LTC6909

13

6909f

pseudorandom modulating signal where the frequency

transitions have been slowed and the corners rounded by

a first order lowpass filter with a corner frequency set to

rate divider setting, which is determined by the state of

the MOD pin. This filtered modulating signal may be ac-

ceptable for many logic systems but the cycle-to-cycle

jitter issues must be considered carefully.

DRIVING SWITCHING REGULATORS

The LTC6909 is designed primarily to provide an accurate

and stable clock for switching regulator systems. The

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

nomenclature varies depending on the part’s family his-

tory. SYNC, PLLIN, SYNC/MODE, 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 LTC6909 should be run

with the MOD pin tied to ground (SSFM enabled, modula-

strictly specified bandwidths and conditions. Modulating

faster than, or as close to, the test 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 synchronization 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 sufficient capture range and

bandwidth.

The frequency hopping transitions of the LTC6909 are

slowed by a lowpass filter. The corner frequency of this

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

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 to 1/20 of the operating frequency. When the clock

frequency’s transition is within the bandwidth of the switch-

ing 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 sufficiently high, beyond

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 with the

regulator’s operating frequency (the main exception be-

ing 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 switching

regulator’s inductor design and triangle wave frequency,

it may even result in an audible noise. The LTC6909 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. Ad-

ditionally, the pseudorandom signal repeats at such a low

rate that it is well below the audible range.

The LTC6909 with the spread spectrum frequency modu-

lation enabled results in improved EMC performance. If

the bandwidth of the switching regulator is sufficient,

not a difficult requirement in most cases, the regulator’s

regulation, efficiency and load response are maintained

APPLICATIONS INFORMATION