ir3500v International Rectifier Corp., ir3500v Datasheet - Page 12

• Manufacturer Part Number: ir3500v
• Description: Xphase3 Vr11.1 Cpu Vtt Control Ic
• Manufacturer: International Rectifier Corp.

IR3500V
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 7. Its gain is
nominally 32.5 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 DAC voltage and sent to the control IC and
other phases through an on-chip 3K 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 sense accuracy, the current sense amplifier continuously calibrates itself. This
calibration algorithm creates ripple on the ISHARE bus with a frequency of fsw / 896 in a multiphase architecture.
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.
IR3500V THEORY OF OPERATION
Block Diagram
The Block diagram of the IR3500V is shown in Figure 8, and specific features are discussed in the following
sections.
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July 03, 2008