MP3397—4-STRING, MAX 350mA/STRING WHITE LED DRIVER

MP3397 Rev. 1.01

www.MonolithicPower.com

11

9/4/2012

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© 2012 MPS. All Rights Reserved.

APPLICATION INFORMATION

Selecting the Switching Frequency

Set the switching frequency of the step-up

converter from 150kHz to 500kHz for most

applications. An oscillator resistor on OSC pin

sets the internal oscillator frequency for the step-

up converter according to the equation:

SW

OSC

62100

f(kHz)

R(k )





to 188kHz.

Setting the LED Current

The LED string currents are identical and set

through the current setting resistor on the ISET

pin.

LED

SET

790 1.23V

I(mA)

(R

+0.4)k







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

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 MP3397 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, larger-

value inductors have 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



 





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 MP3397 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