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LM1042N View Datasheet(PDF) - National ->Texas Instruments

Part NameLM1042N National-Semiconductor
National ->Texas Instruments National-Semiconductor
DescriptionFluid Level Detector
LM1042N Datasheet PDF : 8 Pages
1 2 3 4 5 6 7 8
Application Notes (Continued)
CIRCUIT OPERATION
1) Thermo-Resistive Probes
These probes require measurements to be made of their
resistance before and after power has been dissipated in
them With a probe connected as probe 1 in the connection
diagram the LM1042 will start a measurement when pin 8 is
taken to a logic low level (V8 k 0 5V) and the internal time-
base ramp generator will start to generate the waveform
shown in Figure 5 At 0 7V T1 the probe current drive is
switched on supplying a constant 200 mA via the external
PNP transistor and the probe failure circuit is enabled At 1V
pin 1 is unclamped and C1 stores the probe voltage corre-
sponding to this time T2 The ramp charge rate is now re-
duced as CT charges toward 4V As the 4 1V threshold is
passed a current sink is enabled and CT now discharges
Between 1 3V and 1 0V T3 and T4 the amplified pin 1 volt-
age representing the change in probe voltage since T2 (and
as the current is constant this is proportional to the resist-
ance change) is gated onto the memory capacitor at pin 14
At 0 7V T5 the probe current is switched off and the mea-
surement cycle is complete In the event of a faulty probe
being detected the memory capacitor is connected to the
regulated supply during the gate period The device leakage
at pin 14 is a maximum of 2 nA to give a long memory
retention time The voltage present on pin 14 is amplifed by
1 2 to drive pin 16 with a low impedance g10 mA capabili-
ty between 0 5V and 4 7V A new measurement can only be
started by taking pin 8 to a low level again or by means of
the repeat oscillator
FIGURE 6
TL H 8709 – 9
FIGURE 7
TL H 8709 – 10
3) Second Probe Input
A high impedance input for an alternative sensor is available
at pin 7 The voltage applied to this input is amplified and
output at pin 16 when the input is selected with a high level
on pin 8 The gain is defined by the feedback arrangement
shown in Figure 8 with adjustment possible at pin 10 With
pin 10 open the gain is set at a nominal value of 1 2 and
this may be increased by connecting a resistor between pin
10 and ground up to a maximum of 3 4 with pin 10 directly
grounded A variable resistor may be used to calibrate for
the variations in sensitivity of the sensor used for probe 2
FIGURE 5
TL H 8709–8
2) Repetitive Measurement
With a capacitor connected between pin 9 and ground the
repeat oscillator will run with a waveform as shown in Figure
6 and a thermo-resistive probe measurement will be trig-
gered each time pin 9 reaches a threshold of 4 3V provided
pin 8 is at a logic low level The repeat oscillator runs inde-
pendently of the pin 8 control logic
As the repetition rate is increased localized heating of the
probe and liquid being measured will be the main considera-
tion in determining the minimum acceptable measurement
intervals Measurements will tend to become more depen-
dent on the amount of fluid movement changing the rate of
heat transfer away from the probe The typical repeat time
versus timing capacitor value is shown in Figure 7
FIGURE 8
TL H 8709 – 11
POWER SUPPLY REGULATOR
The arrangement of the feedback for the supply regulator is
shown in Figure 9 The circuit acts to maintain pin 15 at a
constant 6V and when directly connected to pin 11 the reg-
ulated output is held at 6V If required a resistor R may be
connected between pins 15 and 11 to increase the output
voltage by an amount corresponding typically to 1 mA flow-
ing in R In this way a variable resistor may be used to trim
out the production tolerance of the regulator by adjusting for
VREG t 6 2V
6
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