AN1012 STMICROELECTRONICS | Alldatasheet
Document overview
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Technical content
Datasheet sections
- 1 Process technology
- 2 Battery technology
- 3 Battery backup current - pr edicting data retention time
- 3.1 Storage life
- 3.2 Calculating storage life
- 3.3 Capacity consumption
- 3.4 Calculating capacity consumption
- 5 TIMEKEEPER products
- 5.1 TIMEKEEPER ® register map
- 5.2 TIMEKEEPER ® evolution
- 5.2.1 M48T02 and M48T12
- 5.2.2 M48T08 and M48T18
- 5.2.3 M48T58
- 5.2.4 M48T35 and M48T37V/Y
- 6 Supervisor products
- 7 Choosing SRAM
- 8 Industrial temperature devices
- 10 Summary
unexpected glitch or loss of the power supply). trickle current supplied by the internal power cell. cell and, in the case of the TIMEKEEPER, a high-accuracy crystal. highly reliable data storage for the electronics industry. Figure 1. Standard ZEROPOWER, TIME KEEPER, supervisor, and serial RTC
Table 7. Data for ZEROPOWER Table 9. Data from M48Z02/12 devices (available only in CAPHAT™ - BR1225, Table 12. Data from M48T02/12 devices (available only in CAPHAT™ - BR1632, Table 15. Data from M41T56/94, M41ST85W, M41ST87W/Y, and M41ST95W ind. temp. (MH6) Table 16. Data from M41T00/S, M41T11, and M41T81/S industrial temperature (MH6) devices . . . 31
1 Process technology
illustrates a one-bit storage cell from a 4-transistor SRAM cell. typical battery lifetimes in Appendix A: Product data on page 25. Table 1. ZEROPOWER and TIMEKEEPER Figure 2. Four-transistor (4T) SRAM cell over the full commercial temperature range.
2 Battery technology
STMicroelectronics uses both the BR1225 and the BR1632 lithium button cell batteries. sealed with a polypropylene grommet. found to provide up to a two-fold reduction of the electrolyte loss rate. leakage CMOS SRAMs (see Figure 3). Figure 3. (A) BR1225 discharge rate and (B) BR1632 discharge rate
Battery backup current - predicting data retention time AN1012 8/33 Doc ID 6395 Rev 4
3 Battery backup current - predicting data retention
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 1% and SL50%, represent different failure rate distributions for the cell’s storage life. At 60 °C, for example, the SL1% line indicates that the battery has a 1% chance of failure 28 years into its life, and the SL50% line shows that the battery has a 50% chance of failure at the 50 year mark. The SL1% line represents the practical onset of wear out, and can be considered the worst case storage life for the cell. The SL50% line can be considered 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 50% = 14270 x (0.91)T, and SL1% = 8107 x (0.91)T, when 20 °C < T < 90 °C.
Figure 4. Predicted battery storage life versus temperature
3.2 Calculating storage life
- ti /T is the relative proportion (of the total time) during which the device is at ambient temperature TAi;
- SLi is the storage life at ambient temperature TAi as illustrated in Figure 4; and
- T is the total time = t1 + 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 t1% values from Figure 4,
- SL1 is about 1.8 yrs;
- SL2 is about 28 yrs;
- T is 8760 hrs/yr;
- t1 is 600 hrs/yr; and
- t2 is 8160 hrs/yr. The predicted storage life evaluates to: 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. AI01024b 20 30 40 50 60 70 80 90 TEMPERATURE (Degrees Celsius) STORAGE LIFE (Y ears) SL50% (AVERAGE) SL1% 1–t1 T ----- 1 SL1 T ----- 1 SL2 tn T ----- 1 SLn ⎛⎞++ + 1–600 8760 1.8 ⎛⎞ 8160 8760 ⎛⎞+
Battery backup current - predicting data retention time AN1012 10/33 Doc ID 6395 Rev 4
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 VCC falls below the battery backup switchover voltage (VSO), the device goes into battery backup mode and draws all of its current from the battery. The VCC 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 CCDR) current, caused primarily by the current through the Poly-R load resistors in the 4T technology, as well as also by junction leakage, sub-threshold current, and gate-to- substrate leakage. The total current is referred to as IBAT (the current drawn during battery backup mode). For ZEROPOWER® devices, this is the sum of leakage currents plus the current necessary to maintain the SRAM array. For TIMEKEEPER® devices, it is the sum of the array current (including leakage) and the clock current: IBAT = IARRAY + ICLOCK Many factors need to be taken into account when calculating the IBAT current, including process parameters, working temperature, and the VCC duty cycle.
