SC1100 datasheet(83/348 Pages) NSC

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SuperI/O Module (Continued)

4.5.3.3

Power Supply

The device is supplied from two supply voltages, as shown

in Figure 4-8:

• 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 4-8. Power Supply Connections

Figure 4-9 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 4-9. 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 7.0 "Electrical Specifica-

tions" on page 280.

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 4-10 shows typical battery current consumption dur-

ing battery-backed operation, and Figure 4-11 during nor-

mal operation.

Figure 4-10. Typical Battery Current: Battery

Backed Power Mode

Figure 4-11. 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#

ACPI Controller

VBAT

0.1

µF

VSB

BT1

0.1

µF

VPP

RUL

VREF

RTC

Note:

Place a 0.1

µF capacitor on each V

SB and

VSBL power supply pin as close as possible to

the pin, and also on VBAT.

VSBL

0.1

µF

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)

SC1100 Datasheet(PDF) 83 Page - National Semiconductor (TI)

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