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BQ4015YMA-120N Datasheet(PDF) 2 Page - Texas Instruments |
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BQ4015YMA-120N Datasheet(HTML) 2 Page - Texas Instruments |
2 / 13 page 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 VCC supply for a power-fail-detect threshold VPFD. The bq4015 monitors for VPFD = 4.62V typical for use in systems with 5% supply tolerance. The bq4015Y monitors for VPFD = 4.37V typical for use in systems with 10% supply tolerance. 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. 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. 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. 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 VCC, 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. 2 bq4015/Y Block Diagram OE A0–A18 DQ0–DQ7 WE CE Lithium Cell CECON VCC 1024K x 8 SRAM Block Power-Fail Control Power BD4015.eps |
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