TMP01
Rev. E | Page 9 of 20
The hysteresis current is readily calculated. For example, for
2 degrees of hysteresis, I
VREF
= 17 A. Next, the setpoint
voltages, V
SETHIGH
and V
SETLOW
, are determined using the VPTAT
scale factor of 5 mV/K = 5 mV/(°C + 273.15), which is 1.49 V
for 25°C. Then, calculate the divider resistors, based on those
setpoints. The equations used to calculate the resistors are
V
SETHIGH
= (T
SETHIGH
+ 273.15) (5 mV/°C)
V
SETLOW
= (T
SETLOW
+ 273.15) (5 mV/°C)
R1 (kΩ) = (V
VREF
− V
SETHIGH
)/I
VREF
= (2.5 V − V
SETHIGH
)/I
VREF
R2 (kΩ) = (V
SETHIGH
− V
SETLOW
)/I
VREF
R3 (kΩ) = V
SETLOW
/I
VREF
VPTAT
V+
1
2
3
4
8
7
6
5
TMP01
V
VREF
= 2.5V
V
SETHIGH
V
SETLOW
GND
UNDER
OVER
(V
VREF
– V
SETHIGH
)/I
VREF
= R1
(V
SETHIGH
– V
SETLOW
)/I
VREF
= R2
V
SETLOW
/I
VREF
= R3
I
VREF
00333-015
Figure 15. TMP01 Setpoint Programming
The total R1 + R2 + R3 is equal to the load resistance needed to
draw the desired hysteresis current from the reference, or I
VREF
.
The formulas shown above are also helpful in understanding
the calculation of temperature setpoint voltages in circuits other
than the standard two-temperature thermostat. If a setpoint
function is not needed, the appropriate comparator should be
disabled. SET HIGH can be disabled by tying it to V+, SET
LOW by tying it to GND. Either output can be left
unconnected.
PTAT
K
°C
°F
1.09 1.24 1.991.8651.741.6151.491.365
218 248 398373348323298273
–67 –25 257200 21215010050 77320
–55 –25 1251007550250–18
0333-016
Figure 16. Temperature—VPTAT Scale
UNDERSTANDING ERROR SOURCES
The accuracy of the VPTAT sensor output is well characterized
and specified; however, preserving this accuracy in a heating or
cooling control system requires some attention to minimizing
the various potential error sources. The internal sources of
setpoint programming error include the initial tolerances and
temperature drifts of the reference voltage VREF, the setpoint
comparator input offset voltage and bias current, and the
hysteresis current scale factor. When evaluating setpoint
programming errors, remember that any VREF error
contribution at the comparator inputs is reduced by the
resistor divider ratios. The comparator input bias current
(inputs SET HIGH, SET LOW) drops to less than 1 nA (typ)
when the comparator is tripped. This can account for some
setpoint voltage error, equal to the change in bias current times
the effective setpoint divider ladder resistance to ground.
The thermal mass of the TMP01 package and the degree of
thermal coupling to the surrounding circuitry are the largest
factors in determining the rate of thermal settling, which
ultimately determines the rate at which the desired temperature
measurement accuracy may be reached. Thus, allow sufficient
time for the device to reach the final temperature. The typical
thermal time constant for the plastic package is approximately
140 seconds in still air. Therefore, to reach the final temperature
accuracy within 1%, for a temperature change of 60 degrees, a
settling time of 5 time constants, or 12 minutes, is necessary.
The setpoint comparator input offset voltage and zero hyster-
esis current affect setpoint error. While the 7 A zero hysteresis
current allows the user to program the TMP01 with moderate
resistor divider values, it does vary somewhat from device to
device, causing slight variations in the actual hysteresis obtained
in practice. Comparator input offset directly impacts the pro-
grammed setpoint voltage and thus the resulting hysteresis
band, and must be included in error calculations.
External error sources to consider are the accuracy of the pro-
gramming resistors, grounding error voltages, and the overall
problem of thermal gradients. The accuracy of the external
programming resistors directly impacts the resulting setpoint
accuracy. Thus, in fixed-temperature applications, the user
should select resistor tolerances appropriate to the desired
programming accuracy. Resistor temperature drift must be
taken into account also. This effect can be minimized by
selecting good quality components, and by keeping all com-
ponents in close thermal proximity. Applications requiring high
measurement accuracy require great attention to detail
regarding thermal gradients. Careful circuit board layout,
component placement, and protection from stray air currents
are necessary to minimize common thermal error sources.
Also, the user should take care to keep the bottom of the set-
point programming divider ladder as close to GND (Pin 4) as
possible to minimize errors due to IR voltage drops and coup-
ling of external noise sources. In any case, a 0.1 F capacitor for
power supply bypassing is always recommended at the chip.
SAFETY CONSIDERATIONS IN HEATING AND
COOLING SYSTEM DESIGN
Designers should anticipate potential system fault conditions,
which may result in significant safety hazards, which are outside
the control of and cannot be corrected by the TMP01-based
circuit. Observe governmental and industrial regulations
regarding safety requirements and standards for such designs
where applicable.
