LTC6908IDCB-1 Linear Technology, LTC6908IDCB-1 Datasheet - Page 8

• Manufacturer Part Number: LTC6908IDCB-1
• Manufacturer: Linear Technology
• Type: Silicon Oscillatorr

Specifications of LTC6908IDCB-1

• Mounting Style: Surface Mount
• Screening Level: Industrial
• Product Depth (mm): 3mm
• Lead Free Status / RoHS Status: Not Compliant

OPERATIO
LTC6908-1/LTC6908-2
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 fi rst 64 clock cycles after power-up,
ensuring that the fi rst 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 fi xed at 20% (±10%),
where frequency spreading is defi ned as:
Frequency Spreading (in %) = 100 • ( f
The I
multiplying digital to analog converter (MDAC) referenced to
I
noise-like manner between 0.9 • I
causes the output frequency to vary in a pseudorandom
noise-like manner between 0.9 • f
8
SET
and lowpass fi ltered. I
MASTER
current is a dynamic signal generated by a
U
MASTER
varies in a pseudorandom
128 STEPS
OUT
SET
f
f
OUT
OUT
t
STEP
+ 10%
– 10%
and 1.1 • f
and 1.1 • I
MAX
= N/f
OUT
– f
MIN
OUT
SET
)/f
t
STEP
.
. This
OUT
Figure 4
t
REPEAT
TIME
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
(2
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 (2
discrete steps that change every shift register clock cycle.
Note that the shift register clock is the output frequency,
f
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
The output of the MDAC is then fi ltered by a lowpass fi lter
with a corner frequency set to the modulation rate (f
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 fi lter are approximately 0.35/f
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.
OUT
15
, divided by N, where N is the modulation rate divider
– 1) • N clock cycles. This guarantees a repetition
t
REPEAT
= ((2
15
– 1) • N)/f
+
for the N = 64 setting.
OUT
690812 F04
CORNER
. This is benefi cial
OUT
690812fa
/N).
7
)