Datasheet LTC6908-1, LTC6908-2 (Analog Devices) - 8

link to page 9 link to page 8 LTC6908-1/LTC6908-2
OPERATION
very high current applications, a significant improvement To disable the SSFM, connect one of the outputs to the in conducted EMI results due to the reduced levels of input MOD pin. An AC detector circuit shuts down the modu- and output ripple currents. The LTC6908-1 is ideal for use lation circuitry if a frequency in the vicinity of the output with two single output switching regulators. The quadrature frequency is detected at the MOD pin. outputs of the LTC6908-2, together with two dual output As stated previously, the modulating waveform is a pseu- switching regulators, provide the 0°, 90°, 180° and 270° dorandom noise-like waveform. The pseudorandom sig- phased shifted clocks for four-phase control. nal is generated by a linear feedback shift register that The rise and fall times are typically 6ns with a 5V supply is 15 bits long. The pseudorandom sequence will repeat and 11ns with a 3V supply. An internal counter mutes the every (215 – 1) • N clock cycles. This guarantees a repeti- outputs (OUT1 = Low, OUT2 = High) for the first 64 clock tion rate below 20Hz for output frequencies up to 10MHz. cycles after power-up, ensuring that the first clock cycle Seven bits of the shift register are sent in parallel to the is close to the desired operating frequency. MDAC which produces the modulating current waveform. Being a digitally generated signal, the output of the MDAC
Spread Spectrum Frequency Modulation
is not a perfectly smooth waveform, but consists of (27) The LTC6908 provides the additional feature of spread discrete steps that change every shift register clock cycle. spectrum frequency modulation (SSFM). The oscillator’s Note that the shift register clock is the output frequency, frequency is modulated by a pseudorandom noise (PRN) fOUT, divided by N, where N is the modulation rate divider signal to spread the oscillator’s energy over a wide fre- setting, which is determined by the state of the MOD pin. quency band. This spreading decreases the peak electro- The MOD pin should be tied to ground for the N = 16 magnetic radiation levels and improves electromagnetic setting. Floating the MOD pin selects N = 32. The MOD compatibility (EMC) performance. pin should be tied to V+ for the N = 64 setting. The amount of frequency spreading is fixed at 20% The output of the MDAC is then filtered by a lowpass (±10%), where frequency spreading is defined as: filter with a corner frequency set to the modulation rate (fOUT/N). This limits the frequency change rate and soft- Frequency Spreading (in %) = 100 • ( fMAX – fMIN)/fOUT ens corners of the waveform, but allows the waveform The I to fully settle at each frequency step. The rise and fall MASTER current is a dynamic signal generated by a multiplying digital to analog converter (MDAC) referenced times of this single pole filter are approximately 0.35/ to I f SET and lowpass filtered. IMASTER varies in a pseudoran- CORNER. This is beneficial when the LTC6908 is used to dom noise-like manner between 0.9 • I clock switching regulators as will be discussed in the SET and 1.1 • ISET. This causes the output frequency to vary in a pseudoran- Applications Information section. Figure 4 illustrates how dom noise-like manner between 0.9 • f the output frequency varies over time. OUT and 1.1 • fOUT. fOUT + 10% 128 STEPS FREQUENCY fOUT – 10% tSTEP tSTEP = N/fOUT tREPEAT = ((215 – 1) • N)/fOUT tREPEAT TIME 690812 F04
Figure 4.
Rev B 8 For more information www.analog.com Document Outline Features Applications Description Absolute Maximum Ratings Electrical Characteristics Pin Functions Operation Applications Information Typical Applications Package Description Typical Application