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LM122 データシート(PDF) 11 Page - National Semiconductor (TI) |
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LM122 データシート(HTML) 11 Page - National Semiconductor (TI) |
11 / 14 page Application Hints (Continued) Timer Protected Against Damage for up to 50V TLH7768 – 26 FIGURE 15 Comparator with 0V to 3V Threshold Eliminating Timing Cycle Upon Initial Application of Power The LM122 will normally start a timing cycle (with no trigger input) when Va is first turned on If this characteristic is undesirable it can be defeated by tying the timing capacitor to VREF instead of ground as shown in Figure 16 This con- nection does not affect operation of the timer in any other way If an electrolytic timing capacitor is used be sure the negative end is tied to the RC pin and the positive end to VREF A 10 kX resistor should be included in series with the timing capacitor to limit the surge current load on VREF when the capacitor is discharged TLH7768 – 27 FIGURE 16 Eliminating Initial Timing Cycle Using Dual Supplies The LM122 can be operated off dual supplies as shown in Figure 17 The only limitation is that the emitter terminal cannot be tied to ground it must either drive a load referred to Vb or be actually tied to Vb as shown Although capaci- tive coupling is shown for the trigger input (to allow 5V trig- gering) a resistor can be substituted for C1 R2 must be chosen to give proper level shifting between the trigger sig- nal and the trigger pin of the timer Worst case ‘‘lo’’ on the trigger pin (with respect to Vb) is 08V and worst case ‘‘high’’ is 25V R2 may be calculated from the divider equa- tion with R1 to give these levels TLH7768 – 28 Select for Proper Level Shift Emitter Terminal or Emitter Load must be Tied to GND Pin of Timer FIGURE 17 Operating Off Dual Supplies Linearizing the Charging Sweep In some applications (such as a linear pulse width modula- tor) it may be desirable to have the timing capacitor charge from a constant current source A simple way to accomplish this is shown in Figure 18 TLH7768 – 29 FIGURE 18 Temperature Compensated Linear Charging Sweep Q1 converts the current through R1 to a current source in- dependent of the voltage across Ct R2 R3 D1 and D2 are added to make the current through R1 independent of sup- ply variations and temperature changes (D2 is a low TC type) D2 and R3 can be omitted if the Va supply is stable and D1 and R2 can be omitted also if temperature stability is not critical With D1 D2 R2 and R3 omitted the current through R1 will change about 0015% C with a 15V supply and 01% C with a 50V supply 11 |
同様の部品番号 - LM122 |
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同様の説明 - LM122 |
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