TMP01GBC AD [Analog Devices], TMP01GBC Datasheet - Page 7

• Manufacturer Part Number: TMP01GBC
• Description: Low Power, Programmable Temperature Controller
• Manufacturer: AD [Analog Devices]

REV. C
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 hys-
teresis current scale factor. When evaluating setpoint program-
ming errors, remember that any VREF error contribution at the
comparator inputs is reduced by the resistor divider ratios. The
comparator input bias current (inputs SETHIGH, SETLOW)
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 ulti-
mately determines the rate at which the desired temperature
measurement accuracy may be reached. Thus, one must 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 min-
utes, 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 de-
vice, causing slight variations in the actual hysteresis obtained
Figure 5. Supply Current vs. Supply Voltage
550
525
500
475
450
425
400
375
350
0
+125 C
+25 C
–40 C
+85 C
5
SUPPLY VOLTAGE – Volts
10
15
–55 C
20
–7–
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 select-
ing good quality components, and by keeping all components in
close thermal proximity. Applications requiring high measure-
ment 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
setpoint programming divider ladder as close to GND (Pin 4)
as possible to minimize errors due to IR voltage drops and cou-
pling 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. Governmental and industrial regulations regarding
safety requirements and standards for such designs should be
observed where applicable.
Figure 6. Minimum Supply Voltage vs. Temperature
5.0
4.5
4.0
3.5
3.0
–75
–50
–25
TEMPERATURE – C
0
25
50
75
TMP01
100
125