114 AMD Geode™ SC2200 Processor Data Book
SuperI/O Module
32580B
5.5.2.6 Power Supply
The device is supplied from two supply voltages, as shown
in Figure 5-8:
System standby power supply voltage, VSB
Backup voltage, from low capacity Lithium battery
A standby voltage, VSB, from the external AC/DC power
supply powers the RTC under normal conditions.
Figure 5-9 represents a typical battery configuration. No
external diode is required to meet the UL standard, due to
the internal switch and internal serial resistor RUL.
Figure 5-8. Power Supply ConnectionsFigure 5-9. Typical Battery Configuration
The RTC is supplied from one of two power supplies, VSB
or VBAT
, according to their levels. An internal voltage com-
parator delivers the control signals to a pair of switches.
Battery backup voltage VBAT maintains the correct time and
saves the CMOS memory when the VSB voltage is absent,
due to power failure or disconnection of the external AC/DC
input power supply or VSB main battery.
To assure that the module uses power from VSB and not
from VBAT
, the VSB voltage should be maintained above its
minimum, as detailed in Section 9.0 "Electrical Specifica-
tions" on page 369.
The actual voltage point where the module switches from
VBAT to VSB is lower than the minimum workable battery
voltage, but high enough to guarantee the correct function-
ality of the oscillator and the CMOS RAM.
Figure 5-10 shows typical battery current consumption dur-
ing battery-backed operation, and Figure 5-11 during nor-
mal operation.
Figure 5-10. Typical Battery Current: Battery Backed Power Mode @ TC = 25°CFigure 5-11. Typical Battery Current: Normal Operation Mode
RTC
Power
Backup
VDIGITAL
VSB
VDIGITAL
VSB
VBAT
Battery
External AC Power
Supply
VDIGITAL Sense
VSB
VBAT
VSB
PC0
VBAT
ONCTL# ONCTL#
ACPI Controller
VBAT
0.1μF
VSB
VSB
CF
CF
BT1
0.1μF
VPP
RUL
VREF
RTC
Note: Place a 0.1 μF capacitor on each VSB, VSBL
power supply pin as close as possible to the
pin, and also on VBAT
.
VSBL
CF0.1μF
VSBL
IBAT (μA)
10.0
7.5
5.0
2.5
2.4 3.0 3.6 VBAT (V)
IBAT (μA)
0.75
0.50
0.25
3.0 3.3 3.6
VSB
Note:Battery voltage in this test is 3.0V.
(V)