FM3808_06 datasheet(18/27 Pages) RAMTRON

18 / 27 page

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FM3808

Rev. 1.3 (EOL)

Feb. 2006

Page 18 of 27

IBAK vs VBAK

0.0E+00

5.0E-07

1.0E-06

1.5E-06

2.0E-06

2.5E-06

4.7

4.5

4.25

3.75

3.5

3.25

2.75

2.5

VBAK

Figure 9. Backup Current vs. Voltage

Crystal Selection

The second passive component needed for the RTC

function is the timekeeping crystal. A 32.768 kHz

time-base is required, and the FM3808 is designed to

accept a low cost crystal. The major parameters

associated with the crystal are timekeeping accuracy

and backup current. The FM3808 is designed to

accept a crystal with a characteristic capacitance of 6

pF. Deviations from this specification will lead to

different accuracy and IBAK from the specified

values. Though accuracy is unlikely to improve, the

IBAK may go up or down from the specified value as

a function of the capacitive load.

The timekeeping accuracy is also a strong function of

the operating temperature due to errors in crystal

frequency. Temperature behavior of timekeeping

crystals is well known and it follows a curve like the

one shown below. The specific crystal manufacturer

should be consulted for the behavior of their specific

device. Note the error in frequency ppm. One ppm is

roughly 2.6 seconds per month in timekeeping error.

-200

-180

-160

-140

-120

-100

-80

-60

-40

-20

Ambient Temperature

Figure 10. Typical Crystal Error vs. Temperature

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