SP707CU-LTR datasheet(11/17 Pages) EXAR

Part #	SP707CU-LTR

Description	Low Power Microprocessor Supervisory Circuits

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Manufacturer	EXAR [Exar Corporation]

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11 / 17 page

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Rev. 2.0.0

MONITORING VOLTAGES OTHER THAN THE

UNREGULATED DC INPUT

Monitor voltages other than the unregulated

DC by connecting a voltage divider to PFI and

adjusting the ratio appropriately. If required,

add hysteresis by connecting a resistor (with a

value approximately 10 times the sum of the

two resistors in the potential divider network)

between PFI and PFO. A capacitor between PFI

and GND will reduce the power-fail circuit's

sensitivity to high-frequency noise on the line

being monitored. RESET can be used to

monitor voltages other than the +5V VCC line.

Connect PFO to MR to initiate a RESET pulse

when PFI drops below 1.25V. Figure 21 shows

the SP705/706/707/708 configured to assert

RESET when the +5V supply falls below the

RESET threshold, or when the +12V supply

falls below approximately 11V.

Fig. 21: Monitoring +5V and +12V Power Supplies

MONITORING A NEGATIVE VOLTAGE SUPPLY

The power-fail comparator can also monitor a

negative supply rail, shown in Figure 22. When

the negative rail is good (a negative voltage of

large magnitude), PFO is LOW. By adding the

resistors and transistor as shown, a HIGH PFO

triggers RESET. As long as PFO remains HIGH,

the SP705-708/813L will keep RESET asserted

(where RESET = LOW and RESET = HIGH).

Note that this circuit's accuracy depends on

the PFI threshold tolerance, the VCC line, and

the resistors.

Fig. 22: Monitoring a Negative Voltage Supply

INTERFACING TO µPS WITH BIDIRECTIONAL

RESET PINS

µPs with bidirectional RESET pins, such as the

Motorola 68HC11 series, can contend with the

SP705/706/707/708 RESET output. If, for

example, the RESET output is driven HIGH and

the µP wants to pull it LOW, indeterminate

logic levels may result. To correct this,

connect a 4.7KΩ resistor between the RESET

output and the µP reset I/O, as shown if

Figure 23. Buffer the RESET output to other

system components.

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