an1844

Application Note 1844
ISL71590SEH Evaluation Board User Guide
Circuit Comments
The ISL71590SEH is an integrated-circuit temperature sensing
transducer which produces an output current proportional to
absolute temperature. The device acts as a high impedance
constant current regulator passing 1µA/Kelvin for supply
voltages between +4V and +33V.
The ISL71590SEHEV1Z evaluation platform supports the
ISL71590SEH by highlighting four common application
configurations. The evaluation board is composed of two
ISL71590SEH devices, both of which can be individually
heated and with a slotted PCB they are thermally isolated from
each other.
With jumpers for circuit configuration, the four applications
demonstrated are; single temperature sensor; lowest
temperature in an array; average temperature in an array and
differential temperature. These are illustrated in Figures 3
through 11 with supporting text accompanying each figure.
The configuration jumpers are grouped by application circuit
so populating all the jumpers within a defined and labeled
area configures the evaluation board for a particular functional
configuration.
See Figure 1 for the ISL71590SEHEV1Z photograph.
The ISL71590SEHEV1Z sensors are potted with resistor
heaters in Arctic Alumina Thermal Adhesive to simulate an
installed application. In real world applications, as examples
the sensors may be embedded in hollow metal probe sleeves,
bolts or fastened to plate surfaces.
This user guide walks through each of the four applications
illustrating noteworthy observations of each.
All evaluations and results in this document are done in still air.
Table 1 explains the ISL71590SEHEV1Z connections.
TABLE 1. ISL71590SEHEV1Z CONNECTIONS
BOARD
CONNECTION NAME
DESCRIPTION AND FUNCTION
V+
V+ connection to ISL71590SEH and op-amp V+
bias.
V-/GND
Negative voltage connection to ISL71590SEH
and negative bias to op-amp when evaluating
differential temperature configuration.
IOUT * R
Output used for single, average and lowest
temperature configurations, VOUT = IOUT * R.
AMP V-
Connect to 0V when evaluating differential
temperature configuration with op-amp.
DIFFERENTIAL OUT Output used when evaluating differential
temperature configuration.
HEATER 1
U1 heater voltage connection.
HEATER 2
U2 heater voltage connection.
Table2 illustrates the jumper installation definitions.
The ISL71590SEHEV1Z is shipped in single sensor
configuration.
TABLE 2. JUMPER CONFIGURATION
AVERAGE
LOWEST
SINGLE
TEMPERATURE TEMPERATURE
SENSOR
(PARALLEL)
(SERIES)
TEMPERATURE
JP8
Connects U1 to
R3
JP4, JP5
JP3, P8
Connects U1
Connects U1
and U2 in series and U2 in
parallel to R3
to R3
DIFFERENTIAL
TEMPERATURE
JP2, JP6,
JP7, J P9
Connects R2, U2,
op-amp and U1 to a
common node
NOTE: Install HEATER 1 and HEATER 2 jumpers to heat U1 (left) and U2
(right) ISL71590SEH respectively.
FIGURE 1. ISL71590SEHEV1Z EVALUATION BOARD
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Application Note 1844
Typical Applications
373mV, ~+100°C
+4V TO +33V
ISL71590SEH
+
298mV, ~+25°C
VOUT = 1mV/K
1kΩ
FIGURE 2A.
450
FIGURE 3. SERIES CONFIGURATION FOR MINIMUM TEMPERATURE
OUTPUT CURRENT (µA)
400
350
VS = +14V
+
T
300
+
250
T
ISL71590SEH
+
200
218
238
258
278
298 318 338 358 378
TEMPERATURE (K)
398
418
438
T
-
FIGURE 2B.
FIGURE 2. SINGLE SENSOR OUTPUT CURRENT IS
PROPORTIONAL TO ABSOLUTE TEMPERATURE
Connecting just a single ISL71590SEH and resistor as shown in
Figure 2A provides the simplest single temperature sensing
implementation. The resulting over-temperature output current is
shown in Figure 2B illustrating the 1µA/Kelvin (K) linear
performance across the entire operating range of temperature.
For the single sensor configuration, Figure 3 shows the VOUT
increasing once the heater is turned on, the temperature
increases over time rising from the ambient +25°C (298.15K) to
a peak temperature of ~+100°C (373K) when the heater is
turned off and both the VOUT and temperature decreases. The
output current change in this example is 1µA/Kelvin, resulting in a
1mV/°C or 1mV/K change with a 1kΩ VOUT resistor. Increasing
the VOUT resistor value increases the measurement resolution.
