FN6690 datasheet(7/19 Pages) RENESAS

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ISL12008 I2C Real Time Clock with Battery Backup

FN6690 Rev 1.00

Page 7 of 19

Sep 26, 2008

manufacturing or with an external temperature sensor and

microcontroller for active compensation (see Figure 6).

VBAT

This input provides a backup supply voltage to the device.

VBAT supplies power to the device in the event that the VDD

supply fails. This pin can be connected to a battery, a super

capacitor or tied to ground if not used.

FT/OUT (512Hz Frequency Output/Logic Output)

This dual function pin can be used as a 512Hz frequency

output pin for on-chip crystal compensation or a simple digital

output control via I2C. The FT/OUT mode is selected via the

OUT and FT control bits of the control/status register (address

07h). This pin is an open drain output requires the use of a

pull-up resistor.

Serial Clock (SCL)

The SCL input is used to clock all serial data into and out of the

device. The input buffer on this pin is always active (not gated).

It is disabled when the backup power supply on the VBAT pin is

activated to minimize power consumption.

Serial Data (SDA)

SDA is a bidirectional pin used to transfer data into and out of

the device. It has an open drain output and may be ORed with

other open drain or open collector outputs. The input buffer is

always active (not gated) in normal mode.

An open drain output requires the use of a pull-up resistor. The

output circuitry controls the fall time of the output signal with

the use of a slope controlled pull-down. The circuit is designed

for 400kHz I2C bus speeds. It is disabled when the backup

power supply on the VBAT pin is activated.

VDD, GND

Chip power supply and ground pins. The device will operate

with a power supply from 2.7V to 5.5VDC. A 0.1µF decoupling

capacitor is recommended on the VDD pin to ground.

Functional Description

Power Control Operation

The power control circuit accepts a VDD and a VBAT input.

Many types of batteries can be used with Intersil RTC

products. For example, 3.0V or 3.6V Lithium batteries are

appropriate, and battery sizes are available that can power the

ISL12008 for up to 10 years. Another option is to use a super

capacitor for applications where VDD is interrupted for up to a

month. See “Application Section” on page 16 for more

information.

Normal Mode (VDD) to Battery Backup Mode (VBAT)

To transition from the VDD to VBAT mode, both of the following

conditions must be met:

Condition 1:

VDD < VBAT - VBATHYS

where VBATHYS  50mV

Condition 2:

VDD < VTRIP

where VTRIP  2.6V

Battery Backup Mode (VBAT) to Normal Mode (VDD)

The ISL12008 device will switch from the VBAT to VDD mode

when one of the following conditions occurs:

Condition 1:

VDD > VBAT + VBATHYS

where VBATHYS 50mV

Condition 2:

VDD > VTRIP + VTRIPHYS

where VTRIPHYS  30mV

These power control situations are illustrated in Figures 7 and

FIGURE 6. RECOMMENDED CRYSTAL CONNECTION

VBAT - VBATHYS

VBAT

VBAT + VBATHYS

BATTERY BACKUP

MODE

VDD

VTRIP

2.6V

1.8V

FIGURE 7. BATTERY SWITCHOVER WHEN VBAT < VTRIP

FIGURE 8. BATTERY SWITCHOVER WHEN VBAT > VTRIP

VTRIP

VBAT

VTRIP + VTRIPHYS

BATTERY BACKUP

MODE

VDD

VTRIP

3.0V

2.6V

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