MP3384L – 50V, 4-STRING, WHITE LED DRIVER

MP3384L Rev. 1.0

www.MonolithicPower.com

7

12/27/2013

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© 2013 MPS. All Rights Reserved.

OPERATION

The MP3384L is a step-up converter with

peak-current mode that regulates the LED current

in each of its 4 channels. The block diagram

(Figure 1) illustrates its operation.

Internal 5V Regulator

The MP3384L employs an internal linear

regulator (VCC) to supply the internal MOSFET

gate driver and the internal control circuitry. VCC

drops to 0V when the chip shuts down. If VIN is

lower than 5.5V, tie VCC and VIN together. The

MP3384L features under-voltage lockout to

disable the chip until VCC exceeds the UVLO

threshold. UVLO hysteresis is approximately

200mV.

System Startup

When the MP3384L is enabled, the chip checks

the topology connection first. The chip checks for

70mV disables the chip. The MP3384L then

checks for UVLO and OTP threshold crossings

before an internal soft-start drives the step-up

converter.

For bests results, apply the enable signal after

applying both the input voltage and the PWM

dimming signal.

Step-Up Converter

The MP3384L’s selectable operating frequency

(1.25MHz or 625kHz) optimizes for external

component sizes and efficiency.

When each cycle starts, the MOSFET turns ON

after the internal clock. A stabilizing ramp added

to the output of the current-sense amplifier

prevents sub-harmonic oscillations at duty cycles

greater than 50% before it enters the PWM

comparator. When this summed voltage equals

MOSFET turns OFF.

The output of the internal error amplifier is the

amplified

difference

between

the

600mV

reference voltage and the feedback voltage. The

converter automatically chooses the lowest active

LEDx pin voltage as the boost-control loop’s

feedback to regulate the output voltage to all the

LED arrays.

If the feedback voltage drops below the 600mV

reference, the output of the error amplifier

increases, increasing the current flowing through

the MOSFET to increase the output power for

closed-loop, output-voltage regulation.

At light-loads or when Vout≈VIN , the converter

runs in pulse-skipping mode to turn the MOSFET

on for the minimum ON time （approximately

100ns). Then the converter powers the output in

the remaining time. The MOSFET will remain

OFF until the output voltage requires another

boost.

Dimming Control

The MP3384L provides PWM dimming. PWM

dimming uses the PWM signal to determine the

size of the current signals from each of the 4 LED

channels. The brightness of the LED array is

directly proportional to the duty cycle of the PWM

signal.

Open-String Protection

Open-string protection relies on over-voltage

reaches the OVP threshold. Then the part records

the LEDx pin voltages less than 180mV before

forcing the remaining LED strings back into tight

regulation. The string with the highest voltage

drop is the reference string during output

regulation.

The MP3384L tries to light at least one string, and

if all strings in use are open, the MP3384L shuts

down the step-up converter. The part retains the

short information until the part shuts down.

Short-String Protection

The MP3384L monitors the LEDx pin voltages for

LED string shorts. If one or more strings are

shorted, the respective LEDx pin rises to the

boost output and tolerates high voltage stress. If

the LEDx pin voltage exceeds 5.5V, this even is

considered a short string. When a short string

fault continues for more than 1.6ms, the string is

recorded and disabled. Once a string turns OFF,