ISL6398 Datasheet, PDF (28/57 Pages) Intersil Corporation – Programmable soft-start rate and DVID rate

ISL6398
The block diagram of thermal monitoring function is shown in
Figure 20. One NTC resistor should be placed close to the
respective power stage of the voltage regulator VR to sense the
operational temperature, and pull-up resistors are needed to form
the voltage dividers for the TM pins. 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 21 shows the
TM voltage over the temperature for a typical design with a
recommended 6.8k NTC (P/N: NTHS0805N02N6801 from
Vishay, b = 3477) and 1k resistor R_TM. It is recommended to use
those resistors for the accurate temperature compensation since
the internal thermal digital code is developed based upon these
two components. If a different value is used, the temperature
coefficient must be close to 3477 and R_TM must be scaled
accordingly. For instance, NTC = 10k (b = 3477), then R_TM
should be 10k/6.8k*1k = 1.47k.
VCC
RTM
TM
oc
RNTC1
NTC BETA ~ 3477
THERMAL TRIP POINT
LOOKUP TABLE
(+90 TO +120⁰C)
TMAX
+
-
VR_HOT#
ISL6398
FIGURE 20. BLOCK DIAGRAM OF THERMAL MONITORING
FUNCTION
There is a comparator with hysteresis to compare the TM pin
voltage to the threshold set by the TMAX register (programmable
via PMBus E8[2:0]) for VR_HOT# signal. With TMAX set at
+100°C, the VR_HOT# signal is pulled to GND when TM voltage is
lower than 39.12% of V_CC voltage, and is open (pulled high
through TM) when TM voltage increases to above 40.98% of V_CC
voltage. The comparator trip point will be programmable by
TMAX values. Figure 21 shows the operation of those signals.
100
90
80
70
60
50
40
30
20
0
20
40 60
80 100 120 140
TEMPERATURE (^°C)
FIGURE 21. THE RATIO OF TM VOLTAGE TO NTC TEMPERATURE
WITH RECOMMENDED PARTS
Submit Document Feedback ₂₈
TM
40.98%*VCC
39.12%*VCC
VR_HOT#
TEMPERATURE
T1 T2
FIGURE 22. VR_HOT# SIGNAL (TMAX = +100°C) vs TM VOLTAGE
Based on the NTC temperature characteristics and the desired
threshold of the VR_HOT# signal, the pull-up resistor R_TM of TM
pin is given by Equation 19:
R_TM = 1.557xR_{NTCT2}
(EQ. 19)
R_NTC(T2) is the NTC resistance at the VR_HOT# threshold
temperature T2. The VR_HOT# is de-asserted at temperature T1,
as shown in Table 6. The NTC directly senses the temperature of
the PCB and not the exact temperature of the hottest component
on the board due to airflow and varied thermal impedance.
Therefore, the user should select a lower TMAX number,
depending upon the mismatch between NTC and the hottest
components, than such component to guarantee a safe
operation.
TABLE 6. VR_HOT# TYPICAL TRIP POINT AND HYSTERESIS
TMAX
(°C)
85
VR_HOT# LOW (°C;
T2, %V_CC)
83.1; 48.94%
VR_HOT# OPEN
(°C; T1, %V_CC)
80.3; 51.04%
HYSTERESIS
(°C)
2.7
90
88.6; 45.52%
85.9; 47.56%
2.7
95
94.3; 42.26%
91.4; 44.20%
2.9
100
100.0; 39.12%
97.1; 40.98%
2.9
105
106.1; 36.14% 103.0; 37.92%
3.1
110
109.1; 33.32% 106.1; 35.00%
3.0
115
115.5; 30.68% 112.3; 32.24%
3.2
120
118.7; 28.24%
115.5; 29.7%
3.2
In addition, as the temperature increase, the voltage on the TM
pin drops. The controller is disabled when the TM pin voltage drops
below 0.95 (typically) and becomes active again when it is above
1.05V (typically).
Temperature Compensation
The ISL6398 supports inductor DCR sensing, or resistive sensing
techniques. The inductor DCR has a positive temperature
coefficient, which is about +0.385%/°C. Since the voltage across
the inductor is sensed for the output current information, the
sensed current has the same positive temperature coefficient as
the inductor DCR. n order to obtain the correct current
information, there should be a way to correct the temperature
impact on the current sense component.
FN8575.1
August 13, 2015