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LM1042N データシート(PDF) 6 Page - National Semiconductor (TI) |
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LM1042N データシート(HTML) 6 Page - National Semiconductor (TI) |
6 / 8 page 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 05V) and the internal time- base ramp generator will start to generate the waveform shown in Figure 5 At 07V 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 41V threshold is passed a current sink is enabled and CT now discharges Between 13V and 10V 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 07V 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 12 to drive pin 16 with a low impedance g10 mA capabili- ty between 05V and 47V A new measurement can only be started by taking pin 8 to a low level again or by means of the repeat oscillator TLH8709 – 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 trig- gered each time pin 9 reaches a threshold of 43V 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 TLH8709 – 9 FIGURE 6 TLH8709 – 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 12 and this may be increased by connecting a resistor between pin 10 and ground up to a maximum of 34 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 TLH8709 – 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 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 62V 6 |
同様の部品番号 - LM1042N |
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同様の説明 - LM1042N |
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