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
CFx Cathode
Anode Cap
Lithium Anode
Current Cathode
Separator and Electrolyte
AI02531
March 2012
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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
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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.
Figure 2.
(A) BR1225X discharge rate
(B) BR1632 discharge rate
3.5
Voltage (V)
Voltage (V)
3.0
2.5
2.0
1.5
1.0
0
200 400 600 800 1000 1200 1400 1600 1800 2000
Duration (Hrs.)
(A)
15k
30k
100k
3.5
3.0
2.5
2.0
1.5
1.0
0
1000
2000
3000
4000
5000
6000
15k
30k
50k
100k
Duration (Hrs.)
(B)
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:
1.
2.
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.
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 V
DC
.
That the devices are intended to be mounted on a printed wiring board, flame rated to a
minimum of 94 V-1.
That the devices are provided with an appropriate on-chip, reverse current protection
circuit. (See
Figure 3
for the circuit diagram.)
3.
4.
Figure 3.
Battery control circuit
VCC
P
(OFF when VCC is below
(2)
the switchover voltage)
VSO
RA
BATTERY
(1)
RB
(1)
INTERNAL VCC
P
VSO
(ON when VCC is below
the switchover voltage)
(2)
AI02520
1. R
A
is the ESD protection resistor and R
B
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.
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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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