MP3115 – HIGH-EFFICIENCY 1.3MHz SYNCHRONOUS STEP-UP CONVERTER

MP3115 Rev. 0.9

www.MonolithicPower.com

7

4/12/2016

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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 20kΩ for the low-side resistor (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=20kΩ.

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 recommended as they have

extremely low ESR and are available in small

footprints. Use an input capacitor of 4.7μF or

greater, and place it physically close to the

device.

Selecting the Output Capacitor

A single 4.7µF to 10µF ceramic capacitor

normally 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

its

capacitance, so the output voltage ripple is

mostly independent of ESR. The output voltage

(

)

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 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 the efficiency. For the MP3115, 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 for heavy load

transients and extreme startup conditions.

Selecting the Feed-Forward Capacitor

A feed-forward capacitor in parallel with the

high-side resistor R1 can be added to improve

the

output

ripple

at

both

discontinuous

conduction modes and the load transient

response. A 47pF capacitor is recommended

for most applications.

LAYOUT CONSIDERATIONS

High frequency switching regulators require

very careful layout for stable operation and low

noise. All components must be placed as close

to the IC as possible. All feedback components

must be kept close to the FB pin to prevent

noise injection on the FB pin trace. The ground

return of C1 and C2 should be tied close to the

GND pin. See the MP3115 demo board layout

for reference.