LMP7300MA datasheet(15/17 Pages) TI

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INP

INN

HYSTP

HYSTN

OUT

0.1

318.4 k

GND

LED

VREF

9.86 k

VBATT

so, if

VREF

VBATT

D and R2 is known,

The LED turns on when VBATT x

R1 + R2

d VREF,

As an example:

VREF = 2.048V, VBATTLOW = +2.7V, R2 = 1 M:

then R1 = 318.4 k:

then, R1 = R2 ¨¨©

1 -

= R2 ¨©

§ VBATT - VREF

VREF

1 M

VREF

HYSTN

HYSTP

0.1 PF

1 M:

VCC

GND

RH1

1.44 k:

RH2

205 k:

RTH

6.8 PF

LMP7300

VREF

VCC (2.7V ± 12V)

RADJ

3.24 k:

RSET

78.7 k:

RSET

78.7 k:

LMP7300

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SNOSAT7F – AUGUST 2007 – REVISED MARCH 2013

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 3840.2Ω. The trip voltage threshold is established by

one half of the bridge, which is the ratio of RADJ and RSET. The input signal bias is set by the second half, which

is the ratio of the thermistor resistance RTH and RSET. The resistance values are chosen for ∼50 µA 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 independent of

supply voltage variation. Capacitor C1 establishes a low frequency pole at FCORNER = 1/(2πC1*2(RSET//RADJ)).

With the resistance values chosen C1 should be selected for Fc < 10 Hz. 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 k

Ω, and RSET is 78.7 kΩ. The bridge gain becomes 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 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.

Figure 25. Precision High Temperature Switch

MICROPOWER PRECISION BATTERY LOW VOLTAGE DETECTOR

The ability of the LMP7300 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 3 cell 0.9V discharge voltage, battery monitor application. R1 and R2 are chosen to set

the inverting input voltage equal 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 connected

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 VBATT, and turns on

when the circuit detects a minimum battery voltage condition.

Figure 26. Battery Voltage Monitor for 3 Cell Discharge Voltage

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