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MAX690A Datasheet(PDF) 9 Page - Maxim Integrated Products |
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MAX690A Datasheet(HTML) 9 Page - Maxim Integrated Products |
9 / 12 page Microprocessor Supervisory Circuits _______________________________________________________________________________________ 9 connected directly to the VBATT input of the MAX690A/MAX692A/MAX802L/MAX802M/MAX805L with no additional circuitry (see the Typical Operating Circuit). However, batteries with open-circuit voltages that are greater cannot be used for backup, as current is sourced into the substrate through the diode (D1 in Figure 3) when VCC is close to the reset threshold. Operation Without a Backup Power Source If a backup power source is not used, ground VBATT and connect VOUT to VCC. Since there is no need to switch over to any backup power source, VOUT does not need to be switched. A direct connection to VCC eliminates any voltage drops across the switch which may push VOUT below VCC. Replacing the Backup Battery The backup battery can be removed while VCC remains valid, without danger of triggering RESET/RESET. As long as VCC stays above the reset threshold, battery- backup mode cannot be entered. In other switchover PFO VCC GND MAX690A MAX692A MAX802L MAX802M MAX805L +5V R2 R1 PFI +5V 0V PFO VTRIP V- 0V V- NOTE: VTRIP IS NEGATIVE Figure 7. Monitoring a Negative Voltage 5 1.25 R1 1.25 - VTRIP R2 − = ICs where battery-backup mode is entered whenever VBATT gets close to VCC, an unconnected VBATT pin accumulates leakage charge and triggers RESET/RESET in error. Adding Hysteresis to the Power-Fail Comparator Hysteresis adds a noise margin to the power-fail com- parator and prevents repeated triggering of PFO when VIN is close to its trip point. Figure 6 shows how to add hysteresis to the power-fail comparator. Select the ratio of R1 and R2 such that PFI sees 1.25V when VIN falls to its trip point (VTRIP). R3 adds the hysteresis. It will typically be an order of magnitude greater than R1 or R2 (about 10 times either R1 or R2). The current through R1 and R2 should be at least 1µA to ensure that the 25nA (max) PFI input current does not shift the trip point. R3 should be larger than 10kΩ so it does not load down the PFO pin. Capacitor C1 adds additional noise rejection. Monitoring a Negative Voltage The power-fail comparator can be used to monitor a negative supply rail using the circuit of Figure 7. When the negative rail is good (a negative voltage of large magnitude), PFO is low. When the negative rail is degraded (a negative voltage of lesser magnitude), PFO goes high. This circuit’s accuracy is affected by the PFI threshold tolerance, the VCC line, and the resis- tors. Interfacing to µPs with Bidirectional Reset Pins µPs with bidirectional reset pins, such as the Motorola 68HC11 series, can contend with the MAX690A/MAX692A/MAX802L/MAX802M RESET out- put. If, for example, the RESET output is driven high and the µP wants to pull it low, indeterminate logic lev- els may result. To correct this, connect a 4.7k Ω resis- tor between the RESET output and the µP reset I/O, as in Figure 8. Buffer the RESET output to other system components |
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