LTC6908CDCB-1 View Datasheet(PDF) - Linear Technology
Part Name
Description
Manufacturer
LTC6908CDCB-1 Datasheet PDF : 16 Pages
| |||
LTC6908-1/LTC6908-2
APPLICATIO S I FOR ATIO
of the MOD pin. 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 LTC6908 is designed primarily to provide an accurate
and stable clock for switching regulator systems. The
complementary (LTC6908-1) or quadrature (LTC6908-2)
CMOS logic outputs are 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 no-
menclature 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.
For the best EMC performance, the LTC6908 should be
run with the MOD pin tied to ground (SSFM enabled,
modulation rate set to fOUT/16). Regulatory testing is
done with strictly specified bandwidths and conditions.
Modulating faster than the test bandwidth or as close to
the bandwidth as possible gives the lowest readings. The
optimal modulating rate is not as straightforward when
the goal is to lower radiated signal levels interfering with
other circuitry in the system. The modulation rate will
have to be evaluated with the specific system conditions
to determine the optimal rate. Depending on the specific
frequency synchronization method a switching regulator
employs, the modulation rate must be within the synchro-
nization capability of the regulator. Many regulators use
a phase-locked loop (PLL) for synchronization. For these
parts, the PLL loop filter should be designed to have suf-
ficient capture range and bandwidth.
The frequency hopping transitions of the LTC6908 are
slowed by a lowpass filter. The corner frequency of this
filter is set to the modulation rate (fOUT/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. This is an important feature when driving
a switching regulator. The switching regulator is itself a
servo loop with a bandwidth typically on the order of 1/10,
but can vary from 1/50 to 1/2 of the operating frequency.
When the clock frequency’s transition is within the band-
width of the switching regulator, the regulator’s output
stays in regulation. If the transition is too sharp, beyond
the bandwidth of the switching regulator, the regulator’s
output will experience a sharp jump and then settle back
into regulation. If the bandwidth of the regulator is suf-
ficiently high, beyond fOUT/N, then there will not be any
regulation issues.
One aspect of the output voltage that will change is the
output ripple voltage. Every switching regulator has some
output ripple at the clock frequency. For most switching
regulator designs with fixed MOSFET’s, fixed inductor,
fixed capacitors, the amount of ripple will vary some with
the regulators operating frequency (the main exception
being hysteretic architecture regulators). An increase in
frequency results in lower ripple and a frequency decrease
gives more ripple. This is true for static frequencies or
dynamic frequency modulated systems. If the modulating
signal was a triangle wave, the regulator’s output would
have a ripple that is amplitude modulated by the triangle
wave. This repetitive signal on the power supply could
cause system problems by mixing with other desired
signals creating distortion. Depending on the inductor
design and triangle wave frequency, it may even result
in an audible noise. The LTC6908 uses a pseudorandom
noise-like signal. On an oscilloscope, it looks essentially
noise-like of even amplitude. The signal is broadband
and any mixing issues are eliminated. Additionally, the
pseudorandom signal repeats at such a low rate that it is
well below the audible range.
The LTC6908 directly drives many switching regulators. The
LTC6908 with the spread spectrum frequency modulation
results in improved EMC performance. If the bandwidth of
the switching regulator is sufficient, not a difficult require-
ment in most cases, the regulator’s regulation, efficiency
10
690812fa
Share Link: 