Maximum number = (VSmin -VOUTmax)/4V
VOUT = 1mV/K
1kΩ
0.1%
FIGURE 4. LOWEST TEMPERATURE SENSING SCHEME.
AVAILABLE CURRENT IS THAT OF THE “COLDEST”
SENSOR
Connecting several ISL71590SEH temperature sensors in series,
as shown in Figure 4, results in the lowest individual temperature
in the array to be indicated, since the series output current will be
constrained by the sensor exposed to the lowest individual
temperature. The maximum number of sensors in any single
resistor string is limited by the total voltage applied divided by 4V
as each device needs to be adequately biased when the
temperature is the same or similar across all sensors.
As the higher temperature device(s) tries to output more current,
it is prevented from doing so by the series current of the lowest
temperature device and the result is that the voltage across the
higher temperature device(s) decreases to the minimum
operational voltage.
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Application Note 1844
VS
+
R * IOUT
T
- 1
+
+
T
T
-
2
-
n
.
VOUT = (Io1+Io2 -- +Ion) R
n
R
Voltage across U2 as U1 is heated and cooled
where n = number of ISL71590SEH
e.g. with 4 sensors and R = 250Ω VOUT = 1mV/K)
FIGURE 7. AVERAGE TEMPERATURE SENSING SCHEME
FIGURE 5. ISL71590SEH IN SERIES CONFIGURATION SHOWING
VOLTAGE AROSS COOLER SENSOR
In Figure 5 the resultant IOUT x ROUT and the voltage across U2
are shown. As U1 is first heated then cooled to show the
relationship between the coolest sensor which provides the
output current level and the voltage across the cooler of the two
sensors. Note that the output voltage representing the coolest
temperature in the series array does not change. The coolest
sensor will be the one with the highest voltage across it as the
others are in a collapsed state as they attempt to provide more
current than the minimum sensor is providing. Total voltage
across string is 10V.
R * IOUT
VOUT UNCHANGED UNTIL U2 IS HEATED
U1TEMP<U2 TEMP
U1 HEATED
In contrast, connecting several temperature sensors in parallel as
in Figure 7, results in the sum of the individual output currents
flowing through the output resistor. This allows for the average
temperature of the array to be expressed as a voltage. The value of
the resistor must be appropriately low enough to ensure adequate
voltage across the entire array. Keeping the output voltage at the
same scale as in Figures 2 and 4 the value of the resistor R is
chosen by the formula shown in Equation 1:
1k
R = ----------n
(EQ. 1)
where n = the number of sensors.
Figure 8 shows the result of the parallel array configuration that
returns the average temperature of the array as each ISL71590SEH
in the evaluation board array outputs a current relative to its
individual temperature. The sum of these currents flow through the
output resistor, resulting in a VOUT voltage that represents the
average of all sensor temperatures expressed in Kelvin when the
VOUT is divided by the number of sensors in the parallel array.
The VOUT is 596mV when both sensors are at +25°C, each sensor
outputting 298µA of current into the 1kΩ resistor. When one
temperature sensor is heated, the average temperature increases
resulting in the VOUT increasing, representing the average of the two
ISL71590SEH temperatures. In Figure 8 the VOUT rises to 686mV
representing an average temperature of 70°C (343K).
U2 HEATED
VOLTAGE ACROSS U2 AS U1 IS FIRST HEATED THEN COOLED AS
U2 IS HEATED
686mV, 343 K,
AVERAGE TEMPERATURE ~70°C, WITH 1 SENSOR
HEATED TO 110°C, THE OTHER 25°C
FIGURE 6. ISL71590SEH IN SERIES CONFIGURATION SHOWING
VOLTAGE ACROSS COOLER SENSOR
596mV, 296K, AVERAGE TEMPERATURE FOR BOTH 25°C
Figure 6 shows the voltage across U2 and the resultant IOUT x
ROUT. U1 is first heated and then cooled as U2 is heated. Here the
VOUT is unchanged until the second sensor is heated, then it rises
reflecting the lower temperature is rising as U2 is heated. As the
decreasing U1 temperature falls below the U2 increasing
temperature, the VOUT decreases and the voltage across U2
collapses as U1 is now the cooler dominant device. Total voltage
across string is 10V.
