Datasheet

+
-
INP
INN
HYSTP
HYSTN
OUT
0.1 PF
R
1
318.4 k:
1%
GND
V
+
R
5
LED
V
REF
R
3
9.86 k:
1%
V
BATT
so, if
V
REF
V
BATT
= D
and R
2
is known,
The LED turns on when V
BATT
x
R
2
R
1
+ R
2
d
V
REF
,
As an example:
V
REF
= 2.048V, V
BATT
LOW
= +2.7V, R
2
= 1 M:
then R
1
= 318.4 k:
then, R
1
= R
2
¨
¨
©
§
D
1 - D
¨
¨
©
§
= R
2
¨
©
§
V
BATT
- V
REF
¨
©
§
V
REF
R
2
1 M:
1%
R
4
1 M:
1%
-
+
V
REF
HYSTN
HYSTP
0.1 PF
1 M:
V
CC
GND
R
H1
1.44 k:
R
H2
205 k:
R
TH
T
C
1
6.8 PF
LMP7300
V
REF
V
CC
(2.7V ± 12V)
R
ADJ
3.24 k:
R
SET
78.7 k:
R
SET
78.7 k:
LMP7300
www.ti.com
SNOSAT7F AUGUST 2007REVISED 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 and at 80°C is 3840.2. The trip voltage threshold is established by
one half of the bridge, which is the ratio of R
ADJ
and R
SET
. The input signal bias is set by the second half, which
is the ratio of the thermistor resistance R
TH
and R
SET
. 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 C
1
establishes a low frequency pole at F
CORNER
= 1/(2πC
1
*2(R
SET
//R
ADJ
)).
With the resistance values chosen C
1
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, and R
SET
is 78.7 k. 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. R
1
and R
2
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 V
BATT
, 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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