IR3087PBF

When designing for OVP the overall system must be considered. In many cases the over-current protection of the

AC-DC or DC-DC converter supplying the multiphase converter will be triggered thus providing effective protection

without damage as long as all PCB traces and components are sized to handle the worst-case maximum current. If

this is not possible a fuse can be added in the input supply to the multiphase converter. One scenario to be careful

of is where the input voltage to the multiphase converter may be pulled below the level where the ICs can provide

adequate voltage to the low side MOSFET thus defeating OVP.

Dynamic changes in the VID code to a lower output voltage may trigger OVP. For example; a 250mV decrease in

output voltage combined with a light load condition will cause the low side MOSFETs to turn on and interfere with

Body Braking

less than 125mV above the VID voltage.

Since CSIN- pin is also used as the inductor current sensing input, it is usually connected to the local converter

output, which may be far away from the load of the multiphase converter. Excessive distribution impedance

between the converter and load may trigger OVP during normal operation. If the voltage drop across the distribution

impedance exceeds the minimum OVP comparator threshold of 100mV plus VID offset and voltage positioning, the

IR3087 can not be used. The IR3088A Phase IC without OVP should be used instead in applications with excessive

distribution impedance and very small or no AVP. For example, a converter having 25mV of VID offset, 125mV of

AVP at full load, and 100mV of drop in the distribution path at full load would be OK, since 100mV + 25mV + 125mV

= 250mV which is greater than the 100mV drop. However, a converter having 25mV of VID offset, no AVP, and

130mV of drop in the distribution path would require IR3088A, since 100mV + 25mV + 0mV = 125mV which is

smaller than the 130mV drop.

Converter with higher output voltage than VID voltage may also trigger OVP during normal operation, and IR3088A

should be used to replace IR3087.

Thermal Monitoring (VRHOT)

The IR3087 senses its own die temperature and produces a voltage at the input of the VRHOT comparator that is

proportional to temperature. An external resistor divider connected from VBIAS to the HOTSET pin and ground can

be used to program the thermal trip point of the VRHOT comparator. The VRHOT pin is an open-collector output

and should be pulled up to a voltage source through a resistor. If the thermal trip point is reached the VRHOT

output drives low.

Opti-Phase

TM

The number of phases chosen for a particular design is based upon meeting thermal requirements and minimizing

the number of input and output capacitors at the maximum output current. At currents less than the maximum

efficiency will increase if less phases are used. Turning off phases as the output current decreases increases

efficiency by eliminating the gate charging losses, MOSFET switching losses, and circulating currents in the

MOSFETS and output inductors. For each unique design there will be an optimal point were phases should be

turned off, one after another, to achieve the maximum efficiency over the entire output current range.

The IR3087 implements patent pending Opti-Phase

function of converter output current. The Opti-Phase

voltage on the DACIN pin. This voltage provides a direct indication of converter output current. A resistor divider

connected between the BIASIN, OPTIPHS, and LGND pins programs the threshold of the Opti-Phase

TM

comparator. If the converter output current drops below the programmed level the GATEH and GATEL pins will both

drive low turning off the MOSFETs and causing the inductor current to begin to decay. The output voltage will begin

to sag causing the Control IC to immediately increase the duty cycle of the remaining phase(s) to compensate. The

voltage on the ISHARE will remain constant as it represents the converter output current rather than the current in

the individual phases.

Opti-Phase

phase IC per converter to ensure operation under zero load conditions.

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