LM1042.PDF

February 1995

LM1042 Fluid Level Detector

General Description

The LM1042 uses the thermal-resistive probe technique to

measure the level of non-flammable fluids. An output is pro-

vided 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 de-

vice. A second linear input for alternative sensor signals

may also be selected.

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 § Cto a 80 § C operation

Block Diagram

TL/H/8709±1

C 1995 National Semiconductor Corporation

TL/H/8709

RRD-B30M115/Printed in U. S. A.

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

Output Current Pin 11 (source)

25 mA

Output Current Pin 16

g 10 mA

Operating Temperature Range

b 40 § Cto a 80 § C

32V

Storage Temperature Range

b 55 § Cto a 150 § C

Voltage at Pin 8

32V

Lead Temperature (Soldering 10 sec.)

260 § C

Positive Peak Voltage (Pins 6, 8, 3) (Note 1)

10 ms 2A

50V

Package Power Dissipation

T A e 25 § C (Note 8)

1.8W

Output Current Pin 4, (I 4 )(sink)

10 mA

Device Power Dissipation

0.9W

Electrical Characteristics

V CC e 13V, T A within operating range except where stated otherwise. C T e 22 m F, R T e 12k

Tested Limits

Design Limits

Symbol

Parameter

Conditions

(Note 2)

(Note 3)

Units

Min Max Min

Typ Max

V CC

Supply Voltage

7.5

18 V

I S

Supply Current

35 mA

V REG Regulated Voltage

Pins 15 and 11 connected

5.7

6.15

5.65

5.9 6.2 V

Stability Over V CC Range Referred to value at

g 0.5

g 0.5 %

V CC e 13V (Note 4)

V 6 ±V 3 Probe Current

2.15

2.35

2.10

2.25 2.40 V

Reference Voltage

Probe Current Regulation (Note 4)

g 0.5

g 0.8 %

Over V CC Range

T 1

Ramp Timing

SeeFigure5

42 ms

T 2 ±T 1

16 ms

T 4 ±T 1 Ramp Timing

1.4

2.1

1.4

1.75 2.1

T STAB Ramp Timing Stability

Over V CC Range

a 5

g 5 %

Ramp Resistor Range

15.0 k X

V 8

Start Input Logic High Level

1.7

V 8

Start Input Logic Low Level

0.5

0.5 V

I 8

Start Input Current

V 8 e V CC

100

100 nA

I 8

Start Input Current

V 8 e 0V

300

300 nA

V 16 Maximum Output Voltage R L e 600 X from

V REG b 0.3

Pin 16 to V REG

Minimum Output Voltage

0.5

0.2 0.6 V

PROBE 1

G 1

Probe 1 Gain

Pin 1 80 mV to 520 mV

9.9

10.4

10.15

(Notes 6, 7)

Non-linearity of G 1

Pin180mVto520mV b 1 a 1 b 2

2 %

(Note 7)

OS 1

Pin 1 Offset

(Note 7)

g 5

PROBE 2

G 2

Probe 2 Gain

Pin 7 240 mV to 1.562V

3.31

3.49

3.4

(Note 7)

Non-linearity of G 2

Pin 7 240 mV to 1.562V b 1 a 1 b 2

0.2

2 %

(Note 7)

OS 7

Pin 7 Offset

(Note 7)

g 5

R 7

Input impedance

M X

Electrical Characteristics

V CC e 13V, T A within operating range except where stated otherwise. C T e 22 m F, R T e 12k (Continued)

Tested Limits

Design Limits

Symbol

Parameter

Conditions

(Note 2)

(Note 3)

Units

Min Max Min

Typ Max

V 1

Probe 1 Input

V CC e 9V to 18V

Voltage Range

V CC e 7.5V, I 4 k 2.5 mA

3.5

(V REG e 6.0V)

V 5

Probe 1 Open

At Pin 5

V REG b 0.7 V REG b 0.5 V REG b 0.85 V REG b 0.6 V REG b 0.35 V

Circuit Threshold

V 5

Probe 1 Short

0.5

0.7

0.35

0.6

0.85

Circuit Threshold

I 14 Pin 14 Input

Pin 14 e 4V

b 2.0

2.0

Leakage Current

I 1

Pin 1 Input

Pin 1 e 300 mV

b 5.0

5.0

1.5

5.0

Leakage Current

T R Repeat Period

C R e 22 m F (Note 5)

