MP3398E

– 4-STRING, 80V, STEP-UP, WLED CONTROLLER

MP3398E Rev. 1.02

www.MonolithicPower.com

13

7/31/2017

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

© 2017 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 900kHz for most applications. A

resistor on OSC sets the internal oscillator

frequency for the step-up converter according

to Equation (1):

SW

OSC

38200

F

(kHz)

R

( k

Ω )



(1)

set to 382kHz.

Setting the LED Current

The current in each LED string can be set

through the current setting resistor on ISET and

can be calculated with Equation (2):

ISET

20362

ILED(mA)

R

( k

Ω )



(2)

202mA. 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

A larger value inductor results in less ripple

current and lower peak inductor current, which

reduces stress on the N-channel MOSFET.

However, the larger value inductor has a larger

physical size, a higher series resistance, and a

lower saturation current. Choose an inductor

that will 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

Equation (3) and Equation (4):

2

OUT

SW

LOAD

η V

D (1 D)

L

2

f

I





 





(3)

IN

OUT

V

D1

V



(4)

the LED load current, an

d η is the efficiency.

The switching current is used for peak-current-

mode control. To avoid reaching the current

limit, the voltage across the sensing resistor

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

2 L F

V









 



(6)

Selecting the Power MOSFET

The MP3398E is capable of driving a wide

variety of N-channel power MOSFETS. The

critical parameters of selection for a MOSFET

maximum

current

on

resistance

Ideally,

the

off-state

voltage

across

the

MOSFET is equal to the output voltage.

Considering the voltage spike when it turns off,

output voltage.

The maximum current through the power

MOSFET occurs at the minimum input voltage

and the maximum output power. The maximum

RMS current through the MOSFET is given

using Equation (7) and Equation (8):

RMS(MAX)

IN(MAX)

MAX

I

I

D



(7)

OUT

IN(MIN)

MAX

OUT

VV

D

V





(8)

The current rating of the MOSFET should be