MP3378

– 4-CHANNEL WLED CONTROLLER PLUS BUCK CONVERTER

MP3378 Rev. 1.01

www.MonolithicPower.com

13

5/26/2017

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© 2017 MPS. All Rights Reserved.

OPERATION

WLED CONTROLLER SECTION:

The WLED controller employs a programmable

constant frequency, peak-current

–mode, step-

up converter with 4 channel regulated current

sources to drive an array of up to 4 strings of

white LEDs.

Internal 6 V Regulator

When VIN1 is greater than 6.5 V, VCC1 outputs

a 6 V power supply to the external MOSFET

switch gate driver and the internal control

circuitry. The VCC1 voltage drops to 0 V when

the WLED controller shuts down.

System Start-Up

When enabled, the WLED controller checks the

topology connection first. The WLED controller

monitors the over-voltage protection (OVP) pin

to see if the Schottky diode is connected or if

the boost output is shorted to GND. An OVP

voltage of less than 57 mV disables the WLED

controller . Once all the protection tests pass,

the WLED controller 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 by an external resistor on OSC.

300 kHz to 500 kHz is recommended as an

operating frequency. This optimizes efficiency

and the size of external components.

At the beginning of each switching cycle, the

internal clock turns on the external MOSFET (In

normal operation, the minimum turn-on time is

200 ns.) 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 this voltage reaches the

external MOSFET turns 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 automatically the lowest

active LEDX voltage to provide a bus voltage

high enough to power all 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, 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

200 ns, 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 MP3378 allows two dimming methods:

PWM and analog dimming mode.

For PWM dimming, apply a PWM signal to

PWM. The LED current is chopped by this

PWM signal, and the average LED current is

current amplitude.

For analog dimming, either a PWM signal or DC

signal can be applied to ADIM.

When a PWM signal is applied to ADIM, the

signal is filtered by the internal RC filter. The

amplitude. A PWM signal of 20 kHz or higher is

recommended

to

achieve

better

filtering

performance. When a DC signal is applied to

ADIM, the voltage range (0.41 V to 1.49 V) sets

directly the LED current linearly from minimum

to full scale.

Open-String Protection

Open-string protection is achieved through the

OVP pin and LEDX pins (1 to 4). If one or more

strings are open, the respective LEDX pins are

pulled to ground, and the WLED controller

keeps charging the output voltage until it

reaches

the

over-voltage

protection