MP3385A – 4-STRING WLED CONTROLLER WITH I

MP3385A Rev. 1.01

www.MonolithicPower.com

13

8/29/2017

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© 2017 MPS. All Rights Reserved.

OPERATION

The MP3385A employs a programmable,

constant-frequency, peak-current mode, step-

up converter with four regulated current

channels to drive an array of four strings of

for easy communication that sets the operation

modes flexibly.

Internal 6V Regulator

The MP3385A includes an internal linear

regulator (VCC). When VIN is greater than 6.5V,

the regulator outputs a 6V power supply to the

external MOSFET switch gate driver and the

internal control circuitry. The VCC voltage drops

to 0V when the chip shuts down. The MP3385A

features under-voltage lockout (UVLO), in

which the chip is disabled until VCC exceeds

the UVLO threshold. The UVLO hysteresis is

approximately 340mV.

System Start-Up

When enabled, the MP3385A checks the

topology connection first. The chip monitors the

over-voltage protection (OVP) pin to determine

if the Schottky diode is connected or if the boost

output is shorted to GND. An OVP voltage

higher than 2.5V allows the chip to switch

normally. Otherwise, switching is disabled. The

MP3385A

checks

additional

safety

limits

including LED open/short protection, UVLO,

over-temperature protection (OTP), and over-

current protection (OCP) after passing the OVP

test. If all the protection tests pass, the chip

begins boosting the step-up converter with an

internal soft start.

Step-Up Converter

At the beginning of each switching cycle, the

internal clock turns on the external MOSFET. In

normal operation, the minimum turn-on time is

around 150ns. A stabilizing ramp is added to

the output of the current sense amplifier to

prevent sub-harmonic oscillations for duty

cycles greater than 50%. This result is fed into

the PWM comparator. When the summed

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 chooses the lowest active LEDX

voltage automatically to provide a high-enough

bus voltage 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, 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, 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

MP3385A

provides

flexible

dimming

methods according to the dimming mode

settings below.

1) PWM Dimming Mode

MODE bits = 00b. The LED current duty cycle

directly follows the PWM input signal duty cycle

when INTERFACE = 0. The IC works in internal

PWM dimming mode, and the LED current duty

cycle is set by the internal registers 03H and

04H when INTERFACE = 1. The internal

dimming frequency is fixed at 23kHz.

2) Analog Dimming Mode

MODE bits = 01b. The LED current amplitude

follows the duty cycle of the input PWM signal

when INTERFACE = 0. The IC works in internal

analog dimming mode (the LED current

amplitude follows the internal register value of

03H and 04H) if INTERFACE = 1.

3) Mixed Dimming Mode

There are two transfer points from analog to

interface.