MAX708 Application Circuit

Photocoupler

The MAX708 is capable of monitoring a secondary power supply signal to offer a voltage drop warning feature for the processor. This capability allows for executing certain safety procedures before the power supply falls to the reset state. Parameters can be saved to trigger an alarm signal or switch to a backup battery. For instance, using the MAX708 meter, the current power supply can be saved to the E2PROM prior to a power glitch or outage. Multiple power backup algorithms can then be implemented to effectively address the issue of data loss in the E2PROM during a power failure. It’s crucial to choose an appropriate warning voltage point to ensure that the VCC voltage drops from the warning voltage to the reset voltage within a sufficient time frame. The reset voltage’s sustain time, tB, should be long enough. The write cycle of the E2PROM takes approximately 10 to 20 milliseconds, and generally, the data becomes stable when the warning voltage is written with tB > 200 ms. To adjust, when VDC equals the warning voltage, R1 and R2 should be adjusted so that the voltage at PFI is 1.25V. At this point, you can verify the operation of the internal voltage comparator by checking /PFO. It’s important to note that this comparator is a window comparator. Figure 10 shows the application program flowchart.