MP3398DGS datasheet(15/19 Pages) MPS

15 / 19 page

MP3398D – 4-STRING, 350MA/STRING, STEP-UP, WHITE LED CONTROLLER

MP3398D Rev. 1.0

www.MonolithicPower.com

5/23/2016

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

Selecting the Power MOSFET

The MP3398D is capable of driving a wide

variety of N-channel power MOSFETs. The

critical parameters for selecting a MOSFET are

maximum

drain-to-source

voltage

(VDS(MAX)),

maximum current (ID(MAX)), on-resistance (RDS(ON)),

gate-source charge (QGS) and gate-drain charge

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

Ideally, the off-state voltage across the MOSFET

should equal the output voltage. Considering the

voltage spike when the MOSFET 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 (7):

RMS(MAX)

IN(MAX)

MAX

(7)

DMAX can be calculated with Equation (8):

OUT

IN(MIN)

MAX

OUT

−

≈

(8)

The current rating of the MOSFET should be

greater than 1.5 times IRMS. The on resistance of

the MOSFET determines the conduction loss,

which is given by Equation (9):

(ON)

RMS

cond

(9)

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, which can be read in the VGS vs. QG chart in

the MOSFET datasheet. QGD is the charge during

the plateau voltage. These two parameters are

needed to estimate the turn-on and turn-off

losses, and can be calculated with Equation (10):

PLT

GS1

−

(10)

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 loss estimation. The

formula above provides a simplified equation. For

more accurate estimates, the equation becomes

much more complex.

The total gate charge (QG) is used to calculate

the gate drive loss and can be calculated with

Equation (11):

(11)

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 22μF

electrolytic capacitor is sufficient.

Setting the Over-Voltage Protection (OVP)

Open-string protection is achieved through the

detection of the voltage on the OVP pin. In some

cases, an LED string failure results in the

feedback voltage always being zero. The part

then continues boosting the output voltage higher

and higher. If the output voltage reaches the

programmed OVP threshold, the protection is

triggered.

To ensure that the chip functions properly, select

the 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. The OVP voltage can be calculated

with Equation (12):

HIGH

OVP

LOW

1.23 (1

)

× +

(12)

MP3398DGS 数据表(PDF) 15 Page - Monolithic Power Systems

MP3398DGS 数据表(HTML) 15 Page - Monolithic Power Systems

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

数据表下载