LM1042 Fluid Level Detector General Description Features 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. Y Y Y Y Y Y Y Y Y Y Selectable thermal-resistance or linear probe inputs Control circuitry for thermal-resistive probe Single-shot or repeating measurements Switch on reset and delay to avoid transients Output amplifier with 10 mA source and sink capability Short or open probe detection a 50V transient protection on supply and control input 7.5V to 18V supply range Internally regulated supply b 40§ C to a 80§ C operation Block Diagram TL/H/8709 – 1 C1995 National Semiconductor Corporation TL/H/8709 RRD-B30M115/Printed in U. S. A. LM1042 Fluid Level Detector February 1995 Absolute Maximum Ratings Output Current Pin 11 (source) If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/Distributors for availability and specifications. Supply Voltage VCC g 10 mA Operating Temperature Range 32V 32V Voltage at Pin 8 Positive Peak Voltage (Pins 6, 8, 3) (Note 1) 10 ms 2A Output Current Pin 4, (I4)(sink) 25 mA Output Current Pin 16 b 40§ C to a 80§ C Storage Temperature Range Lead Temperature (Soldering 10 sec.) Package Power Dissipation TA e 25§ C (Note 8) Device Power Dissipation 50V 10 mA b 55§ C to a 150§ C 260§ C 1.8W 0.9W Electrical Characteristics VCC e 13V, TA within operating range except where stated otherwise. CT e 22 mF, RT e 12k Symbol Parameter Tested Limits (Note 2) Conditions Min VCC Supply Voltage IS Supply Current VREG Regulated Voltage Pins 15 and 11 connected Stability Over VCC Range Referred to value at VCC e 13V (Note 4) V6 – V3 T1 7.5 Max Min Typ 18 7.5 13 5.7 18 V mA 2.15 (Note 4) Ramp Timing See Figure 5 5.65 6.2 V g 0.5 % 2.40 V g 0.8 % 31 42 ms 16 ms 1.75 2.1 s g5 % 15.0 kX 5.9 g 0.5 Probe Current Reference Voltage Probe Current Regulation Over VCC Range 6.15 2.35 2.10 2.25 g 0.5 20 37 15 1.4 2.1 1.4 3 T4 – T1 Ramp Timing TSTAB Ramp Timing Stability RT Ramp Resistor Range V8 Start Input Logic High Level V8 Start Input Logic Low Level I8 Start Input Current I8 Start Input Current V8 e 0V V16 Maximum Output Voltage RL e 600X from Pin 16 to VREG Minimum Output Voltage PROBE 1 Probe 1 Gain Non-linearity of G1 OS1 Pin 1 Offset G2 PROBE 2 Probe 2 Gain Non-linearity of G2 OS7 Pin 7 Offset R7 Input impedance Over VCC Range a5 3 15 1.7 V8 e VCC 3 1.7 V 0.5 0.5 V 100 100 nA 300 nA 300 VREGb0.3 Pin 1 80 mV to 520 mV (Notes 6, 7) Pin 1 80 mV to 520 mV (Note 7) Units Max 35 35 T2 – T1 G1 Design Limits (Note 3) VREGb0.3 V 0.5 0.2 9.9 10.4 10.15 b1 a1 b2 (Note 7) 0 0.6 V 2 % g5 Pin 7 240 mV to 1.562V (Note 7) Pin 7 240 mV to 1.562V (Note 7) (Note 7) 2 3.31 3.49 b1 a1 mV 3.4 b2 0.2 2 % g5 mV 5 MX Electrical Characteristics VCC e 13V, TA within operating range except where stated otherwise. CT e 22 mF, RT e 12k (Continued) Symbol Parameter Conditions V1 Probe 1 Input Voltage Range VCC e 9V to 18V VCC e 7.5V, I4 k 2.5 mA (VREG e 6.0V) V5 Probe 1 Open Circuit Threshold At Pin 5 V5 Probe 1 Short Circuit Threshold I14 Pin 14 Input Leakage Current Pin 14 e 4V I1 Pin 1 Input Leakage Current Pin 1 e 300 mV TR Repeat Period CR e 22 mF (Note 5) CR Discharge Time CR e 22 mF Tested Limits (Note 2) Design Limits (Note 3) Min Max Min 1 5 1 1 Typ Units Max 5 3.5 VREGb0.7 VREGb0.5 VREGb0.85 VREGb0.6 VREGb0.35 0.5 0.7 b 2.0 2.0 b 5.0 5.0 12 28 0.35 9.1 0.6 V V V 0.85 V 2.0 nA 1.5 5.0 nA 17 36 s 135 ms CM Memory Capacitor Value 70 0.47 mF C1 Input Capacitor Value 0.47 mF 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. 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. TA e b 40§ C to a 80§ C and from VCC 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 Figure 6 . Note 6: Probe 1 amplifier tests are measured with pin 12 ramp voltage held between the T3 and T4 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 VB (VB*) and the measured value. Note 8: Above TA e 25§ C derate with ijA e 70§ C/W. VC b VA For probe 1 and probe 2ÐGain (G) e Vc b Va Input offset e Linearity e Ð ÐG VC b Vc V B* b1 VB ( ( c 100% VB* e VA a G(Vb b Va) TL/H/8709 – 15 3 Typical Performance Characteristics Supply Current vs Supply Voltage Regulated Voltage vs Supply Voltage Output Voltage vs Pin 7 Voltage Probe Reference V vs Supply Voltage Output Voltage vs Pin 14 Voltage TL/H/8709 – 3 Pin Function Description Pin 1 Pin 2 Pin 3 Pin 4 Pin 5 Pin 6 Pin 7 Pin 8 Pin 9 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 VREG 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, normally connected to VREG 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 capable of driving up to g 10 mA. May be connected with a 600X meter to VREG. Input amplifier for thermo-resistive probe with 5 nA maximum leakage. Clamped to ground at the start of a probe 1 measurement. Device ground Ð 0V. 