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IR3553 Datasheet(PDF) 15 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) 15 Page - International Rectifier

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July 16, 2014 | DATASHEET V3.3
15
IR3553
40A Integrated PowIRstage®
SW
VIN
CSIN+
CSIN-
IR3553
VIN
VOUT
CIN
CCS
L
COUT
RCS
RL
-
+
iL
+ vCS -
Current
Sense
Amplifier
+
vL
-
Figure 31: Inductor current sensing
Usually the resistor RCS and capacitor CCS are chosen so that
the time constant of RCS and CCS equals the inductor time
constant, which is the inductance L over the inductor DCR
(RL). If the two time constants match, the voltage across CCS
is proportional to the current through L, and the sense
circuit can be treated as if only a sense resistor with the
value of RL was used. The mismatch of the time constants
does not affect the measurement of inductor DC current,
but affects the AC component of the inductor current.
The advantage of sensing the inductor current versus high
side or low side sensing is that actual output current being
delivered to the load is obtained rather than peak or
sampled information about the switch currents. The
output voltage can be positioned to meet a load line based
on real time information. This is the only sense method
that can support a single cycle transient response. Other
methods provide no information during either load
increase (low side sensing) or load decrease (high side
sensing).
CURRENT SENSE AMPLIFIER
A high speed differential current sense amplifier is located
in the IR3553, as shown in Figure 6. Its gain is nominally
32.5, and the inductor DCR increase with temperature is
not compensated inside the IR3553. The current sense
amplifier output IOUT is referenced to REFIN, which is
usually connected to a reference voltage from the PWM
controller. Figure 27 shows the differential voltage of
V(IOUT) – V(REFIN) versus the inductor current and reflects
the inductor DCR increase with temperature at higher
current.
The current sense amplifier can accept positive differential
input up to 25mV and negative input up to -10mV before
clipping. The output of the current sense amplifier is
summed with the reference voltage REFIN and sent to the
IOUT pin. The REFIN voltage is to ensure at light loads
there is enough output range to accommodate the
negative current ripple shown in Figure 28. In a multiphase
converter, the IOUT pins of all the phases can be tied
together through resistors, and the IOUT voltage
represents the average current through all the inductors
and is used by the controller for adaptive voltage
positioning.
The input offset voltage is the primary source of error for
the current signal. In order to obtain very accurate current
signal, the current sense amplifier continuously calibrates
itself, and the input offset of this amplifier is within +/-
450uV. This calibration algorithm can create a small ripple
on IOUT with a frequency of fsw/128.
If the IR3553 current sense amplifier is required, connect
its output IOUT and the reference voltage REFIN to the
PWM controller and connect the inductor sense circuit as
shown in Figure 4. If the current sense amplifier is not
needed, tie CSIN+, CSIN- and REFIN pins to LGND and float
IOUT pin, as shown in Figure 5.
MAXIMUM OUTPUT VOLTAGE
When the IR3553 current sense amplifier is used, the
maximum output voltage is limited by the VCC voltage
used and should be lower than VCC – 2.5V to ensure
enough headroom for the current sense amplifier. The
maximum voltage is 4.3V when 6.8V VCC is used, but is
only 2.5V when 5V VCC is used. The maximum voltage is
4.3V when 6.8V is used for the VCC.
When the IR3553 current sense amplifier is not used, the
maximum voltage is not limited by the VCC voltage. The
IR3553 can support output voltage up to 5.5V but the
output current must be derated since the MOSFET ratio
was optimized for duty cycles of 10% to 20%.
DESIGN PROCEDURES
POWER LOSS CALCULATION
The single-phase IR3553 efficiency and power loss
measurement circuit is shown in Figure 32.
The IR3553 power loss is determined by,
OUT
SW
VCC
CC
IN
IN
LOSS
I
V
I
V
I
V
P
Where both MOSFET loss and the driver loss are included,
but the PWM controller and the inductor losses are not.


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