

R

FB

FB

OUT

1.2V feedback voltage and R2=100kΩ.

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.

Multi-layer

ceramic

capacitors are the best choice as they have

extremely low ESR and are available in small

footprints. Use an input capacitor value of 4.7µF

or greater. This capacitor must be placed

physically close to the device.

Selecting the Output Capacitor

A single 4.7µF to 10µF ceramic capacitor

usually provides sufficient output capacitance

for most applications. Larger values up to 22µF

may be used to obtain extremely low output

voltage ripple and improve transient response.

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. 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 the minimum

current limit at the duty cycle used (to prevent

loss of regulation due to the current limit

variations).

Calculate the required inductance value L using

the equations:

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. For the MP1543, typically, 4.7µH is

recommended for most applications. Choose an

inductor that does not saturate at the peak

switch current as calculated above with

additional margin to cover heavy load transients

and extreme startup conditions.