SP706P Datasheet, PDF (12/18 Pages) Sipex Corporation – +3.0V/+3.3V Low Power Microprocessor Supervisory Circuits

Ensuring a Valid RESET Output Down to
V_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 100KΩ,
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
V line. Connect PFO to MR to initiate a
CC
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
Regulated +3.3V/+3.0V
Power Supply
Unregulated DC
Power Supply
VCC
0.1µF
µP ^RESET
RESET
INTERRUPT _PFO
I/O LINE
PFI
NMI
WDO
VCC
R1
PFI
R2
MR
GND
GND
PUSHBUTTON
SWITCH
to µP
+3.3V/+3.0V +12V
MR
PFO
RESET
VCC
1MΩ
1%
PFI
130KΩ
1%
GND
Figure 16. Typical Operating Circuit
Figure 17. Monitoring Both +3.3V/+3.0V and +12V
Power Supplies
Rev. 10-17-00
SP706 +3.0/ +3.3 Low Power Microprocessor Circuits
12
© Copyright 2000 Sipex Corporation