AMD SC3200 manual Power Supply, Rtc, BT1

32581C SuperI/O Module

5.5.2.6Power 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 5-9represents a typical battery configuration. No external diode is required to meet the UL standard, due to the internal switch and internal serial resistor RUL.

	External AC Power
ACPI Controller		Power
		Supply
VDIGITAL Sense	VDIGITAL	VDIGITAL

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 351.

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 5-10shows typical battery current consumption dur- ing battery-backed operation, and Figure 5-11during nor- mal operation.

	PC0		ONCTL#	ONCTL#
		VSB
RTC	VSB		VSB
				VSB
	VBAT	VBAT	VBAT	Backup
				Battery

Figure 5-8. Power Supply Connections

IBAT (μA)

10.0

7.5

5.0

2.5

2.4 3.0 3.6

VBAT (V)

	VSB		VSB
	RTC	CF
			0.1 μF
VPP	VSBL	CF	VSBL
	VREF		0.1 μF
	VBAT		BT1
	RUL	C
		F
		0.1 μF

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.

Figure 5-9. Typical Battery Configuration

Figure 5-10. Typical Battery Current: Battery

Backed Power Mode @ TC = 25°C

IBAT (μA)

0.75
0.50
0.25	VSB
3.0 3.3 3.6	(V)

Note: Battery voltage in this test is 3.0V.

Figure 5-11. Typical Battery Current: Normal

Operation Mode

106 AMD Geode™ SC3200 Processor Data Book