Datasheet LTC3545, LTC3545-1 (Analog Devices) - 10

LTC3545/LTC3545-1
OPERATION MAIN CONTROL LOOP PULSE SKIPPING/Burst Mode OPERATION
The LTC3545/LTC3545-1 use a constant frequency, current At light loads, the inductor current may reach zero or mode step-down architecture. Both the main (P-channel reverse on each pulse. The bottom MOSFET is turned off MOSFET) and synchronous (N-channel MOSFET) switches by the current reversal comparator, IRCMP , and the switch are internal. During normal operation, the internal top power voltage will ring. This is discontinuous mode operation, MOSFET is turned on each cycle when the oscillator sets and is normal behavior for the switching regulator. the RS latch, and turned off when the current comparator, At very light loads, the LTC3545/LTC3545-1 will automati- ICOMP , resets the RS latch. The peak inductor current at cally begin operating in either pulse skipping or Burst Mode which ICOMP resets the RS latch, is controlled by the output operation depending on the state of the MODE/SYNC pin of error amplifi er EA. When the load current increases, it (LTC3545). In either case the part will begin to skip cycles causes a slight decrease in the feedback voltage FB rela- in order to maintain regulation. tive to the 0.6V reference, which in turn, causes the EA amplifi er’s output voltage to increase until the average In pulse skip mode, the current pulses are smaller and inductor current matches the new load current. While the more frequent, giving lower output ripple. In this mode, top MOSFET is off, the bottom MOSFET is turned on until internal circuitry remains on and the pulses occur more either the inductor current starts to reverse, as indicated by frequently resulting in lower effi ciency than in Burst Mode the current reversal comparator, I operation at light loads. RCMP , or the beginning of the next clock cycle. In Burst Mode operation, the part supplies fewer, larger current pulses, resulting in higher output ripple, but much higher light load effi ciency than pulse skip mode. Effi ciency is also improved by turning off much of the internal circuitry during the dead time between pulses. 35451fb 10