Mitsubishi Motors DS5000TK - Manual (Page 58)

USER’S GUIDE
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58
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 = V
IH
and V
CC
=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
µ
A of data retention cur-
rent and a CR1632 lithium cell will achieve well over 10
years of data retention in the absence of V
CC
. 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
µ
A
at 25 ° C, it will draw approximately 1.7 µ A 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
µ A 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 V
CC
is assumed absent for the entire period. Actual
performance have a longer lifetime based on the ratio of
time when V
CC
is applied vs. data retention time.