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MAX706AREPA Datasheet(PDF) 9 Page - Maxim Integrated Products |
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MAX706AREPA Datasheet(HTML) 9 Page - Maxim Integrated Products |
9 / 15 page +3V Voltage Monitoring, Low-Cost µP Supervisory Circuits _______________________________________________________________________________________ 9 To build an early-warning power-failure circuit, use the power-fail comparator input (PFI) to monitor the unregu- lated DC supply voltage (see the Typical Operating Circuits). Connect the PFI to a resistive-divider network such that the voltage at PFI falls below 1.25V just before the regulator drops out. Use PFO to interrupt the µP so it can prepare for an orderly power-down. Regulated and unregulated voltages can be monitored by simply adjusting the PFI resistive-divider network values to the appropriate ratio. In addition, the reset signal can be asserted at voltages other that VCC reset threshold, as shown in Figure 5. Connect PFO to MR to initiate a reset pulse when the 12V supply drops below a user-specified threshold (11V in this example) or when VCC falls below the reset threshold. Operation with +3V and +5V Supplies The MAX706P/R/S/T, the MAX706AP/AR/AS/AT, and the MAX708R/S/T provide voltage monitoring at the reset threshold (2.63V to 3.08V) when powered from either +3V or +5V. These devices are ideal in portable-instru- ment applications where power can be supplied from either a +3V battery or an AC-DC wall adapter that gen- erates +5V (a +5V supply allows a µP or a microcon- troller to run faster than a +3V supply). With a +3V supply, these ICs consume less power, but output drive capability is reduced, the MR to RESET delay time increases, and the MR minimum pulse width increases. The Electrical Characteristics table provides specifica- tions for operation with both +3V and +5V supplies. Ensuring a Valid RESET Output Down to VCC = 0V When VCC falls below 1V, the MAX706R/S/T, MAX706AR/AS/AT, and MAX708R/S/T RESET output no longer sinks current; it becomes an open circuit. High- impedance, CMOS logic inputs can drift to undeter- mined voltages if left as open circuit. If a pulldown resistor is added to the RESET pin , as shown in Figure 6, any stray charge or leakage current will flow to ground, holding RESET low. Resistor value R is not criti- cal, but it should not load RESET and should be small enough to pull RESET and the input it is driving to ground. 100kΩ is suggested for R1. Applications Information Adding Hysteresis to the Power-Fail Comparator Hysteresis adds a noise margin to the power-fail com- parator and prevents repeated triggering of the PFO when VIN is near the power-fail comparator trip point. Figure 7 shows how to add hysteresis to the power-fail comparator. Select the ratio of R1 and R2 such that PFI MAX706_ MAX708R/S/T VCC GND PFI *OPTIONAL R2 R3 R1 VIN +3V/+3.3V C1* TO µP PFO VTRIP = 1.25 (R1 + R2) R2 PFO +3V/+3.3V 0V 0V VH VH = 1.25 (1 + R3 + R2 R1) VL = 1.25 + R1 1.25 - VCC - 1.25 R2 × R3 R2 R3 VTRIP VIN VL MAX706_ MAX708R/S/T VCC GND PFI R2 R1 +3V/+3.3V PFO PFO +3V/+3.3V 0V NOTE: VTRIP IS NEGATIVE. 0V VTRIP V- VCC - 1.25 = 1.25 - VTRIP R1 R2 V- Figure 8. Monitoring a Negative Voltage Figure 7. Adding Hysteresis to the Power-Fail Comparator |
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