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LTC6902IMS Datasheet(PDF) 11 Page - Linear Technology |
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LTC6902IMS Datasheet(HTML) 11 Page - Linear Technology |
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11 / 16 page  LTC6902 11 6902f THEORY OF OPERATIO fMAX fMIN 128 STEPS TIME 69012 F05 3200 fMASTER τSTEP τSTEP = τREPEAT 3200 • 512 fMASTER τREPEAT = 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. 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 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. |
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