MP3387A ― 6-STRINGS, MAX 80MA/STRING, STEP-UP, WHITE LED DRIVER

MP3387A Rev. 1.0

www.MonolithicPower.com

10

5/17/2017

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

© 2017 MPS. All Rights Reserved.

OPERATION

The MP3387A employs a peak current as its PWM

mode architecture and has 6-channel current

sources to regulate the array of 6-string white LEDs.

Internal 5V Regulator

The MP3387A 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. If VIN is

smaller than 5.5V, tie VCC and VIN together.

The MP3387A features under-voltage lockout

(UVLO), which disables the chip until VCC

exceeds the UVLO threshold. The hysteresis of

UVLO is 100mV, approximately.

System Start-Up

When enabled, the MP3387A checks the

topology connection first. The chip monitors the

over-voltage protection (OVP) pin to determine

whether the Schottky diode is connected or

whether the boost output is shorted to GND. An

OVP voltage less than 80mV will fail to switch.

The MP3387A also checks other safety limits,

including

UVLO

and

over-temperature

protection (OTP). If detections of all protections

pass, the chip then begins boosting the step-up

converter with an internal soft start.

It is recommended that the enable signal occur

after the establishment of the input voltage and

PWM dimming signal during the start-up

sequence to prevent a large inrush current.

Step-Up Converter

The

converter

operating

frequency

is

programmed

with

an

external

resistor

connected to OSC. This helps optimize

efficiency and the size of the external

components.

At the beginning of each switching cycle, the

internal clock turns on the low side MOSFET. In

normal operation, the minimum turn-on time is

100ns. A stabilizing ramp added to the output of

the

current

sense

amplifier

prevents

subharmonic oscillations for duty cycles greater

than 50%. This result is fed into the PWM

comparator.

When

this

resulting

voltage

reaches the output voltage of the error amplifier

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 chooses the lowest active LEDX

pin voltage automatically to provide a bus

voltage high enough to power all of the LED

arrays.

If the feedback voltage drops below the

reference, the output of the error amplifier

increases. This results in more current flowing

through the MOSFET, which increases 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. In this mode, 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 low side MOSFET

remains off until the output voltage needs to be

boosted again.

Dimming Control

The MP3387A provides two dimming methods:

direct PWM dimming mode or combined analog

and PWM dimming mode.

If MIX is pulled high, the MP3387A works in

combined analog and PWM dimming mode with

25% as its transfer point. By default, MIX is

pulled high. A PWM signal is applied to PWMI.

When the duty cycle of the PWM dimming

signal is ≥25%, the IC dims the LED current

with analog mode and the LED current

LED current set by the resistor connected to

dimming signal). Analog dimming helps reduce

LED power loss and prevent audible noise

issues.

When the duty cycle of the PWM dimming

signal is <25%, the IC dims the LED current in

PWM mode. The LED current amplitude is fixed

current

dimming

frequency

is

switched

automatically from 100Hz ~ 20kHz to 24kHz ~

27.5kHz of the input PWM dimming signal

frequency. By doing this, the MP3387A