MP3378E

– 4-CHANNEL WLED CONTROLLER WITH BUCK CONVERTER

MP3378E Rev. 1.02

www.MonolithicPower.com

13

5/26/2017

MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.

© 2017 MPS. All Rights Reserved.

OPERATION

WLED CONTROLLER SECTION:

The WLED controller employs a programmable,

constant frequency, peak current mode, step-up

converter with four channels that regulate

current sources to drive an array of up to four

strings of white LEDs.

Internal 6V Regulator

When VIN1 is greater than 6.5V, VCC1 outputs

a 6V power supply to the external MOSFET

switch gate driver and the internal control

circuitry. The VCC1 voltage drops to 0V 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) to

see if the schottky diode is connected, or if the

boost output is shorted to GND. An OVP

voltage of less than 75mV will disable the

WLED controller. Once all of the protection

tests pass, the WLED controller then boosts 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

the PWM dimming signal during the start-up

sequence to avoid a large inrush current. The

PWM pin should be used for dimming function

and can not be used for turning on/off the

WLED controller.

If the PWM is used for turning on/off WLED

recommended to parallel with COMP pin as

shown in figure 2. The recommended value for

frequency.

MP3378E

COMP

Figure 2 COMP Connection for turning on/off

WLED Controller with PWM signal

Step-Up Converter

The

converter

operating

frequency

is

programmable through an external resistor on

OSC. The operating frequency is recommended

between

300kHz

to

500kHz.

This

helps

optimize efficiency and the size of the 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

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 this resulted 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.

Automatically, the converter chooses the lowest

active LEDX 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. This results in more current flowing

through the MOSFET, increasing the power

delivered to the output. This forms a closed

loop that regulates the output voltage.

Under light-load operation, especially in the

skipping mode, where the MOSFET turns on for

a minimum on time of approximately 200ns

before 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

Two dimming methods, PWM and analog

dimming mode, are allowed.

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.