ISL12022MR5421 Datasheet, PDF (11/29 Pages) Intersil Corporation – Low Power RTC with Battery Backed SRAM, Integrated 5ppm

ISL12022MR5421
Functional Description
Power Control Operation
The power control circuit accepts a V_DD and a V_BAT
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 ISL12022MR5421 for up to 10 years.
Another option is to use a supercapacitor for applications
where V_DD is interrupted for up to a month. See the
“Application Section” on page 26 for more information.
Normal Mode (V_DD) to Battery Backup Mode
(V_BAT)
To transition from the V_DD to V_BAT mode, both of the
following conditions must be met:
Condition 1:
V_DD < V_BAT - V_BATHYS
where V_BATHYS ≈ 50mV
Condition 2:
V_DD < V_TRIP
where V_TRIP ≈ 2.2V
Battery Backup Mode (V_BAT) to Normal
Mode (V_DD)
The ISL12022MR5421 device will switch from the V_BAT to
V_DD mode when one of the following conditions occurs:
Condition 1:
V_DD > V_BAT + V_BATHYS
where V_BATHYS ≈ 50mV
Condition 2:
V_DD > V_TRIP + V_TRIPHYS
where V_TRIPHYS ≈ 30mV
These power control situations are illustrated in
Figures 12 and 13.
The I²C bus is deactivated in battery backup mode to
reduce power consumption. Aside from this, all RTC
functions are operational during battery backup mode.
Except for SCL and SDA, all the inputs and outputs of
the ISL12022MR5421 are active during battery backup
mode unless disabled via the control register.
V_DD
BATTERY BACKUP
MODE
V_TRIP
V_BAT
2.2V
1.8V
V_BAT - V_BATHYS
V_BAT + V_BATHYS
FIGURE 12. BATTERY SWITCHOVER WHEN V_BAT <
V_TRIP
V_DD
V_BAT
V_TRIP
BATTERY BACKUP
MODE
3.0V
2.2V
V_TRIP
V_TRIP + V_TRIPHYS
FIGURE 13. BATTERY SWITCHOVER WHEN V_BAT >
V_TRIP
The device Time Stamps the switchover from V_DD to
V_BAT and V_BAT to V_DD, and the time is stored in t_SV2B
and t_SB2V registers respectively. If multiple V_DD
power-down sequences occur before the status is read,
the earliest V_DD to V_BAT power-down time is stored and
the most recent V_BAT to V_DD time is stored.
Temperature conversion and compensation can be
enabled in battery backup mode. Bit BTSE in the BETA
register controls this operation, as described in “BETA
Register (BETA)” on page 19.
Power Failure Detection
The ISL12022MR5421 provides a Real Time Clock Failure
Bit (RTCF) to detect total power failure. It allows users to
determine if the device has powered up after having lost
all power to the device (both V_DD and V_BAT).
Brownout Detection
The ISL12022MR5421 monitors the V_DD level
continuously and provides warning if the V_DD level drops
below prescribed levels. There are six (6) levels that can
be selected for the trip level. These values are 85%
below popular V_DD levels. The LVDD bit in the Status
Register will be set to “1” when brownout is detected.
Note that the I²C serial bus remains active unless the
Battery V_TRIP levels are reached.
Battery Level Monitor
The ISL12022MR5421 has a built-in warning feature
once the backup battery level drops first to 85% and
then to 75% of the battery’s nominal V_BAT level. When
the battery voltage drops to between 85% and 75%, the
LBAT85 bit is set in the status register. When the level
drops below 75%, both LBAT85 and LBAT75 bits are set
in the status register.
The battery level monitor is not functional in battery
backup mode. In order to read the monitor bits after
powering up V_DD, instigate a battery level measurement
by setting the TSE bit to "1" (BETA register), and then
read the bits.
There is a Battery Time Stamp Function available. Once
the V_DD is low enough to enable switchover to the
battery, the RTC time/date are written into the TSV2B
register. This information can be read from the TSV2B
registers to discover the point in time of the V_DD
power-down. If there are multiple power-down cycles
before reading these registers, the first values stored in
11
FN7576.1
June 4, 2010