lmp7300max National Semiconductor Corporation, lmp7300max Datasheet - Page 12

• Manufacturer Part Number: lmp7300max
• Description: Micropower Precision Comparator And Precision Reference With Adjustable Hysteresis
• Manufacturer: National Semiconductor Corporation

www.national.com
PRECISION HIGH TEMPERATURE SWITCH
The LMP7300 brings accuracy and stability to simple sensor
switch applications. Figure 9 shows the LMP7300 setup in a
high temperature switch configuration. The input bridge es-
tablishes the trip point at 85°C and the reset temperature at
80°C. The comparator is set up with positive hysteresis of
14.3 mV and no negative hysteresis. When the temperature
is rising it trips at 85°C. The 14.3 mV hysteresis allows the
temperature to drop to 80°C before reset.
The temperature sensor used is an Omega 44008 Precision
NTC Thermistor. The 44008 has an accuracy of ±0.2°C. The
resistance at 85°C is 3270.9Ω and at 80°C is 3840.2Ω. The
trip voltage threshold is established by one half of the bridge,
which is the ratio of R
set by the second half, which is the ratio of the thermistor re-
sistance R
∼
The thermistor specification states it has a 1°C/mW dissipa-
MICROPOWER PRECISION BATTERY LOW VOLTAGE
DETECTOR
The ability of the LMP7300 to operate at very low supply volt-
ages, makes it an ideal choice for low battery detection ap-
plication in portable equipment. The circuit in Figure 9
performs the function of low voltage threshold detection in a
3 cell 0.9V discharge voltage, battery monitor application.
R
50 µA bridge current to minimize the power in the thermistor.
1
and R
2
TH
are chosen to set the inverting input voltage equal
and R
SET
ADJ
. The resistance values are chosen for
and R
FIGURE 9. Battery Voltage Monitor for 3 Cell Discharge Voltage
SET
. The input signal bias is
FIGURE 8. Precision High Temperature Switch
12
tion error. The reference voltage establishes the supply volt-
age for the bridge to make the circuit independent of supply
voltage variation. Capacitor C
pole at F
tance values chosen C
This will limit the thermal noise in the bridge.
The accuracy of the circuit can be calculated from the nearest
resistance values chosen. For 1% resistors RADJ is 3.24
kΩ, and R
2.488 mV/C at 85°C. In general, the higher the bridge current
is allowed to be, the higher the bridge gain will be. The actual
trip point found during simulation is 85.3°C and the reset point
is 80.04°C. With the values chosen the worst case trip tem-
perature uncertainty is ±1.451°C and the reset uncertainty is
±1.548°C. Accuracy could be maximized with resistors cho-
sen to 0.1% values, 0.1% tolerance and by using the 0.1%
model of the Omega 44008 thermistor.
to the non-inverting input voltage when the battery voltage is
equal to the minimum operating voltage of the system. Here,
the very precise reference output voltage is directly connect-
ed to the non-inverting input on the comparator and sets an
accurate threshold voltage. The hysteresis is set to 0 mV
negative and 20 mV positive. The output is off for voltages
higher than the minimum V
detects a minimum battery voltage condition.
CORNER
SET
is 78.7 kΩ. The bridge gain becomes
= 1/(2πC
20175647
1
should be selected for Fc < 10 Hz.
BATT
1
*2(R
1
, and turns on when the circuit
establishes a low frequency
SET
20175631
//R
ADJ
)). With the resis-