參數(shù)資料
型號(hào)： LTC6902IMS
廠商： LINEAR TECHNOLOGY CORP
元件分類(lèi)： XO, clock
英文描述： Multiphase Oscillator with Spread Spectrum Frequency Modulation
中文描述： PLL FREQUENCY SYNTHESIZER, PDSO10
封裝： PLASTIC, MSOP-10
文件頁(yè)數(shù)： 14/16頁(yè)
文件大小： 212K
代理商： LTC6902IMS
LTC6902
14
6902f
APPLICATIOU
W
U
U
generator is driven by the master oscillator frequency, not
the output frequency. This gives some design flexibility in
the choice of the R
SET
and the programmable divider
setting. When making the choice, usually the faster mas-
ter oscillator is the better choice. This is especially true
when the main goal is to lower peak radiated or conducted
signal levels measured during regulatory agency testing.
Regulatory testing is done with strictly specified band-
widths and conditions. Modulating faster than the test
bandwidth or as close to the bandwidth as possible gives
the lowest readings. The optimal modulating rate is not as
straightforward when the goal is to lower radiated signal
levels interfering with other circuitry in the system. The
modulation rate will have to be evaluated with the specific
system conditions to determine the optimal rate. Depend-
ing on the specific frequency synchronization method a
switching regulator employs, the modulation rate must
be within the synchronization capability of the regulator.
Many switching regulators use a phase-locked loop (PLL)
for synchronization. For these parts, the PLL loop filter
should be designed to have sufficient capture range and
bandwidth.
Even when running the LTC6902 at the maximum modu-
lation rate, the frequency hopping transitions are slowed
by the part’s servo loop. The frequency transitions are
slowed by a 25kHz lowpass. This is an important feature
when driving a switching regulator. The switching regula-
tor is itself a servo loop with a bandwidth typically on the
order of 1/10, but can vary from 1/50 to 1/2 of the
operating frequency. When the input clock frequency’s
transition is within the bandwidth of the switching regula-
tor, the regulator’s output stays in regulation. If the tran-
sition is too sharp, beyond the bandwidth of the switching
regulator, the regulator’s output will experience a sharp
jump and then settle back into regulation. If the bandwidth
of the switching regulator is sufficiently high, beyond
25kHz, then there will not be any regulation issues.
One aspect of the output voltage that will change is the
output ripple voltage. Every switching regulator has some
output ripple at the clock frequency. For most switching
regulator designs with fixed MOSFETs, fixed inductor,
fixed capacitors, the amount of ripple will vary some with
the regulator’s operating frequency (the main exception
being hysteresis architecture regulators). An increase in
frequency results in lower ripple and a frequency decrease
gives more ripple. This is true for static frequencies or
dynamic frequency modulated systems. If the modulating
signal was a triangle wave, the regulator’s output would
have a ripple that is amplitude modulated by the triangle
wave. This repetitive signal on the power supply could
cause system problems by mixing with other desired
signals and giving a distorted output. Depending on the
inductor design and triangle wave frequency, it may even
result in an audible noise. The LTC6902 uses a pseudoran-
dom noise-like modulating signal. This results in the
regulator’s output ripple being modulated by the wideband
pseudorandom noise-like signal. On an oscilloscope, it
looks essentially noise-like of even amplitude. The signal
is broadband and any mixing issues are minimized. Addi-
tionally, the pseudorandom signal repeats at such a low
rate that it is well below the audible range.
The LTC6902 directly drives many switching regulators.
The LTC6902 with the spread spectrum frequency modu-
lation results in improved EMC performance. If the band-
width of the switching regulator is sufficient, not a difficult
requirement in most cases, the regulator’s regulation,
efficiency and load response are maintained while peak
electromagnetic radiation (or conduction) is reduced.
Output ripple may be somewhat increased, but its behav-
ior is very much like noise and its system impact is benign.
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