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AN1011
Application note
Battery technology used in NVRAM
and real-time clock (RTC) products from ST
Introduction
Lithium Carbon MonoFlouride (LiCFx) batteries are used by STMicroelectronics in their
ZEROPOWER® and TIMEKEEPER® NVRAM devices and in their serial real-time clock
(RTC) devices. The batteries provide the backup power to maintain the static RAM arrays
and to keep the oscillators running in the TIMEKEEPER and serial RTC devices.
When the discharge rates are low, these non-rechargeable lithium batteries are capable of
maintaining a highly reliable voltage level for many years. Lithium batteries, therefore, are
used for a variety of applications that require battery backup without need of maintenance
for many years.
To be used in electronic components, these cells also need to be composed of constituents
that are nontoxic, non-corrosive, and non-explosive. They also must be chemically and
thermally stable before, during and after discharge. STMicroelectronics utilizes the 48 mAh
BR1225X cell, and the 120 mAh BR1632 cell. The electrolyte of these cells (see Figure 1) is
based on an organic solvent, instead of a corrosive alkaline or acidic solution found in most
conventional batteries. This greatly improves the cell’s leakage resistance and guards
against the negative effects caused by leakage.
Figure 1.
Cell cross-section
(-)
(+)
Cell Can
Gasket
Anode Cap
CFx Cathode
Lithium Anode
Current Cathode
Separator and Electrolyte
AI02531
March 2012
Doc ID 6254 Rev 4
1/11
www.st.com
Contents
AN1011
Contents
Characterization and modeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
UL 1950/60950 validation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Battery status for NVRAMs and serial RTCs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2/11
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AN1011
Characterization and modeling
Characterization and modeling
STMicroelectronics has conducted extensive tests on these cells, a great proportion being
destructive testing. Three main mechanisms of battery degradation were modeled to
establish the effects under various loads, temperatures (up to 85 °C) and other
environmental variables:
●
Active lifetime of the cell, as a result of the consumption of the lithium anode
●
Shelf life (storage lifetime) of the cell, as a result of the electrolyte drying up
(evaporation)
●
Self-discharge, as a result of small leakage currents inherent to the battery chemistry
The weight loss proved to be a valuable measure of electrolyte evaporation. Self-discharge
is specified by the battery manufacturer to be less than 0.3% per year.
Models were then developed to predict the rate of electrolyte loss and what effect
encapsulation of the cell would have. Tests on the cells, when encapsulated in the epoxy
used in the SNAPHAT process, have demonstrated a significant reduction in electrolyte
evaporation.
(A) BR1225X discharge rate
(B) BR1632 discharge rate
3.5
3.5
3.0
3.0
2.5
2.0
15k
30k
100k
2.5
2.0
15k
30k
50k
100k
1.5
1.5
1.0
Voltage (V)
Voltage (V)
Figure 2.
0
200 400 600 800 1000 1200 1400 1600 1800 2000
1.0
0
1000
2000
3000
4000
5000
6000
Duration (Hrs.)
(B)
Duration (Hrs.)
(A)
AI02519
Figure 3 shows the load discharge graphs taken at 20 °C. As can be seen, both cells
produce a nominal 3 V output with a flat discharge curve until the end of their effective lives,
and so confirms that both are suitable for providing battery backup to low leakage CMOS
SRAMs.
The battery lifetime of individual ZEROPOWER, TIMEKEEPER and RTC devices, as
described in AN1012, is a function of the current load, the battery selected and the system
duty cycle. System life, which even with 100% duty cycle can still be greater than ten years,
is specified in the datasheet for each device.
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UL 1950/60950 validation
AN1011
UL 1950/60950 validation
All ZEROPOWER, TIMEKEEPER and RTC devices are recognized by the Underwriters
Laboratory Inc. UL 1950/60950 specification. The conditions for acceptance under this
include the following:
Figure 3.
1.
That the devices are intended for use as components where the replacement of the
battery can be performed by the operator or a trained technician.
2.
That the devices are intended for use as components in low voltage, isolated,
secondary circuits; where the case temperature does not exceed 100 °C, and the
voltage on any pin, relative to ground, does not exceed 7 VDC.
3.
That the devices are intended to be mounted on a printed wiring board, flame rated to a
minimum of 94 V-1.
4.
That the devices are provided with an appropriate on-chip, reverse current protection
circuit. (See Figure 3 for the circuit diagram.)
