Battery lifetime – Maxim Integrated Secure Microcontroller User Manual
Page 60
Secure Microcontroller User’s Guide
60 of 187
ESR rating over the intended operating temperature range to ensure against leakage that may shorten
battery life.
Battery Lifetime
The calculations of data retention lifetime are helpful for chip or module users. They can serve as design
and system reliability guidelines. All microcontroller modules are rated for better than 10 years of data
retention in the absence of V
CC
at +25°C. Following these guidelines, similar performance can be
achieved using chips. It is also not difficult to achieve better than 10 years depending on the user’s actual
environment and design goals.
The system lifetime can be determined from three parameters: data retention current, battery capacity, and
battery self-discharge. Lithium cells have extremely good self-discharge performance, and manufacturer’s
data and Maxim 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, microprocessor, RTC, and other battery-backed circuits,
if any. In a Maxim module, these are screened for combination with the appropriate battery. When
designing with discrete soft/secure microprocessors, 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 most important
specification is data retention current. This is not the same as standby current. 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. The lifetime calculations are illustrated below. The formula for data
retention life in years is as follows:
a year)
in
days
of
(number
*
day)
a
in
hours
of
(number
*
amps)
in
current
retention
(Data
hours
amp
in
capacity
Battery
As an example, a microprocessor rated for 75nA, SRAM for 500nA, RTC for 400nA for a total of 950nA
of current consumption in battery-backed mode. A Panasonic CR1632 lithium cell is selected, which has
a capacity of 120mAh.
Thus, a system with less than 1µA of data retention current and a CR1632 lithium cell achieves 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 finds a variety of RAMs that allow this at room temperature. It makes no
difference if the system operates at +70°C, as long as data retention is at +25°C. If storage is at elevated
temperature, than the data retention current should be derated accordingly. If the manufacturer does not
specify data retention current over temperature, a conservative number is a 70% increase per +10°C.
Thus, if a RAM in data retention mode draws 1µA at +25°C, it draws approximately 1.7µA at +35°C. A
second example illustrates the case of elevated temperature storage.
In this example, the system is constructed using a DS5001FP chip with a Sony CXK581000P-LL 128kB
x 8kB SRAM. The system is stored at +40°C. The data retention current of this RAM is 2.4µA at +40°C.
The DS5001FP data retention current actually drops as temperature increases, so the maximum of 75nA
120 x 10
-3
=
120 x 10
-3
(75 + 500 + 400) x 10
-9
x 24 x 365)
=
8.54 x 10
-3
=
14 years