ST AN1012 Application note

AN1012

Application note

Predicting the battery life and data

retention period of NVRAMs and serial RTCs

Introduction

Standard SRAM devices have the advantage, over EEPROM and Flash memory, of high write-speed when used as main memory for a processor or microcontroller. Their disadvantage is that they are volatile, and lose their contents as soon as the power supply is removed (whether this is for a prolonged period due to being turned off, or due to an unexpected glitch or loss of the power supply).

STMicroelectronics manufactures a line of non-volatile SRAMs (NVRAMs), known as ZEROPOWER best of both worlds: memory devices that are non-volatile like EEPROM, yet have the fast access of SRAM. These devices consist of an array of low-power CMOS SRAM, plus a small long-life lithium power cell (along with a high-accuracy quartz crystal, in the case of the TIMEKEEPER). While the external power supply is within its specified limits, the memory behaves as standard SRAM; but as soon as the external power supply strays out of tolerance, the SRAM becomes write-protected, and its contents are preserved by a small trickle current supplied by the internal power cell.

Unlike EEPROM, where the data contents are guaranteed to be preserved for 10 years (and typically last for much longer), the contents of NVRAM will only be retained while the internal cell is able to supply sufficient current to maintain the array. This document summarizes the factors involved in predicting the battery life, and consequently data retention under various operating conditions.

Many of the ZEROPOWER, TIMEKEEPER, supervisor, and serial RTC devices are packaged in a 600 mil DIP CAPHAT™, a hybrid DIP, or a 330 mil SOIC SNAPHAT SNAPHAT (shown in Figure 1) has a removable top that includes both the long-life lithium cell and, in the case of the TIMEKEEPER, a high-accuracy crystal.

STMicroelectronics has shipped several million SNAPHATs that have been used in a broad range of applications. From PC-based systems to high-end workstations, telecommunications, consumer, and automotive applications, these products have provided highly reliable data storage for the electronics industry.

or TIMEKEEPER® NVRAMs, supervisors, and serial RTCs which offer the

. The

Figure 1. Standard ZEROPOWER, TIMEKEEPER, supervisor, and serial RTC

packages

CAPHAT™

SOIC and SNAPHAT® To p

Hybrid DIP

September 2011 Doc ID 6395 Rev 4 1/33

www.st.com

Contents AN1012

Contents

1 Process technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2 Battery technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

3 Battery backup current - predicting data retention time . . . . . . . . . . . . 8

3.1 Storage life . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

3.2 Calculating storage life . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

3.3 Capacity consumption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

3.4 Calculating capacity consumption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

4 4T cell devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

5 TIMEKEEPER products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

5.1 TIMEKEEPER® register map . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

5.2 TIMEKEEPER

5.2.1 M48T02 and M48T12 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

5.2.2 M48T08 and M48T18 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

5.2.3 M48T58 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

5.2.4 M48T35 and M48T37V/Y . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

evolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

6 Supervisor products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

7 Choosing SRAM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

8 Industrial temperature devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

9 U.L. recognition and recycling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

10 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Appendix A Product data. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Appendix B ZEROPOWER products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

Appendix C TIMEKEEPER

2/33 Doc ID 6395 Rev 4

products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

AN1012 Contents

Appendix D Serial RTC products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

11 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

Doc ID 6395 Rev 4 3/33

List of tables AN1012

List of tables

Table 1. ZEROPOWER and TIMEKEEPER® product categories . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Table 2. Typical TIMEKEEPER (M48T37V/Y) register map . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Table 3. Typical I

Table 4. SNAPHAT part numbers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Table 5. M40Z300W (120mAh SNAPHAT) data retention life vs. SRAM type . . . . . . . . . . . . . . . . . 20

Table 6. M48T201V/Y (120 mAh SNAPHAT) data retention life vs. SRAM type . . . . . . . . . . . . . . . 21

Table 7. Data for ZEROPOWER Table 8. Data from hybrid/module devices (V Table 9. Data from M48Z02/12 devices (available only in CAPHAT™ - BR1225,

48 mAh) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

Table 10. Data from M48Z08/18, M48Z58, and M48Z58Y devices . . . . . . . . . . . . . . . . . . . . . . . . . . 26

Table 11. Data from M48Z35/Y/AV devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

Table 12. Data from M48T02/12 devices (available only in CAPHAT™ - BR1632,

120 mAh) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

Table 13. Data from M48T08/Y/18 and M48T58/Y devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

Table 14. Data from M48T35/Y/AV and M48T37V/Y devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

Table 15. Data from M41T56/94, M41ST85W, M41ST87W/Y, and M41ST95W ind. temp. (MH6)

devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

Table 16. Data from M41T00/S, M41T11, and M41T81/S industrial temperature (MH6) devices . . . 31

Table 17. Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

current for TIMEKEEPER devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

BAT

and TIMEKEEPER® devices . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

duty cycle = 0%) . . . . . . . . . . . . . . . . . . . . . . . . . . 25

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AN1012 List of figures

List of figures

Figure 1. Standard ZEROPOWER, TIMEKEEPER, supervisor, and serial RTC packages. . . . . . . . . 1

