Datasheet

LTC6902
11
6902f
THEORY OF OPERATIO
U
f
MAX
f
MIN
128 STEPS
TIME
69012 F05
3200
f
MASTER
FREQUENCY
τ
STEP
τ
STEP
=
τ
REPEAT
3200 • 512
f
MASTER
τ
REPEAT
=
Figure 5
APPLICATIO S I FOR ATIO
WUUU
SELECTING THE DIVIDER SETTING AND R
SET
VALUE
The LTC6902’s master oscillator has a frequency range
spanning 0.1MHz to 20MHz. However, accuracy may
suffer if the master oscillator is operated at greater than
10MHz with a supply voltage lower than 4V. A program-
mable divider extends the frequency range to greater than
three decades. Additional frequency division may occur
depending on the multiphase mode selected. The
multiphase mode and the parameter M are generally
dependent on the application’s requirement and usually
do not offer any additional design flexibility.
The LTC6902’s master oscillator covers a 200:1 range
while the programmable divider has 10:1 steps (1, 10,
100). This wide frequency range coupled with the part’s
programmable divider yields at least two solutions for any
desired output frequency (the exception being the highest
output frequencies that cannot be divided down). Choos-
ing the best divider setting and the correct R
SET
resistor
value depends on the application.
For spread spectrum frequency modulated (SSFM) appli-
cations, choose the highest divider setting. This forces the
master oscillator to run at its highest frequency. The
pseudorandom signal generator is clocked by the master
oscillator, not the output, and the faster the signal moves
the greater the improvement in EMC performance. For
most applications the multiphase mode is determined by
the specific application’s need. For these applications, the
parameter M is predetermined and fixed. Table 1 lists the
recommended output (f
OUT
) frequency range for each
divider setting when using SSFM.
clock cycle. Note that the shift register clock is the master
oscillator’s output divided by 3200. This results in a some-
what slow moving modulating signal where each step is
separated in time by 3200/f
MASTER
seconds and the pseu-
dorandom sequence repeats every (512 • 3200)/f
MASTER
seconds.
The servo loop in the LTC6902 cannot respond instanta-
neously to each step due to its limited bandwidth. The
V
MOD
voltage steps are converted to frequency steps by
the servo loop. The servo loop has a bandwidth of about
25kHz that limits the frequency change rate and softens
corners of the waveform. This is beneficial when the
LTC6902 is used to clock switching regulators as will be
discussed in the Applications Information section. Fig-
ure␣ 5 illustrates the how the output frequency varies over
time.
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