Panasonic KX-FP207FX-S Service Manual page 20

KX-FP207FX-S/FP218FX-S
6.3.7. RTC Backup Circuit
1. Function
This unit has a lithium battery (BAT501) which works for Real Time Clock IC (RTC: inside IC501).
The RTC continues to work, backed up by a lithium battery even when the power switch is OFF.
The user parameters for autodial numbers, the system setup data and others are in the FLASH MEMORY (IC506).
2. RTC Inside (IC501) Backup Circuit Operation
When the power switch is turned ON, power is supplied through Q505 to the RTC (inside IC501). At this time, the voltage at
pin 14 of the IC501 is +2.5V. When the power switch is turned OFF, the BAT501 supplies power to RTC through D501 and
Q505. The voltage at pin 14 of IC501 is about +2.2V. When the power switch is OFF and the voltage of +3.3V decreases, pin
14 of RTC (IC501) becomes roughly the same voltage as the battery voltage. RTC goes into the backup mode, in which the
power consumption is lower.
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