STEP UP, 4 STRINGS, MAX. 350MA/STRING ANALOG AND PWM DIMMING, WHITE LED CONTROLLER

MP3398A Rev. 1.04

www.MonolithicPower.com

14

5/26/2017

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APPLICATION INFORMATION

Selecting the Switching Frequency

The switching frequency of the step-up converter

is recommended from 100kHz to 500kHz for

most of application. An oscillator resistor on OSC

pin sets the internal oscillator frequency for the

step-up

converter

according

to

the

below

equation:

SW

OSC

67320

F

(kHz)

R

(k )





to 180kHz.

Setting the LED Current

The LED each string current can be set through

the current setting resistor on the ISET pin.

LED

810 1.22V

I

(mA)

Rk

Ω

ISET





For RSET=8.2

kΩ, the LED current is set to

120mA. Please do not leave ISET pin 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 4.7μF ceramic capacitor

in parallel with a 220µF electrolytic capacitor.

Selecting the Inductor and Current Sensing

Resistor

The MP3398A 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 has a larger physical size, higher series

resistance, and 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 value using the

equation:

2

OUT

SW

LOAD

η V

D (1 D)

L

2

f

I





 





IN

OUT

V

D1

V



the LED load current, and η is the efficiency.

The switching current is usually used for the peak

current mode control. In order to avoid hitting the

current limit, the voltage across the sensing

SENSE

L(PEAK)

0.8 VSENSE

R

I





OUT

LOAD

IN

OUT

IN

L(PEAK)

IN

SW

OUT

V

I

V

(V

V )

I

ηV

2 L F

V









 



Figure 2

—V