MP3378E

– 4-CHANNEL WLED CONTROLLER WITH BUCK CONVERTER

MP3378E Rev. 1.02

www.MonolithicPower.com

15

5/26/2017

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

© 2017 MPS. All Rights Reserved.

BUCK CONVERTER SECTION:

The buck converter is a step-down, switch-

mode converter with built-in internal power

MOSFETs and offers a very compact solution.

It operates in a fixed-frequency, peak-current-

control mode to regulate the output voltage. A

PWM cycle is initiated by an internal clock. The

integrated high-side power MOSFET is turned

on and remains on until its current reaches the

value set by the COMP_BUCK voltage (this

COMP_BUCK is one of the buck’s internal

control voltage, not the COMP pin). When the

power switch is off, it remains off until the next

clock cycle starts. If the current in the power

MOSFET does not reach the COMP_BUCK set

current value in 95% of one PWM period, the

power MOSFET is forced to turn off.

Internal Regulator

Most of the internal circuitries are powered by a

5V internal regulator. This regulator takes the

VIN2 input and operates in the full VIN2 range.

When VIN2 is greater than 5.0V, the output of

the regulator is in full regulation. When VIN2 is

lower than 5.0V, the output decreases. A 0.1µF

ceramic capacitor is required for decoupling

purposes.

Error Amplifier

The error amplifier compares the FB voltage

with the internal 0.8V reference (REF) and

outputs a COMP_BUCK voltage, which is used

to control the power MOSFET current. The

optimized

internal

compensation

network

minimizes the external component count and

simplifies the control loop design.

SYNC Control

The buck converter can be synchronized to an

external clock ranging from 250kHz to 2MHz

through SYNC. The internal clock

’s rising edge

is synchronized to the external clock

’s falling

edge. The synchronized logic high voltage

should

be

higher

than

1.8V,

and

the

synchronized logic low voltage should be lower

than 0.6V. The frequency of the external clock

should be higher than the internal clock.

Otherwise the internal clock may turn on the

high-side MOSFET.

Under-Voltage Lockout (UVLO)

Under-voltage lockout (UVLO) is implemented

to protect the buck converter from operating at

insufficient supply voltages by monitoring the

output voltage of the internal regulator (VCC2).

The UVLO rising threshold is about 3.9V while

its falling threshold is consistently 3.25V.

Internal Soft Start (SS)

The soft start is implemented to prevent the

converter output voltage from overshooting

during start-up. When the chip starts up, the

internal circuitry generates a soft-start voltage

(SS) and ramps up from 0V. The soft-start

period lasts until the voltage of the soft-start

capacitor exceeds the reference voltage of 0.8V.

At this point, the reference voltage takes over.

The soft-start time is set internally to around

1.5ms.

Over-Current Protection and Hiccup Mode

The

cycle-by-cycle

over-current

limit

is

implemented when the inductor current peak

value exceeds the set current limit threshold.

The output voltage drops until FB is below the

under-voltage (UV) threshold, typically 50%

below the reference. Once UV is triggered, the

buck converter enters hiccup mode to restart

the part periodically. This protection mode is

especially useful when the output is dead-

shorted to ground. The average short-circuit

current is greatly reduced to alleviate thermal

issues and protect the regulator. The buck

converter exits hiccup mode once the over-

current condition is removed.

Thermal Shutdown

Thermal shutdown is implemented to prevent

the chip from operating at exceedingly high

temperatures. When the silicon die temperature

is higher than 150°C, the buck converter shuts

down. When the temperature is lower than its

lower threshold, typically 130°C, the buck

converter is enabled again.