82C836 Datasheet, PDF (31/205 Pages) List of Unclassifed Manufacturers – Single-Chip 386sx AT

Functional Description
Battery Backup and Power-Up/Down s
Figure 3-2. PS Diode for External Real Time Clock Systems
PWRGOOD
39
82C836B
141
-RTCCS
(MFP5)
1N4148
Battery Backup and Power-Up/Down
Most applications require the Real Time Clock to remain active whenever the system
power is turned off. To accomplish this, the user must provide an alternate source of
power to the 82C836. This alternate source of power is normally provided by connecting
a battery to the V _CC supply pin of the device. A means should be provided for switching
between the system power supply and the battery. A circuit such as the one shown in
Figure 3-3 may be used to eliminate power drain on the battery when system power is
available.
The circuit allows reliable transitions between system and battery power without undue
battery power drain. The battery voltage should not be high enough to cause a net
increase in back-up voltage when normal power is turned off. Otherwise, the 82C836
and 32KHz oscillator will continue to be powered by the battery even when the power
supply is turned on, and battery drain will actually increase when power is on due to
reverse conduction through the PNP transistor (2N3906).
Chips and Technologies, Inc.
PRELIMINARY
Revision 3.0 3-3