IR3513Z International Rectifier Corp., IR3513Z Datasheet - Page 15

• Manufacturer Part Number: IR3513Z
• Description: Xphase3tm Pol 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 included in both the IR3513Z and optional phase ICs, as shown in
Figure 6. Its gain is nominally 33 at 25ºC, and the 3850 ppm/ºC increase in inductor DCR should be compensated in the
voltage loop feedback path.
The current sense amplifier can accept positive differential input up to 50mV and negative up to -10mV before clipping.
The output of the current sense amplifier is summed with the VREF voltage and sent to other phases through an on-
chip 3KΩ 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. The input offset of this amplifier is calibrated to +/- 1mV in order to reduce the
current sense error.
The input offset voltage is the primary source of error for the current share loop. In order to achieve very small input
offset error and superior current sharing performance, the current sense amplifier continuously calibrates itself. This
calibration algorithm creates ripple on ISHARE bus with a frequency of fsw/896.
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 average current at the share bus. If current in a phase is smaller
than the average current, the share adjust amplifier of the phase will pull down the starting point of the PWM ramp
thereby increasing its duty cycle and output current; if current in a phase is larger than the average current, the share
adjust amplifier of the phase will pull up the starting point of the PWM ramp thereby decreasing its duty cycle and output
current. The current share amplifier is internally compensated so that the crossover frequency of the current share loop
is much slower than that of the voltage loop and the two loops do not interact.
Page 15
Figure 6 - Inductor Current Sensing and Current Sense Amplifier
CSOUT
Current
Sense Amp
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L
R
CS
v
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May 11, 2009
IR3513Z