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IR3553 Datasheet(PDF) 14 Page - International Rectifier

Part No. IR3553
Description  40A Integrated PowIRstage
Download  22 Pages
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Maker  IRF [International Rectifier]
Homepage  http://www.irf.com
Logo IRF - International Rectifier

IR3553 Datasheet(HTML) 14 Page - International Rectifier

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July 16, 2014 | DATASHEET V3.3
14
IR3553
40A Integrated PowIRstage®
created by the PWM pin parasitic capacitor with the pull-
up and pull-down resistors of PWM pin. For better
performance, no more than 100pF parasitic capacitive load
should be present on the PWM line of IR3553.
SYNCHRONOUS DIODE EMULATION MODE
An additional feature of the IR3553 is the synchronous
diode emulation mode. This function enables increased
efficiency by preventing negative inductor current from
flowing in the synchronous MOSFET.
As shown in Figure 22, when the PWM input enters the tri-
state region the control MOSFET is turned off first, and the
synchronous MOSFET is initially turned on and then is
turned off when the output current reaches zero. If the
sensed output current does not reach zero within a set
amount of time the gate driver will assume that the output
is de-biased and turn off the synchronous MOSFET,
allowing the switch node to float.
This is in contrast to the Body-Braking® mode shown in
Figure 23, where GATEL follows PWM input. The Schottky
diode in parallel with the synchronous MOSFET conducts
for a longer period of time and therefore lowers the light
load efficiency.
The zero current detection circuit in the IT3553 is
independent of the current sense amplifier and therefore
still functions even if the current sense amplifier is not
used. As shown in Figure 6, an offset is added to the diode
emulation comparator so that a slightly positive output
current in the inductor and synchronous MOSFET is treated
as zero current to accommodate propagation delays,
preventing
any
negative
current
flowing
in
the
synchronous MOSFET. This causes the Schottky diode in
parallel with the synchronous MOSFET to conduct before
the inductor current actually reaches zero, and the
conduction time increases with inductance of the output
inductor.
To set the IR3553 in diode emulation mode, the BBRK# pin
must be toggled low at least once after the VCC passes its
UVLO threshold during power up. One simple way is to use
the internal BBRK# pull-up resistor (200kΩ typical) with an
external capacitor from BBRK# pin to LGND, as shown in
Figure 4. To ensure the diode emulation mode is properly
set, the BBRK# voltage should be lower than 0.8V when the
VCC voltage passes its UVLO threshold (3.3V minimum and
3.7V typical), as shown in Figure 24. A digital signal from
the PWM controller can also be used to set the diode
emulation mode. The BBRK# signal can either be pulled
low for at least 20ns after the VCC passes its UVLO
threshold, as shown in Figure 25, or be pulled low before
VCC power up and then released after the VCC passes its
UVLO threshold, as shown in Figure 26.
Once the diode emulation mode is set, it cannot be reset
until the VCC power is recycled.
PHASE FAULT AND THERMAL FLAG OUTPUT
The phase fault circuit looks at the switch node with
respect to ground to determine whether there is a
defective MOSFET in the phase. The output of the phase
fault signal is high during normal operation and is pulled
low when there is a fault. Each driver monitors the
MOSFET it drives. If the switch node is less than a certain
voltage above ground when the PWM signal goes low or
if the switch node is a certain voltage above ground when
the PWM signal rises, this gives a fault signal. If there are a
number of consecutive faults the phase fault signal is
asserted.
Thermal flag circuit monitors the temperature of the
IR3553. If the temperature goes above a threshold (160°C
typical) the PHSFLT# pin is pulled low after a maximum
delay of 100us.
The PHSFLT# pin can be pulled low by either the phase
fault circuit or the thermal flag circuit, but the IR3553 relies
on the system to take protective actions. The phase fault
signal could be used by the system to turn off the AC/DC
converter or blow a fuse to disconnect the DC/DC
converter input from the supply.
If PHSFLT# is not used it can be floated or connected to
LGND.
LOSSLESS AVERAGE INDUCTOR CURRENT
SENSING
Inductor current can be sensed by connecting a series
resistor and a capacitor network in parallel with the
inductor and measuring the voltage across the capacitor,
as shown in Figure 31.
The equation of the current sensing network is as follows.
CS
CS
L
L
L
CS
CS
L
CS
C
sR
R
L
s
R
s
i
C
sR
s
v
s
v
1
1
)
(
1
1
)
(
)
(
L
L
R
s
i
)
(
CS
CS
L
C
R
R
L
when


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