LTC6902IMS#TR Linear Technology, LTC6902IMS#TR Datasheet - Page 13

• Manufacturer Part Number: LTC6902IMS#TR
• Description: IC OSC MULTIPHASE PREC LP 10MSOP
• Manufacturer: Linear Technology
• Type: Oscillator, Siliconr

Specifications of LTC6902IMS#TR

• Frequency: 20MHz
• Voltage - Supply: 2.7 V ~ 5.5 V
• Current - Supply: 2.5mA
• Operating Temperature: -40°C ~ 85°C
• Package / Case: 10-MSOP, Micro10™, 10-uMAX, 10-uSOP
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Count: -

APPLICATIO S I FOR ATIO
spreading is determined, the R
using the simple equation below:
The only limitations for this formula are in the R
range and the spreading percentage range. The range of
the R
ranging from 10k to 400k. The LTC6902 is tested and
specified for spreading of 10% and 40%. These are
practical limits that would apply to many systems but they
are not the actual limits of the part. The lower end limit is
set by internal offsets and mismatches. At lower spreading
percentages, these mismatches become more significant
and the error from the calculated, desired spreading
increases. A practical lower end limit would be about 5%
spreading. At the higher end internal mismatching be-
comes less significant, however other factors come into
play and a theoretical limit approaching 100% (f
proaching zero) cannot be reliably achieved. A practical
upper limit would be about 80% spreading.
To disable the SSFM, connect the MOD pin to ground.
Grounding the MOD pin disables the modulation and shuts
down the modulation circuitry. While leaving the MOD pin
open, R
is not a good method of disabling the modulation. The
open pin is susceptible to external noise coupling that can
affect the output frequency accuracy. Grounding the MOD
pin is the best way to disable the SSFM.
DRIVING LOGIC CIRCUITS
The outputs of the LTC6902 are suitable for driving
general digital logic circuits. The CMOS output drivers
have an ON resistance that is typically less than 100 and
are very similar in performance to HCMOS logic outputs.
However, the form of frequency spreading used in the
LTC6902 may not be suitable for many logic designs.
Many logic designs have fairly tight timing and cycle-to-
cycle jitter requirements. These systems often benefit
from a spread spectrum clocking system where the fre-
quency is slowly and linearly modulated by a triangular
R
MOD
MOD
MOD
resistor value is the same as that for R
20 •
= , gives a frequency spreading of 0%, this
Spreading Percentage
U
R
U
SET
MOD
W
value is calculated
U
MOD
MIN
value
ap-
SET
waveform, not a pseudorandom waveform. This type of
frequency spreading maintains a minimal difference in the
timing from one clock edge to the next adjacent clock edge
(cycle-to-cycle jitter). The LTC6902 uses a pseudorandom
modulating signal where the frequency transitions have
been slowed and the corners rounded by a 25kHz lowpass
filter. This filtered modulating signal may be acceptable for
many logic systems but the cycle-to-cycle jitter issues
must be considered carefully.
DRIVING SWITCHING REGULATORS
The LTC6902 is designed primarily to provide an accurate
and stable clock for switching regulator systems, espe-
cially those systems with multiple switching regulators
where all of the regulators are interleaved and are run at the
same frequency. This lowers the input capacitor require-
ments and prevents beat notes formed by mixing numer-
ous clock frequencies and their harmonics. The multiphase
outputs have CMOS drivers with an ON resistance that is
typically less than 100 and are very similar in perfor-
mance to HCMOS logic outputs. This is suitable for
directly driving most switching regulators and switching
controllers. Linear Technology has a broad line of fully
integrated switching regulators and switching regulator
controllers designed for synchronization to an external
clock. All of these parts have one pin assigned for external
clock input. The nomenclature varies depending on the
part’s family history. SYNC, PLLIN, SYNC/MODE, SHDN,
EXTCLK, FCB and S/S (shorthand for SYNC/SHDN) are
examples of clock input pin names used with Linear
Technology ICs. The exact operating details depend on the
switching regulator in use, but generally switching is
synchronized to the rising edge of the clock. Since the
LTC6902’s master oscillator is passed through inverters
or flip-flops to generate its multiphase outputs, coincident
rising edges (or falling edges) cannot occur. This is true
even when the LTC6902 is used with a high percentage of
spreading.
For the best EMC performance, the LTC6902 should be
run with SSFM enabled and the master oscillator at its
highest frequency. The pseudorandom modulation signal
LTC6902
13
6902f