MP3399-12-STRING, CASCADING WHITE LED DRIVER WITH STEP-UP CONTROLLER

MP3399 Rev. 1.0

www.MonolithicPower.com

13

3/27/2013

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© 2013 MPS. All Rights Reserved.

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 MP3399 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 (See Figure 3).

Figure 3—Direct PWM Dimming

2. DC Input PWM Dimming

To apply DC input PWM dimming, apply an

analog signal (range from 0 V to 1.2V) to the

DBRT pin to modulate the LED current directly. If

the PWM is applied with a zero DC voltage, the

PWM duty cycle will be 0%. If the DBRT pin is

applied with a DC voltage>1.2V, the output will

be 100% (See Figure 4). The capacitor on BOSC

pin set the frequency of internal triangle

waveform according to the equation,:

The dimming frequency is recommended set

from 100Hz to 2kHz.