MP3394SGS datasheet(12/17 Pages) MPS

12 / 17 page

MP3394S—4-STRING, MAX 200mA/STRING WHITE LED DRIVER

MP3394S Rev. 1.03

www.MonolithicPower.com

3/21/2013

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

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 of the output voltage.

The maximum current through the power

MOSFET occurs at the maximum input voltage

and the maximum output power. The maximum

RMS current through the MOSFET is given by

RMS(MAX)

IN(MAX)

MAX

=×

, where:

OUT

IN(MIN)

MAX

OUT

−

≈

The current rating of the MOSFET should be

greater than 1.5xIRMS

The ON resistance of the MOSFET determines

the conduction loss, which is given by:

(on)

RMS

cond

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 chart of VGS vs. QG

of 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.

GS1

PLT

=×

× ×

−

×× ×

−

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. 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. The expression is

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 will suffice.

Setting the Over Voltage Protection

The 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 zero. The part then

keeps boosting 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

appropriate

set

voltage.

The

recommended OVP point is about 1.1 to 1.2

times higher than the output voltage for normal

operation.

HIGH

OVP

LOW

V1.23 (1

)

=× +

Selecting Dimming Control Mode

The MP3394S provides two different dimming

methods

1. Direct PWM Dimming

An external PWM dimming signal is employed to

achieve PWM dimming control. Connect a 100kΩ

resistor from BOSC pin to GND and apply a

PWM dimming signal—in the range of 100Hz to

20kHz—to

the

DBRT

pin.

The

minimum

recommended amplitude of the PWM signal is

1.2V. The low level should be less than 0.4V

(See Figure 4).

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