3.4 Calculating capacity consumption
Capacity consumption is simply calculated by: where:
- Battery capacity is measured in ampere-hours;
- 8760 is the constant for the number of hours there are in a year;
- VCC duty cycle is measured as a percentage; and
- IBAT is measured in amperes. For the M48T35Y , a 32K x 8 TIMEKEEPER® device with a 0.048 Ah (48 mAh) M4T28- BR12SH1 battery, the typical battery current is approximately 2666 nA at 70 °C. So, if the VCC duty cycle is 50%, the predicted capacity life is: and therefore is about 4.11 years at 70 °C. BatteryCapacity 0.048 8760 0.5 2666 10 9–×××
In moving to the newer process technologies (e.g., M48Z58 (8K x 8) device), STMicroelectronics has chosen to reduce the active current as well as decrease the die size. The STMicroelectronics HCMOS4PZ process is a 0.6 μm, double-level metal process. In the standard SRAM memory cell, 6 transistors are formed into a pair of cross-coupled inverters. In the 4T memory cell, the top two p-channel devices are replaced by poly-silicon load resistors (poly-R). This combination allows for significant die size reduction because the poly-R structures can be stacked on top of the active n-channel devices. There is always at least one direct path constantly leaking current to ground in each cell because of the poly-R structures in each SRAM cell. However, the value of the resistor is extremely high (about 3TΩ at 25 °C), so at a cell voltage of 3 V, this leads to a leakage current of 1 pA. Multiplying by the number of cells within the array, the array standby current can be calculated (i.e. 65.5 nA for a 65536-cell array). The poly-R structure values are dependent on temperature, so the entire array current is very strongly temperature-dependent. Appendix B: ZEROPOWER products on page 26 shows the expected battery lifetime of an M48Z58 device versus working temperature with a V CC duty cycle of 0%. The original specification was an expected lifetime of greater than 10 years at 25 °C but, in fact, this target is typically achieved even at 70 °C. By reducing the temperature, the expected lifetime rises to greater than 20 years (i.e., when the device is operated at 50 °C). This change is defined entirely by the temperature sensitivity of the poly-R structures within each SRAM cell. The M48Z35 also employs the STMicroelectronics HCMOS4PZ process, 4T SRAM cell technology. Appendix B shows the expected battery lifetime of an M48Z35 device versus working temperature with a V CC duty cycle of 0%. From this we can see that expected lifetime is typically greater than 20 years when operated at 30 °C with no external VCC applied, and falls to approximately 2.6 years for continuous battery backup at 70 °C. This is to be expected, due to the increased current consumption inherent in the 4T SRAM cell architecture. It should be noted that this data is based on usage of the SNAPHAT ® product which includes a 48 mAh battery.
5 TIMEKEEPER products
- a CMOS RAM array;
- voltage sense and switching circuitry;
- an analog oscillator and clock chain;
- a lithium power cell; and
- a high-accuracy quartz crystal.