Figure 2. Four-transistor (4T) SRAM cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Figure 3. (A) BR1225 discharge rate and (B) BR1632 discharge rate. . . . . . . . . . . . . . . . . . . . . . . . . 7

Figure 4. Predicted battery storage life versus temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Figure 5. Block diagram of a TIMEKEEPER

Figure 6. M48T02/12 data retention lifetime vs. temperature (120 mAh, 100% battery backup). . . . 14

Figure 7. M48T08/18 data retention lifetime vs. temperature (120 mAh, 100% battery backup). . . . 15

Figure 8. M48T58 data retention lifetime vs. temperature (48 mAh, 100% battery backup) . . . . . . . 16

Figure 9. M48T58 data retention lifetime vs. temperature (120 mAh, 100% battery backup) . . . . . . 16

Figure 10. M48T35/37V/37Y data retention lifetime vs. temperature (48 mAh, 100% battery backup)17 Figure 11. M48T35/37V/37Y data retention lifetime vs. temperature (120 mAh, 100% battery

backup) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

device . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

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Process technology AN1012

1 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 Ta bl e 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

Category Devices

ZEROPOWER (4T cell) M48Z02, M48Z12, M48Z08, M48Z18, M48Z58/Y, M48Z35/Y/AV

ZEROPOWER Hybrid M48Z128/Y, M48Z129V, M48Z512A/AY, M48Z2M1V/Y

TIMEKEEPER (4T cell) M48T08/Y, M48T58/Y, M48T35/Y/AV, M48T37V/Y

TIMEKEEPER Hybrid M48T128Y, M48T129V, M48T512Y

Supervisors M40Z111/W, M40Z300W, M48T201V/Y

Serial RTCs (6T cell) M41T00/S, M41T11, M41T56, M41T81/S, M41T94, M41ST85W, M41ST87W

Figure 2. Four-transistor (4T) SRAM cell

SUPPLY VOLTAGE

POLY-LOAD

BIT-LINE

RESISTORS

Q3 Q4

GND

ROW SELECT

BIT-LINE

AI02457

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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AN1012 Battery technology

2 Battery technology

STMicroelectronics uses both the BR1225 and the BR1632 lithium button cell batteries.

These have charge capacities of 48 mAh and 120 mAh, respectively. Their constituents

have non-toxic and non-corrosive characteristics, and are chemically and thermally stable

before, during, and after discharge. This makes these cells particularly attractive for use in

electrical components.

They contain a solid carbon cathode that is pressed into a tablet of predetermined weight

and height. The anode consists of high-purity lithium metal. The electrolyte is based on an

organic solvent instead of the corrosive alkaline or acidic solution found in most

conventional batteries. This greatly reduces the likelihood of internally-induced cell leakage,

and reduces the ill effects in cases of externally-induced cell leakage. The cell is then crimp-

sealed with a polypropylene grommet.

ST has conducted extensive tests on these cells, at temperatures up to 85 °C. Destructive

analysis was conducted (post-stress), in order to measure such factors as weight loss and

remaining charge capacity. The analysis determined that the cells were drying out, and that

the weight loss was due to electrolyte evaporation. Models were developed to predict the

nominal rate of electrolyte loss, and how this would be reduced by adding a second level of

encapsulation. This proprietary secondary seal encapsulation, adopted by ST, has been

found to provide up to a two-fold reduction of the electrolyte loss rate.

Both cells produce a nominal 2.9 V output with a flat discharge curve until the end of their

effective lives, and thus confirms that both are suitable for providing battery backup to low

leakage CMOS SRAMs (see Figure 3).

Figure 3. (A) BR1225 discharge rate and (B) BR1632 discharge rate

LOAD CHARACTERISTICS Temp: 20°C

3.5

3.0

2.5

2.0

Voltage (V)

15kΩ 30kΩ

1.5

1.0 0

200 400 600 1200 1600 1800

800 1000 1400 2000

Duration (Hrs.)

(A)

100kΩ

LOAD CHARACTERISTICS Temp: 20°C

3.5

3.0

2.5

2.0

Voltage (V)

15kΩ 30kΩ 50kΩ 100kΩ

1.5

1.0 1000 2000 3000 4000 5000 6000

Duration (Hrs.)

(B)

AI02519

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Battery backup current - predicting data retention time AN1012

3 Battery backup current - predicting data retention

time

A ZEROPOWER®, TIMEKEEPER®, supervisor, or serial RTC device will reach the end of its useful life for one of two reasons:

● Capacity consumption

It becomes discharged, having provided current to the SRAM (and to the oscillator in the case of the TIMEKEEPER) in the battery backup mode.

● Storage life

The effects of aging will have rendered the cell inoperative before the stored charge has been fully consumed by the application.