Battery control circuit
VCC
P
(OFF when VCC is below
(2)
the switchover voltage)
RA
(1)
RB
VSO
(1)
BATTERY
INTERNAL VCC
P
VSO
(ON when VCC is below
the switchover voltage)(2)
AI02520
1. RA is the ESD protection resistor and RB is the battery protection resistor required by UL.
2. Depending on the device, switchover will be a function of battery voltage or set by a fixed reference.
4/11
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AN1011
Battery status for NVRAMs and serial RTCs
Battery status for NVRAMs and serial RTCs
For the devices listed in Table 1 and 2, the battery can be internal or external to the IC
package. In the external cases, the battery will either be a user-supplied battery or it may
be incorporated into ST's SNAPHAT package which mates to the underlying IC as depicted
in Figure 6 on page 6.
Figure 4 shows the CAPHAT IC package with the battery integrated with the IC. In this
package, the battery is encapsulated within the device.
Figure 5 is representative of several IC packages all of which use a customer-supplied,
external battery. None of these device types include the battery in the IC package.
Figure 6 on page 6 shows ST's SNAPHAT SOH28 (SOIC) package with mating SNAPHAT
battery top. Here, there is no battery in the IC package. Instead, it is a separate,
removable/replaceable unit stacked on top of the IC which reduces board space.
Figure 4.
CAPHAT IC package
CAPHAT and
Hybrid DIPs:
Internal battery
encapsulated in
device package
Figure 5.
External battery, user supplied
External,
user supplied
battery
Serial RTC
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Battery status for NVRAMs and serial RTCs
Figure 6.
AN1011
External battery, SNAPHAT battery top
SNAPHAT
Battery top
RTC AM
NVR
Table 1.
SOH28
SNAPHAT SOIC
Battery status for serial RTCs
Part number
Package
Battery status
M41ST84WMQ6E
SO-16
External, user supplied
M41ST84WMQ6F
M41ST85WMH6E
SOH28 (330 mil)
External, SNAPHAT
SOX28 (300 mil)
External, user supplied
SOX28 (300 mil)
External, user supplied
SSOP20
External, user supplied
SOX28 (300 mil)
External, user supplied
SOX28 (300 mil)
External, user supplied
M41T00AUDD1F
DFN16
External, user supplied
M41T00CAPPC1
CAPHAT DIP-24
M41ST85WMH6F
M41ST85WMX6
M41ST85WMX6TR
M41ST87WMX6
M41ST87WMX6TR
M41ST87WSS6F
M41ST87YMX6
M41ST87YMX6TR
M41ST95WMX6
M41ST95WMX6TR
Internal, BR1632
M41T00M6E
SO-8
External, user supplied
SO-8
External, user supplied
SO-8
External, user supplied
M41T00M6F
M41T00SM6E
M41T00SM6F
M41T11M6E
M41T11M6F
M41T11MH6E
SOH28 (330 mil)
External, SNAPHAT
M41T11MH6F
M41T56M6E
SO-8
M41T56M6F
6/11
Doc ID 6254 Rev 4
External, user supplied
AN1011
Battery status for NVRAMs and serial RTCs
Table 1.
Battery status for serial RTCs (continued)
Part number
Package
Battery status
M41T81M6E
SO-8
External, user supplied
SO-8
External, user supplied
SOX18 (300 mil)
External, user supplied
SO-8
External, user supplied
SOX18 (300 mil)
External, user supplied
QFN16
External, user supplied
SOX18 (300 mil)
External, user supplied
QFN16
External, user supplied
SOX18 (300 mil)
External, user supplied
QFN16
External, user supplied
SOX18 (300 mil)
External, user supplied
QFN16
External, user supplied
SOX18 (300 mil)
External, user supplied
QFN16
External, user supplied
SOX18 (300 mil)
External, user supplied
QFN16
External, user supplied
M41T81M6F
M41T81SM6E
M41T81SM6F
M41T81SMY6E
M41T81SMY6F
M41T82RM6E
M41T82RM6F
M41T82SM6E
M41T82SM6F
M41T82ZM6E
M41T82ZM6F
M41T83RMY6E
M41T83RMY6F
M41T83RQA6F
M41T83SMY6E
M41T83SMY6F
M41T83SQA6F
M41T83ZMY6E
M41T83ZMY6F
M41T83ZQA6F
M41T93RMY6E
M41T93RMY6F
M41T93RQA6F
M41T93SMY6E
M41T93SMY6F
M41T93SQA6F
M41T93ZMY6E
M41T93ZMY6F
M41T93ZQA6F
M41T94MH6E
SOH28 (330 mil)
External, SNAPHAT
M41T94MH6F
M41T94MQ6E
SO-16
External, user supplied
M41T94MQ6F
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Battery status for NVRAMs and serial RTCs
Table 2.
