SP706PCP datasheet(12/18 Pages) SIPEX

12 / 18 page

Rev. 10-17-00

SP706 +3.0/ +3.3 Low Power Microprocessor Circuits

Ensuring a Valid RESET Output Down to

CC = 0V

When V

CC falls below 1V, the RESET output no

longer sinks current, it becomes an open circuit.

High-impedance CMOS logic inputs can drift to

undetermined voltages if left undriven. If a pull-

down resistor is added to the RESET pin, any

stray charge or leakage currents will be shunted

to ground, holding RESET LOW. The resistor

value is not critical. It should be about 100K

Ω,

large enough not to load RESET and small

enough to pull RESET to ground.

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 +3.3V/+3.0V

CC line. Connect PFO to MR to initiate a

RESET pulse when PFI drops below 1.25V.

Figure 17 shows the SP706R/S/T-SP708R/

S/T series configured to assert RESET when the

+3.3V/+3.0V supply falls below the RESET

threshold, or when the +12V supply falls below

approximately 11V.

Monitoring a Negative Voltage Supply

The power-fail comparator can also monitor a

negative supply rail, shown in Figure 18.

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 SP706P/R/S/T-SP708R/S/

T series will keep RESET asserted (where

RESET = LOW and RESET = HIGH). Note that

this circuit's accuracy depends on the PFI

threshold tolerance, the V

CC line, and the resis-

tors.

Interfacing to mPs with Bidirectional

RESET Pins

µPs with bidirectional RESET pins, such as the

Motorola 68HC11 series, can contend with the

RESET output. If, for example, the RESET

PFI

PFO

VCC

+3.3V/+3.0V

GND

RESET

µP

+12V

Ω

130K

Ω

Figure 17. Monitoring Both +3.3V/+3.0V and +12V

Power Supplies

GND

RESET

INTERRUPT

I/O LINE

VCC

RESET

PFO

PFI

Unregulated DC

Power Supply

Regulated +3.3V/+3.0V

Power Supply

VCC

0.1

µF

PFI

µP

NMI

WDO

PUSHBUTTON

SWITCH

Figure 16. Typical Operating Circuit

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