IR3086A

L

i

V

L

O

R

C

CS

CS

Current

C

c

Sense Amp

CSOUT

Figure 5. 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 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

This is a high speed differential current sense amplifier, 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 being delivered to the load 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 error 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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