MP4430

– 36V, 3.5A, LOW IQ, SYNCHRONOUS, STEP-DOWN CONVERTER

MP4430 Rev. 1.01

www.MonolithicPower.com

20

1/12/2018

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

© 2018 MPS. All Rights Reserved.

SYNC

CLK

SW1

SW2

t

SW1: Phase high

SW2: Phase low

Figure 4: In-Phase and 180°Out-of-Phase

Soft Start (SS)

Soft start (SS) is implemented to prevent the

converter output voltage from overshooting

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

internal current source begins charging the

external soft-start capacitor. When the soft-start

voltage

is

lower

than

the

internal

capacitor can be calculated with Equation (2):







SS

REF

SS

SS

C (nF) V

(V)

t (ms)

I ( A)

(2)

current.

SS can be used for tracking and sequencing.

Pre-Bias Start-Up

means the output has a pre-bias voltage,

neither the HS-FET nor the LS-FET are turned

Over-Current Protection (OCP) and Hiccup

The MP4430 has cycle-by-cycle peak current-

limit protection with valley-current detection and

hiccup mode.

The power MOSFET current is accurately

sensed via a current sense MOSFET. It is then

fed to the high-speed current comparator for

current-mode control purposes. During the HS-

FET on state, if the sensed current exceeds the

peak-current limit value set by the COMP high-

clamp

voltage,

the

HS-FET

turns

off

immediately. Then the LS-FET turns on to

discharge the energy, and the inductor current

decreases. The HS-FET remains off unless the

inductor valley current is lower than a certain

current threshold (the valley current limit), even

though the internal clock pulses high. If the

inductor current does not drop below the valley

current limit when the internal clock pulses high,

the HS-FET misses the clock, and the switching

frequency decreases to half the nominal value.

Both the peak and valley current limits assist in

keeping the inductor current from running away

during an overload or short-circuit condition.

When the output is shorted to ground, causing

the output voltage to drop below 55% of its

nominal output, the peak current limit is kicked,

and the device considers this to be an output

dead

short

and

triggers

hiccup

mode

immediately to restart the part periodically.

In hiccup mode, the MP4430 disables its output

power stage and discharges the soft-start

capacitor slowly. The MP4430 restarts with a

full soft-start when the soft-start capacitor is

fully discharged. If the short-circuit condition still

remains after the soft start ends, the device

repeats this operation until the fault is removed

and output returns to the regulation level. This

protection mode reduces the average short

circuit current greatly to alleviate thermal issues

and protect the regulator.

Floating Driver and Bootstrap Charging

A 0.1

μF to 1μF external bootstrap capacitor

powers the floating power MOSFET driver. The

floating driver has its own UVLO protection with

a rising threshold of 2.5V and hysteresis of

200mV.

The bootstrap capacitor voltage is charged to

~5V from VCC through a PMOS pass transistor

when the LS-FET is on.

At a high duty cycle operation or sleep mode

condition, the time period available to the

bootstrap charging is less, so the bootstrap

capacitor may not be charged sufficiently. In

case

the

external

circuit

does

not

have

sufficient

voltage

or

time

to

charge

the

bootstrap capacitor, extra external circuitry can

be used to ensure that the bootstrap voltage is

in the normal operation region.