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TL7715 Arkusz danych(PDF) 6 Page - Texas Instruments |
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TL7715 Arkusz danych(HTML) 6 Page - Texas Instruments |
6 / 17 page Circuit Description 2 Literature Number: SLVAE04 In larger computers several features are provided to prevent such errors: a power-fail interrupt signals dangerous conditions in time, the content of the memory is protected by a battery back-up, and so on. In small microcomputer systems this amount of effort is too expensive, and in most applications also not required. It is usually sufficient if, after a serious voltage drop, the microcomputer is forced into a defined initial condition. To implement this function, whilst preventing the problems mentioned above, the following circuit features are required: − Accurate detection of a serious voltage drop. − Generation of a reset signal while as the supply voltage is not in the operational range, to prevent undefined operations of the microcomputer. − Maintenance of the reset signal for a certain time after the supply voltage has returned to its nominal value, to ensure proper initialization of the circuit. For these applications, Texas Instruments has developed a series of integrated circuits which, with a minimum of external components and without additional adjustment, will fulfill the requirements described above. 2. Circuit Description The main part of this circuit is a reference voltage source, which consists of a very stable, temperature- compensated band gap reference. An external capacitor (typ. 0.1 µF) must be connected to the voltage output V ref, to reduce the influence of fast transients in the supply voltage. The voltage at the SENSE input is divided by a resistor divider and compared with the reference voltage by a comparator. To achieve high accuracy, this divider is adjusted at wafer probe. When the input voltage is sensed to be lower than the threshold voltage, the thyristor is triggered, which discharges the timing capacitor C t. It is also possible to fire the thyristor via the RESIN input by a logic level (TTL level, active low). The thyristor is turned off again when either the voltage at the SENSE input (or RESIN input) increases beyond the threshold, or - during short supply voltage drops - the discharge current of the capacitor becomes lower than the hold current of the thyristor. Thereafter, the capacitor is recharged by a current source 100 µA, the charge time being calculated as follows: t C d t = ⋅ ⋅ 1 3 10 4 . Ct in F, t in s The magnitude of charge current and therefore also the delay t d time is determined by the tolerance of the resistors in the integrated circuit. These tolerances, caused by the semiconductor manufacturing process, are not negligible. Therefore the delay time may vary -50% to +100 %. However, for the applications discussed here this will not be a restriction. The diagram in figure 3 shows the typical delay time t d versus the capacitance of the external capacitor C t. |
Podobny numer części - TL7715 |
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Podobny opis - TL7715 |
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