Datasheet


SLVS340D − DECEMBER 2000 − REVISED JULY 2008
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5
detailed description
backup-battery switchover
In case of a brownout or power failure, it may be
necessary to preserve the contents of RAM. If a
backup battery is installed at V
BAT
, the device
automatically switches the connected RAM to
backup power when V
DD
fails. In order to allow the
backup battery (for example, 3.6-V lithium cells)
to have a higher voltage than V
DD
, these
supervisors do not connect V
BAT
to V
OUT
when
V
BAT
is greater than V
DD
. V
BAT
only connects to
V
OUT
(through a 15-Ω switch) when V
DD
falls
below V
IT
and V
BAT
is greater than V
DD
. When
V
DD
recovers, switchover is deferred either until
V
DD
crosses V
BAT
, or when V
DD
rises above the
reset threshold V
IT
. V
OUT
connects to V
DD
through
a 1-Ω (max) PMOS switch when V
DD
crosses the
reset threshold.
V
DD
>V
BAT
V
DD
>V
IT
V
OUT
1 1 V
DD
1 0 V
DD
0 1 V
DD
0 0 V
BAT
V
DD
− Mode
V
BAT
− Mode
VBSW Hysteresis
V
IT
Hysteresis
V
BAT
− Backup-Battery Supply Voltage
− Normal Supply Voltage
V
DD
Undefined
Figure 1. V
DD
− V
BAT
Switchover