LTC6908-1/LTC6908-2

10

Rev C

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to the modulation rate (fOUT/N), where N is the modula-

tion rate divider setting, which is determined by the state

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 accu-

rate 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 nomenclature varies depending on the part’s fam-

ily 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 syn-

chronization 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 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 pseu-

dorandom signal repeats at such a low rate that it is well

below the audible range.

The LTC6908 directly drives many switching regula-

tors. The LTC6908 with the spread spectrum frequency

modulation results in improved EMC performance. If

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