LTC6902

12

6902f

Table 1. Recommended Frequency Range vs Programmable

Divider Setting for SSFM Applications or for Low Jitter Constant

Frequency Applications

DIVIDER SETTING

N = 1

DIV (Pin 2) = GND

2MHz to 20MHz

N = 10

DIV (Pin 2) = Open

200kHz to 2MHz

N = 100

< 200kHz

Note: The frequency range numbers are for a 5V supply where a 20MHz

output is the maximum frequency supported. For low supply applications

(2.7V

of the above numbers should be halved.

For constant frequency applications, where SSFM is dis-

abled, the best operating position depends on which

parameter is most important in the application. For the

lowest clock jitter it is best to set the divider to its highest

setting as done above. The divider reduces the master

oscillator’s jitter. The higher the division number the

greater the reduction in the master oscillator’s jitter. For

the best frequency accuracy it is best to run the program-

mable divider at its lowest setting, and thus, the master

oscillator runs at a lower frequency. The lower master

oscillator frequencies are more accurate and use less

power. To determine a tradeoff between frequency accu-

racy and jitter consult the Typical Performance Character-

istics curves. Table 2 lists the recommended output fre-

quency range for each divider setting for continuous

frequency applications where frequency accuracy is the

primary specification.

Table 2. Recommended Frequency Range vs Programmable

Divider Setting for Best Frequency Accuracy, Constant Frequency

Applications (SSFM disabled)

DIVIDER SETTING

N = 1

DIV (Pin 2) = GND

> 500kHz*

N = 10

DIV (Pin 2) = Open

50kHz to 500kHz

N = 100

< 50kHz

10MHz for low supply applications (2.7V

For some applications, the multiphase circuit is also useful

in forcing the master oscillator to run at a higher or lower

frequency. If the application requires a single clock source,

the multiphase circuit can be set in whatever mode gives

the highest or lowest divider number (M) and thus the

highest or lowest master oscillator frequency. Addition-

ally, if the application requires just two phases, the 4-phase

APPLICATIO S I FOR ATIO

mode can be selected with only the OUT1 and OUT3

outputs are used (or alternatively the OUT2 and OUT4

outputs).

For instance, a 500kHz, 2-phase clock can be obtained in

four different ways. Table 3 lists the possible solutions.

For an SSFM application, the preferred solution for best

EMC performance is the last alternative where the master

oscillator is at 20MHz. For a constant frequency applica-

tion, the preferred solution is the first alternative with the

master oscillator at 500kHz.

Table 3. Four Possible Ways to Obtain a 500kHz, 2-Phase Clock

N

MULTIPHASE MODE

M

OUTPUTS

400k

1

2

1

500kHz

OUT1, OUT2

100k

1

4

4

2MHz

OUT1, OUT3

40k

10

2

1

5MHz

OUT1, OUT2

10k

10

4

4

20MHz

OUT1, OUT3

After choosing the proper divider setting, determine the

correct frequency-setting resistor. Because of the linear

correspondence between oscillation period and resis-

tance, a simple equation relates resistance with frequency.

Rk

MHz

MN f

SET

OUT

=

























20

10

100

10

1

4

3

1

•

••

, N =

M =

SETTING THE SPREAD SPECTRUM MODULATION

SPREADING PERCENTAGE WITH THE LTC6902

Setting the spread spectrum modulation percentage on

the LTC6902 is very simple and straightforward. Since the

spreading is ratiometric, in percentage, the program-

mable divider and multiphase mode selection have no

influence on the spreading percentage. In general, for

greatest EMC improvement, each application should apply

as much spreading as possible. The amount of spreading

that any particular application can tolerate is dependent on