-

+

HYSTN

HYSTP

0.1 PF

1 M:

GND

1.44 k:

205 k:

T

6.8 PF

LMP7300

3.24 k:

78.7 k:

78.7 k:

LMP7300

SNOSAT7G – AUGUST 2007 – REVISED OCTOBER 2015

www.ti.com

Typical Applications (continued)

8.2.2 Precision High-Temperature Switch

The LMP7300 brings accuracy and stability to simple sensor switch applications. Figure 26 shows the LMP7300

setup in a high temperature switch configuration. The input bridge is used to establish the temperature at which

the LMP7300 will trip and the temperature at which it resets.

Figure 25. Precision High-Temperature Switch

8.2.2.1 Design Requirements

In Figure 25, the input bridge establishes 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 3270.9

Ω and at 80°C is 3840.2 Ω. The trip voltage threshold is established by

bridge current to minimize the power in the thermistor. The thermistor specification states it has a 1°C/mW

dissipation error. The reference voltage establishes the supply voltage for the bridge to make the circuit

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 k

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 temperature uncertainty is ±1.451°C

and the reset uncertainty is ±1.548°C. Accuracy could be maximized with resistors chosen to 0.1% values, 0.1%

tolerance and by using the 0.1% model of the Omega 44008 thermistor.

8.2.3 Micropower Precision-Battery Low-Voltage Detector

The ability of the LMP7300 device to operate at very low supply voltages makes it an ideal choice for low battery

detection application in portable equipment. The circuit in Figure 26 performs the function of low voltage

threshold detection in a battery monitor application.

18

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