AD22105 Просмотр технического описания (PDF) - Analog Devices
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AD22105 Datasheet PDF : 11 Pages
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AD22105
THEORY OF OPERATION
The AD22105 is a single-supply semiconductor thermostat
switch that uses a circuit architecture to realize the combined
functions of a temperature sensor, setpoint comparator, and output
stage all in one IC. By using one external resistor, the AD22105
can be programmed to switch at any temperature selected by
the system designer in the −40°C to +150°C range. The internal
comparator is designed to switch accurately as the ambient
temperature rises past the setpoint temperature. When the ambient
temperature falls, the comparator relaxes its output at a somewhat
lower temperature than that at which the comparator originally
switched. The difference between the switch and unswitched
temperatures, known as the hysteresis, is nominally 4°C.
THE SETPOINT RESISTOR
Determine the setpoint resistor by the following equation:
RSET
=
39 MΩ °C
TSET (°C) + 281.6°C
− 90.3 kΩ
(1)
Connect the setpoint resistor directly between the RSET pin and
the GND pin. If a ground plane is used, connect the resistor
directly to this plane at the closest available point.
The setpoint resistor, RSET, can be almost any resistor type.
However, the resistor initial tolerance and thermal drift affects
the accuracy of the programmed switching temperature. For most
applications, a 1% metal film resistor provides the best tradeoff
between cost and accuracy. Calculations for computing an error
budget are found in the Effect of Resistor Tolerance and Thermal
Drift on Setpoint Accuracy section.
After RSET is calculated, the calculated value does not agree with
readily available standard resistors of the chosen tolerance. To
achieve an RSET value as close as possible to the calculated value,
a compound resistor can be constructed by connecting two
resistors in series or in parallel. To conserve cost, one moderately
precise resistor and one lower precision resistor can be combined.
If the moderately precise resistor provides most of the necessary
resistance, the lower precision resistor can provide a fine
adjustment. Consider an example where the closest standard
1% resistor has only 90% of the value required for RSET. If a
5% series resistor is used for the remainder, the tolerance of
the resistor only adds 5% of 10% or 0.5% additional error to
the combination. Likewise, the 1% resistor only contributes
90% of 1% or 0.9% error to the combination. These two
contributions are additive, resulting in a total compound
resistor tolerance of 1.4%.
Data Sheet
EFFECT OF RESISTOR TOLERANCE AND THERMAL
DRIFT ON SETPOINT ACCURACY
Figure 4 shows the typical accuracy error in setpoint
temperature as a function of the programmed setpoint
temperature. This curve assumes an ideal resistor for RSET.
Figure 5 can be used to calculate additional setpoint error as a
function of resistor tolerance. Figure 5 shows additional error
beyond the initial accuracy error of the device and must be
added to the value found in Table 1. For example, consider
using the AD22105 programmed to switch at 125°C. Figure 5
indicates that at +125°C, the additional error is approximately
−0.2°C/% of RSET. If a 1% resistor (of exactly correct value) is
chosen, the additional error is −0.2°C/% × 1% or −0.2°C. If the
closest standard resistor value is 0.6% away from the calculated
value, the total error is 0.6% for the nominal value and 1% for
the tolerance or 1.006 × 1. 01 or 1.01606 (about 1.6%). The
closest resistor value differing slightly from the calculated value
can lead to an additional setpoint error as high as 0.32°C.
For additional accuracy considerations, take the thermal drift of
the setpoint resistor into account. For example, consider that
the drift of the metal film resistor is 100 ppm/°C. Because this
drift is usually referred to 25°C, the setpoint resistor can be in
error by an additional 100 ppm/°C × (125°C − 25°C) or 1%.
Using a setpoint temperature of 125°C, this error source adds an
additional −0.2°C (for positive drift) making the overall
setpoint error potentially −0.52°C higher than the original
accuracy error.
To combine and calculate the initial tolerance and thermal drift
effects of the setpoint resistor use the following equation:
RMAX = RNOM × (1 + ε) × (1 + TC × (TSET − 25°C))
where:
RMAX is the worst case value that the setpoint resistor can be at TSET.
RNOM is the standard resistor with a value closest to the desired RSET.
ε is the 25°C tolerance of the chosen resistor (usually 1%, 5%, or
10%).
TC is the temperature coefficient of the available resistor.
TSET is the desired setpoint temperature.
After calculation, compare RMAX to the desired RSET from
Equation 1. The required value of RSET at a TSET of 125°C is
5.566 kΩ. If the nearest standard resistor value is 5.600 kΩ, its worst
case maximum value at +125°C is 5.713 kΩ, which is +2.6%
higher than RSET, leading to a total additional error of −0.52°C
beyond that given in Table 1.
Rev. A | Page 8 of 11
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