MP38894 – 4.5A, 42V, 420KHZ STEP-DOWN WITH SYNCHRONIZABLE GATE DRIVER
MP38894 Rev.1.0 www.MonolithicPower.com 6
9/28/2009 MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.
© 2009 MPS. All Rights Reserved.
OPERATION
IN
EN/SYNC
FB
SW
VCC
BG
GND
BST
REGULATOR
OSCILLATOR
420KHz
DRIVER
CURRENT SENSE
AMPLIFIER
ERROR
AMPLIFIER
COMP
CURRENT
LIMIT
COMPARATOR
PWM
COMPARATOR
DRIVER
D
REGULATOR
REFERENCE
--
+
--
+
--
+
S
R
R
Q
Q
--
+
V
CC
V
CC
Figure 1—Functional Block Diagram
The MP38894 is a fixed frequency,
synchronous, step-down switching regulator
with an integrated high-side power MOSFET
and a gate driver for a low-side external
MOSFET. It achieves 4.5A continuous output
current over a wide input supply range with
excellent load and line regulation. It provides a
single highly efficient solution with current mode
control for fast loop response and easy
compensation.
The MP38894 operates in a fixed frequency,
peak current control mode to regulate the
output voltage. A PWM cycle is initiated by the
internal clock. The integrated high-side power
MOSFET is turned on and remains on until its
current reaches the value set by the COMP
voltage. When the power switch is off, it
remains off until the next clock cycle starts. If, in
90% of one PWM period, the current in the
power MOSFET does not reach the COMP set
current value, the power MOSFET will be
forced to turn off.
Error Amplifier
The error amplifier compares the FB pin voltage
with the internal 0.8V reference (REF) and
outputs a current proportional to the difference
between the two. This output current is then
used to charge or discharge the internal
compensation network to form the COMP
voltage, which is used to control the power
MOSFET current. The optimized internal
compensation network minimizes the external
component counts and simplifies the control
loop design.
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.
