MP3391—8-CHANNEL, 120mA/CH STEP-UP WLED DRIVER CONTROLLER

MP3391 Rev. 1.11

www.MonolithicPower.com

12

3/14/2013

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APPLICATION INFORMATION

Selecting the Switching Frequency

The switching frequency of the step-up converter

is programmable from 150kHz to 500kHz. An

oscillator resistor on OSC pin sets the internal

oscillator frequency for the step-up converter

according to the equation:

283 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.

The Number of LED Strings Selection

The MP3391 can drive 8 strings, 6 strings or 4

strings of LEDs. Set the NUMSEL high level for

driving 6 strings of LEDs (LED1~LED6). Set the

NUMSEL low level for driving 4 strings of LEDs

(LED1~LED4). Float the NUMSEL pin for driving

8 strings of LEDs.

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

high frequency switching current from passing

through the input. Ceramic capacitors with X5R

or X7R dielectrics are highly recommended

because of their low ESR and small temperature

coefficients. For most applications, a 4.7μF

ceramic capacitor paralleled a 220μF electrolytic

capacitor is sufficient.

Selecting the Inductor and Current Sensing

Resistor

The inductor is required to force the 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. A good rule for

determining the inductance is to allow the peak-

to-peak ripple current to be approximately 30% to

40% of the maximum input current. Calculate the

required inductance value by the equation:

IN

OUT

IN

OUT

SW

V(V

V )

L

Vf

ΔI

×−

=

××

OUT

LOAD(MAX)

IN(MAX)

IN

VI

I

V

η

×

=

×

IN(MAX)

I

40%)

~

(30%

I

×

=

Δ

load current, ∆I is the peak-to-peak inductor

ripple current and η is the efficiency.

The switch 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

×

=

Current Limit(Vsense) vs.

Duty Cycle

0

50

100

150

200

250

300

350

400

450

500

0 1020304050 60708090 100

DUTY CYCLE(%)

Selecting the Power MOSFET

The MP3391 is capable of driving a wide variety

of N-Channel power MOSFETS. The critical

parameters of selection of a MOSFET are:

2.