MP3394S—4-STRING, MAX 200mA/STRING WHITE LED DRIVER

MP3394S Rev. 1.03

www.MonolithicPower.com

11

3/21/2013

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APPLICATION INFORMATION

Selecting the Switching Frequency

The switching frequency of the step-up converter

is recommended from 100kHz to 500kHz for

most of application. An oscillator resistor on OSC

pin sets the internal oscillator frequency for the

step-up converter according to the equation:

SW

OSC

69190

f(kHz)

R(k )

=

Ω

to 185 kHz.

Setting the LED Current

The LED string currents are identical and set

through the current setting resistor on the ISET

pin. The ISET pin can not be open.

LED

SET

800 1.22V

I(mA)

(R

0.5)k

×

=

+Ω

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

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, use a 4.7μF ceramic capacitor

in parallel with a 220µF electrolytic capacitor.

Selecting the Inductor and Current Sensing

Resistor

The MP3394S 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 (1D)

L

2

f

I

××

× −

≥

××

OUT

IN

V

V

1

D

−

=

the LED load current, and η is the efficiency.

The switching current is usually used for the peak

current mode control. In order to avoid hitting the

current limit, the voltage across the sensing

SENSE

SENSE

L(PEAK)

0.8 V

R

I

×

=

OUT

LOAD

IN

OUT

IN

L(PEAK)

IN

SW

OUT

VI

V

(V

-V )

I

ηV

2 L f

V

××

=+

××

×

DUTY CYCLE (%)

Vsense vs. Duty Cycle

0

100

200

300

400

500

0 1020 3040 50 6070 8090100

Selecting the Power MOSFET

The MP3394S is capable of driving a wide variety

of N-channel power MOSFETS. The critical

parameters of selection of a MOSFET are:

3.