Mitsubishi DS907x SIP, DS5000TK manual 54 * 10±3, 180 * 10±3, 2400 + 75 * 10±9 * 24 21.68 * 10±3

USER'S GUIDE

10 years depending on the user's actual environment and design goals.

The system lifetime can be determined from three parameters: 1) Data retention current, 2) Lithium cell capacity, 3) Lithium self±discharge. Current production lithium cells have extremely good self±discharge per- formance. Manufacturer's data and Dallas Semicon- ductor characterization has determined that the self± discharge of a coin cell lithium battery is less than 0.5% per year at 25°C. Consequently, even after 15 years of shelf life, the lithium cell would have 90% of its capacity remaining. Therefore when using a lithium coin cell, the self±discharge mechanism is not a consideration for rating equipment life.

Data retention current is a combination of RAM, micro- processor, Real±time clock (RTC), and other lithium

backed circuits, if any. In a Dallas module, these are screened for combination with the appropriate battery. In using a chip, the user must balance the size/cost of a larger lithium cell with the data retention current/cost of SRAMs.

When designing a chip±based system and selecting the appropriate SRAM, the important specification is data retention current. This is not the same as standby cur- rent. Data retention current should be specified with CE = VIH and VCC=3V. This specification is usually available at 25°C, and possibly for other temperatures. Selected RAMs have been provided in chapter 5 with the manufacturer specified data retention current. The lifetime calculations are illustrated below. The formula for data retention life in years is as follows:

Battery capacity in amp hours

Data retention current in amps * # days in a year * # of hours in a day

As an example, the Microprocessor rated for 75 nA, SRAM for 500 nA, RTC for 400 nA for a total of 950 nA.

A Panasonic CR1632 lithium cell is used with a capacity of 120 mAh.

120 * 10±3		120 * 10±3
	=		=	14 years
(75 + 500 + 400) * 10±9 * 24 * 365)		8.54 * 10±3

Thus a system with less than 1 μA of data retention cur- rent and a CR1632 lithium cell will achieve well over 10 years of data retention in the absence of VCC. Referring to the recommended RAM chart in the previous section, the user will find a variety of RAMs that allow this at room temperature. It makes no difference if the system oper- ates at 70°C, as long as data retention is at 25°C. If stor- age is at elevated temperature, than the data retention current should be derated accordingly. If the manufac- turer does not specify data retention current over tem- perature, a conservative number is a 70% increase per 10°C. Thus if a RAM in data retention mode draws 1 μA at 25°C, it will draw approximately 1.7 μA at 35°C.

A second example illustrates the case of elevated tem- perature storage.

In this example, the system is constructed using a DS5001FP chip with a Sony CXK581000P±LL 128K x 8 SRAM. The system will be stored at 40°C. As shown in the table in chapter 5, the data retention current of this RAM is 2.4 μA at 40°C. The DS5001FP data retention current will actually drop as temperature increases, so the maximum of 75 nA is conservative. This gives a total data retention current of 2475 nA. In this system, a Rayovac BR2325 with a capacity of 180 mAh is used.

180 * 10±3		180 * 10±3
	=		=	8.3 years
(2400 + 75) * 10±9 * 24 * 365)		21.68 * 10±3

Note that these ratings are for continuous data retention so VCC is assumed absent for the entire period. Actual

performance have a longer lifetime based on the ratio of time when VCC is applied vs. data retention time.

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