MPQ2451-5/33―36V, 2.2MHz, 0.6A, STEP-DOWN CONVERTER, AEC-Q100 QUALIFIED
Thermal Shutdown
VCOMP. When the sensed value exceeds VCOMP,
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.
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.
Startup and Shutdown
Floating Driver and Bootstrap Charging
If both VIN and VEN exceed their respective
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.
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
about 300mV. During this UVLO, VSS resets to
0V. When the UVLO is removed, the controller
enters soft-start.
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.
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
pass transistor connected from VIN to BST turns
on. The charging current path goes from VIN→
Three events shut down the chip: EN low, VIN low,
thermal shutdown. During shutdown, the
signaling path is blocked first to avoid any fault
triggering. VCOMP and the internal supply rail are
then pulled low. The floating driver is not subject
to this shutdown command but its charging path
is disabled.
BST → SW. The external circuit must provide
enough voltage headroom to facilitate charging.
If VIN is sufficiently higher than VSW, the bootstrap
capacitor will charge. When the power MOSFET
is ON, VIN=VSW so the bootstrap capacitor does
not charge. Optimal charging occurs when
VIN−VSW reaches its apex when the external
freewheeling diode is on. When there is no
current in the inductor, VSW=VOUT so VIN−VOUT can
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
MPQ2451-5/33 Rev 1.0
5/25/2016 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2016 MPS. All Rights Reserved.
www.MonolithicPower.com
12
MPS [ MONOLITHIC POWER SYSTEMS ]
