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TMP12FS View Datasheet(PDF) - Analog Devices

Part Name
Description
Manufacturer
TMP12FS
ADI
Analog Devices ADI
TMP12FS Datasheet PDF : 0 Pages
TMP12
LOW, deactivating the open-collector output and disabling the
hysteresis current buffer output. The scale factor for the pro-
grammed hysteresis current is:
I = IVREF = 5 µA/°C ؉ 7 µA
Thus, since VREF = 2.5 V, a reference load resistance of 357 k
or greater (output current of 7 µA or less) will produce a tem-
perature setpoint hysteresis of zero degrees. For more details, see
the temperature programming discussion below. Larger values of
load resistance will only decrease the output current below 7 µA,
but will have no effect on the operation of the device. The
amount of hysteresis is determined by selecting an appropriate
value of load resistance for VREF, as shown below.
Programming the TMP12
The basic thermal monitoring application only requires a simple
three-resistor ladder voltage divider to set the high and low
setpoints and the hysteresis. These resistors are programmed in
the following sequence:
1. Select the desired hysteresis temperature.
2. Calculate the hysteresis current, IVREF
3. Select the desired setpoint temperatures.
4. Calculate the individual resistor divider ladder values needed
to develop the desired comparator setpoint voltages at the
Set High and Set Low inputs.
The hysteresis current is readily calculated, as shown above. For
example, to produce 2 degrees of hysteresis IVREF should be set
to 17 µA. Next, the setpoint voltages VSETHIGH and VSETLOW are
determined using the VPTAT scale factor of 5 mV/K = 5 mV/
(°C ؉ 273.15), which is 1.49 V for ؉25°C. Finally, the divider
resistors are calculated, based on the setpoint voltages.
The setpoint voltages are calculated from the equation:
VSET = (TSET ؉ 273.15)(5 mV/°C)
This equation is used to calculate both the VSETHIGH and the
VSETLOW values. A simple 3-resistor network, as shown in Figure
18, determines the setpoints and hysteresis value. The equations
used to calculate the resistors are:
R1 (k) = (VREF ؊ VSETHIGH)/IVREF = (2.5 V ؊ VSETHIGH)/IVREF
R2 (k) = (VSETHIGH ؊ VSETLOW)/IVREF
R3 (k) = VSETLOW/IVREF
VREF = 2.5 V
(VREF – VSETHIGH) / IVREF = R1
VSETHIGH
(VSETHIGH – VSETLOW) / IVREF = R2
VSETLOW
VSETLOW / IVREF = R3
GND
1
IVREF
2
3
TMP12
4
8 V+
7 OVER
6 UNDER
5 HEATER
Figure 18. TMP12 Setpoint Programming
For example, setting the high setpoint for ؉80°C, the low
setpoint for ؉55°C, and hysteresis for 3°C produces the
following values:
IHYS = IVREF = (3°C ؋ 5 µA/°C) ؉ 7 µA = 15 µA ؉ 7 µA =
22 µA
VSETHIGH = (TSETHIGH ؉ 273.15)(5 mV/°C) = (80°C ؉
273.15)(5 mV/°C) = 1.766 V
VSETLOW = (TSETLOW ؉ 273.15)(5 mV/°C) = (55°C ؉ 273.15)
(5 mV/°C) = 1.641 V
R1 (k) = (VREF ؊ VSETHIGH)/IVREF = (2.5 V ؊ 1.766 V)/
22 µA = 33.36 k
R2 (k) = (VSETHIGH ؊ VSETLOW)/IVREF = (1.766 V ؊ 1.641 V)/
22 µA = 5.682 k
R3 (k) = VSETLOW/ IVREF = (1.641 V)/22 µA = 74.59 k
The total of R1 ؉ R2 ؉ R3 is equal to the load resistance
needed to draw the desired hysteresis current from the
reference, or IVREF.
The nomograph of Figure 19 provides an easy method of
determining the correct VPTAT voltage for any temperature.
Simply locate the desired temperature on the appropriate scale
(K, °C or °F) and read the corresponding VPTAT value from
the bottom scale.
218
K
248
273
298
323
348
373
398
–55
°C
–25 –18 0
25
50
75
100
125
–67
°F
–25 0
32 50 77 100
150
200 212 257
1.09
VPTAT
1.24 1.365 1.49 1.615 1.74 1.865 1.99
Figure 19. Temperature ؊ VPTAT Scale
The formulas shown above are also helpful in understanding the
calculations of temperature setpoint voltages in circuits other
than the standard two-temperature thermal/airflow monitor. If a
setpoint function is not needed, the appropriate comparator in-
put should be disabled. SETHIGH can be disabled by tying it
to V؉ or VREF, SETLOW by tying it to GND. Either output
can be left disconnected.
Selecting Setpoints
Choosing the temperature setpoints for a given system is an em-
pirical process, because of the wide variety of thermal issues in
any practical design. The specific setpoints are dependent on
such factors as airflow velocity in the system, adjacent compo-
nent location and size, PCB thickness, location of copper
ground planes, and thermal limits of the system.
The TMP12’s temperature rise above ambient is proportional to
airflow (Figures 1, 2 and 16). As a starting point, the low
setpoint temperature could be set at the system ambient temp-
erature (inside the enclosure) plus one half of the temperature
rise above ambient (at the actual airflow in the system). With
this setting, the low limit will provide a warning either if the fan
output is reduced or if the ambient temperature rises (for ex-
ample, if the fan’s cool air intake is blocked). The high setpoint
could then be set for the maximum system temperature to pro-
vide a final system shutdown control.
–8–
REV. 0
 

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