MP3313 Rev1.0

www.MonolithicPower.com

24

8/2/2017

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APPLICATION INFORMATION

Selecting the Input Capacitor

The input capacitor reduces the surge current

drawn from the input supply and the switching

noise from the device. The input capacitor

impedance at the switching frequency should be

much less than the input source impedance to

prevent the high-frequency switching current

from passing through to the input. Ceramic

capacitors with X5R or X7R dielectrics are

recommended for their low ESR and small

temperature coefficients. For most applications,

a 1 ~ 4.7

μF ceramic capacitor is sufficient.

Selecting the Inductor

The converter requires an inductor to supply a

high output voltage while being driven by the

input voltage. A larger value inductor results in

less ripple current, lower peak inductor current,

and less stress on the internal N-channel

MOSFET. However, the larger inductor also has

a larger physical size, higher series resistance,

and lower saturation current.

Choose an inductor that will not saturate under

the worst-case load conditions. Select the

minimum inductor value to ensure that the boost

converter works in continuous conduction mode

(CCM) with high efficiency and good EMI

performance.

Calculate the required inductance value using

Equation (35) and Equation (36):

2

OUT

SW

LOAD

η V

D (1 D)

L

2

f

I





 





(35)

IN

OUT

V

D1

V



(36)

the LED load current

, and η is the efficiency.

The

switching

current

is

used

for

peak-current-mode control. To prevent hitting

the current limit, the worst-case inductor peak

current should be less than 80% of the current

inductor is sufficient.

Selecting the Output Capacitor

The output capacitor keeps the output voltage

ripple small and ensures feedback loop stability.

The output capacitor impedance must be low at

the switching frequency. Ceramic capacitors

with X7R dielectrics are recommended for their

low ESR characteristics. Note that the ceramic

capacitance is dependent on the voltage rating.

With a DC bias voltage, the capacitance can

lose as much as 50% of its value at its rated

voltage rating. Leave a large enough voltage

rating margin when selecting the component.

Too low a capacitance value causes loop

instability. For most applications, a 10μF

ceramic capacitor is sufficient.

Selecting the External Schottky Diode

To optimize the efficiency, a high-speed and low

reverse-recovery current Schottky diode are

recommended. Make sure the diode

’s average

and peak current ratings exceed the output

average LED current and the peak inductor

current. In addition, the diode

’s break-down

voltage rating should be large than the maximum

voltage across the diode. Usually, unexpected

high-frequency voltage spikes can be seen

across the diode when the diode turns off.

Therefore, leaving some voltage rating margin is

always needed to guarantee normal long-term

operation when selecting a diode.

PCB Layout Guidelines

Efficient

PCB

layout

is

critical for

stable

operation. Proper layout of the high-frequency

switching path is critical to prevent noise and

electromagnetic interference problems. For best

results, refer to the guidelines below.

1. Minimize the loop of MP3313

’s internal

LS-FET,

Schottky

diode,

and

output

capacitor,

since

it

is

flowing

with

high-frequency ripple current.

2. Place the input and output capacitors as

close to the IC as possible.