AN1011
Battery status for TIMEKEEPER and ZEROPOWER NVRAMs
Part number
Package
Battery status
M40Z111MH6E
M40Z111MH6F
SOH28 (330 mil)
External, SNAPHAT
M40Z111WMH6E
M40Z111WMH6F
M40Z300AVMQ6TR
SO-16
External, user supplied
M40Z300WMH6E
SOH28 (330 mil)
External, SNAPHAT
M40Z300WMH6F
M40Z300WMQ6E
SO-16
External, user supplied
M40Z300WMQ6F
M48T02-150PC1
CAPHAT DIP-24
Internal, BR1225
CAPHAT DIP-28
Internal, BR1225
SOH28 (330 mil)
External, SNAPHAT
CAPHAT DIP-24
Internal, BR1225
Hybrid DIP-32
Internal, BR1632
CAPHAT DIP-28
Internal, BR1632
SOH28 (330 mil)
External, SNAPHAT
CAPHAT DIP-28
Internal, BR1632
SOH28 (330 mil)
External, SNAPHAT
CAPHAT DIP28
Internal, BR1632
SOH28 (330 mil)
External, SNAPHAT
M48T02-70PC1
M48T08-100PC1
M48T08-150PC1
M48T08Y-10MH1E
M48T08Y-10MH1F
M48T12-150PC1
M48T12-70PC1
M48T128Y-70PM1
M48T129V-85PM1
M48T129Y-70PM1
M48T18-100PC1
M48T18-150PC1
M48T201V-85MH1E
M48T201V-85MH1F
M48T201Y-70MH1E
M48T201Y-70MH1F
M48T35-70PC1
M48T35Y-70PC1
M48T35AV-10MH1E
M48T35AV-10MH1F
M48T35AV-10PC1
M48T35Y-70MH1E
M48T35Y-70MH1F
8/11
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AN1011
Battery status for NVRAMs and serial RTCs
Table 2.
Battery status for TIMEKEEPER and ZEROPOWER NVRAMs (continued)
Part number
Package
Battery status
M48T37V-10MH1E
M48T37V-10MH1F
SOH28 (330 mil)
External, SNAPHAT
M48T37Y-70MH1E
M48T37Y-70MH1F
M48T512Y-70PM1
Hybrid DIP-32
Internal, BR1632
CAPHAP DIP-28
Internal, BR1225
SOH28 (330 mil)
External, SNAPHAT
CAPHAP DIP-28
Internal, BR1225
CAPHAP DIP-28
Internal, BR1632
CAPHAP DIP-28
Internal, BR1225
Hybrid DIP-32
Internal, BR1632
CAPHAP DIP-28
Internal, BR1632
M48T58-70PC1
M48T58Y-70PC1
M48T58Y-70MH1E
M48T58Y-70MH1F
M48Z02-150PC1
M48Z02-70PC1
M48Z08-100PC1
M48Z12-150PC1
M48Z12-70PC1
M48Z128-70PM1
M48Z128Y-70PM1
M48Z128Y-85PM1
M48Z129V-85PM1
M48Z18-100PC1
M48Z2M1V-85PL1
Hybrid DIP-36
Internal, BR1632 (two)
M48Z2M1Y-70PL1
M48Z35-70PC1
CAPHAP DIP-28
Internal, BR1225
SOH28 (330 mil)
External, SNAPHAT
M48Z35Y-70PC1
M48Z35AV-10MH1E
M48Z35Y-70MH1E
M48Z35Y-70MH1F
M48Z512A-70PM1
M48Z512AV-85PM1
Hybrid DIP-32
Internal, BR1632
CAPHAP DIP-28
Internal, BR1225
SOH28 (330 mil)
External, SNAPHAT
M48Z512AY-70PM1
M48Z512BV-85PM1
M48Z58-70PC1
M48Z58Y-70PC1
M48Z58Y-70MH1E
M48Z58Y-70MH1F
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Revision history
AN1011
Revision history
Table 3.
10/11
Document revision history
Date
Revision
Changes
Dec-1998
1
Initial release.
28-Dec-2007
2
Reformatted document; updated UL 1950/60950 validation and
technical support URL.
02-Nov-2009
3
Updated Introduction; Characterization and modeling; Figure 3;
reformatted document.
07-Mar-2012
4
Added Battery status for NVRAMs and serial RTCs, Figure 4, 5, 6
Table 1, 2; updated title of datasheet; minor textual updates.
Doc ID 6254 Rev 4
AN1011
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