MP3378

– 4-CHANNEL WLED CONTROLLER PLUS BUCK CONVERTER

MP3378 Rev. 1.01

www.MonolithicPower.com

17

5/26/2017

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APPLICATION INFORMATION

WLED CONTROLLER SECTION:

Selecting the Switching Frequency

The switching frequency of the step-up converter

is recommended from 300 kHz to 500 kHz 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)

to 300 kHz.

Setting the LED Current

Each LED string current is set through the

current-setting resistor on ISET. See Equation (2):

 

SET

795 1.23

ILED(mA)

RK

Ω





(2)

120 mA. Please 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. Use ceramic

capacitors with X5R or X7R dielectrics for their

low ESR and small temperature coefficients. For

most

applications,

use

a

4.7

μF ceramic

capacitor in parallel with a 220 µF electrolytic

capacitor.

Selecting the Inductor and Current-Sensing

Resistor

A larger value inductor results in less ripple

current and lower peak inductor current, reducing

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

(CCM)

with high efficiency and good EMI

performance.

Calculate the required inductance value using

Equation (3):

2

OUT1

SW1

LOAD1

η V

D (1 D)

L1

2 f

I



  





IN1

OUT1

V

D1

V



(3)

the LED load current, and η is the efficiency.

Usually the switching current is used for peak-

current-mode control. In order to avoid hitting the

current limit, the voltage across the sensing

Equation (4):

SENSE

SENSE

L1(PEAK)

0.8 V

R

I





OUT1

LOAD1

IN1

OUT1

IN1

L1(PEAK)

IN1

SW1

OUT1

V

I

V

(V

V )

I

ηV

2 L1 F

V









 



(4)

Figure 4

—V