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LM1042 Fluid Level Detector
LM1042 Fluid Level Detector
February 1995
General Description
The LM1042 uses the thermal-resistive probe technique to
measure the level of non-flammable fluids. An output is provided proportional to fluid level and single shot or repeating
measurements may be made. All supervisory requirements
to control the thermal-resistive probe, including short and
open circuit probe detection, are incorporated within the device. A second linear input for alternative sensor signals
may also be selected.
Block Diagram
Features
Y
Selectable thermal-resistance or linear probe inputs
Y
Control circuitry for thermal-resistive probe
Y
Single-shot or repeating measurements
Y
Switch on reset and delay to avoid transients
Y
Output amplifier with 10 mA source and sink capability
Y
Short or open probe detection
Y
a
50V transient protection on supply and control input
Y
7.5V to 18V supply range
Y
Internally regulated supply
Y
b
40§Ctoa80§C operation
TL/H/8709– 1
C
1995 National Semiconductor Corporation RRD-B30M115/Printed in U. S. A.
TL/H/8709
Absolute Maximum Ratings
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales
Office/Distributors for availability and specifications.
Supply Voltage V
CC
32V
Voltage at Pin 8 32V
Positive Peak Voltage (Pins 6, 8, 3) (Note 1)
10 ms 2A 50V
Output Current Pin 4, (I
)(sink) 10 mA
4
Output Current Pin 11 (source) 25 mA
Output Current Pin 16
Operating Temperature Range
Storage Temperature Range
b
g
b
10 mA
40§Ctoa80§C
55§Ctoa150§C
Lead Temperature (Soldering 10 sec.) 260§C
Package Power Dissipation
e
T
25§C (Note 8) 1.8W
A
Device Power Dissipation 0.9W
Electrical Characteristics
e
V
13V, TAwithin operating range except where stated otherwise. C
CC
Symbol Parameter Conditions
V
I
V
CC
S
REG
Supply Voltage 7.5 18 7.5 13 18 V
Supply Current 35 35 mA
Regulated Voltage Pins 15 and 11 connected 5.7 6.15 5.65 5.9 6.2 V
Stability Over VCCRange Referred to value at
e
13V (Note 4)
V
CC
V6–V3Probe Current
Reference Voltage
Probe Current Regulation (Note 4)
CC
Range
Figure 5
Over V
T
1
T2–T
T
4
T
STAB
Ramp Timing See
1
–T1Ramp Timing 1.4 2.1 1.4 1.75 2.1 s
Ramp Timing Stability Over VCCRange
RTRamp Resistor Range 3 15 3 15.0 kX
V
8
V
8
I
8
I
8
V
16
Start Input Logic High Level 1.7 1.7 V
Start Input Logic Low Level 0.5 0.5 V
Start Input Current V
Start Input Current V
Maximum Output Voltage R
Minimum Output Voltage
e
V
8
CC
e
0V 300 300 nA
8
e
600X from V
L
Pin 16 to V
REG
PROBE 1
G
OS
Probe 1 Gain Pin 1 80 mV to 520 mV 9.9 10.4 10.15
1
Non-linearity of G
Pin 1 Offset (Note 7)
1
1
(Notes 6, 7)
Pin180mVto520mV
(Note 7)
PROBE 2
G
OS
R
7
Probe 2 Gain Pin 7 240 mV to 1.562V 3.31 3.49 3.4
2
Non-linearity of G
Pin 7 Offset (Note 7)
7
2
(Note 7)
Pin 7 240 mV to 1.562V
(Note 7)
Input impedance 5MX
T
e
22 mF, R
e
12k
T
Tested Limits Design Limits
(Note 2) (Note 3)
Min Max Min Typ Max
g
0.5
g
0.5 %
2.15 2.35 2.10 2.25 2.40 V
g
0.5
g
0.8 %
20 37 15 31 42 ms
316ms
a
5
g
5%
100 100 nA
b
0.3 V
REG
b
0.3 V
REG
0.5 0.2 0.6 V
b
b
a
1
a
1
b
1
1
202%
g
5mV
b
2 0.2 2 %
g
5mV
Units
2
Electrical Characteristics
e
V
13V, TAwithin operating range except where stated otherwise. C
CC
Tested Limits Design Limits
Symbol Parameter Conditions
Min Max Min Typ Max
V
Probe 1 Input V
1
Voltage Range V
V
Probe 1 Open At Pin 5
5
Circuit Threshold
V
Probe 1 Short
5
Circuit Threshold
I
Pin 14 Input Pin 14e4V
14
Leakage Current
I
Pin 1 Input Pin 1e300 mV
1
Leakage Current
T
Repeat Period C
R
CRDischarge Time C
C
Memory Capacitor Value 0.47 mF
M
C
Input Capacitor Value 0.47 mF
1
Sensitivity fo Electrostatic DischargeÐ
Pins 7, 10, 13, and 14 will withstand greater than 1500V when tested using 100 pF and 1500X in accordance with National Semiconductor standard ESD test
procedures.
