Panasonic KX-MB771GR Service Manual page 24

KX-MB771GR
6.3.2. RTC BACKUP CIRCUIT
1. Function
This unit has a lithium battery (BAT300) which works for the Real Time Clock IC (RTC: inside IC300).
The RTC continues to work, backed up by a lithium battery even when the power switch is OFF.
2. RTC Inside (IC300) Backup Circuit Operation
When the power switch is turned ON, power is supplied to the RTC (inside IC300). At this time, the voltage at pin AD4 of the
IC300 is +3.3V. When the power switch is turned OFF, the BAT300 supplies power to RTC through DA300.
When the power switch is OFF and the voltage of +3.3V decreases, pin AD4 of RTC (IC300) becomes roughly the same volt-
age as the battery voltage. RTC goes into the backup mode, in which the power consumption is lower.
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