MP3387L ― STEP UP, 6STRINGS,WHITE LED DRIVER

MP3387L Rev. 1.11

www.MonolithicPower.com

12

3/29/2016

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Selecting Dimming Control Mode

The MP3387L provides two different dimming

methods.

1. Direct PWM Dimming

Pull MIX pin to low to make MP3387L work in

direct PWM dimming mode. Apply a PWM

dimming signal to PWMI pin, the LED current is

following to the direct PWM dimming signal duty

cycle and frequency. The average output current

frequency depends on the LED current dimming

ratio. Lower input PWM dimming frequency

results in better LED dimming ratio. Usually, a

200Hz to 2kHz PWM signal is ok to cover most

applications.

2. Combined Analog and PWM Dimming

If MIX pin is pulled to high (In default, this pin is

pulled to high.), MP3387L works in combined

analog and PWM dimming mode with 25% as its

transfer point. A PWM signal is applied to the

PWMI pin. When the duty cycle of PWM dimming

signal is >= 25%, the IC dims the LED current

with analog mode and the LED current amplitude

duty cycle of PWM dimming signal.) Analog

dimming helps to reduce the LED power loss and

avoid the audible noise issue. When the duty

cycle of PWM dimming signal is <25%, the IC

dims the LED current in PWM mode, the LED

The actual LED current dimming frequency is

automatically

transferred

to

the

range

of

24kHz~27.5kHz from 100Hz~20kHz of the input

PWM dimming signal frequency. By doing this,

MP3387L can not only improve the low LED

current dimming ratio but also avoid the

troublesome audible noise issue.

Layout Considerations

Careful attention must be paid to the PCB board

layout and components placement. Proper layout

of the high frequency switching path is critical to

prevent noise and electromagnetic interference

problems. The loop of LX to PGND pin (U1),

output diode (D1), and output capacitor (C2) is

flowing with high frequency pulse current. It must

be as short as possible (See figure below).

LX

PGND

GND

D1

C2

R1

Iset

Rset

U1

R2

Layout Consideration

The IC exposed pad is internally connected to

GND pin, and all logic signals are referred to the

GND. The PGND should be externally connected

to GND and is recommended to keep away from

the logic signals.