MP3385A datasheet(22/25 Pages) MPS

22 / 25 page

MP3385A – 4-STRING WLED CONTROLLER WITH I

2C INTERFACE

MP3385A Rev. 1.01

www.MonolithicPower.com

8/29/2017

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

Usually, the switching current is used for peak-

current-mode control. To avoid reaching the

current limit, the voltage across the sensing

resistor (RSENSE) must be less than 70% of the

current-limit voltage (VSENSE), in the worst cases.

Calculate RSENSE and IL(PEAK) with Equation (7)

and Equation (8):

SENSE

L(PEAK)

0.7 V





(7)

OUT

LOAD

OUT

L(PEAK)

OUT

V )

ηV2 L F











(8)

The current limit setting voltage (VSENSE) is set

by the OCP2:0 bits in register 05H.

Selecting the Power MOSFET

The MP3385A is capable of driving a wide

variety of N-channel power MOSFETS. The

critical parameters of selection for 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 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 (9) and Equation (10):

RMS(MAX)

IN(MAX)

MAX



(9)

OUT

IN(MIN)

MAX

OUT





(10)

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 can be calculated

with Equation (11):

(on)

RMS

cond





(11)

Where k is the temperature coefficient of the

MOSFET.

The switching loss is related to QGD and QGS,

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 turn-on and

turn-off losses and can be calculated with

Equation (12):

PLT

GS1

















(12)

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 of 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, shown in Equation (13):





(13)

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 4.7μF 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

MP3385A continues boosting the output voltage

higher and higher. If the output voltage reaches

the programmed OVP threshold, the protection

is triggered.

MP3385A 数据表(PDF) 22 Page - Monolithic Power Systems

MP3385A 数据表(HTML) 22 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 20 21 22 23 24 25

数据表下载