ir3084a International Rectifier Corp., ir3084a Datasheet - Page 11

• Manufacturer Part Number: ir3084a
• Description: Xphase Tm Vr 10/11 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 6. 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 10K
phases are tied together and the voltage on the share bus represents the average inductor 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 nominal 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 6 – Inductor Current Sensing and Current Sense Amplifier
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IR3084A
10/30/2006