Datasheet

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SLVS335C − DECEMBER 2000 − REVISED JANUARY 2007
5
detailed description
battery freshness seal
The battery freshness seal of the TPS3606 family disconnects the backup battery from the internal circuitry until
it is needed. This ensures that the backup battery connected to V
BAT
is fresh when the final product is put to
use. The following steps explain how to enable the freshness seal mode:
1. Connect V
BAT
(V
BAT
> V
BAT(min)
)
2. Ground PFO
3. Connect PFI to V
DD
or PFI > V
(PFI)
4. Connect V
DD
to power supply (V
DD
> V
IT
)
5. Ground MR
6. Power down V
DD
7. The freshness seal mode is entered and pins PFO and MR can be disconnected.
The battery freshness seal mode is disabled by the positive-going edge of RESET
when V
DD
is applied.
power-fail comparator (PFI and PFO
)
An additional comparator is provided to monitor voltages other than the nominal supply voltage. The power-fail
input (PFI) is compared with an internal voltage reference of 1.15 V. If the input voltage falls below the power-fail
threshold (V
(PFI)
) of 1.15 V typical, the power-fail output (PFO) goes low. If it goes above V
(PFI)
plus about 12-mV
hysteresis, the output returns to high. By connecting two external resistors, it is possible to supervise any
voltages above V
(PFI)
. The sum of both resistors should be about 1 MΩ, to minimize power consumption and
also to ensure that the current in the PFI pin can be neglected compared with the current through the resistor
network. The tolerance of the external resistors should be not more than 1% to ensure minimal variation of
sensed voltage.
If the power-fail comparator is unused, connect PFI to ground and leave PFO
unconnected.
backup-battery switchover
In the event of a brownout or power failure, it may be necessary to keep a processor running. If a backup battery
is installed at V
BAT
, the devices automatically connect the processor to backup power when V
DD
fails. In order
to allow the backup battery (e.g., a 3.6-V lithium cell) to have a higher voltage than V
DD
, this family of supervisors
does not connect V
BAT
to V
OUT
when V
BAT
is greater than V
DD
. V
BAT
only connects to V
OUT
(through a 2-Ω
switch) when V
OUT
falls below V
(SWN)
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 threshold (V
(SWP)
.
V
DD
> V
BAT
V
DD
> V
(SWN)
V
OUT
1 1 V
DD
1 0 V
DD
0 1 V
DD
0 0 V
BAT
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