MP3384- 50V, 4-STRING WHITE LED DRIVER

MP3384 Rev. 1.01

www.MonolithicPower.com

8

1/29/2013

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© 2013 MPS. All Rights Reserved.

OPERATION

The MP3384 employs a constant frequency, peak

current mode step-up converter and 4-channels

regulated current sources to regulate the array of

4 strings white LEDs. The operation of the

MP3384 can be understood by referring to the

block diagram of Figure 1.

Internal 5V Regulator

The MP3384 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 lower than 5.5V, tie VCC and VIN together.

The MP3384 features Under Voltage Lockout.

The chip is disabled until VCC exceeds the UVLO

threshold. And the hysteresis of UVLO is

approximately 200mV.

System Startup

When the MP3384 is enabled, the chip checks

the topology connection first. the chip monitors

the OVP pin to see if the Schottky diode is not

connected. If the OVP voltage is lower than 70mV,

the chip will be disabled. The MP3384 will also

check other safety limits, including UVLO and

OTP after the OVP test is passed. If they are all in

function, it then starts boosting the step-up

converter with an internal soft-start.

It is recommended on the start up sequence that

the enable signal comes after input voltage and

PWM dimming signal established.

Step-up Converter

The converter operation frequency is selectable

(1.25MHz or 625kHz), which is helpful for

optimizing the external components sizes and

improving the efficiency.

At the beginning of each cycle, the power FET is

turned on with the internal clock. To prevent

sub-harmonic oscillations at duty cycles greater

than 50 percent, a stabilizing ramp is added to the

output of the current sense amplifier and the

result is fed into the PWM comparator. When this

result voltage reaches the output voltage of the

off.

The voltage at the output of the internal error

amplifier is an amplified signal of the difference

between the 600mV reference voltage and the

feedback voltage. The converter automatically

chooses the lowest active LEDX pin voltage for

providing enough bus voltage to power all the

LED arrays.

If the feedback voltage drops below the 600mV

reference, the output of the error amplifier

increases. It results in more current flowing

through the power FET, thus increasing the

power delivered to the output. In this way it forms

a close loop to make the output voltage in

regulation.

At light-load or Vout near to Vin operation, the

converter runs into the pulse-skipping mode, the

FET is turned on for a minimum on-time of

approximately 100ns, and then the converter

discharges the power to the output in the remnant

period. The FET will keep off until the output

voltage needs to be boosted again.

Dimming Control

The MP3384 provides PWM Dimming mode with

external PWM signal on PWM pin. The method

results in PWM chopping of the current in the

LEDs for all 4 channels to provide LED control.

The brightness of the LED array is proportional to

the duty cycle of the PWM signal.

Open String Protection

The open string protection is achieved through

the over voltage protection. If one or more strings

are open, the respective LEDX pins are pulled to

ground and the IC keeps charging the output

voltage until it reach OVP threshold. Then the

part will mark off the open strings whose LEDX

pin voltage are less than 180mV. Once the

mark-off operation completes, the remaining LED

strings will force the output voltage back into tight

regulation. The string with the highest voltage

drop is the ruling string during output regulation.

The MP3384 always tries to light at least one

string and if all strings in use are open, the

MP3384 shuts down the step-up converter. The