LTC6908CS6-2 datasheet(8/16 Pages) LINER

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LTC6908-1/LTC6908-2

690812fa

OPERATIO

in conducted EMI results due to the reduced levels of input

and output ripple currents. The LTC6908-1 is ideal for use

with two single output switching regulators. The quadrature

outputs of the LTC6908-2, together with two dual output

switching regulators, provide the 0°, 90°, 180° and 270°

phased shifted clocks for four-phase control.

The rise and fall times are typically 6ns with a 5V supply

and 11ns with a 3V supply. An internal counter mutes

the outputs for the ﬁrst 64 clock cycles after power-up,

ensuring that the ﬁrst clock cycle is close to the desired

operating frequency.

Spread Spectrum Frequency Modulation

The LTC6908 provides the additional feature of spread

spectrum frequency modulation (SSFM). The oscillator’s

frequency is modulated by a pseudorandom noise (PRN)

signal to spread the oscillator’s energy over a wide fre-

quency band. This spreading decreases the peak electro-

magnetic radiation levels and improves electromagnetic

compatibility (EMC) performance.

The amount of frequency spreading is ﬁxed at 20% (±10%),

where frequency spreading is deﬁned as:

Frequency Spreading (in %) = 100 • ( fMAX – fMIN)/fOUT

The IMASTER current is a dynamic signal generated by a

multiplyingdigitaltoanalogconverter(MDAC)referencedto

ISETandlowpassﬁltered.IMASTERvariesinapseudorandom

noise-like manner between 0.9 • ISET and 1.1 • ISET. This

causes the output frequency to vary in a pseudorandom

noise-like manner between 0.9 • fOUT and 1.1 • fOUT.

To disable the SSFM, connect one of the outputs to the

MOD pin. An AC detector circuit shuts down the modula-

tion circuitry if a frequency in the vicinity of the output

frequency is detected at the MOD pin.

As stated previously, the modulating waveform is a pseu-

dorandom noise-like waveform. The pseudorandom signal

is generated by a linear feedback shift register that is 15

bits long. The pseudorandom sequence will repeat every

(215 – 1) • N clock cycles. This guarantees a repetition

rate below 20Hz for output frequencies up to 10MHz.

Seven bits of the shift register are sent in parallel to the

MDAC which produces the modulating current waveform.

Being a digitally generated signal, the output of the MDAC

is not a perfectly smooth waveform, but consists of (27)

discrete steps that change every shift register clock cycle.

Note that the shift register clock is the output frequency,

fOUT, divided by N, where N is the modulation rate divider

setting, which is determined by the state of the MOD pin.

The MOD pin should be tied to ground for the N = 16 set-

ting. Floating the MOD pin selects N = 32. The MOD pin

should be tied to V+ for the N = 64 setting.

The output of the MDAC is then ﬁltered by a lowpass ﬁlter

with a corner frequency set to the modulation rate (fOUT/N).

This limits the frequency change rate and softens corners

of the waveform, but allows the waveform to fully settle at

each frequency step. The rise and fall times of this single

pole ﬁlter are approximately 0.35/fCORNER.Thisisbeneﬁcial

when the LTC6908 is used to clock switching regulators

as will be discussed in the Applications Information sec-

tion. Figure 4 illustrates how the output frequency varies

over time.

Figure 4

tREPEAT = ((2

15 – 1) • N)/f

OUT

tSTEP = N/fOUT

fOUT + 10%

128 STEPS

tREPEAT

tSTEP

fOUT – 10%

TIME

690812 F04

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