MP38873 – 15A, 16V, HIGH FREQUENCY STEP-DOWN WITH SYNCHRONOUS GATE DRIVER
MP38873 Rev. 0.9 www.MonolithicPower.com 8
9/17/2009 MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.
© 2009 MPS. All Rights Reserved.
Internal Regulator
Most of the internal circuitries are powered from
the 5V internal regulator. This regulator takes the
VIN input and operates in the full VIN range.
When VIN is greater than 5.0V, the output of the
regulator is in full regulation. When VIN is lower
than 5.0V, the output decreases. Since this
internal regulator provides the bias current for the
bottom gate driver that requires significant
amount of current depending upon the external
MOSFET selection, a 1µF ceramic capacitor for
decoupling purpose is required.
Under-Voltage Lockout (UVLO)
Under-voltage lockout (UVLO) is implemented to
protect the chip from operating at insufficient
supply voltage. The MP38873 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.
Soft-Start
The soft-start is implemented to prevent the
converter output voltage from overshooting
during startup. When the chip starts, the internal
current source (10µA) charges up an external
soft start capacitor C
SS
from 0V t o 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.
Over-Current-Protection and Latch off
The MP38873 has cycle-by-cycle over current
limit. If the soft start Voltage is greater than 1.2V,
and inductor current exceeds the current limit
threshold, and FB voltage drops below 50% of
reference Voltage, then the MP38873 goes into
latch off until En or IN is recycled. This protection
mode is especially useful when the output is
dead-short to ground.
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 V
IN
through D1, M3, C4,
L1 and C2 (Figure 3). If (V
IN
-V
SW
) is more than 5V,
U2 will regulate M3 to maintain a 5V BST voltage
across C4.
--
+
--
+
V
IN
5V
U2
D1
M3
BST
SW
C4
C2
L1
V
OUT
Figure 3—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.
Synchin/Synchout Control
The MP38873 has a dedicated synchin control
pin (SYNCIN) that allows MP38873 be
synchronized to external clock ranging from
300KHz up to 1 MHz. the MP38873 also has a
synchout pin (SYNCOUT) generating a 50% duty
cycle, 180° out of phase logic signal. The
Synchout signal can be used to synchronize a
slave phase by connecting the SYNCOUT pin of
the master MP38873 with the SYNCIN pin of the
slave MP38873. The 180° interleaving operation
greatly reduces the requirement of the input
decoupling capacitors.
