MP3378 datasheet(18/25 Pages) MPS

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MP3378

– 4-CHANNEL WLED CONTROLLER PLUS BUCK CONVERTER

MP3378 Rev. 1.01

www.MonolithicPower.com

5/26/2017

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

Selecting the Power MOSFET

The critical parameters for the selection of a

MOSFET are as follows:

1. Maximum drain-to-source voltage, VDS(MAX)

2. Maximum current, ID(MAX)

3. On-resistance, RDS(ON)

4. Gate-source charge (QGS) and gate-drain

charge (QGD)

5. Total gate charge, QG

Ideally, the off-state voltage across the MOSFET

is equal to the output voltage. Considering the

voltage spike when it turns off, VDS(MAX) should be

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 (5):

RMS(MAX)

IN1(MAX)

MAX



, where:

OUT1

IN1(MIN)

MAX

OUT1





(5)

The current rating of the MOSFET should be

greater than 1.5 x IRMS.

The on resistance of the MOSFET determines

the conduction loss, which is given by Equation

(6):

(on)

RMS

cond





(6)

Where k is the temperature coefficient of the

MOSFET.

The switching loss is related to QGD and QGS1,

which determine the commutation time. QGS1 is

the charge between the threshold voltage and

the plateau voltage when a driver charges the

gate (see the chart of VGS vs. QG of the MOSFET

datasheet). QGD is the charge during the plateau

voltage. These two parameters are needed to

estimate

turn-on

and

turn-off

losses.

See

Equation (7):

SW1

IN1

PLT

SW1

IN1

GS1

















(7)

Where VTH is the threshold voltage, VPLT is the

plateau voltage, RG is the gate resistance, and

VDS is the drain-source voltage. Please note that

calculating the switching loss is the most difficult

part in the loss estimation. Equation (7) provides

simplified

equation.

For

accurate

estimates, the equation becomes much more

complex.The total gate charge (QG) is used to

calculate the gate drive loss. See Equation (8)

SW1





(8)

Where VDR is the drive voltage.

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 22 μF

electrolytic capacitor will suffice.

Setting the Over-Voltage Protection

Open-string protection detects the voltage on

OVP. In some cases, an LED string failure

results in the feedback voltage equaling zero.

The part then keeps boosting the output voltage

higher and higher. If the output voltage reaches

the programmed OVP threshold, the protection is

triggered.

To ensure the chip functions properly, select

resistor values for the OVP resistor divider to

provide

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 (9):

HIGH

OVP

LOW

1.23

)







(9)

Selecting 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 (in the range of 100 Hz to 20 kHz)

to PWM. The minimum recommended amplitude

of the PWM signal is 1.5 V, and the low-level

amplitude should be less than 0.4 V (see Table

1).

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