3.0V or 3.3V
VCC
+5V
3.0V or 3.3V
VCC
A)
B)
1N4148
1N4148
CONNECT TO
STATIC RAM
VOUT
CONNECT TO
STATIC RAM
VOUT
VBATTERY
VBATTERY
pin 7*
pin 7*
CONNECT
CONNECT
0.1F
0.1F
TO µP
TO µP
GND
GND
RESET for the SP690T/S/R and the SP802T/S/R
RESET for the SP804T/S/R and the SP805T/S/R
*
Figure 21. Using a High Capacity Capacitor as a Backup Power Source
SwitchoveratVSW (2.40V)ensuresthatbattery-
backup mode is entered before VOUT gets too
close to the 2.0V minimum required to reliably
retaindatainCMOSRAM.Switchoverathigher
VCC voltages would decrease backup-battery
life. WhenVCC recovers, switchoverisdeferred
Replacing the Backup Battery
IfVBATTERY isdecoupledwitha0.1µFcapacitor
to ground, the backup battery can be removed
while VCC remains valid without danger of
triggering RESET/RESET. As long as VCC
stays above VSW, battery-backup mode cannot
be entered.
until VCC rises above the reset threshold, VRST
,
to ensure a stable supply. VOUT is connected to
VCC through a 1.5Ω PMOS power switch.
Adding Hysteresis to the Power-Fail
Comparator
Using a High Capacity Capacitor as a
Backup Power Source
The power-fail comparator has a typical input
hysteresis of 10mV. This is sufficient for most
applications where a power-supply line is being
monitored through an external voltage divider
(refertotheMonitoringanAdditionalPower
Supply section).
Figure 21 shows two ways to use a High Value
Capacitor as a backup power source. The High
Value Capacitor may be connected through a
diode to the 3V input as in Figure 21A or, if a
5V supply is also available, the High Value
Capacitor may be charged up to the 5V supply
as in Figure 21B allowing a longer backup
period. Since VBATTERY can exceed VCC while VCC
is above the reset threshold, there are no
special precautions when using these µP
supervisors with a High Value Capacitor.
If additional noise margin is desired, connect a
resistor between PFO and PFI as shown in
Figure 22A. Select the ratio of R1 and R2 such
that PFI sees 1.25V when VIN falls to its trip
point (VTRIP). R3 adds the hysteresis and will
typically be more than 10 times the value of R1
or R2. The hysteresis window extends both
above (VH) and below (VL) the original trip
point (VTRIP).
Operation Without a Backup Power
Source
These µP supervisors were designed for
battery-backed applications. If a backup power
source is not used, connect both VBATTERY
and VOUT to VCC. Since there is no need to
switch over to any backup power source, VOUT
does not need to be switched. A direct connec-
tion to VCC eliminates any voltage drops across
the switch which may push VOUT below VCC
.
SP690T/S/R DS/10 SP690T/S/R, 802/T/S/R, 804T/S/R, 805T/S/R Low Power Microprocessor Supervisory
© Copyright 2000 Sipex Corporation
14
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