MP3312 － 2.7V-5.5V INPUT, 38V OVP, DUAL-CHANNEL WHITE LED DRIVER

MP3312 Rev. 1.02

www.MonolithicPower.com

13

1/13/2015

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APPLICATION INFORMATION

Setting the LED Current

The full scale LED current can be set through the

current setting resistor on the FB pin.

=

Ω

ISET

ISET

V(V)

ILED(mA)

*1020

R(k )

is set to 20mA. Please do not leave ISET pin

open.

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. Use ceramic

capacitors with X5R or X7R dielectrics for their

low ESR and small temperature coefficients. For

most

applications,

a

1uF~4.7μF

ceramic

capacitor is ok.

Selecting the Inductor

The MP3312 requires an inductor to supply a

higher output voltage while being driven by the

input voltage. A larger value inductor results in

less ripple current, resulting in lower peak

inductor current and reducing stress on the

internal N-channel MOSFET. However, the larger

value inductor has a larger physical size, higher

series resistance, and lower saturation current.

Choose an inductor that does not saturate under

the worst-case load conditions. Select the

minimum inductor value to ensure that the boost

converter works in continuous conduction mode

with high efficiency and good EMI performance.

Calculate the required inductance value using the

equation:

2

OUT

SW

LOAD

η V

D (1 D)

L

2

f

I

××

× −

≥

××

IN

OUT

V

D1

V

=−

the total LED load current, and η is the efficiency.

The switching current is used for the peak current

mode control. In order to avoid hitting the current

limit, the worst-case inductor peak current should

Generally, a 4.7uH~10uH inductor is ok to cover

most of the applications. Note that the system

efficiency is dependent on the DC resistance of

inductor, and larger DC resistance causes larger

power loss.

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. Care must be taken that

ceramic capacitance is also dependent on the

voltage rating; DC bias voltage and the value can

loss as much as 50% of its capacitance at its

rated voltage rating. Please leave enough

voltage rating margin when select the component.

In addition, too low capacitance will cause the

loop

instability.

For

most

applications,

a

1μF~4.7μF ceramic capacitor is ok.

Selecting the External Schottky Diode

To optimize the efficiency, a high-speed and low

reverse recovery current schottky diode is

recommended. Make sure the diode’s average

and peak current rating exceeds 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 spike voltage can be seen across

the diode when the diode turns off. So, leaving

some voltage rating margin is always needed to

guarantee normal long term operation when

selecting a diode.

Layout Considerations

Careful attention must be paid to the PCB board

layout and components placement. Proper layout

of the high frequency switching path is critical to

prevent noise and electromagnetic interference

problems. The loop of MP3312’s internal low side

MOSFET, schottky diode, and output capacitor is

flowing with high frequency ripple current, it must

be minimized. So the input and output capacitor

should be placed to IC as close as possible.