ISL6263B Datasheet, PDF (10/18 Pages) Intersil Corporation – 5-Bit VID Single-Phase Voltage Regulator with Current Monitor for IMVP-6+ Santa Rosa GPU Core

ISL6263B
VR_ON
~100µs
V_SOFT/ V_CCGFX
90%
PGOOD
13 SWITCHING CYCLES
FIGURE 4. ISL6263B START-UP TIMING
Static Regulation
The V_CCGFX output voltage will be regulated to the value set
by the VID inputs per Table 2. A true differential amplifier
connected to the VSEN and RTN pins implements processor
socket Kelvin sensing for precise core voltage regulation at
the GPU voltage sense points.
As the load current increases from zero, the V_CCGFX output
voltage will droop from the VID set-point by an amount
proportional to the IMVP-6+ load line. The ISL6263B can
accommodate DCR current sensing or discrete resistor
current sensing. The DCR current sensing uses the intrinsic
series resistance of the output inductor as shown in the
application circuit of Figure 2. The discrete resistor current
sensing uses a shunt connected in series with the output
inductor as shown in the application circuit of Figure 3. In
both cases the signal is fed to the non-inverting input of the
DROOP amplifier at the VSUM pin, where it is measured
differentially with respect to the output voltage of the
converter at the VO pin and amplified. The voltage at the
DROOP pin minus the output voltage measured at the VO
pin, is proportional to the total inductor 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 within
the R³ modulator.
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
ISL6263B 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 ISL6263B will shut down when the voltage between the
VO and VSS pins exceeds the severe overvoltage protection
threshold V_OVPS of 1.55V. To prevent this issue from
occurring, it is recommended to install resistors R_opn1 and
R_opn2 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 20Ω to
100Ω.
High Efficiency Diode Emulation Mode
The ISL6263B 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. An improvement in light-load efficiency is achieved
by allowing the converter to operate in diode-emulation
mode (DEM) where the low-side MOSFET behaves as a
smart-diode, forcing the device 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 into 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 when the output DC load current is
less than ½ the inductor ripple current. Sinking negative
inductor current through the low-side MOSFET lowers
efficiency through unnecessary conduction losses. Efficiency
can be further improved with a reduction of unnecessary
switching losses by reducing the PWM frequency. The PWM
frequency can be configured to automatically make a
step-reduction upon entering DEM by forcing a
step-increase of the window voltage V_W. The window
voltage can be configured to increase approximately 30%,
50%, or not at all. The characteristic PWM frequency
reduction, coincident with decreasing load, is accelerated by
the step-increase of the window voltage. An audio filter can
be enabled that briefly turns on the low-side MOSFET gate
driver LGATE approximately every 35μs.
The converter will enter DEM after detecting three
consecutive PWM pulses with negative inductor current. The
negative inductor current is detected during the time that the
high-side MOSFET gate driver output UGATE is low, with the
exception of a brief blanking period. The voltage between
the PHASE pin and VSS pin is monitored by a comparator
that latches upon detection of positive phase voltage. The
converter will return to CCM after detecting three
consecutive PWM pulses with positive inductor current.
The inductor current is considered positive if the phase
comparator has not been latched while UGATE is low.
10
FN6388.3
July 8, 2010