MP3394EY datasheet(11/17 Pages) MPS

11 / 17 page

MP3394—4-STRING, MAX 200mA/STRING WHITE LED DRIVER

MP3394 Rev. 1.07

www.MonolithicPower.com

3/21/2013

MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.

APPLICATION INFORMATION

Selecting the Switching Frequency

The switching frequency of the step-up converter

is recommended from 150kHz 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:

fSW(kHz) = 62100 / ROSC（kΩ）

For ROSC=330kΩ, the switching frequency is set

to 188 kHz.

Setting the LED Current

The LED string currents are identical and set

through the current setting resistor on the ISET

pin.

ILED(mA) = 790 x 1.23V / (RSET+0.4)(kΩ)

For RSET=7.68kΩ, the LED current is set to

120mA. The ISET pin can not be 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

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 220uF 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

larger

physical

size,

higher

series

resistance,and lower saturation current.

Choose an inductor that does not saturate under

the worst-case load conditions. The minimum

value of inductor is selected to ensure that the

boost converter works in continuous conduction

mode for high efficiency and good EMI

performance.

Calculate the required inductance value by the

equation:

OUT

LOAD

η V

D (1 D)

××

× −

≥

××

OUT

−

Where VIN and VOUT is the input and output

voltage, fSW is the switching frequency, ILOAD is

the LED load 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

resistor RSENSE should be less than 80% of the

worst case current limit voltage, VSENSE.

SENSE

L(PEAK)

0.8 V

OUT

LOAD

OUT

L(PEAK)

OUT

-V )

ηV

2 L f

××

Where IL(PEAK) is the peak value of the inductor

current. VSENSE is shown in Figure 3.

DUTY CYCLE (%)

Vsense vs. Duty Cycle

100

200

300

400

500

0 1020 3040 50 6070 8090100

Figure 3—VSENSE vs Duty Cycle

Selecting the Power MOSFET

The MP3394 is capable of driving a wide variety

of N-Channel power MOSFETS. The critical

parameters of selection of a MOSFET are:

1. Maximum drain to source voltage, VDS(MAX)

2. Maximum current, ID(MAX)

3. On-resistance, RDS(ON)

4. Gate source charge QGS and gate drain

charge QGD

5. Total gate charge, QG

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