FIGURE 8. ISL71590SEH IN PARALLEL CONFIGURATION SHOWING
VOLTAGE REPRESENTING THE TOTAL ARRAY CURRENT
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Application Note 1844
For measuring a maximum temperature, individual sensors can
be employed in an array but must then be multiplexed to a
common output resistor and periodically polled for the individual
temperatures to be monitored.
+
U2 HEATED, 7V ON HEATER UNTIL IT SETTLES TO A 43.7°C
DIFFERENTIAL TEMPERATURE = 437mV
U1
V+
50kΩ
(R2)
(8V MIN)
V-
U1 is then heated as U2 is cooling driving the output in the
opposite polarity as U1 temperature now exceeds U2. Both
sensors are then allowed to cool, returning to within 0.2°C
differential temperature at the end of the trace and eventually to
0.01°C where it started.
5MΩ (R1)
10kΩ
(R3)
+
+
VOUT = (T2 TO T1) x
(10mV/°C)
U2
-
10kΩ
(R4)
FIGURE 9. DIFFERENTIAL THERMOMETER
Figure 9 illustrates a simple circuit useful for measuring differential
temperatures. R2 is used to trim the output of the op-amp to a
desired differential temperature reference.
Any output voltage deviation in either polarity from that reference
voltage is then an indication of a change in the differential
temperature between the two sensors.
For example, with the output trimmed to 0mV and U2 sensing the
reference temperature when U1 is cooled below the temperature of
the U2 reference temperature, the op-amp output will decrease to a
negative polarity indicating that the measured temperature is less
than the reference. Conversely an increase in U1 temperature would
result in the op-amp output voltage moving in a positive direction.
U1 HEATED
U1 COOLED
0.1V /DIV = 10°C /DIV, 50s /DIV
ISL71590SEH LEADS
FIGURE 11. ISL71590SEH DIFFERENTIAL CONFIGURATION
ACCURACY
The Figure 9 circuit has an output voltage deflection of 10mV/°C
when either U1 or U2 is held as the reference. Figure 11
illustrates that with both the differential circuit output voltage
and with an IR thermal image this is true. In the IR image, the
ISL71590SEH location is shown to be 66°C and the ambient
shown in top left corner to be 23°C. To optimize the accuracy of the
temperature measurements, high precision resistors with a low
temperature coefficient are recommended such as 0.01% metal
film or metal strip types for the output resistor.
When in the differential temp configuration connect +10V to V+
test point, -10V to V- test point and connect GND (0V) to AMP Vtest point.
U2 COOLED
U2 HEATED
IR thermal measurement
showing 43°C differential
temp between ISL71590SEH
and ambient temp
BOTH SENORS
COOLED TO AMBIENT
On the ISL71590SHEV1Z, each ISL71590SEH device has an on
board heater allowing each to be independently heated to different
temperatures by adjusting the heater voltage for each. The heaters
are two 200Ω SMD resistors mounted on each side of the
temperature sensor. Table 3 provides a guide for heater voltage to
approximate temperature increase that the sensor will be exposed
to in the epoxy embedded assembly.
TABLE 3. HEATER VOLTAGE GUIDE
0.5V/DIV = +50°C /DIV 100s/DIV
FIGURE 10. ISL71590SEH DIFFERENTIAL CONFIGURATION
OPERATION
Figure 10 displays the output of the differential temperature
configuration as shown in Figure 9 with U1 held as the
temperature reference and U2 then U1 being alternately heated.
Here the op-amp output moves in proportion to and in the
direction of the change in differential temperature relative to the
reference sensor. Starting with the differential temperature set to
0°C, at an ambient room temperature U2 is first heated until it
reaches a temperature of ~ 200°C when the heater is turned off,
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HEATER VOLTAGE
(V)
APPROXIMATE TEMPERATURE
INCREASE
(°C)
3
9
5
24
7
43
9
73
Using the PCB heaters allows a quick demonstration of the four
functional configurations, observing the VOUT changes with a
voltmeter or oscilloscope as appropriate.
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Application Note 1844
ISL71590SEHEV1Z
Differential
Out
( OPAMP -V when in
differential configuration)
AMP V(GND when in differential
configuration)
Iout x R
FIGURE 12. ISL71590SEHEV1Z SCHEMATIC
FIGURE 13. ISL71590SEHEV1Z TOP LEVEL PCB PATTERN
Intersil Corporation reserves the right to make changes in circuit design, software and/or specifications at any time without notice. Accordingly, the reader is
cautioned to verify that the document is current before proceeding.
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