LTC6908-1/LTC6908-2 APPLICATIONS INFORMATION to the modulation rate (fOUT/N), where N is the modula- the modulation rate divider setting, which is determined tion rate divider setting, which is determined by the state by the state of the MOD pin. The MOD pin should be tied of the MOD pin. This filtered modulating signal may be to ground for the N = 16 setting. Floating the MOD pin acceptable for many logic systems but the cycle-to-cycle selects N = 32. The MOD pin should be tied to V+ for the jitter issues must be considered carefully. N = 64 setting. This is an important feature when driving a switching regulator. The switching regulator is itself a DRIVING SWITCHING REGULATORS 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. The LTC6908 is designed primarily to provide an accu- When the clock frequency’s transition is within the band- rate and stable clock for switching regulator systems. The width of the switching regulator, the regulator’s output complementary (LTC6908-1) or quadrature (LTC6908- stays in regulation. If the transition is too sharp, beyond 2) CMOS logic outputs are suitable for directly driving the bandwidth of the switching regulator, the regulator’s most switching regulators and switching controllers. output will experience a sharp jump and then settle back Linear Technology has a broad line of fully integrated into regulation. If the bandwidth of the regulator is suf- switching regulators and switching regulator controllers ficiently high, beyond f designed for synchronization to an external clock. All of OUT/N, then there will not be any regulation issues. these parts have one pin assigned for external clock input. The nomenclature varies depending on the part’s fam- One aspect of the output voltage that will change is the ily history. SYNC, PLLIN, SYNC/MODE, SHDN, EXTCLK, output ripple voltage. Every switching regulator has some FCB and S/S (shorthand for SYNC/SHDN) are examples of output ripple at the clock frequency. For most switching clock input pin names used with Linear Technology ICs. regulator designs with fixed MOSFET’s, fixed inductor, fixed capacitors, the amount of ripple will vary some with For the best EMC performance, the LTC6908 should be the regulators operating frequency (the main exception run with the MOD pin tied to ground (SSFM enabled, being hysteretic architecture regulators). An increase in modulation rate set to fOUT/16). Regulatory testing is frequency results in lower ripple and a frequency decrease done with strictly specified bandwidths and conditions. gives more ripple. This is true for static frequencies or Modulating faster than the test bandwidth or as close to dynamic frequency modulated systems. If the modulating the bandwidth as possible gives the lowest readings. The signal was a triangle wave, the regulator’s output would optimal modulating rate is not as straightforward when have a ripple that is amplitude modulated by the triangle the goal is to lower radiated signal levels interfering with wave. This repetitive signal on the power supply could other circuitry in the system. The modulation rate will cause system problems by mixing with other desired have to be evaluated with the specific system conditions signals creating distortion. Depending on the inductor to determine the optimal rate. Depending on the specific design and triangle wave frequency, it may even result frequency synchronization method a switching regulator in an audible noise. The LTC6908 uses a pseudorandom employs, the modulation rate must be within the syn- noise-like signal. On an oscilloscope, it looks essentially chronization capability of the regulator. Many regulators noise-like of even amplitude. The signal is broadband and use a phase-locked loop (PLL) for synchronization. For any mixing issues are eliminated. Additionally, the pseu- these parts, the PLL loop filter should be designed to have dorandom signal repeats at such a low rate that it is well sufficient capture range and bandwidth. below the audible range. The frequency hopping transitions of the LTC6908 are The LTC6908 directly drives many switching regula- slowed by a lowpass filter. The corner frequency of this tors. The LTC6908 with the spread spectrum frequency filter is set to the modulation rate (fOUT/N), where N is modulation results in improved EMC performance. If Rev B 10 For more information www.analog.com Document Outline Features Applications Description Absolute Maximum Ratings Electrical Characteristics Pin Functions Operation Applications Information Typical Applications Package Description Typical Application
