ISL28288FUZ データシートの表示（PDF） - Intersil
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ISL28288FUZ Datasheet PDF : 13 Pages
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ISL28288, ISL28488
noise injection into the channel being used. The proper way
to prevent this oscillation is to short the output to the
negative input and ground the positive input (as shown in
Figure 34).
-
ISL28288
+
FIGURE 34. PREVENTING OSCILLATIONS IN UNUSED
CHANNELS
Proper Layout Maximizes Performance
To achieve the maximum performance of the high input
impedance and low offset voltage of the ISL28288, care
should be taken in the circuit board layout. The PC board
surface must remain clean and free of moisture to avoid
leakage currents between adjacent traces. Surface coating
of the circuit board will reduce surface moisture and provide
a humidity barrier, reducing parasitic resistance on the
board. When input leakage current is a concern, the use of
guard rings around the amplifier inputs will further reduce
leakage currents. Figure 35 shows a guard ring example for
a unity gain amplifier that uses the low impedance amplifier
output at the same voltage as the high impedance input to
eliminate surface leakage. The guard ring does not need to
be a specific width, but it should form a continuous loop
around both inputs. For further reduction of leakage
currents, components can be mounted to the PC board
using Teflon standoff insulators.
HIGH IMPEDANCE INPUT
V+
IN
FIGURE 35. GUARD RING EXAMPLE FOR UNITY GAIN
AMPLIFIER
Example Application
Thermocouples are the most popular temperature-sensing
device because of their low cost, interchangeability, and
ability to measure a wide range of temperatures. The
ISL28288 (Figure 36) is used to convert the differential
thermocouple voltage into single-ended signal with 10X gain.
The ISL28288's rail-to-rail input characteristic allows the
thermocouple to be biased at ground and the amplifier to run
from a single 5V supply.
.
R3 10kΩ
R2
K TYPE
THERMOCOUPLE
10kΩ
R4
100kΩ
+ V+
ISL28X88
-
V-
+
5V
410µV/°C
R1
100kΩ
FIGURE 36. THERMOCOUPLE AMPLIFIER
Current Limiting
The ISL28288 has no internal current-limiting circuitry. If the
output is shorted, it is possible to exceed the Absolute
Maximum Rating for output current or power dissipation,
potentially resulting in the destruction of the device.
Power Dissipation
It is possible to exceed the +150°C maximum junction
temperatures under certain load and power-supply
conditions. It is therefore important to calculate the
maximum junction temperature (TJMAX) for all applications
to determine if power supply voltages, load conditions, or
package type need to be modified to remain in the safe
operating area. These parameters are related in Equation 1:
TJMAX = TMAX + (θJAxPDMAXTOTAL)
(EQ. 1)
where:
• PDMAXTOTAL is the sum of the maximum power
dissipation of each amplifier in the package (PDMAX)
• PDMAX for each amplifier is calculated in Equation 2:
PDMAX
=
2*VS × ISMAX + (VS
-
VO
U
T
MA
X
)
×
V-----O----U----T----M-----A----X--
RL
(EQ. 2)
where:
• TMAX = Maximum ambient temperature
• θJA = Thermal resistance of the package
• PDMAX = Maximum power dissipation of 1 amplifier
• VS = Supply voltage (Magnitude of V+ and V-)
• IMAX = Maximum supply current of 1 amplifier
• VOUTMAX = Maximum output voltage swing of the
application
• RL = Load resistance
11
FN6339.1
June 28, 2007
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