LTC6902 Datasheet, PDF (12/16 Pages) Linear Technology – Multiphase Oscillator with Spread Spectrum Frequency Modulation

LTC6902
APPLICATIO S I FOR ATIO
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
FREQUENCY RANGE (f_OUT • M)
2MHz to 20MHz
N = 10
N = 100
DIV (Pin 2) = Open
DIV (Pin 2) = V⁺
200kHz to 2MHz
< 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 ≤ V⁺ ≤ 4V), the maximum rated output frequency is 10MHz and all
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
FREQUENCY RANGE (f_OUT • M)
> 500kHz*
N = 10
DIV (Pin 2) = Open
50kHz to 500kHz
N = 100 DIV (Pin 2) = V⁺
< 50kHz
*The maximum frequency (f_OUT • M) is 20MHz for 5V applications and is
10MHz for low supply applications (2.7V ≤ V⁺ ≤ 4V).
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
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
R_SET N MULTIPHASE MODE M f_MASTER
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.
R_SET
=
20k
•


10MHz
M • N • f_OUT


,
N
=
11000
¹
M
=
₃₄
¹
(R_SETMIN = 10k, R_SETMAX = 2M)
Any resistor, R_SET, tolerance adds to the inaccuracy of the
oscillator, f_OUT.
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
the specific nature of that application. Once the R_SET
resistor value is calculated to set f_MAX and the desired
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