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TL7702B Fiches technique(PDF) 8 Page - Texas Instruments |
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TL7702B Fiches technique(HTML) 8 Page - Texas Instruments |
8 / 17 page Circuit Description 4 Literature Number: SLVAE04 An additional comparator compares the voltage at the capacitor with the reference voltage and forces the outputs into the active state as long as the voltage at the capacitor is lower than the reference voltage. Figure 4 shows the timing of the various signals. In this example the SENSE input is connected to the supply voltage V cc as in typical applications of this device. The minimum supply voltage for which the function of this device is guaranteed is 3.6 V. After power-on the outputs are undefined until the minimum supply voltage V res is reached. For the TL77xxA the minimum supply voltage is V res = 3.0 V (typical 2.5 V), for the TL77xxB is V res = 1.0 V. Also, when using the TL77xxB it has to be noted, that with such low supply voltages the function of the reset input of the following circuit may not be guaranteed. Beyond the voltage V res the capacitor C t is first kept discharged, and the outputs stay in the active state(RESET = High, RESET = Low). When the input voltage becomes higher than the threshold voltage V t, the thyristor is turned off and the capacitor is charged. After a delay, t d, the voltage at the capacitor passes the trigger level of the output comparator and the outputs become inactive. The circuit to be initialized is now set to a defined state and starts the correct operation. Note: SENSE Input connected to V cc Figure 4. Timing diagram The thyristor is triggered again during voltage drops below the threshold voltage V t. and the reset sequence starts again. Also now the outputs stays in the active state for the time t d after the return of the supply voltage to its required value. A hysteresis V h at the input comparator prevents oscillation of the input circuit when the input voltage rises or falls slowly. The time t d - and also the capacitor Ct - are determined by the requirements of the following circuitry. In TTL or CMOS logic circuits, theoretically a reset time of 20 to 50ns is sufficient. For proper operation, microcomputers require a reset signal which lasts for several machine cycles and is thus of the order of 10 to 200 µs, according to the type of microcomputer in use. In a practical application, the delay time will be determined by characteristics of the power supply. Care has to be taken, that during and shortly after |
Numéro de pièce similaire - TL7702B |
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Description similaire - TL7702B |
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