BQ4285E_14 Datasheet, PDF (8/31 Pages) Texas Instruments – Enhanced RTC With NVRAM Control

bq4285E/L
Power-Down/Power-Up Cycle
The bq4285E/L power-up/power-down cycles are differ-
ent. The bq4285L continuously monitors VCC for out-of-
tolerance. During a power failure, when VCC falls below
VPFD (2.53V typical), the bq4285L write-protects the clock
and storage registers. The power source is switched to BC
when VCC is less than VPFD and BC is greater than VPFD,
or when VCC is less than VBC and VBC is less than VPFD.
RTC operation and storage data are sustained by a valid
backup energy source. When VCC is above VPFD, the power
source is VCC. Write-protection continues for tCSR time af-
ter VCC rises above VPFD.
The bq4285E continuously monitors VCC for out-of-
tolerance. During a power failure, when VCC falls below
VPFD (4.17V typical), the bq4285E write-protects the clock
and storage registers. When VCC is below VBC (3V typical),
the power source is switched to BC. RTC operation and
storage data are sustained by a valid backup energy source.
When VCC is above VBC, the power source is VCC. Write-
protection continues for tCSR time after VCC rises above
VPFD.
An external CMOS static RAM is battery-backed using
the VOUT and chip enable output pins from the
bq4285E/L. As the voltage input VCC slows down during
a power failure, the chip enable output, CEOUT, is forced
inactive independent of the chip enable input CEIN.
This activity unconditionally write-protects the external
SRAM as VCC falls below VPFD. If a memory access is in
process to the external SRAM during power-fail detec-
tion, that memory cycle continues to completion before
the memory is write-protected. If the memory cycle is
not terminated within time tWPT (30µs maximum), the
chip enable output is unconditionally driven high,
write-protecting the controlled SRAM.
As the supply continues to fall past VPFD, an internal
switching device forces VOUT to the external backup energy
source. CEOUT is held high by the VOUT energy source.
During power-up, VOUT is switched back to the main
supply as VCC rises above the backup cell input voltage
sourcing VOUT. If VPFD < VBC on the bq4285L, the
switch to the main supply occurs at VPFD. CEOUT is held
inactive for time tCER (200ms maximum) after the power
supply has reached VPFD, independent of the CEIN in-
put, to allow for processor stabilization.
During power-valid operation, the CEIN input is passed
through to the CEOUT output with a propagation delay
of less than 10ns.
Figure 4 shows the hardware hookup for the external RAM.
A primary backup energy source input is provided on
the bq4285E/L. The BC input accepts a 3V primary bat-
tery, typically some type of lithium chemistry. To pre-
vent battery drain when there is no valid data to retain,
VOUT and CEOUT are internally isolated from BC by the
initial connection of a battery. Following the first appli-
cation of VCC above VPFD, this isolation is broken, and
the backup cell provides power to VOUT and CEOUT for
the external SRAM.
Figure 4. External RAM Hookup to the bq4285E/L RTC
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