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STM704 Datasheet(PDF) 12 Page - STMicroelectronics |
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STM704 Datasheet(HTML) 12 Page - STMicroelectronics |
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12 / 37 page  STM690A/692A/703/704/802/805/817/818/819 12/37 Power-fail Input/Output (NOT available on STM818) The Power-fail Input (PFI) is compared to an inter- nal reference voltage (independent from the VRST comparator). If PFI is less than the power-fail threshold (VPFI), the Power-Fail Output (PFO) will go low. This function is intended for use as an un- dervoltage detector to signal a failing power sup- ply. Typically PFI is connected through an external voltage divider (see Figure 11., page 8) to either the unregulated DC input (if it is available) or the regulated output of the VCC regulator. The voltage divider can be set up such that the voltage at PFI falls below VPFI several milliseconds before the regulated VCC input to the STM690A/692A/703/ 704/802/805/817/818/819 Supervisor or the mi- croprocessor drops below the minimum operating voltage. During battery back-up, the power-fail comparator turns off and PFO goes (or remains) low (see Fig- ure 14 and Figure 15., page 13). This occurs after VCC drops below 2.4V (or VSO). When power re- turns, PFO is forced high (STM817/819 only), irre- spective of VPFI for the WRITE protect time (trec). At the end of this time, the power-fail comparator is enabled and PFO follows PFI. If the comparator is unused, PFI should be connected to VSS and PFO left unconnected. PFO may be connected to MR on the STM703/704/818 so that a low voltage on PFI will generate a reset output. Applications Information These Supervisor circuits are not short-circuit pro- tected. Shorting VOUT to ground - excluding pow- er-up transients such as charging a decoupling capacitor - destroys the device. Decouple both VCC and VBAT pins to ground by placing 0.1µF ca- pacitors as close to the device as possible. Figure 14. Power-fail Comparator Waveform (STM817/818/819) AI08804a VCC VRST VSO (or 2.4V) trec RST to ECON Delay (STM818) RST ECON (STM818) PFO (STM817/819) PFO follows PFI PFO follows PFI |
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