MP3393—8-STRING WHITE LED DRIVER WITH STEP-UP CONTROLLER

MP3393 Rev. 1.05

www.MonolithicPower.com

11

3/26/2013

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© 2013 MPS. All Rights Reserved.

OPERATION

The MP3393 is a step-up controller with 8

internal constant current control circuit to regulate

an array of 8 strings of white LEDs. It employs

peak-current mode with programmable operation

frequency. Refer to the block diagram in Figure 1

for some of the operational details.

Internal 10V Regulator

The MP3393 has an internal linear regulator,

VCC. When VIN exceeds 10V, this regulator

provides a 10V 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. The MP3393 features

under-voltage lockout that disables the chip until

VCC exceeds the UVLO threshold. The UVLO

hysteresis is approximately 400mV.

System Startup

When the MP3393 is enabled, the chip monitors

the OVP pin to see whether the Schottky diode is

connected and whether the boost output is

shorted to GND. The chip is disabled if the OVP

voltage drops below 70mV. The MP3393 also

checks for other safety limits, including under-

voltage, over-voltage, and over-temperature

conditions. If the device functions within normal

working parameters, it boosts the output voltage

with an internal soft-start.

The start-up sequence consists of establishing

the input voltage and the PWM dimming signal

before the enable signal.

Step-up Converter

The

device’s

operating

frequency

is

programmable from 100kHz to 500kHz through

an external set resistor on the OSC pin. This

customizable frequency is helpful to optimize

external components for size and efficiency.

At the beginning of each cycle, the internal clock

turns on the external MOSFET. The addition of a

stabilizing ramp at the output of the current-sent

amplifier prevents sub-harmonic oscillations at

duty cycles greater than 50%; the output is fed

into the PWM comparator. When this voltage

reaches the output voltage level of the error

The voltage at the output of the internal error

amplifier is the amplified difference between the

320mV reference voltage and the feedback

voltage. The converter automatically chooses the

lowest active CTLx pin voltage to provide the bus

voltage to power all the LED arrays.

If the feedback voltage drops below the 300mV

reference, the output of the error amplifier

increases. This results in more current flowing

through the power FET, thus increasing the

power delivered to the output. In this way it forms

a closed loop to regulate the output voltage.

Internal Accurate Constant Current Control

The MP3393 employs an internal current-control

circuit to provide accurate constant current

regulation. The FBx pin detects the LED current

and provides the signal to the current-control

circuit. The CTLx pin controls the external PNP

transistor to accurately regulate the LED current.

Both features improve thermal performance for

high-current LED applications.

Dimming Control

The MP3393 provides both analog dimming

mode and external PWM dimming mode. For

analog dimming mode, apply a 0.4V-to-1.5V DC

signal on the PWM pin to linearly program the

LED current from 0% to 100%. For PWM

dimming, apply an external PWM signal: The

brightness of the LED array is proportional to the

duty cycle of the external PWM signal. Use a

PWM frequency range from 50Hz to 2kHz.

Open-String Protection

The MP3393 uses over-voltage protection (OVP)

to provide open string protection: If one or more

strings are open, then the voltage on the

respective CTLx pin is low and the IC keeps

raising the output voltage until it reaches the

OVP threshold. The part can then discern the

open strings and shut down it because their

CTLx pin voltages are less than 210mV. Once

the part finishes determining the open strings, the

remaining LED strings force the output voltage

back into tight regulation. The string with the

highest voltage drop is the feedback string that

regulates the output voltage.

The MP3393 always tries to light at least one

string: If all strings in use are open, the MP3393

shuts down the step-up converter. The open