ISL6263

FN9213 Rev 2.00

Page 11 of 19

June 10, 2010

current. This information is used exclusively to achieve the

IMVP-6+ load line as well as the overcurrent protection. It is

important to note that this current measurement should not be

confused with the synthetic current ripple information created

When using inductor DCR current sensing, an NTC element is

used to compensate the positive temperature coefficient of the

copper winding thus maintaining the load-line accuracy.

Processor Socket Kelvin Voltage Sensing

The remote voltage sense input pins VSEN and RTN of the

ISL6263 are to be terminated at the die of the GPU through

connections that mate at the processor socket. (The signal

names are Vcc_sense and Vss_sense respectively.) Kelvin

sensing allows the voltage regulator to tightly control the

processor voltage at the die, compensating for various

resistive voltage drops in the power delivery path.

Since the voltage feedback is sensed at the processor die,

removing the GPU will open the voltage feedback path of the

regulator, causing the output voltage to rise towards VIN. The

ISL6263 will shut down when the voltage between the VO and

VSS pins exceeds the severe overvoltage protection threshold

recommended to install resistors Ropn1 and Ropn2 as shown

in Figure 5. These resistors provide voltage feedback from the

regulator local output in the absence of the GPU. These

resistors should be in the range of 20

 to 100

High Efficiency Diode Emulation Mode

The ISL6263 operates in continuous-conduction-mode (CCM)

during heavy load for minimum conduction loss by forcing the

low-side MOSFET to operate as a synchronous rectifier.

Depending upon the VID and FDE pin states, an improvement

in light-load efficiency can be achieved by operating in

discontinuous-conduction-mode (DCM) where the low-side

MOSFET is operated in diode-emulation-mode (DEM), forcing

the low-side MOSFET to block negative inductor current flow.

Positive-going inductor current flows from either the source of

the high-side MOSFET, or the drain of the low-side MOSFET.

Negative-going inductor current flows into the source of the

high-side MOSFET, or the drain of the low-side MOSFET.

When the low-side MOSFET conducts positive inductor

current, the phase voltage will be negative with respect to the

VSS pin. Conversely, when the low-side MOSFET conducts

negative inductor current, the phase voltage will be positive

with respect to the VSS pin. Negative inductor current occurs

in CCM when the output load current is less than ½ the

inductor ripple current. Sinking negative inductor through the

low-side MOSFET lowers efficiency through unnecessary

conduction losses. Upon entering DEM the PWM switching

frequency is automatically shifted downward by an increase of

will continue to decrease as the load continues to decrease.

The reduction of PWM frequency further improves efficiency by

reducing switching losses. The converter will automatically

enter DEM after eight consecutive PWM pulses where the

PHASE pin has detected positive voltage shortly after the

LGATE pin has gone high. The converter will return to CCM on

the following cycle after the PHASE pin detects negative

FIGURE 5. SIMPLIFIED VOLTAGE DROOP CIRCUIT WITH GPU SOCKET KELVIN SENSING AND INDUCTOR DCR CURRENT SENSING

VSUM

DFB



















VO

VSEN

RTN





VDIFF

OCP

OCSET

DROOP

DROOP

ESR

DCR

PHASE

To

Processor

Socket

Kelvin

Connections



VDD

10µA