bq4015/Y
As VCC falls past VPFD and approaches 3V, the control
circuitry switches to the internal lithium backup supply,
which provides data retention until valid VCC is applied.
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.
When VCC returns to a level above the internal backup
cell voltage, the supply is switched back to VCC
. After
VCC ramps above the VPFD threshold, write-protection
continues for a time tCER (120ms maximum) to allow for
processor stabilization. Normal memory operation may
resume after this time.
Power-down/power-up control circuitry constantly moni-
tors the VCC supply for a power-fail-detect threshold
V
PFD. The bq4015 monitors for VPFD = 4.62V typical for
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.
use in systems with 5% supply tolerance. The bq4015Y
monitors for VPFD = 4.37V typical for use in systems
with 10% supply tolerance.
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
When VCC falls below the VPFD threshold, the SRAM
automatically write-protects the data. All outputs be-
come high impedance, and all inputs are treated as
“don’t care.” 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 tWPT, write-protection takes place.
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
A –A
0
18
1024K x 8
SRAM
WE
Block
DQ –DQ
0
7
CE
CON
Power
CE
V
Power-Fail
Control
CC
Lithium
Cell
BD4015.eps
2
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