IR3084 datasheet(11/45 Pages) IRF

11 / 45 page

IR3084

Page 11 of 45

07/20/2005

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

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 inductor current through all the

inductors and is used by the Control IC for voltage positioning and current limit protection.

Figure 6 – Inductor Current Sensing and Current Sense Amplifier

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.

CSA

IR3084 Datasheet(PDF) 11 Page - International Rectifier