LM57B10EB/NOPB National Semiconductor, LM57B10EB/NOPB Datasheet - Page 2

• Manufacturer Part Number: LM57B10EB/NOPB
• Description: Temperature Switch/Sensor Eval. Board
• Manufacturer: National Semiconductor

Specifications of LM57B10EB/NOPB

• Silicon Manufacturer: National
• Application Sub Type: Temperature Sensor
• Kit Application Type: Sensing - Temperature
• Silicon Core Number: LM57
• Kit Contents: Board And Literature

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The analog temperature signal is derived directly from the
voltage divider, producing a voltage level that is an analog of
the thermistor's temperature (V
necessary to set the gain of the circuit and to keep the ther-
mistor operating within its optimal power dissipation, which
minimizes temperature-induced error in the resistance. The
over-temperature alarm (T
the output of the thermistor to the input of the comparator in
order to set the voltage (over-temperature level) at which the
comparator output is to go active. A hysteresis feedback loop
is included to keep the comparator from rapidly switching
back and forth when V
The discrete thermistor solution does have a number of de-
sign drawbacks such as the accumulated error of all the
components, power consumption, linearity, quantization
noise, operating temperature range, board area, and produc-
tion cost. The LM57's integrated analog temperature sensor
and temperature switch addresses these design drawbacks
and improves the performance of the system.
The Integrated LM57 Circuit
National Semiconductor developed the LM57 integrated cir-
cuit to provide a complete temperature sensor and over-
temperature alarm solution in one component. It integrates all
the features of the discrete circuit shown in Figure 1, plus it
includes additional features – all while providing better per-
formance specifications. The LM57 single-chip solution is
illustrated in Figure 2. Since the whole sensor solution is in-
tegrated, fewer components are necessary, and extra fea-
tures are included, like the active-low and active-high trip
point output and an input pin that allows a system to test the
TEMP
OVER
is equal to the V
) is generated by connecting
TEMP
). The R
FIGURE 2. LM57 Integrated Circuit Application
REF
BIAS1
.
resistor is
2
LM57 functionality while still in the system (in situ). The tem-
perature trip point (T
lated values of the sense resistors, R
Accuracy
One of the most important measurements in any temperature
sensor circuit is the accuracy (or error) of the total circuit.
When designing a discrete solution, the error from each com-
ponent ust be added to determine the worst-case total circuit
error of the measurement. For example, the accuracy of the
analog temperature output (V
circuit (Figure 1) will be affected by the sum of the accuracies
of the thermistor and of resistor R
T
of V
(R
resistor and the hysteresis resistor. If this circuit were used to
control, for example, a large building climate control system,
these errors could mean motors and elements will be running
when they don't need to be — resulting in excessive power
usage by the system.
The LM57 does not include these multiple sources of error
since it is fully integrated (Figure 3) and is calibrated at the
time of production. Thus the designer does not have to add
up all of the sources of error due to the constituent compo-
nents. The LM57's guaranteed maximum error of the V
alalog output is ±0.7°C, and the maximum error of the
T
the system engineer can design without calculating the sum
of all the error sources in the circuit.
OVER
OVER
BIAS2
TEMP
alarm output is ±1.5°C. These are worst-case limits that
digital alarm will be affected not only by the accuracy
), the shunt reference, the comparator, the feedback
, but also the error inherent in the reference bias
TRIP
) is easily set by choosing the corre-
TEMP
) in the discrete thermistor
BIAS1
SENSE1
30100402
. The accuracy of the
and R
SENSE2
TEMP
.