MP3378

– 4-CHANNEL WLED CONTROLLER PLUS BUCK CONVERTER

MP3378 Rev. 1.01

www.MonolithicPower.com

16

5/26/2017

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Over-Current Protection (OCP) and Hiccup

The

cycle-by-cycle

over-current

limit

is

implemented when the inductor current peak

value exceeds the set current-limit threshold.

Meanwhile, the output voltage starts to drop

until FB is below the under-voltage (UV)

threshold (50 percent below the reference,

typically). Once a UV is triggered, the buck

converter enters hiccup mode to re-start the

part periodically. This protection mode is useful

when the output is dead shorted to ground. The

average short-circuit current is reduced greatly

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 prevents the chip from

operating at exceedingly high temperatures.

When the die temperature is higher than 150°C,

it shuts down the buck converter. When the

temperature is lower than its lower threshold

(130°C, typically) the buck converter is enabled

again.

Floating Driver and Bootstrap Charging

The floating power MOSFET driver is powered

by an external bootstrap capacitor. This floating

driver has its own UVLO protection. The

UVLO’s rising threshold is 2.2 V with a

hysteresis of 150 mV. The bootstrap capacitor

voltage is regulated internally by VIN2 through

DB, R6, C7, L2, and C5 (see Figure 3). If

VIN2-VSW is more than 5 V, U1 regulates MB

to maintain a 5 V BST voltage across C7.

5V

U1

BST

R6

C7

L2

SW

C5

Figure 3

—Internal bootstrap charging circuit

start-up and shutdown

If VIN2 is higher than its appropriate thresholds,

the buck converter starts up. The reference

block starts first, generating stable reference

voltage and currents and then the internal

regulator is enabled. The regulator provides a

stable supply for the remaining circuitries.

Two events can shut down the buck converter:

VIN2 UVLO and thermal shutdown. During the

shutdown procedure, the signaling path is

blocked first to avoid any fault triggering. The

COMP voltage and the internal supply rail are

then pulled down. The floating driver is not

subject

to

this

shutdown

command.