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ASM690AESA Datasheet(PDF) 6 Page - PulseCore Semiconductor |
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ASM690AESA Datasheet(HTML) 6 Page - PulseCore Semiconductor |
6 / 15 page ASM690A/692A ASM802L/802M April 2008 ASM805L rev 1.7 µP Power Supply Supervisor With Battery Backup Switch 6 of 15 Notice: The information in this document is subject to change without notice. Power-Fail Comparator The Power Fail feature is an independent voltage monitoring function that can be used for any number of monitoring activities. The PFI function can provide an early sensing of power supply failure by sensing the voltage of the unregulated DC ahead of the regulated supply sensing seen by the backup-battery switchover circuitry. The PFI pin is compared to a 1.25V internal reference. If the voltage at the PFI pin is less than this reference voltage, the PFO pin goes low. By sensing the voltage of the raw DC power supply, the microprocessor system can prepare for imminent power-loss, especially if the battery backup supply is not enabled. The input voltage at the PFI pin results from a simple resistor voltage divider as shown in Figure 6. Power Fail Hysteresis A noise margin can be added to the simple monitoring circuit of Figure 6 by adding positive feedback from the PFO pin. The circuit of Figure 7 adds this positive “latching” effect by means of an additional resistor R3 connected between PFO and PFI which helps in pulling PFI in the direction of PFO and eliminating an indecision at the trip point. Resistor R3 is normally about 10 times higher in resistance than R2 to keep the hysteresis band reasonable and should be larger than 10k Ω to avoid excessive loading on the PFO pin. The calculations for the correct values of resistors to set the hysteresis thresholds are given in Figure 7. A capacitor can be added to offer additional noise rejection by low-pass filtering. Monitoring Capabilities Of The Power-fail Input: Although designed for power supply failure monitoring, the PFI pin can be used for monitoring any voltage condition that can be scaled by means of a resistive divider. An example is the negative power supply monitor configured in Figure 8. In this case a good negative supply will hold the PFI pin below 1.25V and the PFO pin |
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