FN6667 datasheet(21/34 Pages) RENESAS

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ISL12020M

FN6667 Rev 6.00

Page 21 of 34

January 9, 2015

FREQUENCY OF TEMPERATURE SENSING AND

CORRECTION BIT (BTSR)

This bit controls the frequency of Temp Sensing and Correction.

BTSR = 0 default mode is every 10 minutes, BTSR = 1 is every

1.0 minute. Note that BTSE has to be enabled in both cases. See

Table 16.

The temperature measurement conversion time is the same for

battery mode as for VDD mode, approximately 22ms. The battery

mode current will increase during this conversion time to

typically 68µA. The average increase in battery current is much

lower than this due to the small duty cycle of the ON-time versus

OFF-time for the conversion.

To figure the average increase in battery current, we take the

change in current times the duty cycle. For the 1 minute

temperature period the average current is as shown in

Equation 1:

For the 10 minute temperature period the average current is as

shown in Equation 2:

If the application has a stable temperature environment that

doesn’t change quickly, the 10 minute option will work well and

the backup battery lifetime impact is minimized. If quick

temperature variations are expected (multiple cycles of more

than 10° within an hour), then the 1 minute option should be

considered and the slightly higher battery current figured into

overall battery life.

GAIN FACTOR OF AT BIT (BETA<4:0>)

Beta is specified to take care of the Cm variations of the crystal.

Most crystals specify Cm around 2.2fF. For example, if Cm > 2.2fF,

the actual AT steps may reduce from 1ppm/step to approximately

0.80ppm/step. Beta is then used to adjust for this variation and

restore the step size to 1ppm/step.

BETA values are limited in the range from 01000 to 11111 as

shown in Table 17

. To use Table 17, the device is tested at two AT

settings in Equation 3:

Where:

AT(max) = FOUT in ppm (at AT = 00H) and

AT(min) = FOUT in ppm (at AT = 3FH).

The BETA VALUES result is indexed in the right hand column and

the resulting Beta factor (for the register) is in the same row in

the left column.

The ISL12020M has a preset BETA value corresponding to the

crystal in the module. This value is recalled on initial power-up

and should never be changed for best temperature

compensation performance, although the user may override this

preset value if so desired.

The value for BETA should only be changed while the TSE (Temp

Sense Enable) bit is “0”. The procedure for writing the BETA

TSE = 0. Then write the same value of BETA with TSE = 1. This will

insure the next temp sense cycle will use the new BETA value

TABLE 16. FREQUENCY OF TEMPERATURE SENSING AND

CORRECTION BIT

BTSE

BTSR

TC PERIOD IN

BATTERY MODE

OFF

10 Minutes

1 Minute

I

BAT

0.022s

60s

------------------

A 250nA



(EQ. 1)

I

BAT

0.022s

600s

------------------

A25nA



(EQ. 2)

BETA VALUES

AT max

 AT min



–

/63

(EQ. 3)

TABLE 17. BETA VALUES

BETA<4:0>

AT STEP ADJUSTMENT

01000

0.5000

00111

0.5625

00110

0.6250

00101

0.6875

00100

0.7500

00011

0.8125

00010

0.8750

00001

0.9375

00000

1.0000

10000

1.0625

10001

1.1250

10010

1.1875

10011

1.2500

10100

1.3125

10101

1.3750

10110

1.4375

10111

1.5000

11000

1.5625

11001

1.6250

11010

1.6875

11011

1.7500

11100

1.8125

11101

1.8750

11110

1.9375

11111

2.0000

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