ISL6326B Datasheet, PDF (22/30 Pages) Intersil Corporation – 4-Phase PWM Controller with 8-Bit DAC Code Capable of Precision DCR Differential Current Sensing

ISL6326B
The diagram of thermal monitoring function block is shown in
Figure 11. One NTC resistor should be placed close to the
power stage of the voltage regulator to sense the operational
temperature, and one pull-up resistor is needed to form the
voltage divider for the TM pin. As the temperature of the
power stage increases, the resistance of the NTC will
reduce, resulting in the reduced voltage at the TM pin.
Figure 12 shows the TM voltage over the temperature for a
typical design with a recommended 6.8kΩ NTC (P/N:
NTHS0805N02N6801 from Vishay) and 1kΩ resistor RTM1.
We recommend using those resistors for the accurate
temperature compensation.
There are two comparators with hysteresis to compare the
TM pin voltage to the fixed thresholds for VR_FAN and
VR_HOT signals respectively. The VR_FAN signal is set to
high when the TM voltage is lower than 33% of VCC voltage,
and is pulled to GND when the TM voltage increases to
above 39% of VCC voltage. The VR_FAN signal is set to
high when the TM voltage goes below 28% of VCC voltage,
and is pulled to GND when the TM voltage goes back to
above 33% of VCC voltage. Figure 13 shows the operation
of those signals.
100%
90%
80%
70%
60%
50%
40%
30%
20%
0
V_TM / V_CC vs . Te m pe rature
20 40 60 80 100 120 140
Te m pe rature (^oC)
FIGURE 12. THE RATIO OF TM VOLTAGE TO NTC
TEMPERATURE WITH RECOMMENDED PARTS
TM
0.39*Vcc
0.33*Vcc
0.28*Vcc
VCC
VR_FAN
R_TM1
TM
0.33V_CC
VR_HOT
o_c R_NTC
0.28V_CC
FIGURE 11. BLOCK DIAGRAM OF THERMAL MONITORING
FUNCTION
VR_FAN
VR_HOT
T1 T2 T3
Temperature
FIGURE 13. VR_HOT AND VR_FAN SIGNAL vs TM VOLTAGE
Based on the NTC temperature characteristics and the
desired threshold of the VR_HOT signal, the pull-up resistor
RTM1 of TM pin is given by:
R_TM1 = 2.75xR_NTC(T3)
(EQ. 18)
R_NTC(T3) is the NTC resistance at the VR_HOT threshold
temperature T3.
The NTC resistance at the set point T2 and release point T1
of VR_FAN signal can be calculated as:
R_NTC(T2) = 1.267xR_NTC(T3)
(EQ. 19)
R_NTC(T1) = 1.644xR_NTC(T3)
(EQ. 20)
With the NTC resistance value obtained from Equations 19
and 20, the temperature value T2 and T1 can be found from
the NTC datasheet.
22
FN9286.0
April 21, 2006