MP3393 datasheet(14/19 Pages) MPS

14 / 19 page

MP3393—8-STRING WHITE LED DRIVER WITH STEP-UP CONTROLLER

MP3393 Rev. 1.05

www.MonolithicPower.com

3/26/2013

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Where

• hFE-MIN is minimum value of current transfer

ratio on BJT manufacturer’s datasheet.

• ICTL-MIN is minimum threshold of maximum

sink current of CTRLx pin. It is 5.5mA on EC

table.

Selecting the Power MOSFET

The MP3393 is capable of driving a wide variety

of N-Channel power MOSFETS. The critical

parameters of selection of a MOSFET are:

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

equals the output voltage. Given the turn-off

voltage spike, VDS(MAX) should exceed 1.5×VOUT.

The maximum current through the power

MOSFET happens when the input voltage is at a

minimum and the output power is at a maximum

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 1.5×IRMS.

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 when the gate driver charges VG from

VTH to VPLT; QGD is the charge during the period

when VG stays at the plateau voltage. QGS1 can

be read in the chart of VGS vs. QG of the MOSFET

datasheet. These two parameters are needed to

estimate the turn-on and turn-off losses to

estimate the total switching loss, PSW.

PLT

GS1

−

Where

• VTH is the threshold voltage,

• VPLT is the plateau voltage,

• RG is the gate resistance,

• VDS is the drain-source voltage.

Please note that the switching loss is the most

difficult part in the loss estimation. The equation

above

only

provides

simple

estimate

expression.

The total gate charge, QG, is used to calculate

the gate drive loss, where:

Where VDR is the drive voltage

Selecting the Output Capacitor

The output capacitor limits the output voltage

ripple and ensures feedback loop stability. Select

a capacitor with low impedance at the switching

frequency. For best results, use ceramic

capacitors with X7R dielectrics for their low ESR

characteristics. For most applications, use a

4.7μF ceramic capacitor in parallel with an

electrolytic capacitor. Select the electrolytic

capacitor using the following equation:

OUT

I* D

f* P

Where

• COUT is the value of output electrolytic

capacitor,

• IOUT is total output current, and

• PV is ΔVOUT/VOUT.

Aim for a PV in the range of 0.05% to 0.1%.

Setting the Over-Voltage Protection

The MP3393 uses OVP to achieve open-string

protection. In some cases, an LED string open-

failure results in low feedback voltage. The chip

then keeps boosting the output voltage higher. If

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