All other pins will withstand in excess of 2 kV.
Note 1: Test circuit for over voltage capability at pins 3, 6, 8.
e
9V to 18V 1 5 1 5 V
CC
e
CC
(V
REG
7.5V, I
e
6.0V)
k
2.5 mA 1 3.5 V
4
b
V
REG
0.5 0.7 0.35 0.6 0.85 V
b
2.0 2.0 2.0 nA
b
5.0 5.0 1.5 5.0 nA
e
22 mF (Note 5) 12 28 9.1 17 36 s
R
e
22 mF 70 135 ms
R
T
e
22 mF, R
e
12k (Continued)
T
(Note 2) (Note 3)
0.7 V
REG
b
0.5 V
REG
b
0.85 V
REG
b
0.6 V
REG
b
0.35 V
Units
Note 2: Guaranteed and 100% production tested at 25§C. These limits are used to calculate outgoing quality levels.
Note 3: Limits guardbanded to include parametric variations. T
figures.
Note 4: Variations over temperature range are not production tested.
Note 5: Time for first repeat period, see
Note 6: Probe 1 amplifier tests are measured with pin 12 ramp voltage held between the T
4.1V to simulate ramp action. See Figure 5.
Note 7: When measuring gain separate ground wire sensing is required at pin 2 to ensure sufficiently accurate results.
Linearity is defined as the difference between the predicted value of V
Note 8: Above T
e
25§C derate with i
A
Figure 6
e
jA
.
70§C/W.
eb
A
TL/H/8709– 15
40§Ctoa80§C and from V
and T4conditions (pin 12&1.1V) having previously been held above
3
*) and the measured value.
B(VB
For probe 1 and probe 2ÐGain (G)
Input offset
Linearity
V
*eV
B
e
7.5V to 18V. These limits are not used to calculate AOQL
CC
V
C
e
b
V
c
G
Ð
(
VB*
e
b
c
1
V
Ð
a
G(V
A
100%
(
B
b
Va)
b
3
TL/H/8709– 2
b
V
V
C
e
b
V
V
c
A
a
Typical Performance Characteristics
Supply Current vs
Supply Voltage
Regulated Voltage vs
Supply Voltage
Probe Reference V vs
Supply Voltage
Output Voltage vs
Pin 7 Voltage
Pin Function Description
Pin 1 Input amplifier for thermo-resistive probe with 5 nA
maximum leakage. Clamped to ground at the start of
a probe 1 measurement.
Pin 2 Device ground Ð 0V.
Pin 3 This pin is connected to the emitter of an external
PNP transistor to supply a 200 mA constant current
to the thermo-resistive probe. An internal reference
maintains this pin at V
Pin 4 Base connection for the external PNP transistor.
Pin 5 This pin is connected to the thermo-resistive probe
for short and open circuit probe detection.
Pin 6 Supply pin,
a
50V transients.
a
7.5V toa18V, protected against
Pin 7 High Impedance input for second linear voltage
probe with an input range from 1V to 5V. The gain
may be set externally using pin 10.
