11
LTC3733/LTC3733-1
3733f
OPERATIO
U
(Refer to Functional Diagram)
Main Control Loop
The IC uses a constant frequency, current mode step-
down architecture. During normal operation, each top
MOSFET is turned on each cycle when the oscillator sets
the RS latch, and turned off when the main current
comparator, I
1
, resets each RS latch. The peak inductor
current at which I
1
resets the RS latch is controlled by the
voltage on the I
TH
pin, which is the output of the error
amplifier EA. The EAIN pin receives a portion of the voltage
feedback signal via the DIFFOUT pin through the internal
VID DAC and is compared to the internal reference voltage.
When the load current increases, it causes a slight de-
crease in the EAIN pin voltage
relative to the 0.6V refer-
ence, which in turn causes the I
TH
voltage to increase until
each inductor’s average current matches one third of the
new load current (assuming all three current sensing
resistors are equal). In Burst Mode operation and stage
shedding mode, after each top MOSFET has turned off, the
bottom MOSFET is turned on until either the inductor
current starts to reverse, as indicated by current compara-
tor I
2
, or the beginning of the next cycle.
The top MOSFET drivers are biased from floating boot-
strap capacitor C
B
, which is normally recharged during
each off cycle through an external Schottky diode. When
V
IN
decreases to a voltage close to V
OUT
, however, the
loop may enter dropout and attempt to turn on the top
MOSFET continuously. The dropout detector counts the
number of oscillator cycles that the bottom MOSFET
remains off and periodically forces a brief on period to
allow C
B
to recharge.
The main control loop is shut down by pulling the RUN pin
low. Releasing RUN allows an internal 1.5µA current
source to charge soft-start capacitor C
SS
at the SS pin. The
internal I
TH
voltage is then clamped to the SS voltage when
C
SS
is slowly charged up. This “soft-start” clamping
prevents abrupt current from being drawn from the input
power source. When the RUN pin is low, all functions are
kept in a controlled state.
Low Current Operation
The FCB pin is a multifunction pin: 1) an analog compara-
tor input to provide regulation for a secondary winding by
forcing temporary forced PWM operation and 2) a logic
input to select between three modes of operation.
When the FCB pin voltage is below 0.6V, the controller
performs as a continuous, PWM current mode synchro-
nous switching regulator. The top and bottom MOSFETs
are alternately turned on to maintain the output voltage
independent of direction of inductor current. When the
FCB pin is below V
CC
–␣ 1V but greater than 0.6V, the
controller performs as a Burst Mode switching regulator.
Burst Mode operation sets a minimum output current level
before turning off the top switch and turns off the synchro-
nous MOSFET(s) when the inductor current goes nega-
tive. This combination of requirements will, at low current,
force the I
TH
pin below a voltage threshold that will
temporarily shut off both output MOSFETs until the output
voltage drops slightly. There is a burst comparator having
60mV of hysteresis tied to the I
TH
pin. This hysteresis
results in output signals to the MOSFETs that turn them on
for several cycles, followed by a variable “sleep” interval
depending upon the load current. The resultant output
voltage ripple is held to a very small value by having the
hysteretic comparator after the error amplifier gain block.
When the FCB pin is tied to the V
CC
pin, Burst Mode
operation is disabled and the forced minimum inductor
current requirement is removed. This provides constant
frequency, discontinuous current operation over the wid-
est possible output current range. At approximately 10%
of maximum designed load current, the second and third
output stages are shut off and the first controller alone is
active in discontinuous current mode. This “stage shed-
ding” optimizes efficiency by eliminating the gate charging
losses and switching losses of the other two output
stages. Additional cycles will be skipped when the output
load current drops below 1% of maximum designed load
current in order to maintain the output voltage. This
constant frequency operation is not as efficient as Burst
Mode operation at very light loads, but does provide lower
noise, constant frequency operating mode down to very
light load conditions.
Tying the FCB pin to ground will force continuous current
operation. This is the least efficient operating mode, but
may be desirable in certain applications. The output can
