MP3398D – 4-STRING, 350MA/STRING, STEP-UP, WHITE LED CONTROLLER

MP3398D Rev. 1.0

www.MonolithicPower.com

14

5/23/2016

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© 2016 MPS. All Rights Reserved.

APPLICATION INFORMATION

Selecting the Switching Frequency

The switching frequency of the step-up converter

is recommended to be between 100kHz and

500kHz for most applications. An oscillator

resistor on OSC sets the internal oscillator

frequency for the step-up converter according to

Equation (1):

SW

OSC

67320

F

(kHz)

R

(k )

=

W

(1)

kΩ, the switching frequency is

set to 180kHz.

Setting the LED Current

Each LED string current can be set through the

current setting resistor on ISET and can be

calculated with Equation (2):

LED

ISET

810 1.24(V)

I

(mA)

R

(k

Ω)

×

=

(2)

kΩ, the LED current is set to

120mA. Do not leave ISET 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. Ceramic capacitors

with X5R or X7R dielectrics are recommended

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 MP3398D 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 N-

channel MOSFET. However, the larger value

inductor also has a larger physical size, higher

series resistance, and lower saturation current.

Choose an inductor that will not saturate under

worst-case load conditions. Select a minimum

inductor value that ensures that the boost

converter works in continuous conduction mode

with high efficiency and good EMI performance.

Calculate the required inductance value using

Equation (3) and Equation (4):

2

OUT

SW

LOAD

ηV

D (1 D)

L

2

f

I

×

×

×−

≥

××

(3)

IN

OUT

V

D1

V

= −

(4)

the LED load current, and η is the efficiency.

The switching current is used for peak-current-

mode control, typically. To avoid hitting the

current limit, the voltage across the sensing

and Equation (6):

SENSE

L(PEAK)

0.8 VSENSE

R

I

×

=

(5)

OUT

LOAD

IN

OUT

IN

L(PEAK)

IN

SW

OUT

V

I

V

(V

V )

I

ηV

2L F

V

×

×−

=

+

× ×

×

(6)