Datasheet

LTC6909
11
6909fa
Spread Spectrum Frequency Modulation
The LTC6909 can operate with spread spectrum frequency
modulation (SSFM). In this mode, the oscillator’s frequency
is modulated by a pseudorandom noise (PRN) signal
to spread the oscillator’s energy over a wide frequency
band. This spreading decreases the peak electromagnetic
radiation levels and improves electromanetic compatibility
(EMC) performance.
The amount of frequency spreading is fixed at 20% (±10%),
where frequency spreading is defined as:
Frequency Spreading (in %) = 100 • (f
MAX
– f
MIN
)/f
OUT
The I
MASTER
current is a dynamic signal generated by
a multiplying digital-to-analog converter (MDAC) refer-
enced to I
SET
and lowpass filtered. I
MASTER
varies in a
psuedorandom noise-like manner between 0.9 • I
SET
and
1.1 • I
SET
. This causes the output frequency to vary in a
pseudorandom noise-like manner between 0.9 • f
OUT
and
1.1 f
OUT
.
To disable the SSFM, connect one of the active outputs
to the MOD pin. An AC detector circuit shuts down the
modulation circuitry if a frequency in the vicinity of the
output frequency is detected at the MOD pin.
As stated previously, the modulating waveform is a pseu-
dorandom noise-like waveform. The pseudorandom signal
is generated by a linear feedback shift register that is 15
bits long. The pseudorandom sequence will repeat every
(2
15
– 1) • N clock cycles. This guarantees a repetition
rate below 13Hz for output frequencies up to 6.67MHz.
Seven bits of the shift register are sent in parallel to the
MDAC which produces the modulating current waveform.
Being a digitally generated signal, the output of the MDAC
is not a perfectly smooth waveform, but consists of (2
7
)
discrete steps that change every shift register clock cycle.
Note that the shift register clock is the output frequency,
f
OUT
, divided by N, where N is the modulation rate divider
setting, which is determined by the state of the MOD pin.
The MOD pin should be tied to ground for the N = 16 set-
ting. Floating the MOD pin selects N = 32. The MOD pin
should be tied to V
+
for the N = 64 setting.
The output of the MDAC is then filtered by a lowpass filter
with a corner frequency set to the modulation rate (f
OUT
/N).
This limits the rate of frequency change and softens the
corners of the frequency control signal, but allows the
waveform to fully settle at each frequency step. The rise
and fall times of this single pole filter are approximately
0.35/f
CORNER
. This is beneficial for clocking switching
regulators, as discussed in the Applications Information
section. Figure 3 illustrates how the output frequency
varies over time.
Figure 3
operAtion
t
REPEAT
= ((2
15
– 1) • N)/f
OUT
t
STEP
= N/f
OUT
f
OUT
+ 10%
128 STEPS
t
REPEAT
t
STEP
f
OUT
– 10%
TIME
FREQUENCY
6909 F03