LTC6902C データシートの表示（PDF） - Linear Technology

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LTC6902C

Multiphase Oscillator with Spread Spectrum Frequency Modulation

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THEORY OF OPERATIO

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/fMASTER seconds and the pseu-

dorandom sequence repeats every (512 • 3200)/fMASTER

seconds.

The servo loop in the LTC6902 cannot respond instanta-

neously to each step due to its limited bandwidth. The

LTC6902

VMOD 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.

fMAX

128 STEPS

fMIN

τSTEP

τREPEAT

TIME

τSTEP

=

3200

fMASTER

τREPEAT

=

3200 • 512

fMASTER

69012 F05

Figure 5

APPLICATIO S I FOR ATIO

SELECTING THE DIVIDER SETTING AND RSET 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 RSET 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 (fOUT) frequency range for each

divider setting when using SSFM.

6902f

11

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