AN1012 datasheet(6/33 Pages) STMICROELECTRONICS

6 / 33 page

Process technology

AN1012

6/33

Doc ID 6395 Rev 4

Process technology

The ZEROPOWER®, TIMEKEEPER®, supervisor, and serial RTC families consist of a

broad range of products that encompass various technologies. These products can be

divided into six categories, as shown in Table 1. The SRAM array is generally based on a

6-transistor or 4-transistor cell, as indicated by the categories (6T and 4T). Figure 2

illustrates a one-bit storage cell from a 4-transistor SRAM cell.

The hybrid devices (also known as module devices) contain individually packaged analog

circuitry and SRAM. They are not covered in this document, except for the table of values for

typical battery lifetimes in Appendix A: Product data on page 25.

Table 1.

ZEROPOWER and TIMEKEEPER® product categories

Figure 2.

Four-transistor (4T) SRAM cell

The first devices, released in 1982, were based on a conventional 6T, full-CMOS, SRAM

design. These were specified for low-voltage data retention, and were built to stringent

manufacturing and test specifications. With data retention currents of less than 150 nA at

70 °C, these devices were designed to retain data in battery backup for at least 10 years

over the full commercial temperature range.

Newer devices have since been released. They use 4T, CMOS SRAM arrays. By using two

poly-R resistors in place of the pull-up transistors of full-CMOS design, the 4T cell is much

smaller than the 6T equivalent. Die size is dramatically reduced because the poly-R

resistors can be stacked on top of n-channel pull-down MOSFETs in the cell. This leads to a

net reduction in the device costs. Although the current drawn from the lithium cell is

increased, the devices have been specified to outlast the useful life of most equipment in

which they are used.

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