IR3521

Page 14

V3.03

Figure 7 Inductor Current Sensing and Current Sense Amplifier

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 34 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 DAC voltage and sent to the control IC and

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.

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.

L

Current

Sense Amp

CSOUT

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