LTC3728L/LTC3728LX
12
3728lxff
operaTion
Main Control Loop
The IC uses a constant-frequency, current mode step-down
architecture with the two controller channels operating
180 degrees out of phase. During normal operation, each
top MOSFET is turned on when the clock for that channel
sets the RS latch, and turned off when the main current
comparator, I
1
, resets the 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 each
error amplifier EA. The V
OSENSE
pin receives the voltage
feedback signal, which is compared to the internal refer-
ence voltage by the EA. When the load current increases,
it causes a slight decrease in V
OSENSE
relative to the 0.8V
reference, which in turn causes the I
TH
voltage to increase
until the average inductor current matches the new load
current. After the top MOSFET has turned off, the bottom
MOSFET is turned on until either the inductor current
starts to reverse, as indicated by current comparator I
2
,
or the beginning of the next cycle.
The top MOSFET drivers are biased from floating bootstrap
capacitor C
B
, which normally is recharged during each off
cycle through an external diode when the top MOSFET
turns off. As V
IN
decreases to a voltage close to V
OUT
,
the loop may enter dropout and attempt to turn on the
top MOSFET continuously. The dropout detector detects
this and forces the top MOSFET off for about 400ns every
tenth cycle to allow C
B
to recharge.
The main control loop is shut down by pulling the RUN/SS
pin low. Releasing RUN/SS allows an internal 1.2µA cur-
rent source to charge soft-start capacitor C
SS
. When C
SS
reaches 1.5V, the main control loop is enabled with the I
TH
voltage clamped at approximately 30% of its maximum
value. As C
SS
continues to charge, the I
TH
pin voltage is
gradually released allowing normal, full-current operation.
When both RUN/SS1 and RUN/SS2 are low, all control-
ler functions are shut down, including the 5V and 3.3V
regulators.
Low Current Operation
The FCB pin is a multifunction pin providing two func-
tions: 1) to provide regulation for a secondary winding by
temporarily forcing continuous PWM operation on both
controllers; and 2) to select between
two
modes of low
current operation. When the FCB pin voltage is below
0.8V, the controller forces continuous PWM current mode
operation. In this mode, 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
INTVCC
– 2V but greater than 0.8V,
the controller enters Burst Mode operation. Burst Mode
operation sets a minimum output current level before
inhibiting the top switch and turns off the synchronous
MOSFET(s) when the inductor current goes negative. This
combination of requirements will, at low currents, force
the I
TH
pin below a voltage threshold that will temporarily
inhibit turn-on of both output MOSFETs until the output
voltage drops. There is 60mV of hysteresis in the burst
comparator B tied to the I
TH
pin. This hysteresis produces
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.
Frequency Synchronization
The phase-locked loop allows the internal oscillator to
be synchronized to an external source via the PLLIN pin.
The output of the phase detector at the PLLFLTR pin is
also the DC frequency control input of the oscillator that
operates over a 260kHz to 550kHz range corresponding
to a DC voltage input from 0V to 2.4V. When locked, the
PLL aligns the turn on of the top MOSFET to the rising
edge of the synchronizing signal. When PLLIN is left
open, the PLLFLTR pin goes low, forcing the oscillator to
minimum frequency.
(Refer to Functional Diagram)
