ADT14GP AD [Analog Devices], ADT14GP Datasheet - Page 7

• Manufacturer Part Number: ADT14GP
• Description: Quad Setpoint, Programmable Temperature Monitor and Controller
• Manufacturer: AD [Analog Devices]

across an internal 1 k resistor at the comparator input. The
comparator output remains on until the voltage at the compara-
tor input, now equal to VPTAT plus the hysteresis offset, has
returned to the setpoint voltage. At this point the comparator
turns off, the open-collector output is deactivated, and the hys-
teresis current buffer is disabled.
While Table I shows simple pin-programmable hysteresis val-
ues, the user may design for intermediate values of hysteresis
between 0.65 C and 5 C using a resistive divider. To set the
hysteresis in the range 0.65 C < T
connected from the HYS pin (Pin 11) to V
value of the resistor is given by the following equation:
For various values of T
R1:
To set hysteresis in the range of 1.5 C < T
is connected from the HYS pin (Pin 11) to the ADT14’s com-
mon (Pin 5). The value of the resistor is given by:
For various values of T
REV. 0
T
1.6
1.75
2.0
2.25
2.5
2.75
3.0
3.25
3.50
3.75
4.0
4.25
4.5
4.75
HYS
T
0.7
0.8
0.9
1.0
1.1
1.2
1.3
1.4
HYS
R2
R1
( C)
( C)
HYS
HYS
1.5 Hysteresis
Hysteresis – 1.5
, the table below shows the values for
, the table below shows values for R2:
Table III
Table II
201.1
57.5
HYS
R2 (k )
1953
746.9
344.7
210.6
143.6
103.4
76.6
57.4
43.1
31.8
22.9
15.6
9.5
4.4
< 1.5 C, a resistor is
R1 (k )
14.4
24.6
38.3
57.5
86.3
134.2
230
517.5
– 57.5
– 57.5
HYS
REF
(Pin 14). The
< 5 C, a resistor
–7–
The schematics for the two circuits are shown below. As the
resistor values are approximate, it is a good idea to add a poten-
tiometer for fine adjustments.
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
potential error sources. The internal sources of setpoint pro-
gramming error include the initial tolerances and temperature
drifts of the reference voltage V
input offset voltage and bias current, and the hysteresis current
scale factor. When evaluating setpoint programming errors,
remember that any V
inputs is reduced by the resistor divider ratios. Each comparator’s
input bias current drops to less than 1 nA (typ) when the com-
parator is tripped. This change accounts for some setpoint volt-
age error, equal to the change in bias current multiplied by the
effective setpoint divider ladder resistance to ground.
The thermal mass of the ADT14 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 (see Figure 2). Thus,
one must allow sufficient time for the device to reach the final
temperature. The typical thermal time constant for the plastic
SOIC package is approximately 70 seconds in still air. There-
fore, to reach the final temperature accuracy within 1%, a set-
tling time of five time constants, or six minutes, is necessary.
External error sources to consider are the accuracy of the exter-
nal programming resistors, ground voltage errors, and 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 toler-
ances appropriate to the desired programming accuracy. Resistor
temperature drift must also be taken into account. This effect
can be minimized by selecting high quality components, and by
keeping all components 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. The
user should also take care to keep the bottom of the setpoint
programming divider ladder as close as possible to GND (Pin 5)
to minimize errors due to IR voltage drops and coupling of
external noise sources. In any case, a 0.1 F capacitor for power
supply bypassing is always recommended at the chip.
Figure 17. Hysteresis Connections for Miscellaneous
Values
V
0.65 C < T
REF
R1
ADT14
HYS
< 1.5 C
REF
P1
HYS
error contribution at the comparator
REF
, the setpoint comparator
1.5 C < T
ADT14
HYS
< 5 C
HYS
ADT14
R2
P2