Pin 8 Probe select and control input. If this pin is taken to
a logic low level, probe 1 is selected and the timing
cycle is initiated. The selection logic is subsequently
latched low until the end of the measurement. If kept
at a low level one shot or repeating probe 1 measurements will be made depending upon pin 9 conditions. A high input level selects probe 2 except during a probe 1 measurement period.
Pin 9 The repeat oscillator timing capacitor is connected
from this pin to ground. A 2 mA current charges up
the capacitor towards 4.3V when the probe 1 measurement cycle is restarted. If this pin is grounded
the repeat oscillator is disabled and only one probe
1 measurement will be made when pin 8 goes low.
SUPPLY
b
2V.
Output Voltage vs
Pin 14 Voltage
TL/H/8709– 3
Pin 10 A resistor may be connected to ground to vary the
gain of the probe 2 input amplifier. Nominal gain
when open circuit is 1.2 and when shorted to ground
3.4. DC conditions may be adjusted by means of a
resistor divider network to V
REG
and ground.
Pin 11 Regulated voltage output. Requires to be connected
to pin 15 to complete the supply regulator control
loop.
Pin 12 The capacitor connected from this pin to ground
sets the timing cycle for probe 1 measurements.
Pin 13 The resistor connected between this pin and ground
defines the charging current at pin 12. Typically 12k,
the value should be within the range 3k to 15k.
Pin 14 A low leakage capacitor, typical value 0.1 mF and
not greater than 0.47 mF, should be connected from
this pin to the regulated supply at pin 11 to act as a
memory capacitor for the probe 1 measurement.
The internal leakage at this pin is 2 nA max for a
long memory retention time.
Pin 15 Feedback input for the internal supply regulator, nor-
mally connected to V
be connected in series to adjust the regulated output
at pin 11. A resistor may
REG
voltage by an amount corresponding to the 1 mA
current into pin 15.
Pin 16 Linear voltage output for probe 1 and probe 2 capa-
ble of driving up to
a 600X meter to V
g
10 mA. May be connected with
.
REG
4
Application Notes
THERMO-RESISTIVE PROBES Ð OPERATION AND
CONSTRUCTION
These probes work on the principle that when power is dissipated within the probe, the rise in probe temperature is
dependent on the thermal resistance of the surrounding material and as air and other gases are much less efficient
conductors of heat than liquids such as water and oil it is
possible to obtain a measurement of the depth of immersion
of such a probe in a liquid medium. This principle is illustrated in
Figure 1
.
FIGURE 1
TL/H/8709– 4
During the measurement period a constant current drive I is
applied to the probe and the voltage across the probe is
sampled both at the start and just before the end of the
measurement period to give DV. R
sent the different thermal resistances from probe to ambient
in air or oil giving rise fo temperature changes DT
respectively. As a result of these temperature changes the
probe resistance will change by DR
sponding voltage changes DV
Air and RTHOil repre-
TH
or DR2and give corre-
1
or DV2per unit length.
1
and DT
1
Hence
L
e
DV
L
and for DV
the probe length in air increases. For best results the probe
l
DV2,RTHAirlRTHOil, DV will increase as
1
(LbLA)
A
a
DV
1
DV
2
L
needs to have a high temperature coefficient and low thermal time constant. One way to achieve this is to make use
of resistance wires held in a suitable support frame allowing
free liquid access. Nickel cobalt iron alloy resistance wires
are available with resistivity 50 mXcm and 3300 ppm temperature coefficient which when made up into a probe with 4
c
2 cm 0.08 mm diameter strands between supports (10
cm total) can give the voltage vs time curve shown in
2
for 200 mA probe current. The effect of varying the probe
current is shown in
failure detection circuits the probe voltage must be between
0.7V and 5.3V (V
sible probe resistance range is from 3.5X to 24X. The ex-
Figure 3.
REG
To avoid triggering the probe
b
6V), hence for 200 mA the permis-
Figure
ample given has a resistance at room temperature of 9X
which leaves plenty of room for increase during measurements and changes in ambient temperature.
Various arrangements of probe wire are possible for any
given wire gauge and probe current to suit the measurement
range required, some examples are illustrated schematically
in
Figure 4.
