MP3387L ― STEP UP, 6STRINGS,WHITE LED DRIVER

MP3387L Rev. 1.11

www.MonolithicPower.com

9

3/29/2016

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

© 2016 MPS. All Rights Reserved.

OPERATION

The MP3387L employs a peak current as its

PWM mode architecture and has 6-channels

current sources to regulate the array of 6

strings white LEDs. The operation of the

MP3387L can be understood by referring to the

block diagram.

Internal 5V Regulator

The MP3387L includes an internal linear

regulator (VCC). When VIN is greater than 5.5V,

this regulator offers a 5V power supply for the

internal MOSFET switch gate-driver and the

internal control circuitry. The VCC voltage drops

to 0V when the chip shuts down. In the

application of VIN smaller than 5.5V, tie VCC

and VIN together. Also, the MP3387L features

Under Voltage Lockout and the chip is disabled

until VCC exceeds the UVLO threshold. The

hysteresis of UVLO is approximately 200mV.

System Startup

When enabled, the MP3387L checks the

topology connection first. The chip monitors the

over-voltage protection (OVP) pin to see

whether the Schottky diode is connected or

whether the boost output is shorted to GND. An

OVP voltage less than 80mV will fail to the

switching. The MP3387L also checks other

safety limits, including UVLO and over-

temperature protection (OTP). If detections to

all the protections pass, the chip then starts

boosting the step-up converter with an internal

soft-start.

It is recommended that the enable signal occurs

after the establishment of the input voltage and

PWM dimming signal during the start-up

sequence to avoid large inrush current.

Step-Up Converter

The

converter

operating

frequency

is

programmed

with

an

external

resistor

connected to the OSC pin. This helps to

optimize the size of external components and

the efficiency.

At the beginning of each switching cycle, the

internal clock turns on the external MOSFET (In

normal operation, the minimum turning-on time

is 100ns). A stabilizing ramp added to the

output of the current sense amplifier prevents

sub-harmonic

oscillations

for

duty

cycles

greater than 50 percent. This result is fed into

the PWM comparator. When this resulting

voltage reaches the output voltage of the error

off.

The output voltage of the internal error amplifier

is an amplified signal of the difference between

the reference voltage and the feedback voltage.

The converter automatically chooses the lowest

active LEDX pin voltage to provide a high-

enough bus voltage to power all the LED arrays.

If the feedback voltage drops below the

reference, the output of the error amplifier

increases. It results in more current flowing

through the MOSFET, thus increasing the

power delivered to the output. This forms a

closed loop that regulates the output voltage.

Under light-load operation, especially in the

skipping mode where the MOSFET turns on for

a minimum on-time of approximately 100ns,

and then the converter discharges the power to

the output for the remaining period. The

external MOSFET remains off until the output

voltage needs to be boosted again.

Dimming Control

The MP3387L provides two dimming methods:

direct PWM dimming mode or combined analog

and PWM dimming mode.

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

current set by resistor connected to ISET pin;

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

make the actual average LED current equal to

frequency is automatically transferred to the