82C836 Datasheet, PDF (32/205 Pages) List of Unclassifed Manufacturers – Single-Chip 386sx AT

s Battery Backup and Power-Up/Down
Figure 3-3. Battery Backup Circuit
Functional Description
A similar precaution is needed during Vcc ramp up or ramp down. The Vcc
‘‘changeover ’’ threshold, determined by R1 and R2 should be high enough so that the
2N3906 cannot remain on when Vcc is less than 4VDC. Otherwise, depending on
battery voltage, the 2N3906 may again reverse conduct and load down the battery
momentarily as Vcc is ramping up or down.
The user should also ensure that the Vin maximum specification for the 82C836 is never
exceeded when powering the system up or down. Failure to observe this specification
may result in damage to the device. The three diodes in series with the battery provide
roughly 1.8VDC voltage drop between the battery and the 82C836, as well as protecting
the battery from possible explosion in the event of accidental polarity reversal. (Safety
agencies generally require a minimum of two series diodes or one diode and a series
resistor.)
A pin is provided in the 82C836 to protect the contents of the Real Time Clock and
reduce power consumption whenever the system is powered down. The PWRGOOD
pin should be low whenever the system power supply is not within specifications for
proper operation of the system. This signal may be generated by circuitry either in the
power supply or on the system board. The PWRGOOD input disables all unnecessary
3-4 Revision 3.0
PRELIMINARY
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