LMP7300MM datasheet(17/30 Pages) TI1

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LMP7300

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SNOSAT7G – AUGUST 2007 – REVISED OCTOBER 2015

Typical Applications (continued)

8.2.1.1 Design Requirements

Figure 23 monitors the ambient temperature change. If the temperature rises outside the 15°C to 35°C window,

either comparator 1 for high temperature, or comparator 2 for low temperature, sets low, indicating a fault

condition has occurred. The open collector outputs are pulled up separately but can be wire-OR’d for a single

fault indication. If the temperature returns inside the window, it must overcome the 22-mV asymmetric hysteresis

band established on either comparator. For the high side, the temperature must drop below 34°C, and for the low

side the temperature must rise above 16°C for the outputs to reset high and remove the fault indication. The

temperature is sensed by a 30 k

Ω @ 25°C Omega Precision NTC Thermistor #44008 (±0.2% tol).

8.2.1.2 Detailed Design Procedure

To set a fixed temperature threshold, the thermistor resistance ( RT*) must first be approximated at the specified

temperatures. For a temperature of 35°C, RT* =19.74 kΩ from Figure 23. A resistor divider with R1 =61.9 kΩ and

VREF can be formed on INN of comparator 1 to set the high side tripping voltage according to Equation 1. An

equivalent resistor divider must be formed on INP of comparator 1 by using the nearest 1% matching resistors of

R2=19.6 kΩ and a combination of R3=15.4 kΩ and R4=46.4 kΩ.

NOTE

a combination of resistors was chosen with R4=46.4 kΩ to set the low side tripping

threshold where RT* = 46.67 kΩ at 15°C .

A similar technique can be applied for comparator 2 to set the low side temperature of 15°C. The total change in

Volts can be computed by subtracting the two tripping thresholds to get a range of approximately 390 mV. Given

a total temperature change of 20°C, the hysteresis should be set to 19.5 mV to give an equivalent hysteresis of

1°C. A hysteresis value of 22.9 mV is calculated through the resistor divider of VREF, 2.32 kΩ, and 205 kΩ.

8.2.1.3 Application Curve

The results of the circuit shown in Figure 23 above can be plotted for various temperatures. A temperature

change of ±100°C/hour was chosen to demonstrate the functionality. Figure 24 shows that when the temperature

drops to 15°C the output of comparator 2 trips, which signifies a low temperature fault. When the temperature

returns and crosses the 22-mV hysteresis, comparator 2 returns to its normally high-output state. Similarly, when

the temperature reaches 35°C, comparator 1 trips and signifies a high temperature fault. When the temperature

returns below the hysteresis, comparator 1 returns to its normally high-output state.

Figure 24. Window Detector Output Response

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