ISL6377 Datasheet, PDF (31/36 Pages) Intersil Corporation – Multiphase PWM Regulator for AMD Fusion™ Desktop CPUs Using SVI 2.0

ISL6377
T2(s), that describe the entire system. Figure 28 conceptually
shows T1(s) measurement set-up, and Figure 29 conceptually
shows T2(s) measurement set-up. The VR senses the inductor
current, multiplies it by a gain of the load-line slope, adds it on top
of the sensed output voltage, and then feeds it to the
compensator. T1 is measured after the summing node, and T2 is
measured in the voltage loop before the summing node. The
spreadsheet gives both T1(s) and T2(s) plots. However, only T2(s)
can actually be measured on an ISL6377 regulator.
Q1
VIN
GATE Q2
DRIVER
L
VO
COUT
iO
LOAD LINE SLOPE
20
MOD.
EA ^-
Ω
COMP +
VID
CHANNEL B
LOOP GAIN = _{CHANNEL A}
+
+
ISOLATION
TRANSFORMER
CHANNEL A
NETWORK
ANALYZER
EXCITATION OUTPUT
CHANNEL B
FIGURE 28. LOOP GAIN T1(s) MEASUREMENT SET-UP
T1(s) is the total loop gain of the voltage loop and the droop loop.
It always has a higher crossover frequency than T2(s), therefore
has a higher impact on system stability.
T2(s) is the voltage loop gain with closed droop loop, thus having
a higher impact on output voltage response.
Design the compensator to get stable T1(s) and T2(s) with sufficient
phase margin and an output impedance equal to or smaller than
the load-line slope.
Q1
V_IN
GATE Q2
DRIVER
L
V_O
C_O
I_O
LOAD LINE SLOPE
MOD.
COMP
-
EA
+
VID
++ ₂₀
Ω
CHANNEL B
LOOP GAIN =
CHANNEL A
ISOLATION
TRANSFORMER
CHANNEL A
NETWORK
ANALYZER
CHANNEL B
EXCITATION OUTPUT
FIGURE 29. LOOP GAIN T2(s) MEASUREMENT SET-UP
Current Balancing
Refer to Figures 14 through 20 for information on current
balancing. The ISL6377 achieves current balancing through
matching the ISEN pin voltages. R_isen and C_isen form filters to
remove the switching ripple of the phase node voltages. It is
recommended to use a rather long R_isenC_isen time constant such
that the ISEN voltages have minimal ripple and represent the DC
current flowing through the inductors. Recommended values are
R_s = 10kΩ and C_s = 0.22µF.
Thermal Monitor Component Selection
The ISL6377 features two pins, NTC and NTC_NB, which are used
to monitor motherboard temperature and alert the AMD CPU if a
thermal issues arises. The basic function of this circuitry is
outlined in the “Thermal Monitor [NTC, NTC_NB]” section on
page 26. Figure 30 shows the basic configuration of the NTC
resistor, R_NTC, and offset resistor, R_S, used to generate the
warning and shutdown voltages at the NTC pin.
NTC
330kΩ R_NTC
8.45kΩ _Rs
INTERNAL TO
ISL6377
30µA
VR_HOT_L
+V
R
MONITOR
WARNING SHUTDOWN
640mV 580mV
FIGURE 30. THERMAL MONITOR FEATURE OF THE ISL6377
As the board temperature rises, the NTC thermistor resistance
decreases and the voltage at the NTC pin drops. When the
voltage on the NTC pin drops below the thermal warning
threshold of 0.640V, then VR_HOT_L is pulled low. When the
AMD CPU detects VR_HOT_L has gone low, it will begin throttling
back load current on both outputs to reduce the board
temperature.
If the board temperature continues to rise, the NTC thermistor
resistance will drop further and the voltage at the NTC pin could
drop below the thermal shutdown threshold of 0.580V. Once this
threshold is reached, the ISL6377 shuts down both Core and
Northbridge VRs indicating a thermal fault has occurred prior to
the thermal fault counter triggering a fault.
Selection of the NTC thermistor can vary depending on how the
resistor network is configured. The equivalent resistance at the
typical thermal warning threshold voltage of 0.64V is defined in
Equation 42.
0_----._--6_---4_----V_--
30 μ A
=
21.3 k Ω
(EQ. 42)
31
FN8336.0
August 6, 2012