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Zakładka z wyszukiwarką danych komponentów |
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LM1815M Arkusz danych(PDF) 4 Page - National Semiconductor (TI) |
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LM1815M Arkusz danych(HTML) 4 Page - National Semiconductor (TI) |
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4 / 6 page  TLH7893 – 3 FIGURE 2 LM1815 Oscillograms Application Hints Input Clamp The signal input at pin 3 is internally clamped Current limit is provided by an external resistor which should be selected to allow a peak current of 3 mA in normal operation Positive inputs are clamped bya1kX resistor and series diode while an active clamp limits pin 3 to b350 mV for negative inputs (see R4 Q12 Q11 in internal schematic diagram) Operation of Zero Crossing Detector The LM1815 is designed to operate as a zero crossing de- tector triggering an internal one shot on the negative-going edge of the input signal Unlike other zero crossing detec- tors the LM1815 cannot be triggered until the input signal has crossed an ‘‘arming’’ threshold on the positive-going portion of the waveform The arming circuit is reset when the chip is triggered and subsequent zero crossings are ignored until the arming threshold is exceeded again This threshold varies depending on the connection at pin 5 Three different modes of operation are possible MODE 1 Pin 5 open The adaptive mode is selected by leaving pin 5 open circuit For input signals of less than 135 mVp-p the input arming threshold is typically 45 mV Under these conditions the input signal must first cross the 45 mV threshold in the positive direction to arm the zero crossing detector and then cross zero in the negative direc- tion to trigger it If the signal is less than 30 mV peak (mini- mum rating in Electrical Characteristics) the one shot is guaranteed to not trigger Input signals of greater than 230 mVp-p cause the arming threshold to track at 80% of the peak input voltage A peak detector (pin 7) stores a value relative to the positive input peaks to establish the arming threshold Input signals must exceed this threshold in the positive direction to arm the zero crossing detector which can then be triggered by a negative-going zero crossing The peak detector tracks rap- idly as the input signal amplitude increases and decays by virtue of the resistor connected externally at pin 7 to track decreases in the input signal Note that since the input is clamped the waveform ob- served at pin 3 is not identical to the waveform observed at the variable reluctance sensor Similarly the voltage stored at pin 7 is not identical to the peak voltage appearing at pin 3 MODE 2 Pin 5 connected to Va The input arming thresh- old is fixed at 200 mV minimum when pin 5 is connected to the positive supply The chip has no output for signals of less than 200 mV peak and triggers on the next negative- going zero crossing when the threshold is exceeded MODE 3 Pin 5 grounded With pin 5 grounded the input arming threshold is set to 0V (g25 mV maximum) Positive- going zero crossings arm the chip and the next negative zero crossing triggers it The one shot timing is set by a resistor and capacitor con- nected to pin 14 The output pulse width is pulse width e 0673 RC (1) In some systems it is necessary to externally generate puls- es such as during stall conditions when the variable reluc- tance sensor has no output External pulse inputs at pin 9 are gated through to pin 10 when Input Select (pin 11) is pulled high Pin 12 is a direct output for the one shot and is unaffected by the status of pin 11 Inputoutput pins 9 11 10 and 12 are all CMOS logic com- patible In addition pins 9 11 and 12 are TTL compatible Pin 10 is not guaranteed to drive a TTL load Pins 1 4 6 and 13 have no internal connections and can be grounded 4 |
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