ISL62773 Datasheet, PDF (32/37 Pages) Intersil Corporation – Multiphase PWM Regulator for AMD Fusion™ Desktop CPUs Using SVI 2.0

ISL62773
Thermal Monitor Component Selection
The ISL62773 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]” on page 27.
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.
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 39.
NTC
330k R_NTC
8.45k _Rs
INTERNAL TO
ISL62773
30µA
VR_HOT_L
+V
R
MONITOR
Warning Shutdown
640mV 580mV
FIGURE 32. THERMAL MONITOR FEATURE OF THE ISL62773
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 that 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 ISL62773 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 37.
0_----._--6_---4_----V_--
30  A
=
21.3 k 
(EQ. 37)
The equivalent resistance at the typical thermal shutdown
threshold voltage of 0.58V required to shutdown both outputs is
defined in Equation 38.
0_-3---.0_--5_---8_---A-V_{-- = 19.3k}
(EQ. 38)
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
--^--²----¹---^.--³----^k---^------–-----¹---⁹---^.--³----^k---^-----^--
0.03939 – 0.03308
=
317 k 
(EQ. 39)
The closest standard thermistor to the value calculated with
B = 4700 is 330k. 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 40.
330k  0.03939 – 330k  0.03308 = 2.082k
(EQ. 40)
Since the NTC thermistor resistance at +105°C is less than the
required resistance from Equation 38, 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 41.
19.3k – 10.916k = 8.384k
(EQ. 41)
The closest, standard 1% tolerance resistor is 8.45k.
The NTC thermistor is placed in a hot spot on the board, typically
near the upper MOSFET of Channel 1 of the respective output.
The standard resistor is placed next to the controller.
Layout Guidelines
PCB Layout Considerations
POWER AND SIGNAL LAYERS PLACEMENT ON THE PCB
As a general rule, power layers should be close together, either
on the top or bottom of the board, with the weak analog or logic
signal layers on the opposite side of the board. The ground-plane
layer should be adjacent to the signal layer to provide shielding.
COMPONENT PLACEMENT
There are two sets of critical components in a DC/DC converter;
the power components and the small signal components. The
power components are the most critical because they switch
large amount of energy. The small signal components connect to
sensitive nodes or supply critical bypassing current and signal
coupling.
The power components should be placed first and these include
MOSFETs, input and output capacitors, and the inductor. It is
important to have a symmetrical layout for each power train,
preferably with the controller located equidistant from each
power train. Symmetrical layout allows heat to be dissipated
equally across all power trains. Keeping the distance between
32
March 7, 2012
FN8263.0