SP690T Datasheet, PDF (14/20 Pages) Sipex Corporation – 3.0V/3.3V Low Power Microprocessor Supervisory with Battery Switch-Over

A)
3.0V or 3.3V
1N4148
VBATTERY
VCC
VOUT
CONNECT TO
STATIC RAM
0.1F
pin 7*
GND
CONNECT
TO µP
B) ^{+5V 3.0V or 3.3V}
1N4148
VBATTERY
VCC
VOUT
CONNECT TO
STATIC RAM
0.1F
pin 7*
CONNECT
TO µP
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
Switchover at V_SW (2.40V) ensures that battery-
backup mode is entered before V_OUT gets too
close to the 2.0V minimum required to reliably
retain data in CMOS RAM. Switchover at higher
V_CC voltages would decrease backup-battery
life. When V_CC recovers, switchover is deferred
until V_CC rises above the reset threshold, V_RST,
to ensure a stable supply. V_OUT is connected to
V_CC through a 1.5Ω PMOS power switch.
Using a High Capacity Capacitor as a
Backup Power Source
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 V_BATTERY can exceed V_CC while V_CC
is above the reset threshold, there are no
special precautions when using these µP
supervisors with a High Value Capacitor.
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 V_OUT to V_CC. Since there is no need to
switch over to any backup power source, V_OUT
does not need to be switched. A direct connec-
tion to V_CC eliminates any voltage drops across
the switch which may push V_OUT below V_CC.
Replacing the Backup Battery
If V_BATTERY is decoupled with a 0.1µF capacitor
to ground, the backup battery can be removed
while V_CC remains valid without danger of
triggering RESET/RESET. As long as V_CC
stays above V_SW, battery-backup mode cannot
be entered.
Adding Hysteresis to the Power-Fail
Comparator
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
(refer to the Monitoring an Additional Power
Supply section).
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 V_IN falls to its trip
point (V_TRIP). R3 adds the hysteresis and will
typically be more than 10 times the value of R1
or R2. The hysteresis window extends both
above (V_H) and below (V_L) the original trip
point (V_TRIP).
SP690T/S/R DS/10 SP690T/S/R, 802/T/S/R, 804T/S/R, 805T/S/R Low Power Microprocessor Supervisory
14
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