M48T18-100PC 查看數據表（PDF） - STMicroelectronics

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M48T18-100PC

CMOS 8K x 8 TIMEKEEPER SRAM

STMicroelectronics 

M48T08, M48T18

SYSTEM BATTERY LIFE

The useful life of the battery in the M48T08,18 is

expected to ultimately come to an end for one of

two reasons: either because it has been discharged

while providing current to the RAM and clock in the

battery back-up mode, or because the effects of

aging render the cell useless before it can actually

be completely discharged. The two effects are vir-

tually unrelated allowing discharge, or Capacity

Consumption, and the effects of aging, or Storage

Life, to be treated as two independent but simulta-

neous mechanisms. The earlier occurring failure

mechanism defines the battery system life of the

M48T08,18.

Cell Storage Life

Storage life is primarily a function of temperature.

Figure 9 illustrates the approximate storage life of

the M48T08,18 battery over temperature. The re-

sults in Figure 9 are derived from temperature

accelerated life test studies performed at SGS-

THOMSON. For the purpose of the testing, a cell

failure is defined as the inability of a cell stabilized

at 25°C to produce a 2.4V closed circuit voltage

across a 250 kΩ load resistor. The two lines, t1%

and t50%, represent different failure rate distribu-

tions for the cell’s storage life. At 70°C, for example,

the t1% line indicates that an M48T08,18 has a 1%

chance of having a battery failure 11 years into its

life while the t50% shows the part has a 50% chance

of failure at the 20 year mark. The t1% line repre-

sents the practical onset of wear out and can be

considered the worst case Storage Life for the cell.

The t50% can be considered the normal or average

life.

Calculating Storage Life

The following formula can be used to predict stor-

age life:

1

{[(TA1/TT)/SL1]+[(TA2/TT)/SL2]+...+[(T AN/TT)/SLN]}

where,

– TA1, TA2, TAN = time at ambient temperature

1, 2, etc.

– TT = total time = TA1+TA2+...+TAN

– SL1, SL2, SLN = storage life at temperature 1,

2, etc.

For example an M48T08,18is exposed to tempera-

tures of 55°C or less for 8322 hrs/yr, and tempera-

tures greater than 60°C but less than 70°C for the

remaining 438 hrs/yr. Reading predicted t1% values

from Figure 9,

Figure 9. Predicted Battery Storage Life versus Temperature

50

40

30

20

t1%

10

8

6

5

4

3

2

AI01024

t50% (AVERAGE)

1

20

30

40

50

60

70

80

90

TEMPERATURE (Degrees Celsius)

10/18

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