Figure 5. Block diag ram of a TIMEKEEPER® device
5.1 TIMEKEEPER ® register map
currents drawn as a function of technology and working temperature. Table 2. Typical TIMEKEEPER (M48T37V/Y) register map Table 3. Typical I BAT current for TIMEKEEPER devices
64 Kbit 4T Cell 40 nA 497 nA 511 nA 619 nA
5.2 TIMEKEEPER ® evolution
5.2.1 M48T02 and M48T12
2K x 8 RAM and employed the STMicroelectronics 2.0 μm Spectrum™ CMOS technology. increased the expected battery life to 19 years at 60°C. battery life approaching 20 years under most conditions. Figure 6. M48T02/12 data retention lifetime vs. temperature (120 mAh, 100% battery
5.2.2 M48T08 and M48T18
temperature range (0 °C to 70 °C, see Figure 7). Figure 7. M48T08/18 data retention lifetime vs. temperature (120 mAh, 100% battery
5.2.3 M48T58
double-level metal HCMOS4PZ process for 4T SRAM cells. higher currents) because of the negative temperature coefficient of the poly-R resistors. CC duty cycle and temperature. Figure 8. M48T58 data retention lifetime vs. temperature (48 mAh, 100% battery Figure 9. M48T58 data retention lifetime vs. temperature (120 mAh, 100% battery
5.2.4 M48T35 and M48T37V/Y
than twice this (almost 17 years, see Figure 11). Figure 10. M48T35/37V/37Y data retention lifetime vs. temperature (48 mAh, 100% Figure 11. M48T35/37V/37Y data retention lifetime vs. temperature (120 mAh, 100%
which SNAPHAT top part numbers are available. Table 4. SNAPHAT part numbers
AN1012 Supervisor products Doc ID 6395 Rev 4 19/33
6 Supervisor products
STMicroelectronics also has a family of ZEROPOWER® and TIMEKEEPER® supervisor devices. Supervisors are self-contained units that allow standard low-power SRAMs to be turned into non-volatile memory devices. They monitor and provide VCC input for one or more external SRAMs the same way ZEROPOWER and TIMEKEEPER products do. They use a precision voltage reference and comparator to monitor the VCC input for going out-of- tolerance. When VCC becomes invalid, the supervisor’s conditioned chip-enable outputs (ECON) are forced to their “inactive” state, thereby putting each external SRAM into its own write-protect state. During the power failure, the supervisor provides the power for the SRAM from the lithium cell within its SNAPHAT top. The supervisor switches the power source back to the V CC supply as soon as the voltage returns to specified levels.
7 Choosing SRAM
finally choosing which SRAM to use.
- The chip enable input, when taken inactive, must disable all the other inputs to the SRAM. This allows inputs to the external SRAMs to be treated as “Don’t care” once V CC falls below VPFD(min).
- The SRAM should guarantee data retention when working at VCC = 2.0 volts.
- The chip-enable access time must be sufficient to meet the system needs, taking into account propagation delays on chip enable and output enable. Most SRAMs specify a data retention current (I CCDR) at 3.0 V. Manufacturers generally specify a typical condition for room temperature along with a worst case condition (generally at elevated temperatures). The system level requirements will determine the choice of which value to use. The data retention current value of the SRAMs can then be added to the I BAT value of the supervisor to determine the total current requirements for data retention. The available battery capacity for the SNAPHAT ® of your choice can then be divided by this current to determine the data retention period (see Section 3.3: Capacity consumption on page 10). For example, the M48T201V/Y has an IBAT value of 575 nA at 25 °C, and 800 nA at 70 °C. The M40Z300W has an IBAT value of 5 nA at 25 °C, and 100 nA at 70 °C. Table 5 indicates typical data retention lifetimes for the M40Z300W ZEROPOWER supervisor when it is used with a number of commercially available 1 Mbit and 4 Mbit SRAMs. T a b l e6 o n p a g e2 1 shows the same kind of information for the M48T201V/Y TIMEKEEPER supervisors.
Table 5. M40Z300W (120mAh SNAPHAT) data retention life vs. SRAM type
- According to the respective manufactu rer’s datasheets at the time of writing.
4 Renesas
Table 6. M48T201V/Y (120 mAh SNAPHAT) data retention life vs. SRAM type
- According to the respective manufactu rer’s datasheets at the time of writing.
