MP3383

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

MP3383 Rev. 1.0

MonolithicPower.com

16

5/2/2022

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

© 2022 MPS. All Rights Reserved.

APPLICATION INFORMATION

Selecting the Switching Frequency (fSW)

The step-up converter

’s fSW is recommended to

be between 100kHz and 900kHz for most

applications. A resistor on OSC sets the internal

fSW for the step-up converter. fSW can be

estimated using Equation (1):

=

SW

OSC

50000

f

(kHz)

R

( k

Ω )

(1)

For ROSC is 100kΩ, fSW is set to 500kHz.

Setting the LED Current

The current in each LED string can be set

through the current-setting resistor on ISET,

which can be calculated using Equation (2):

=

ISET

1200

ISET(mA)

R

( k

Ω )

(2)

For RISET is 12kΩ, ILED is set to 100mA. Do not

leave ISET floating.

Selecting the Input Capacitor

The input capacitor (CIN) reduces the surge

current drawn from the input supply and the

switching noise from the device. CIN impedance

at fSW should be below 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 reduced ripple

current and a lower peak inductor current

(IL(PEAK)), which reduces stress on the N-channel

MOSFET. However, a 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 inductance (L)

that ensures the boost converter works in

continuous conduction mode (CCM) with high

efficiency and good EMI performance.

The required inductance (L) can be calculated

using Equation (3):

2

OUT

SW

LOAD

η V

D (1 D)

L

2

f

I





 −





(3)

The duty cycle (D) can be calculated using

Equation (4):

IN

OUT

V

D1

V

=−

(4)

Where ILOAD is the LED load current, and η is the

efficiency.

The switching current is used for peak current

control mode. To avoid reaching the current limit,

the voltage across the sensing resistor (RSENSE)

must be below 80% of the current limit voltage

(VSENSE) in the worst-case scenario. RSENSE can

be calculated using Equation (5):

SENSE

SENSE

L(PEAK)

0.8 V

R

I



=

(5)

The peak inductor current (IL(PEAK)) can be

calculated using Equation (6):

OUT

LOAD

IN

OUT

IN

L(PEAK)

IN

SW

OUT

V

I

V

(V

V )

I

η

V

2 L f

V





−

=+



 



(6)

Selecting the Power MOSFET

The MP3383 is capable of driving a wide variety

of N-channel power MOSFETs. The critical

MOSFET selection parameters include the

maximum drain-to-source voltage (VDS(MAX)),

maximum

current

(ID(MAX)),

on

resistance

(RDS(ON)), gate source charge (QGS), gate drain

charge (QGD), and total gate charge (QG).

Ideally, the off-state voltage across the MOSFET

is equal to VOUT. Considering the voltage spike

when the MOSFET turns off, VDS(MAX) should be

1.5 times greater than VOUT.

The maximum current flowing through the power

MOSFET occurs at the minimum VIN and

maximum output power (POUT). The maximum

RMS current through the MOSFET (IRMS(MAX))

can be calculated using Equation (7):

RMS(MAX)

IN(MAX)

MAX

I

I

D

=

(7)