ISL6277A Datasheet, PDF (33/38 Pages) Intersil Corporation – Multiphase PWM Regulator for AMD Fusion Mobile CPUs Using SVI 2.0

ISL6277A
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.
L
VO
Q1
VIN
GATE Q2
DRIVER
CO
IO
LOAD LINE SLOPE
MOD.
COMP
-
EA
+
VID
++ _20Ω
CHANNEL B
LOOP GAIN =
CHANNEL A
ISOLATION
TRANSFORMER
CHANNEL A
NETWORK
ANALYZER
CHANNEL B
EXCITATION OUTPUT
FIGURE 31. LOOP GAIN T2(s) MEASUREMENT SET-UP
Current Balancing
Refer to Figures 15 through 22 for information on current
balancing. The ISL6277A 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 ISL6277A 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 section. Figure 32 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
ISL6277A
30µA
VR_HOT_L
+V
R
MONITOR
WARNING SHUTDOWN
640mV 580mV
FIGURE 32. THERMAL MONITOR FEATURE OF THE ISL6277A
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 ISL6277A 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 38.
0_----._--6_---4_----V_{-- = 21.3kΩ}
30 μ A
(EQ. 38)
The equivalent resistance at the typical thermal shutdown
threshold voltage of 0.58V required to shutdown both outputs is
defined in Equation 39.
0_-3---.0_--5_--μ-8_---A-V_{-- = 19.3kΩ}
(EQ. 39)
The NTC thermistor value correlates to the resistance change
between the warning and shutdown thresholds and the required
temperature change. If the warning level is designed to occur at a
board temperature of +100°C and the thermal shutdown level at
a board temperature of +105°C, then the resistance change of
the thermistor can be calculated. For example, a Panasonic NTC
thermistor with B = 4700 has a resistance ratio of 0.03939 of its
nominal value at +100°C and 0.03308 of its nominal value at
+105°C. Taking the required resistance change between the
thermal warning threshold and the shutdown threshold and
dividing it by the change in resistance ratio of the NTC thermistor
at the two temperatures of interest, the required resistance of
the NTC is defined in Equation 40.
(^--(-0^-2--.^{--10---.3--3--}9^--k-3^--Ω-9^{-----––-----10---9.--}0^-.--33---3^-k---0^Ω---8^--)--) ^{= 317kΩ}
(EQ. 40)
The closest standard thermistor to the value calculated with
B = 4700 is 330kΩ. The NTC thermistor part number is
ERTJ0EV334J. The actual resistance change of this standard
thermistor value between the warning threshold and the
shutdown threshold is calculated in Equation 41.
(330kΩ ⋅ 0.03939) – (330kΩ ⋅ 0.03308) = 2.082kΩ
(EQ. 41)
Since the NTC thermistor resistance at +105°C is less than the
required resistance from Equation 39, additional resistance in
series with the thermistor is required to make up the difference.
A standard resistor, 1% tolerance, added in series with the
thermistor will increase the voltage seen at the NTC pin. The
additional resistance required is calculated in Equation 42.
19.3kΩ – 10.916kΩ = 8.384kΩ
(EQ. 42)
The closest, standard 1% tolerance resistor is 8.45kΩ.
33
FN8322.0
December 19, 2012