LTC3543
8
3543fa
OPERATION
Main Control Loop
The LTC3543 uses current mode step-down architecture
with both the main (P-channel MOSFET) and synchronous
(N-channel MOSFET) switches internal. During normal
operation, the internal top power MOSFET is turned on
each cycle as the oscillator sets the RS latch, and turned
off when the current comparator, I
COMP
, resets the RS
latch. The peak inductor current at which I
COMP
resets
the RS latch, is controlled by the output of error ampli-
fi er, EA. When the load current increases, it causes a
slight decrease in the feedback voltage, V
FB
, relative to
an internal reference voltage 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
RCMP
, or the beginning
of the next clock cycle.
Burst Mode Operation
The LTC3543 is capable of Burst Mode operation in which
the internal power MOSFETs operate intermittently based
on load demand. Burst Mode operation is enabled by con-
necting the MODE pin to ground.
During Burst Mode operation, the LTC3543’s internal
circuits sense when the inductor peak current falls below
100mA. When below this level, the power MOSFETs and any
unneeded circuitry are turned off, reducing the quiescent
current to 45μA, and holding the peak current reference
level at 100mA. The LTC3543 remains in this sleep state
until the feedback voltage falls below its internal reference.
Once this occurs, the regulator wakes up and allows the
inductor to develop 100mA current pulses. In light loads,
this will cause the output voltage to increase and the internal
peak current reference to decrease. When the peak current
reference falls to below 100mA, the part re-enters sleep
mode and the cycle is repeated. This process repeats at
a rate dependent on the load demand.
Pulse Skip Mode Operation
Connecting the MODE pin to V
IN
places the LTC3543 in
pulse skip mode. During light loads, the inductor can
reach zero amps or reverse current on each pulse. This
is caused by the bottom MOSFET being turned off by the
current reversal comparator, I
RCMP
, at which time the switch
voltage will ring. This is discontinuous mode operation,
and is normal behavior for a switching regulator. At very
light loads, the LTC3543 will automatically skip pulses in
order to maintain output regulation.
Spread Spectrum Operation
Setting the MODE pin from 0.55V to 0.8V will place the
part in pulse skip mode with spread spectrum; an easy
way to do this is to connect the MODE pin to the V
FB
pin.
In this mode, an external capacitor is required between
CAP and GND. The external capacitor assists in smoothing
frequency transitions.
The spread spectrum architecture randomly varies the
LTC3543’s switching frequency from 2MHz to 3MHz,
signifi cantly reducing the peak radiated and conducting
noise on both the input and output supplies, making it
easier to comply with electromagnetic interference (EMI)
standards.
Switching regulators can be particularly troublesome in
applications where electromagnetic interference (EMI) is a
concern. Switching regulators operate on a cycle-by-cycle
basis to transfer power to an output. In most cases, the
frequency of operation is either fi xed or constant, based
on the output load. This method of conversion creates
large components of noise at the frequency of operation
(fundamental) and multiples of the operating frequency
(harmonics). Figure 1a depicts the output noise spectrum
of a conventional buck switching converter (LTC3543
with spread spectrum operation disabled) with V
IN
= 3.6V,
V
OUT
= 1.5V and I
OUT
= 300mA.
(Refer to Functional Diagram)
