SP707 EXAR [Exar Corporation], SP707 Datasheet - Page 11

• Manufacturer Part Number: SP707
• Description: Low Power Microprocessor Supervisory Circuits
• Manufacturer: EXAR [Exar Corporation]

Power-Fail Comparator
The power-fail comparator can be used for
various purposes because its output and
noninverting input are not internally connected.
The inverting input is internally connected to a
1.25V reference.
To build an early-warning circuit for power
failure, connect the PFI pin to a voltage divider
as shown in Figure 16. Choose the voltage
divider ratio so that the voltage at PFI falls
below 1.25V just before the +5V regulator drops
out. Use PFO to interrupt the P so it can
prepare for an orderly power-down.
Manual Reset
The manual-reset input (MR) allows RESET to
be triggered by a pushbutton switch. The switch
is effectively debounced by the 140ms mini-
mum RESET pulse width. MR is TTL/CMOS
logic compatible, so it can be driven by an
external logic line. MR can be used to force a
watchdog timeout to generate a RESET pulse
in the SP705/706/813L. Simply connect
WDO to MR.
June 2008 Rev C
Figure 16. Typical Operating Circuit
V
P
GND
CC
RESET
INTERRUPT
I/O LINE
NMI
Regulated +5V
Power Supply
RESET
PUSHBUTTON
0.1 F
WDO
PFO
PFI
SWITCH
SP705
SP706
SP813L
GND
V
Unregulated DC
CC
SP705 Low Power Microprocessor Supervisory Circuits
Power Supply
MR
PFI
R
R
1
2
11
Ensuring a Valid RESET Output Down to
V
When V
708 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 , 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
Figure 17. Monitoring Both +5V and +12V Power
Supplies
CC
to P
= 0V
CC
falls below 1.1V, the SP705/706/707/
RESET
PFO
MR
© 2008 Exar Corporation
+5V
V
GND
CC
PFI
+12V
1M
1%
130K
1%