LTC6902

11

6902f

THEORY OF OPERATIO

128 STEPS

TIME

69012 F05

3200

3200 • 512

Figure 5

APPLICATIO S I FOR ATIO

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-

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

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

seconds.

The servo loop in the LTC6902 cannot respond instanta-

neously to each step due to its limited bandwidth. The

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.