PC87413 datasheet(146/257 Pages) NSC

146 / 257 page

8.0 Real-Time Clock (RTC) (Continued)

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146

Revision1.2

8.2.9

Power Supply

The device is supplied from three supply voltages, as shown in Figure 42:

System power supply voltage, VDD.

System standby power supply voltage, VSB.

Backup voltage, from low-capacity Lithium battery VBAT.

A standby voltage (VSB) from the external AC/DC power supply powers the RTC under normal conditions.

Figure 42. Power Supply Connections

Figure 43 shows a typical battery configuration. No external diode is required to meet the UL standard due to the internal

switch and internal serial resistor RUL(see the MCRS parameter in Section 11.1.2 on page 231).

Figure 43. Typical Battery Conﬁguration

The RTC is supplied from one of two power supplies, VSB or VBAT, depending on their voltage levels. An internal voltage

comparator 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 ensure that the module uses power from VSB and not from VBAT, the VSB voltage must be maintained above its minimum,

as detailed in Section 11.1.5 on page 232.

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 functionality of the oscillator and the CMOS RAM.

Figure 44 shows typical battery current consumption during battery-backed operation; Figure 45 shows the same during nor-

mal operation.

RTC

Power

Backup

VDD

VBAT

VDD

VSB

VBAT

Battery

External AC Power

Supply

VDD Sense

VSB

VBAT

VSB

VBAT

ONCTL

On/Off

Power Management

Control

ONCTL

VDD

VSB

VBAT

0.1

µF

VSB

BT1

0.1

µF

VPP

RUL

VREF

RTC

1. Place a 0.1

µF capacitor on each V

SB power supply

pin and on VBAT as close to the pin as possible.

2. Place a 10-47

µF capacitor on the common V

SB power

supply net as close to the device as possible.

µF

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