MPQ2451-5/33―36V, 2.2MHz, 0.6A, STEP-DOWN CONVERTER, AEC-Q100 QUALIFIED

MPQ2451-5/33 Rev 1.0

www.MonolithicPower.com

12

5/25/2016

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

© 2016 MPS. All Rights Reserved.

Thermal Shutdown

Thermal shutdown prevents thermal runaway.

When the silicon die temperature exceeds its

upper threshold, the entire chip shuts down.

When the temperature drops below its lower

threshold, the chip is enabled again.

Floating Driver and Bootstrap Charging

An external bootstrap capacitor powers the

floating-power-MOSFET

driver.

This

floating

driver has its own UVLO protection with a rising

threshold of about 2.4V and a falling threshold of

0V. When the UVLO is removed, the controller

enters soft-start.

The

dedicated

internal-bootstrap

regulator

charges and regulates the bootstrap capacitor to

about 5V. When the voltage between the BST

and SW nodes falls below its regulation, a PMOS

BST → SW. The external circuit must provide

enough voltage headroom to facilitate charging.

capacitor will charge. When the power MOSFET

not

charge. Optimal charging occurs when

freewheeling diode is on. When there is no

charge the bootstrap capacitor.

At a higher duty cycle, the bootstrap capacitor

may not be charged sufficiently because of a

shorter charging period. If there is insufficient

voltage

and

time

to

charge

the

bootstrap

capacitor, add an extra external circuit to ensure

the bootstrap voltage in normal operation region.

The floating driver’s UVLO is not communicated

to the controller.

Make sure the bleed-through current at the SW

node is at least higher than

the floating driver’s

DC quiescent current of about 20µA.

Current Comparator and Current Limit

A current-sense MOSFET senses the power

MOSFET current. This value is the input to the

high-speed-current comparator for current-mode

control. When the power MOSFET turns on, the

comparator is first blanked to limit noise, and

then compares the power switch current to the

the comparator output goes LOW to turn off the

power MOSFET. The maximum current of the

internal power MOSFET is internally limited

cycle-by-cycle.

Startup and Shutdown

thresholds, the chip starts. The reference block

starts first to generate a stable reference voltage

and current, and then the internal regulator

operates to provide a stable supply for the rest

circuit.

While the internal supply rail is up, an internal

timer turns the power MOSFET off for about 50µs

to blank startup noise. When the internal soft-

start block is enabled, it first holds its SS output

low to ensure the rest of the circuit is ready

before ramping up.

thermal

shutdown.

During

shutdown,

the

signaling path is blocked first to avoid any fault

then pulled low. The floating driver is not subject

to this shutdown command but its charging path

is disabled.