BQ32002 Datasheet, PDF (9/29 Pages) Texas Instruments – BQ32002 Real-Time Clock (RTC)

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BQ32002
SLUSA96B – AUGUST 2010 – REVISED APRIL 2016
7.3.2 V_BACK Switchover
The BQ32002 has an internal switchover circuit that causes the device to switch from main power supply to
backup power supply when the voltage of the main supply pin V_CC drops below a minimum threshold. The V_BACK
switchover circuit uses an internal reference voltage V_REF derived from the on-chip bandgap reference; V_REF is
approximately 1.8 V. The device switches to the V_BACK supply when V_CC is less than the lesser of V_BACK or V_REF.
Similarly, the device switches to the V_CC supply when V_CC is greater than either V_BACK or V_REF.
Some registers are reset to default values when the RTC switches from main power supply to backup power
supply. See the register definitions to determine what register bits are effected by a backup switchover (effected
bits have their reset value (1/0) shown for Cycle, bits that are unchanged by backup are marked UC).
The time-keeping registers can take up to 1 second to update after the RTC switches from backup power supply
to main power supply.
V_BACK > V_REF
V_BACK > V_REF
3.3V
VB AC K
V_{R E F}
V_CC
V_CC
–5 V/ms (max)
3.3V
VB AC K
V_{R E F}
T ime
Time
On VCC
On V BAC K
On V BAC K
On VCC
V_REF > V_BACK
3.3V
V_{R E F}
V_CC
V_{B AC K}
–5 V/ms (max)
T ime
V_REF > V_BACK
V_CC
3.3V
V_{R E F}
V_{B AC K}
Time
On VCC
On V BAC K
On V BAC K
Figure 4. Switchover Diagram
On VCC
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