MP38894 – 4.5A, 42V, 420KHZ STEP-DOWN WITH SYNCHRONIZABLE GATE DRIVER
MP38894 Rev.1.0 www.MonolithicPower.com 7
9/28/2009 MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.
© 2009 MPS. All Rights Reserved.
Enable/Synch Control
The MP38894 has a dedicated Enable/Synch
control pin (EN/SYNC). By pulling it high or low,
the IC can be enabled and disabled by EN. Tie
EN to VIN for automatic start up. To disable the
part, EN must be pulled low for at least 5µs.
The MP38894 can be synchronized to external
clock range from 300KHz up to 1.5MHz through
the EN/SYNC pin. The internal clock rising
edge is synchronized to the external clock rising
edge.
Under-Voltage Lockout (UVLO)
Under-voltage lockout (UVLO) is implemented
to protect the chip from operating at insufficient
supply voltage. The MP38894 UVLO
comparator monitors the output voltage of the
internal regulator, VCC. The UVLO rising
threshold is about 4.1V 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.
Over-Current-Protection (OCP)
The MP38894 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
output UV is triggered, the MP38894 enters
hiccup mode, which is especially useful to
ensure system safety under fault condition. The
MP38894 exits the hiccup mode once the EN or
input power is re-cycled.
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 2). 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 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.
