LTC6908-1/LTC6908-2

8

Rev C

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very high current applications, a significant improvement

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 (OUT1 = Low, OUT2 = High) 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

multiplying digital to analog converter (MDAC) referenced

to ISET and lowpass filtered. IMASTER varies in a pseudoran-

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

This causes the output frequency to vary in a pseudoran-

dom 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 modu-

lation 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 sig-

nal 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 repeti-

tion 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

setting. 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 soft-

ens 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. This is beneficial when the LTC6908 is used to

clock switching regulators as will be discussed in the

Applications Information section. Figure 4 illustrates how

the output frequency varies over time.

Figure 4.

tREPEAT = ((215 – 1) • N)/fOUT

tSTEP = N/fOUT

fOUT + 10%

128 STEPS

tREPEAT

tSTEP

fOUT – 10%

TIME

690812 F04

OPERATION