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BQ4015X-85 Datasheet

  • BQ4015X-85

  • Texas Instruments [512Kx8 Nonvolatile SRAM]

  • TI

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bq4015/Y
Functional Description
When power is valid, the bq4015/Y operates as a stan-
dard CMOS SRAM. During power-down and power-up
cycles, the bq4015/Y acts as a nonvolatile memory, auto-
matically protecting and preserving the memory con-
tents.
Power-down/power-up control circuitry constantly moni-
tors the V
CC
supply for a power-fail-detect threshold
V
PFD
. The bq4015 monitors for V
PFD
= 4.62V typical for
use in systems with 5% supply tolerance. The bq4015Y
monitors for V
PFD
= 4.37V typical for use in systems
with 10% supply tolerance.
When V
CC
falls below the V
PFD
threshold, the SRAM
automatically write-protects the data. All outputs be-
come high impedance, and all inputs are treated as
鈥渄on鈥檛 care.鈥?If a valid access is in process at the time of
power-fail detection, the memory cycle continues to com-
pletion. If the memory cycle fails to terminate within
time t
WPT
, write-protection takes place.
As V
CC
falls past V
PFD
and approaches 3V, the control
circuitry switches to the internal lithium backup supply,
which provides data retention until valid V
CC
is applied.
When V
CC
returns to a level above the internal backup
cell voltage, the supply is switched back to V
CC
. After
V
CC
ramps above the V
PFD
threshold, write-protection
continues for a time t
CER
(120ms maximum) to allow for
processor stabilization. Normal memory operation may
resume after this time.
The internal coin cells used by the bq4015/Y have an ex-
tremely long shelf life and provide data retention for
more than 10 years in the absence of system power.
As shipped from Unitrode, the integral lithium cells of
the MT-type module are electrically isolated from the
memory. (Self-discharge in this condition is approxi-
mately 0.5% per year.) Following the first application of
V
CC
, this isolation is broken, and the lithium backup
provides data retention on subsequent power-downs.
The LIFETIME LITHIUM package option is shipped as
two parts.
Block Diagram
OE
1024K x 8
SRAM
Block
A
0
鈥揂
18
WE
DQ
0
鈥揇Q
7
Power
CE
CON
CE
Power-Fail
Control
V
CC
Lithium
Cell
BD4015.eps
2

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