+3V Voltage Monitoring, Low-Cost µP

Supervisory Circuits

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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

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

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

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

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

GND

PFI

*OPTIONAL

R2

R3

R1

+3V/+3.3V

C1*

TO µP

PFO

(R1 + R2)

R2

PFO

+3V/+3.3V

0V

0V

R3 + R2

1.25

-

R2 × R3

R2

R3

MAX706_

MAX708R/S/T

GND

PFI

R2

R1

+3V/+3.3V

PFO

PFO

+3V/+3.3V

0V

0V

V-

R1

R2

V-

Figure 8. Monitoring a Negative Voltage

Figure 7. Adding Hysteresis to the Power-Fail Comparator