The two effects have very little influence on each other, allowing them to be treated as two independent but simultaneous mechanisms. The data retention lifetime of the device is determined by which ever failure mechanism occurs first.

3.1 Storage life

Storage life, resulting from electrolyte evaporation, is primarily a function of temperature.

Figure 4 illustrates the predicted storage life of the BR1225 battery versus temperature. The

results are derived from temperature-accelerated life test studies performed at STMicroelectronics. For the purpose of testing, a cell failure is defined as the inability of a cell, stabilized at 25 °C, to produce a 2.4 V closed-circuit voltage across a 250 kΩ load resistor.

The two lines, SL storage life. At 60 °C, for example, the SL of failure 28 years into its life, and the SL of failure at the 50 year mark. The SL can be considered the worst case storage life for the cell. The SL

and SL

, represent different failure rate distributions for the cell’s

50%

line indicates that the battery has a 1% chance

line shows that the battery has a 50% chance

50%

line represents the practical onset of wear out, and

line can be considered

50%

to be the normal, or average, life. As indicated by the curves in Figure 4 on page 9, storage life does not become a limiting factor to overall battery life until temperatures in excess of 60 °C to 70 °C are involved.

As an approximation, SL

= 14270 x (0.91)T, and SL1% = 8107 x (0.91)T, when

50%

20 °C < T < 90 °C.

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AN1012 Battery backup current - predicting data retention time

Figure 4. Predicted battery storage life versus temperature

(AVERAGE)

STORAGE LIFE (Years)

20 30 40 50 60 70 80 90

TEMPERATURE (Degrees Celsius)

50%

AI01024b

3.2 Calculating storage life

Only the user can estimate predicted storage life in a given design because the ambient temperature profile is dependent upon application-controlled variables. As long as the ambient temperature is held reasonably constant, the expected storage life can be read directly from Figure 4 on page 9. If the battery spends an appreciable amount of time at a variety of temperatures, the following formula can be used to estimate predicted storage life:

⎛⎞

-----

⎜⎟

⎝⎠

⎛⎞

-----

---------SL

+++

⎜⎟

⎝⎠

---------SL

⎛⎞

-----

…

⎜⎟

⎝⎠

---------SL

1–

where,

● t

/T is the relative proportion (of the total time) during which the device is at ambient

temperature TA

● SL

● T is the total time = t

is the storage life at ambient temperature TAi as illustrated in Figure 4; and

;

+ t2 + ... + tn.

For example, consider a battery exposed to temperatures of up to 90 °C for 600 hrs/yr, and temperatures of 60 °C or less for the remaining 8160 hrs/yr. Reading predicted t

values

from Figure 4,

● SL

● T is 8760 hrs/yr;

● t

is about 1.8 yrs;

is about 28 yrs;

is 600 hrs/yr; and

is 8160 hrs/yr.

The predicted storage life evaluates to:

600

⎛⎞

---------------

⎝⎠

8760

⎛⎞

---------

⎝⎠

1.8

8160

---------------

8760

------

1–

This predicts that the storage life, in this particular case, is at least 14 years. This is, therefore, better than the normally accepted life time of 10 years.

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Battery backup current - predicting data retention time AN1012

3.3 Capacity consumption

When VCC is being held by the external power supply within its specified range, the current drawn from the battery is zero. When V (V

), the device goes into battery backup mode and draws all of its current from the

battery.

falls below the battery backup switchover voltage

The V

duty cycle represents the proportion of time, expressed as a percentage, that the

device is supplied with power from the external supply, and therefore not drawing current from the battery.

In its battery backup mode, the array of SRAM cells can be characterized by its data retention (I

) current, caused primarily by the current through the Poly-R load resistors

CCDR

in the 4T technology, as well as also by junction leakage, sub-threshold current, and gate-tosubstrate leakage. The total current is referred to as I backup mode). For ZEROPOWER

devices, this is the sum of leakage currents plus the current necessary to maintain the SRAM array. For TIMEKEEPER the array current (including leakage) and the clock current:

BAT

= I

ARRAY

+ I

Many factors need to be taken into account when calculating the I process parameters, working temperature, and the V

3.4 Calculating capacity consumption

Capacity consumption is simply calculated by:

-----------------------------------------------------------------------------------------------------

8760 1 V

where:

● Battery capacity is measured in ampere-hours;

● 8760 is the constant for the number of hours there are in a year;

● V

● I

duty cycle is measured as a percentage; and

is measured in amperes.

BAT

For the M48T35Y, a 32K x 8 TIMEKEEPER BR12SH1 battery, the typical battery current is approximately 2666 nA at 70 °C.

BatteryCapacity

DutyCycle 100⁄–()I×

device with a 0.048 Ah (48 mAh) M4T28-

(the current drawn during battery

BAT

devices, it is the sum of

CLOCK

current, including

duty cycle.

BAT

So, if the V

duty cycle is 50%, the predicted capacity life is:

------------------------------------------------------------ -

8760 0.5 2666 10

0.048

and therefore is about 4.11 years at 70 °C.

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