Memory Back-Up Circuit - Ricoh FAX3700l Service Manual

August 2nd, 1995 OVERALL MACHINE INFORMATION
POWER DISTRIBUTION

1.6.2. Memory Back-up Circuit

[A]
[B]
H516V512.wmf
The +5VBAT supply from the lithium battery [A] backs up the system RAM
which contains system parameters and programmed telephone numbers,
and the real time clock in the main cpu. The 5RTCCS signal tells the main
cpu whether the back-up power (+5VBAT) is coming from the battery or from
the +5V power supply.
A rechargeable lithium battery [B] and the dc/dc converter on the FDU back
up the DRAM (SAF memory) for one hour, if there is data in the SAF memory
and the power is switched off. While the main power is on, the +5VE supply
recharges the battery. The battery recharges in one or two days.
The battery [B] generates about 3 volts (max. 3.2 volts). The dc/dc converter
(IC12) lifts this voltage to 5 volts so it can be used as the +5VD supply for
SAF backup. The CPU monitors the voltage of the +5VD supply with the
1VDET signal. When the battery has run down, and the voltage is lower than
4.4 volts, the CPU stops the dc/dc converter by dropping 1SAFFG to low and
the machine stops backing up the memory.
There is no battery switch for the battery [B].
1-21