MP3378E

– 4-CHANNEL WLED CONTROLLER WITH BUCK CONVERTER

MP3378E Rev. 1.02

www.MonolithicPower.com

17

5/26/2017

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

© 2017 MPS. All Rights Reserved.

APPLICATION INFORMATION

WLED CONTROLLER SECTION:

Selecting the Switching Frequency

The switching frequency of the step-up converter

is recommended to be between 300kHz to

500kHz for most applications. An oscillator

resistor on OSC sets the internal oscillator

frequency for the step-up converter, according to

Equation (1):





 

SW1

67320

F

KHz

Rosc K

Ω



(1)

set to 300kHz.

Setting the LED Current

Each LED string current can be set through the

current setting resistor on ISET using Equation

(2):

 

SET

795 1.23

ILED(mA)

RK

Ω





(2)

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

because of 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,

lower

peak

inductor

currents,

and

reduced 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 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 values using

Equation (3) and Equation (4):

2

OUT1

SW1

LOAD1

η V

D (1 D)

L1

2 f

I



  





(3)

IN1

OUT1

V

D1

V



(4)

the total

LED load current, and η is the efficiency.

Usually, the switching current is used for peak-

current-mode control. To avoid hitting the current

limit, the voltage across the sensing resistor

and Equation (6):

SENSE

SENSE

L1(PEAK)

0.8 V

R

I





(5)

OUT1

LOAD1

IN1

OUT1

IN1

L1(PEAK)

IN1

SW1

OUT1

V

I

V

(V

V )

I

ηV

2 L1 F

V









 



(6)