Industrial temperature devices AN1012 22/33 Doc ID 6395 Rev 4
8 Industrial temperature devices
Due to ever increasing requirements for portability and operation under extreme environmental conditions, STMicroelectronics offers industrial temperature versions (–40°C to +85°C) of our serial RTC devices. This expanded operating range allows these products to perform under more extreme temperatures for applications such as:
- cell phone base stations;
- traffic control;
- portable equipment;
- land, water, and aircraft instrumentation; and
- industrial control equipment. These products are indicated by the digit ‘6’ at the end of the sales-type. The industrial temperature TIMEKEEPER® SNAPHAT® top is also designated by the suffix “6.” Predicted data retention lifetimes are listed in Appendix B: ZEROPOWER products on page 26 and Appendix C: TIMEKEEPER® products on page 28.
AN1012 U.L. recognition and recycling Doc ID 6395 Rev 4 23/33 9 U.L. recognition and recycling While providing innovative, leading edge products, STMicroelectronics remains committed to safety, including its products, its customers, and the environment. Each device contains reverse-charge protection circuitry, and uses safe lithium mono-fluoride batteries. All ZEROPOWER ®, TIMEKEEPER, supervisor, and serial RTC components are recognized by Underwriter’s Laboratory under file number E89556, and are compliant to the LL-94-VO flammability rating. The unique SNAPHAT packaging consists of a 330 mil SOIC device and a separate, “snap- on” SNAPHAT, which includes both the lithium power cell, and in the case of TIMEKEEPER product, a high accuracy crystal. The SNAPHAT is removable and can be replaced, providing the added benefit of proper disposal or recycling that has not been available before with NVRAMs. Various companies offer recycling and safe disposal of scrap lithium cells.
10 Summary
Battery life and data retention for ZEROPOWER® and TIMEKEEPER® products are primarily functions of two factors:
- Capacity consumption, and
- Storage life of the lithium button cell battery. Due to the fact that storage life (caused by electrolyte evaporation) has little effect at temperatures below 60 °C, the data retention of most applications will be dependent upon the ICCDR of the SRAM being backed-up, as well as the VCC duty cycle. This allows a fairly simple calculation (see Section 3.4: Calculating capacity consumption on page 10) to be used to determine the lifetime. All ST ZEROPOWER products are able to offer at least a 10 year data retention life, typically at 40 °C. This may be increased by reducing the temperature, increasing the VCC duty cycle, or in the case of the surface mount SNAPHAT® products, using the larger 120 mAh SNAPHAT top. For the TIMEKEEPER family, battery lifetimes are also affected by the percentage of time the oscillator is in operation. Commercial devices fabricated in 4T technologies provide 7 years of continuous operation at 20 °C using the 48 mAh M4T28-BR12SH SNAPHAT top, and typically greater than 15 years with the 120 mAh M4T32-BR12SH SNAPHAT top. The ZEROPOWER and TIMEKEEPER supervisor families allow the user to purchase commodity SRAMs at the best available market price. However, overall data retention life will be determined by the I CCDR of the SRAM selected.
Table 7. Data for ZEROPOWER ® and TIMEKEEPER® devices Table 8. Data from hybrid/module devices (V CC duty cycle = 0%)
- The data retention lifetime can be significantly increa sed by using the SNAPHAT (ZEROPOWER or TIMEKEEPER, as
appropriate) with the higher capacity BR1632 battery.
- The larger capacity BR1632 (120 mAh) battery is also available in the SNAPHAT package.
Table 10. Data from M48Z08/18, M48Z58, and M48Z58Y devices
Table 11. Data from M48Z35/Y/AV devices
Table 13. Data from M48T08/Y/18 and M48T58/Y devices
- Only available in M48T08 and M48T18 CAPHAT™.
Table 14. Data from M48T35/Y/AV and M48T37V/Y devices
Table 15. Data from M41T56/94, M41ST85W, M41ST87W/Y, and M41ST95W ind.
Table 16. Data from M41T00/S, M41T11, and M41T81/S industrial temperature (MH6)
Table 17. Document revision history 15-Sep-2011 4 Product updates; minor textual updates; revised document presentation.