MPQ4432 - 36V, 2.2A, LOW IQ, SYNCHRONOUS STEP-DOWN CONVERTER

MPQ4432 Rev. 1.0

www.MonolithicPower.com

21

9/19/2016

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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 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

has a pre-bias voltage), neither the HS-FET or

Over-Current Protection (OCP) and Hiccup

The MPQ4432 has cycle-by-cycle peak current

limit protection with valley-current detection and

hiccup mode.

The

power

MOSFET

current

is

sensed

accurately via a current sense MOSFET. The

current 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 keep the inductor current from running

away

during

an

overload

or

short-circuit

condition.

When the output is shorted to ground, the

output voltage drops below 50% of its nominal

output. Meanwhile, 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 MPQ4432 disables its

output power stage and slowly discharges the

soft-start capacitor. The MPQ4432 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 the 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 0.1µF 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 a 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 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.