MP3378E

– 4-CHANNEL WLED CONTROLLER WITH BUCK CONVERTER

MP3378E Rev. 1.02

www.MonolithicPower.com

18

5/26/2017

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© 2017 MPS. All Rights Reserved.

Selecting the Power MOSFET

The critical parameters for selecting a MOSFET

are listed below:

Ideally, the off-state voltage across the MOSFET

is equal to the output voltage. Considering the

greater than 1.5 times the output voltage.

The

maximum

current

through

the

power

MOSFET occurs at the minimum input voltage

and the maximum output power. The maximum

RMS current through the MOSFET is given by

Equation (7) and Equation (8):

RMS(MAX)

IN1(MAX)

MAX

I

I

D



, where:

(7)

OUT1

IN1(MIN)

MAX

OUT1

VV

D

V





(8)

The current rating of the MOSFET should be

The on resistance of the MOSFET determines

the conduction loss, which is given by Equation

(9):

k

R

I

P

(on)

DS

2

RMS

cond







(9)

Where k is the temperature coefficient of the

MOSFET.

the charge between the threshold voltage and

the plateau voltage when a driver charges the

the plateau voltage. These two parameters are

needed to estimate the turn-on and turn-off

losses and can be calculated with Equation (10):

SW1

IN1

DS

PLT

DR

GD

SW1

IN1

DS

TH

DR

G

GS1

I

V

V

V

R

Q

I

V

V

V

R

Q

f

f

P

G

SW

























(10)

calculating the switching loss is the most difficult

part in the loss estimation. The formula above is

a simplified equation. For a more accurate

estimation, the equation becomes much more

complex.

the gate drive loss. See Equation (11):

DR

G

DR

SW1

P

Q

V

f







(11)

Selecting the Output Capacitor

The output capacitor keeps the output voltage

ripple small and ensures feedback loop stability.

The output capacitor impedance must be low at

the switching frequency. Ceramic capacitors with

X7R dielectrics are recommended for their low

ESR characteristics. For most applications, a

4.7

μF ceramic capacitor in parallel with a 22μF

electrolytic capacitor is sufficient.

Setting the Over-Voltage Protection

The open-string protection is achieved through

the detection of the voltage on OVP. In some

cases, an LED string failure results in a feedback

voltage of zero. The part then boosts the output

voltage higher and higher. If the output voltage

reaches the programmed OVP threshold, the

protection will be triggered.

To ensure the chip functions properly, select the

resistor values for the OVP resistor divider to

provide

an

appropriate

set

voltage.

The

recommended OVP point is about 1.1 to 1.2

times higher than the output voltage for normal

operation. See Equation (12):

HIGH

OVP

LOW

R

V

1.23

(1

)

R







(12)

Selecting the Dimming Control Mode

Two different dimming methods are provided:

1. Direct PWM Dimming

An external PWM dimming signal is employed to

achieve PWM dimming control. Apply a PWM

dimming signal (ranging from 100Hz to 20kHz) to

PWM. The minimum recommended amplitude of

the PWM signal is 1.5V, and the low level should

be less than 0.4V (see Table 1).