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 VSUPPLY b 2V. Base connection for the external PNP transistor. This pin is connected to the thermo-resistive probe for short and open circuit probe detection. Supply pin, a 7.5V to a 18V, protected against a 50V transients. 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. 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. 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. 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 . 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 noticeable length changes in the wire used and more sophisticated 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 DV. RTH Air and RTH Oil represent the different thermal resistances from probe to ambient in air or oil giving rise fo temperature changes DT1 and DT2 respectively. As a result of these temperature changes the probe resistance will change by DR1 or DR2 and give corresponding voltage changes DV1 or DV2 per unit length. Hence LA (L b LA) DV1 a DV2 DV e L L l l and for DV1 DV2, RTH Air RTH Oil, DV will increase as the probe length in air increases. For best results the probe 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 Figure 2 for 200 mA probe current. The effect of varying the probe current is shown in Figure 3. To avoid triggering the probe failure detection circuits the probe voltage must be between 0.7V and 5.3V (VREG b 6V), hence for 200 mA the permissible probe resistance range is from 3.5X to 24X. The example 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 TL/H/8709 – 6 FIGURE 3 Probes need not be limited to resistance wire types as any 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 CT to suit. TL/H/8709 – 7 FIGURE 4 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 (V8 k 0.5V) and the internal timebase 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 corresponding to this time, T2. The ramp charge rate is now reduced 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 voltage, representing the change in probe voltage since T2 (and as the current is constant this is proportional to the resistance change) is gated onto the memory capacitor at pin 14. At 0.7V, T5, the probe current is switched off and the measurement 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, g 10 mA capability, 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. TL/H/8709 – 9 FIGURE 6 TL/H/8709 – 10 FIGURE 7 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. TL/H/8709–8 FIGURE 5 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 triggered 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. TL/H/8709 – 11 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 VREG t 6.2V. 6 Application Notes (Continued) TL/H/8709 – 12 FIGURE 9 PROBE CURRENT REFERENCE CIRCUIT The circuit defining the probe circuit is given in Figure 10. A 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. TL/H/8709 – 13 FIGURE 10 TYPICAL APPLICATIONS CIRCUIT A typical automotive application circuit is shown in Figure 11 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 C5 holds pin 8 high in the event of a stalled engine so that a second probe 1 measurement can not occur in disturbed oil. Non-automotive applications may drive pin 8 directly with a logic signal. TL/H/8709 – 14 FIGURE 11. Typical Application Circuit 7 LM1042 Fluid Level Detector Ordering Information Order Number LM1042N See NS Package Number N16A Physical Dimensions inches (millimeters) Lit. Ý 107305 Order Number LM1042N NS Package Number N16A LIFE SUPPORT POLICY NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein: 1. Life support devices or systems are devices or systems which, (a) are intended for surgical implant into the body, or (b) support or sustain life, and whose failure to perform, when properly used in accordance with instructions for use provided in the labeling, can be reasonably expected to result in a significant injury to the user. National Semiconductor Corporation 1111 West Bardin Road Arlington, TX 76017 Tel: 1(800) 272-9959 Fax: 1(800) 737-7018 2. A critical component is any component of a life support device or system whose failure to perform can be reasonably expected to cause the failure of the life support device or system, or to affect its safety or effectiveness. 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