9.1

C R Discharge Time C R e 22 m F

135 ms

C M Memory Capacitor Value

0.47 m F

C 1

Input Capacitor Value

0.47 m F

Sensitivity fo Electrostatic DischargeÐ

Pins 7, 10, 13, and 14 will withstand greater than 1500V when tested using 100 pF and 1500 X 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.

TL/H/8709±2

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 A eb 40 § Cto a 80 § C and from V CC e 7.5V to 18V. These limits are not used to calculate AOQL

figures.

Note 4: Variations over temperature range are not production tested.

Note 5: Time for first repeat period, see Figure6.

Note 6: Probe 1 amplifier tests are measured with pin 12 ramp voltage held between the T 3 and T 4 conditions (pin 12 & 1.1V) having previously been held above

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 B (V B *) and the measured value.

Note 8: Above T A e 25 § C derate with i jA e 70 § C/W.

For probe 1 and probe 2ÐGain (G) e

V C b V A

V c b V a

Input offset e

V C

G b V c

(

Linearity e

V B *

V B b 1

(

c 100%

V B * e V A a G(V b b V a )

TL/H/8709±15

Typical Performance Characteristics

Supply Current vs

Regulated Voltage vs

Probe Reference V vs

Supply Voltage

Output Voltage vs

Pin 7 Voltage

Pin 14 Voltage

TL/H/8709±3

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 SUPPLY b 2V.

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 7.5V to a 18V, protected against

a 50V transients.

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 mea-

surements will be made depending upon pin 9 condi-

tions. A high input level selects probe 2 except dur-

ing a probe 1 measurement period.

Pin 9 The repeat oscillator timing capacitor is connected

from this pin to ground. A 2 m A current charges up

the capacitor towards 4.3V when the probe 1 mea-

surement 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.

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 m F and

not greater than 0.47 m F, 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 REG at pin 11. A resistor may

be connected in series to adjust the regulated output

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 g 10 mA. May be connected with

a 600 X meter to V REG .

Application Notes

THERMO-RESISTIVE PROBES Ð OPERATION AND

CONSTRUCTION

These probes work on the principle that when power is dis-

sipated within the probe, the rise in probe temperature is

dependent on the thermal resistance of the surrounding ma-

terial 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 illustrat-

ed in Figure1.

TL/H/8709±5

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 no-

ticeable length changes in the wire used and more sophisti-

cated holders with tensioning devices may be devised to

allow for this.

TL/H/8709±4

FIGURE 1

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 D V. R TH Air and R TH Oil repre-

sent the different thermal resistances from probe to ambient

in air or oil giving rise fo temperature changes D T 1 and D T 2

respectively. As a result of these temperature changes the

probe resistance will change by D R 1 or D R 2 and give corre-

sponding voltage changes D V 1 or D V 2 per unit length.

Hence

TL/H/8709±6

FIGURE 3

Probes need not be limited to resistance wire types as any

device with a positive temperature coefficient and sufficient-

ly 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 like-

ly to be longer than wire the measurement time may be

increased by changing C T to suit.

L D V 2

and for D V 1 l D V 2 ,R TH Air l R TH Oil, D V will increase as

the probe length in air increases. For best results the probe

needs to have a high temperature coefficient and low ther-

mal 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 mX cm and 3300 ppm tem-

perature 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 Figure

2 for 200 mA probe current. The effect of varying the probe

current is shown in Figure3. To avoid triggering the probe

failure detection circuits the probe voltage must be between

0.7V and 5.3V (V REG b 6V), hence for 200 mA the permis-

sible probe resistance range is from 3.5 X to 24 X . The ex-

ample given has a resistance at room temperature of 9 X

which leaves plenty of room for increase during measure-

ments 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

TL/H/8709±7

FIGURE 4

D V e L A

L D V 1 a (L b L A )

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