IR3081 International Rectifier Corp., IR3081 Datasheet - Page 10

• Manufacturer Part Number: IR3081
• Description: Xphasetm Vr 10 Control Ic
• Manufacturer: International Rectifier Corp.

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. Except for a sense resistor in
series with the inductor, 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).
An additional problem associated with peak or valley current mode control for voltage positioning is that they suffer
from peak-to-average errors. These errors will show in many ways but one example is the effect of frequency
variation. If the frequency of a particular unit is 10% low, the peak to peak inductor current will be 10% larger and
the output impedance of the converter will drop by about 10%. Variations in inductance, current sense amplifier
bandwidth, PWM prop delay, any added slope compensation, input voltage, and output voltage are all additional
sources of peak-to-average errors.
Current Sense Amplifier
A high speed differential current sense amplifier is located in the Phase IC, as shown in Figure 5. Its gain decreases
with increasing temperature and is nominally 34 at 25ºC and 29 at 125ºC (-1470 ppm/ºC). This reduction of gain
tends to compensate the 3850 ppm/ºC increase in inductor DCR. Since in most designs the Phase IC junction is
hotter than the inductor these two effects tend to cancel such that no additional temperature compensation of the
load line is required.
The current sense amplifier can accept positive differential input up to 100mV and negative up to -20mV before
clipping. The output of the current sense amplifier is summed with the DAC voltage and sent to the Control IC and
other Phases through an on-chip 10KΩ resistor connected to the ISHARE pin. The ISHARE pins of all the phases
are tied together and the voltage on the share bus represents the average current through all the inductors and is
used by the Control IC for voltage positioning and current limit protection.
Average Current Share Loop
Current sharing between phases of the converter is achieved by the average current share loop in each Phase IC.
The output of the current sense amplifier is compared with the share bus less a 20mV offset. If current in a phase is
smaller than the average current, the share adjust amplifier of the phase will activate a current source that reduces
the slope of its PWM ramp thereby increasing its duty cycle and output current. The crossover frequency of the
current share loop can be programmed with a capacitor at the SCOMP pin so that the share loop does not interact
with the output voltage loop.
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Figure 5. Inductor Current Sensing and Current Sense Amplifier
CSOUT
Current
Sense Amp
i
L
L
R
CS
v
L
R
C
v
c
C
L
CS
S
V
O
C
O
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