LTC3545 LINER [Linear Technology], LTC3545 Datasheet - Page 10

• Manufacturer Part Number: LTC3545
• Description: Triple 800mA Synchronous Step-Down Regulator-2.25MHz
• Manufacturer: LINER [Linear Technology]

LTC3545/LTC3545-1
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,
I
which I
of error amplifi er 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
amplifi er’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
the current reversal comparator, I
of the next clock cycle.
10
COMP
, resets the RS latch. The peak inductor current at
COMP
resets the RS latch, is controlled by the output
RCMP
, or the beginning
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
by the current reversal comparator, I
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 effi ciency 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 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.
RCMP
, and the switch
35451fb