Naturally it is necessary to reduce the probe
FIGURE 2
current with very fine wires to avoid excessive heating and
this current may be optimized to suit a particular type of
wire. The temperature changes involved will give rise to noticeable length changes in the wire used and more sophisticated holders with tensioning devices may be devised to
allow for this.
2
Probes need not be limited to resistance wire types as any
FIGURE 3
device with a positive temperature coefficient and sufficiently low thermal resistance to the encapsulation so as not to
mask the change due to the different surrounding mediums,
could be used. Positive temperature coefficient thermistors
are a possibility and while their thermal time constant is likely to be longer than wire the measurement time may be
increased by changing C
to suit.
T
FIGURE 4
TL/H/8709– 5
TL/H/8709– 6
TL/H/8709– 7
5
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 (V
base ramp generator will start to generate the waveform
shown in
Figure 5.
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 C
sponding to this time, T
duced as C
passed a current sink is enabled and C
charges toward 4V. As the 4.1V threshold is
T
Between 1.3V and 1.0V, T
age, representing the change in probe voltage since T
as the current is constant this is proportional to the resistance change) is gated onto the memory capacitor at pin 14.
At 0.7V, T
surement cycle is complete. In the event of a faulty probe
, the probe current is switched off and the mea-
5
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,
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.
k
0.5V) and the internal time-
8
At 0.7V, T1, the probe current drive is
stores the probe voltage corre-
1
. The ramp charge rate is now re-
2
now discharges.
and T4, the amplified pin 1 volt-
3
T
g
10 mA capabili-
2
(and
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
6
and a thermo-resistive probe measurement will be trig-
Figure
gered each time pin 9 reaches a threshold of 4.3V, provided
pin 8 is at a logic low level. The repeat oscillator runs independently 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 consideration in determining the minimum acceptable measurement
intervals. Measurements will tend to become more dependent 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
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 regulated 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 flowing in R. In this way a variable resistor may be used to trim
out the production tolerance of the regulator by adjusting for
t
V
6.2V.
REG
6
TL/H/8709– 11
Application Notes (Continued)
FIGURE 9
TL/H/8709– 12
PROBE CURRENT REFERENCE CIRCUIT
The circuit defining the probe circuit is given in
Figure 10.
reference voltage is obtained from a bandgap regulator derived current flowing in a diode resistor chain to set up a
voltage 2 volts below the supply. This is applied to an amplifier driving an external PNP transistor to maintain pin 3 at 2V
below supply. The emitter resistance from pin 3 to supply
defines the current which, less the base current, flows in the
probe. Because of the sensitivity of the measurement to
probe current evident in
Figure 3
the current should be adjusted by means of a variable resistor to the desired value.
This adjustment may also be used to take out probe tolerances.
A
FIGURE 10
TYPICAL APPLICATIONS CIRCUIT
A typical automotive application circuit is shown in
where the probe selection signal is obtained from the oil
pressure switch. At power up (ignition on) the oil pressure
switch is closed and pin 8 is held low by R4 causing a probe
1 (oil level) measurement to be made. Once the engine has
started the oil pressure switch opens and D1 pulls pin 8 high
changing over to the second auxiliary probe input. The capacitor C
so that a second probe 1 measurement can not occur in
holds pin 8 high in the event of a stalled engine
5
disturbed oil. Non-automotive applications may drive pin 8
directly with a logic signal.
TL/H/8709– 13
Figure 11
FIGURE 11. Typical Application Circuit
7
TL/H/8709– 14
Ordering Information
Order Number LM1042N
See NS Package Number N16A
Physical Dimensions inches (millimeters) Lit.
LM1042 Fluid Level Detector
Order Number LM1042N
NS Package Number N16A
Ý
107305
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systems which, (a) are intended for surgical implant support device or system whose failure to perform can
into the body, or (b) support or sustain life, and whose be reasonably expected to cause the failure of the life
failure to perform, when properly used in accordance support device or system, or to affect its safety or
with instructions for use provided in the labeling, can effectiveness.
be reasonably expected to result in a significant injury
to the user.
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