LTC3545/LTC3545-1

10

35451fb

OPERATION

MAIN CONTROL LOOP

The LTC3545/LTC3545-1 use a constant frequency, current

mode step-down architecture. Both the main (P-channel

MOSFET) and synchronous (N-channel MOSFET) switches

are internal. During normal operation, the internal top power

MOSFET is turned on each cycle when the oscillator sets

the RS latch, and turned off when the current comparator,

of error amplifier EA. When the load current increases, it

causes a slight decrease in the feedback voltage FB rela-

tive to the 0.6V reference, which in turn, causes the EA

amplifier’s output voltage to increase until the average

inductor current matches the new load current. While the

top MOSFET is off, the bottom MOSFET is turned on until

either the inductor current starts to reverse, as indicated by

of the next clock cycle.

PULSE SKIPPING/Burst Mode OPERATION

At light loads, the inductor current may reach zero or

reverse on each pulse. The bottom MOSFET is turned off

voltage will ring. This is discontinuous mode operation,

and is normal behavior for the switching regulator.

At very light loads, the LTC3545/LTC3545-1 will automati-

cally begin operating in either pulse skipping or Burst Mode

operation depending on the state of the MODE/SYNC pin

(LTC3545). In either case the part will begin to skip cycles

in order to maintain regulation.

In pulse skip mode, the current pulses are smaller and

more frequent, giving lower output ripple. In this mode,

internal circuitry remains on and the pulses occur more

frequently resulting in lower efficiency than in Burst Mode

operation at light loads.

In Burst Mode operation, the part supplies fewer, larger

current pulses, resulting in higher output ripple, but much

higher light load efficiency than pulse skip mode. Efficiency

is also improved by turning off much of the internal circuitry

during the dead time between pulses.