MP3389—12-STRING WHITE LED DRIVER WITH STEP-UP CONTROLLER

MP3389 Rev. 1.04

www.MonolithicPower.com

12

3/27/2013

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© 2013 MPS. All Rights Reserved.

Ideally, the off-state voltage across the MOSFET

is equal to the output voltage. Considering the

greater than 1.5 times of the output voltage.

The maximum current through the power

MOSFET happens when the input voltage is

minimum and the output power is maximum. The

maximum RMS current through the MOSFET is

given by

MAX

IN(MAX)

)

MAX

(

RMS

D

I

I

×

=

Where:

OUT

)

MIN

(

IN

OUT

MAX

V

V

V

D

−

≈

The current rating of the MOSFET should be

The on resistance of the MOSFET determines

the conduction loss, which is given by:

k

R

I

P

(on)

DS

2

RMS

cond

×

×

=

Where k is the temperature coefficient of the

MOSFET.

the charge between the threshold voltage and

the plateau voltage when a driver charges the

during

the

plateau

voltage.

These

two

parameters are needed to estimate the turn on

and turn off loss.

SW

IN

DS

PLT

DR

G

GD

SW

IN

DS

TH

DR

G

GS1

SW

f

I

V

V

V

R

Q

f

I

V

V

V

R

Q

P

×

×

×

−

×

+

×

×

×

−

×

=

the drain-source voltage. Please note that the

switching loss is the most difficult part in the loss

estimation. The formula above provides a simple

physical expression. If more accurate estimation

is required, the expressions will be much more

complex.

For extended knowledge of the power loss

estimation, readers should refer to the book

“Power MOSFET Theory and Applications”

written by Duncan A. Grant and John Gowar.

the gate drive loss. The expression is

SW

DR

G

DR

f

V

Q

P

×

×

=

Selecting the Output Capacitor

The output capacitor keeps the output voltage

ripple small and ensures feedback loop stability.

The output capacitor impedance should 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

paralleled

10uF

electrolytic capacitor will be sufficient.

Setting the Over Voltage Protection

The open string protection is achieved through

the over voltage protection (OVP). 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 make sure the chip functions properly, the

OVP setting resistor divider must be set with a

proper value. The recommended OVP point is

about 1.2 times higher than the output voltage for

normal operation.

Selecting Dimming Control Mode

The MP3389 provides 2 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 the

100Hz to 2kHz PWM dimming signal to DBRT

pin. The minimum recommended amplitude of

the PWM signal is 1.2V. The low level should

less than 0.4V. (See Figure 4).