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MP8718 Datasheet(PDF) 11 Page - Monolithic Power Systems

Part No. MP8718
Description  High Efficiency 4A, 21V, 500kHz Synchronous Step-down Converter
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Manufacturer  MPS [Monolithic Power Systems]
Direct Link  http://www.monolithicpower.com
Logo MPS - Monolithic Power Systems

MP8718 Datasheet(HTML) 11 Page - Monolithic Power Systems

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MP8718 – SYNCHRONOUS STEP-DOWN CONVERTER WITH INTERNAL MOSFETS
MP8718 Rev. 0.9
www.MonolithicPower.com
11
2/9/2010
MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.
© 2010 MPS. All Rights Reserved.
internal clock rising edge synchronized to the
external clock rising edge.
Figure 4—Startup Sequence Using External
Sync Clock Signal
Under-Voltage Lockout (UVLO)
Under-voltage lockout (UVLO) is implemented to
protect the chip from operating at insufficient
supply voltage. The MP8718 UVLO comparator
monitors the output voltage of the internal
regulator, VCC. The UVLO rising threshold is
about 4.0V while its falling threshold is a
consistent 3.2V.
Internal Soft-Start
The soft-start is implemented to prevent the
converter output voltage from overshooting
during startup. When the chip starts, the internal
circuitry generates a soft-start voltage (SS)
ramping up from 0V to 1.2V. When it is lower
than the internal reference (REF), SS overrides
REF so the error amplifier uses SS as the
reference. When SS is higher than REF, REF
regains control. The SS time is internally fixed to
4ms.
Over-Current-Protection and Hiccup
The MP8718 has cycle-by-cycle over current limit
when the inductor current peak value exceeds
the set current limit threshold. Meanwhile, output
voltage starts to drop until FB is below the Under-
Voltage (UV) threshold, typically 30% below the
reference. Once a UV is triggered, the MP8718
enters hiccup mode to periodically restart the
part. This protection mode is especially useful
when the output is dead-short to ground. The
average short circuit current is greatly reduced to
alleviate the thermal issue and to protect the
regulator. The MP8718 exits the hiccup mode
once the over current condition is removed.
Thermal Shutdown
Thermal shutdown is implemented to prevent the
chip
from
operating
at
exceedingly
high
temperatures. When the silicon die temperature
is higher than 150
°C, it shuts down the whole
chip. When the temperature is lower than its
lower threshold, typically 140
°C, the chip 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. This UVLO’s
rising threshold is 2.2V with a hysteresis of
150mV. The bootstrap capacitor voltage is
regulated internally by VIN through D1, M3, C4,
L1 and C2 (Figure 2). If (VIN-VSW) is more than
5V, U2 will regulate M3 to maintain a 5V BST
voltage across C4.
SW
Figure 2—Internal Bootstrap Charging Circuit
Startup and Shutdown
If both VIN and EN are higher than their
appropriate thresholds, the chip starts. The
reference block starts first, generating stable
reference voltage and currents, and then the
internal regulator is enabled. The regulator
provides
stable
supply
for
the
remaining
circuitries.
Three events can shut down the chip: EN low,
VIN low and thermal shutdown. In the shutdown
procedure, the signaling path is first blocked 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
.


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