MP3385A – 4-STRING WLED CONTROLLER WITH I

MP3385A Rev. 1.01

www.MonolithicPower.com

21

8/29/2017

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

© 2017 MPS. All Rights Reserved.

APPLICATION INFORMATION

Selecting the Switching Frequency

The operation frequency of the converter

depends on both the resistor at OSC and the

FS3:0 bit in register 01H. If a 100kΩ resistor is

switching frequency is set from 100kHz to

corresponds to 100kHz to 900kHz, respectively.

Table 8 shows the list of switching frequencies.

Table 8: Switching Frequencies

FS3:0

Switching Frequency

Unit

0000b

Converter off

0001b

100

kHz

0010b

200

0011b

300

0100b

400

0101b

500

0110b

600

0111b

700

1000b

800

1001b

900

on OSC sets the internal oscillator frequency for

the step-up converter according to Equation (1):

SW

OSC

40000

F(kHz)

R(k )





(1)

Setting the LED Current

The current of each LED string is set through

the current setting resistor on ISET and the full

scale LED current setting bits ILED7:0 in Table

3. When the INTERFACE bit in 00H is 0, the

LED current is dependent on the input PWM

dimming duty cycle. Calculate the setting

formula using Equation (2):

FullScale

ISET

20196

ILED(mA)

*K

R(k )





(2)

When the INTERFACE bit in 00H is 1, calculate

the setting formula using Equation (3):

DIM

FullScale

ISET

20196

ILED(mA)

* K

*K

R(k )





(3)

full scale LED current setting bits ILED7:0 in

dimming current setting bits DIM9:0 in 03H and

04H.

LED string is set through the resistor on ISET

according to Equation (4):

ISET

4633

ILED(mA)

R(k )





(4)

100mA. 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 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 (CCM) with high

efficiency and good EMI performance.

Calculate the required inductance value using

Equation (5) and Equation (6):

2

OUT

SW

LOAD

η V

D (1D)

L

2

f

I



 





(5)

IN

OUT

V

D1

V



(6)

efficiency.