MP3373, 8 STRINGS, WLED CONTROLLER WITH EXTERNAL TRANSISTOR

MP3373 Rev. 1.02

www.MonolithicPower.com

11

8/16/2016

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OPERATION

The

MP3373

employs

a

programmable

constant-frequency, peak current mode step-up

converter with 8 channels regulated current

control circuit to drive an array of up to 8 strings

of white LEDs.

Internal 10V Regulator

The MP3373 includes an internal linear

regulator (VCC). When VIN is greater than 10V,

this regulator offers 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 MP3373

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 enabled, the MP3373 checks the

topology connection first. The chip monitors the

over-voltage protection (OVP) pin to see if the

Schottky diode is not connected or if the boost

output is shorted to GND. An OVP voltage of

higher than 80mV makes the chip switching

normally, otherwise the switching is disabled. In

addition, the MP3373 also checks other safety

limits, including UVLO and over-temperature

protection (OTP), over-current protection after

passing the OVP test. If all the protection tests

pass, the chip then starts boosting the step-up

converter with an internal soft-start.

It is recommended that the enable signal occurs

after the establishment of the input voltage and

PWM dimming signal during the start-up

sequence to avoid large inrush current.

Step-Up Converter

The

converter

operating

frequency

is

programmable (from 100kHz to 1MkHz) with an

external resistor connected to the OSC pin.

This helps to optimize the size of external

components and the efficiency.

At the beginning of each switching cycle, the

internal clock turns on the external MOSFET (In

normal operation, the minimum turn on time is

200ns). A stabilizing ramp added to the output

of the current sense amplifier prevents sub-

harmonic oscillations for duty cycles greater

than 50 percent. This result is fed into the PWM

comparator.

When

the

summed

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 automatically chooses the lowest

active LEDX pin voltage to provide a high-

enough bus voltage to power all the LED arrays.

If the feedback voltage drops below the

reference, the output of the error amplifier

increases. It will result in more current flowing

through the MOSFET, thus increasing the

power delivered to the output. This forms a

closed loop that regulates the output voltage.

Under light-load operation, especially in the

pulse-skipping mode where the MOSFET turns

on for a minimum on-time of approximately

200ns, 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 MP3373 provides two dimming methods:

PWM and analog dimming mode.

For PWM dimming, apply a PWM signal to

EN/PWM pin or DIM pin. The LED current is

chopped by this PWM signal and the average

LED current is equal to Iset*D_dim where

D_dim is the duty cycle of PWM dimming signal

and Iset is the LED current amplitude.

For analog dimming, apply a DC voltage range

from 0.4V to 1.5V on this pin linearly to set LED

current from 0% to 100%.

Open String Protection

The open string protection is achieved through

the over voltage protection. If one or more

strings are open, the respective CTLx pin

voltages are low and the IC keeps charging the

output voltage until it reaches OVP threshold.

Then the part will mark off the open strings