POWER DISTRIBUTION
21 May, 1999
1-20
1.6.2 MEMORY BACK-UP CIRCUIT
The +5VBAT supply from the lithium battery [A] backs up the system RAM, which
contains system parameters, 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 FCU 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 5 or 6 days.
The battery [B] generates about 3 volts (max. 3.2 volts). The DC/DC converter
raises this voltage to 5 volts so it can be used as the +5VD supply for the 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.
NOTE: There is no battery switch for the battery [B].
F C U
D R A M
+ 5 V
Q 2 9
I C C a r d
D C / D C
Converter
+ 5 V E
+ 5 V
+ 5 V B A T
+ 5 V D
P O W E R S
5 R T C C S
+5VBAT
S y s t e m
R A M
Switching
Circuit
1 V D E T
1 S A F F G
FCIP05
+
-
+ 5 V D
H547V513.WMF
[A]
[B]
