ADT14

–7–

REV. 0

across an internal 1 k

Ω 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

°C, a resistor is

value of the resistor is given by the following equation:

R1

=

57.5

1.5

− Hysteresis

–57.5

R1:

Table II

R1 (k )

0.7

14.4

0.8

24.6

0.9

38.3

1.0

57.5

1.1

86.3

1.2

134.2

1.3

230

1.4

517.5

To set hysteresis in the range of 1.5

°C < T

°C, a resistor

is connected from the HYS pin (Pin 11) to the ADT14’s com-

mon (Pin 5). The value of the resistor is given by:

R2

=

201.1

Hysteresis –1.5

–57.5

Table III

R2 (k )

1.6

1953

1.75

746.9

2.0

344.7

2.25

210.6

2.5

143.6

2.75

103.4

3.0

76.6

3.25

57.4

3.50

43.1

3.75

31.8

4.0

22.9

4.25

15.6

4.5

9.5

4.75

4.4

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.

HYS

ADT14

0.65

°C < T

HYS

ADT14

1.5

°C < T

R2

P2

R1

P1

Figure 17. Hysteresis Connections for Miscellaneous

Values

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

input offset voltage and bias current, and the hysteresis current

scale factor. When evaluating setpoint programming errors,

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

µF capacitor for power

supply bypassing is always recommended at the chip.