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IR3551 Datasheet(PDF) 13 Page - International Rectifier
IRF [International Rectifier]
IR3551 Datasheet(HTML) 13 Page - International Rectifier
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July 16, 2013 | DATASHEET V3.2
50A Integrated PowIRstage®
THEORY OF OPERATION
The IR3551 PowIRstage® is a synchronous buck driver with
co-packed MOSFETs with integrated Schottky diode, which
provides system designers with ease of use and flexibility
required in cutting edge CPU, GPU and DDR memory
power delivery designs and other high-current low-profile
The IR3551 is designed to work with a PWM controller.
It incorporates a continuously self-calibrated current sense
amplifier, optimized for use with inductor DCR sensing.
The current sense amplifier provides signal gain and noise
immunity, supplying multiphase systems with a superior
design toolbox for programmed impedance designs.
The IR3551 provides a phase fault signal capable
of detecting open or shorted MOSFETs, or an over-
temperature condition in the vicinity of the power stage.
The IR3551 accepts an active low Body-Braking® input
which disables both MOSFETs to enhance transient
performance or provide a high impedance output.
The IR3551 provides diode emulation feature which avoids
negative current in the synchronous MOSFET and improves
light load efficiency.
The IR3551 PWM input is compatible with 3.3V logic signal
and 7V tolerant. It accepts 3-level PWM input signals with
BBRK# PIN FUNCTIONS
The BBRK# pin has two functions. During normal
operation, it accepts direct control signal from the PWM
controller to enable Body-Braking®, which turns off both
control and synchronous MOSFETs to improve the load
The second function of BBRK# pin is to select Body-
Braking® (tristate) or diode emulation mode when PWM
pin receives a tri-state signal. The seletion is recongnized
right after VCC passes its UVLO threshold during the VCC
power up. If the BBRK# input is always high, the default
operation mode is Body- Braking®, in which both MOSFETs
will be turned off when the PWM input is in tri-state. If the
BBRK# input has been pulled low for at least 20ns after the
VCC passes its UVLO threshold during power up, the diode
emulation mode is set. PWM input in tri-state will activate
a synchronous diode emulation feature allowing designers
to maximize system efficiency at light loads without
compromising transient performance. Once the diode
emulation mode is set, it cannot be reset until the VCC
power is recycled.
PWM TRI-STATE INPUT
The IR3551 PWM accepts 3-level input signals. When PWM
input is high, the synchronous MOSFET is turned off and
the control MOSFET is turned on. When PWM input is low,
control MOSFET is turned off and synchronous MOSFET is
turned on. Figures 16-18 show the PWM input and the
corresponding SW and GATEL output. If PWM pin is floated
, the built-in resistors pull the PWM pin into a tri-state
region centered around 1.65V.
When PWM input voltage is in tri-state region, the IR3551
will go into either Body-Braking® mode or diode emulation
mode depending on BBRK# selection during VCC power up.
International Rectifier’s Body-Braking® is an operation
mode in which two MOSFETs are turned off. When the
synchronous MOSFET is off, the higher voltage across the
Shottky diode in parallel helps discharging the inductor
current faster, which reduces the output voltage
overshoot. The Body-Braking® can be used either to
enhance transient response of the converter after load
release or to provide a high impedance output.
There are two ways to place the IR3551 in Body-Braking®
mode, either controlling the BBRK# pin directly or through
a PWM tri-state signal. Both control signals are usually
from the PWM controller.
Pulling BBRK# low forces the IR3551 into Body-Braking®
mode rapidly, which is usually used to enhance converter
transient response after load release, as shown in Figure
19. Releasing BBRK# forces the IR3551 out of Body-
Braking® mode quickly.
The BBRK# low turns off both MOSFETs and therefore can
also be used to disable a converter. Please note that soft
start may not be available when BBRK# is pulled high to
enable the converter.
If the BBRK# input is always high, the Body-Braking® is
activated when the PWM input enters the tri-state region,
as shown in Figures 20 and 21. Comparing to pulling down
the BBRK# pin directly, the Body-Braking® response to
PWM tri-state signal is slower due to the hold-off time
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