April 2008
rev 1.7
ASM690A/692A
ASM802L/802M
ASM805L
the ASM692A/802M or within ±10% of 5V for the
Table 2: Maximum Battery Voltages
ASM690A/802L/805L to insure that the storage capacitor
does not achieve an over voltage state.
Note: SuperCap
TM
is a trademark of Baknor Industries
Part Number
ASM690A
ASM802L
ASM805L
ASM692A
ASM802M
MAXIMUM Battery Voltage (V)
4.80
4.80
4.80
4.55
4.55
Although most batteries that meet the requirements of
Table2
are acceptable, lithium batteries are very effective
backup source due to their high-energy density and very
low selfdischarge rates.
Battery replacement while Powered
Batteries can be replaced even when the device is in a
powered state as long as V
CC
remains above the reset
threshold voltage V
RT
. In the ASM devices, a floating
V
BATT
pin will not cause a powersupply switchover as can
occur in some other supervisory circuits. If V
BATT
is not
used, the pin should be grounded.
Backup Power Sources - SuperCap™
Capacitor storage, with very high values of capacitance,
can be used as a back-up power source instead of
batteries. SuperCap
™
are capacitors with capacities in
the fractional farad range. A 0.1 farad SuperCap™ would
provide a useful backup power source. Like the battery
supply, it is important that the capacitor voltage remain
below the maximum voltages shown in
Table 2.
Although
the circuit of
Figure 4
shows the most simple way to
connect the SuperCap™, this circuit cannot insure that an
over voltage condition will not occur since the capacitor
will ultimately charge up to V
CC
. To insure that an over
voltage condition does not occur, the circuit of
Figure 5
is
preferred. In this circuit configuration, the diode-resistor
pair clamps the capacitor voltage at one diode drop below
V
CC
. V
CC
itself should be regulated within ±5% of 5V for
Operation without a Backup Power Source
When operating without a back-up power source, the
V
BATT
pin should be connected to GND and V
OUT
should
be connected to V
CC
, since power source switchover will
not occur. Connecting V
OUT
to V
CC
eliminates the voltage
drop due to the ON-resistance of the PMOS switch.
µP Power Supply Supervisor With Battery Backup Switch
Notice: The information in this document is subject to change without notice.
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