SL15316
General Description
The primary source of EMI from digital circuits is the
system clock and all the other synchronous clocks and
control signals derived from the system clock. The well
know techniques of filtering (suppression) and shielding
(containment), while effective, can cost money, board
space and longer development time.
A more effective and efficient technique to reduce EMI is
Spread Spectrum Clock Generator (SSCG) technique.
Instead of using constant clock frequency, the SSCG
technique modulates (spreads) the system clock with a
much smaller frequency, to reduce EMI emissions at its
source: The System Clock.
The SL15316 is designed using SpectraLinear
proprietary phase-locked loop (PLL) and Spread
Spectrum Technologies (SST) to synthesize and
modulate (spread) the system clock such that the energy
is spread out over a wider bandwidth. This reduces the
peak value of the radiated emissions at the fundamental
and the harmonics. This reduction in radiated energy
can significantly reduce the cost of complying with
regulatory agency requirements and improve time-to-
market without degrading system performance.
The SL15316 operates from 3.3V to 2.5V power supply
range. Separate VDDO power supply is provided for thre
(3) output clock drivers. The supply voltage on these pins
could be 3.3V to 25V or 1.8V as long as VDDO≤VDD
condition is met.
The SL15316 is available in 16-pin TSSOP package with
Commercial Temperature range of 0 to 70°C and
Industrial Temperature range of –40 to 85°C.
Input Frequency Range
The input frequency range is from 8.0 to 48.0 MHz for
crystals and ceramic resonators. If an external clock is
used, the input frequency range is from 3 to 166 MHz.
Output Frequency Range and Outputs
Up to five (5) outputs can be programmed as SSCLK or
REFCLK. SSCLK output can be synthesized to any value
from 3 to 200 MHz with spread using input frequency.
The spread at SSCLK pins can be stopped by SSON#
input control pin, If SSON# pin is HIGH (VDD), the
frequency at this pin is the synthesized to the nominal
value of the input frequency and there is no spread.
REFOUT is the buffered output of the oscillator and is
the same frequency as the input frequency without
spread. However, REFOUT value can also be divided by
using the output divider. The SSCLK is the programmed
and synthesized value of the input clock. The remaining
SSCLKs could be the same value providing fanout of up
to 5 or the frequency can be divided from also 2 to 32. In
this case, the spread % value is the same as the original
programmed spread % value. By using only first order
crystals, SL15316 can synthesize output frequency up to
200 MHz, eliminating the need for higher order Crystals
(Xtals) and Crystal Oscillators (XOs). This reduces the
cost while improving the system clock accuracy,
performance and reliability.
Rev 1.0, August 7, 2008
Programmable CL (Crystal Load)
The SL15316 provides programmable on-chip capacitors
at XIN/CLKIN (Pin-3) and XOUT (Pin-2). The resolution of
this programmable capacitor is 6-bits with LSB value of
0.5pF. When all bits are off the pin capacitance is
CXIN=CXOUT =7.0pF (minimum value). When all bits are
on the pin capacitance is CXIN=CXOUT=38pF (maximum
value). The values of C
XIN
and C
XOUT
based on the C
L
(Crystal Load Capacitor) can be calculated as:
C
XIN
=C
XOUT
=2C
L
-C
PCB
. Refer to the Page-10 for
additional information on crystal load (C
L
).
In addition, if an external clock is used, the capacitance at
Pin-4 (XIN/CLKIN) can programmed to control the edge
rate of this input clock, providing additional EMI control.
Programmable Modulation Frequency
The Spread Spectrum Clock (SSC) modulation default
value is 31.5 kHz. The higher values of 60, 90 and 120
kHz can also be programmed. Less than 25 kHz
modulation frequency is not recommended to stay out of
the range audio frequency bandwidth since this frequency
could be detected as a noise by the audio receivers within
the vicinity.
Programmable Spread Percent (%)
The spread percent (%) value is programmable from +/-
0.125% to +/-2.5% (center spread) or -0.25% to -5.0%
(down spread) for all SSCLK frequencies. It is possible to
program smaller or larger non-standard values of spread
percent. Contact SLI if these non-standard spread percent
values are required in the application.
SSON# or Function Select (FS)
The SL15316 Pins 7, 12 and 15 could be programmed as
either SSON# to enable or disable the programmed
spread percent value or as Frequency Select (FS). If
SSON# is used, when this pin is pulled high (VDD), the
spread is stopped and the frequency is the nominal value
without spread. If low (GND), the frequency is the nominal
value with the spread.
If FS function is used, the output pins could be
programmed for different set of frequencies or spread %
as selected by FS. SSCLK value can be any frequency
from 3 to 200MHz, but the spread % is the same percent
value. REFOUT is the same frequency as the input
reference clock or divide by from 2 to 32 without spread.
The SL15316 allows a fan-out of up to 5 clocks, meaning
that Pins 7, 11, 12, 13 and 15 can be programmed to the
same frequencies with or without spread.
Power Down (PD#) or Output Enable (OE)
The SL15316 Pins 7, 11, 12, 13 and 15 could also be
programmed as either PD# or OE. PD# powers down the
entire chip whereas OE only disables the output buffers to
Hi-Z.
Spread Spectrum Clock Modulation Frequency
The modulation frequency of spread spectrum clock can
be programmed from 25 to 120kHz.
Page 4 of 12
SPECTRALINEAR [ SPECTRALINEAR INC ]
