LTC3728_15 Datasheet, PDF (11/36 Pages) Linear Technology – Dual, 550kHz, 2-Phase Synchronous Step-Down Switching Regulator

LTC3728
OPERATION (Refer to Functional Diagram)
Main Control Loop
The LTC3728 uses a constant-frequency, current mode
step-down architecture with the two controller channels
operating 180 degrees out of phase. During normal opera-
tion, each top MOSFET is turned on when the clock for
that channel sets the RS latch, and turned off when the
main current comparator, I1, resets the RS latch. The peak
inductor current at which I1 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 I2,
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
current 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 LTC3728 controller 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) 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 – 1V 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 250kHz 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.
Constant-Frequency Operation
When the FCB pin is tied to INTV_CC, Burst Mode opera-
tion is disabled and the forced minimum output current
requirement is removed. This provides constant-frequency,
discontinuous (preventing reverse inductor current)
current operation over the widest possible output current
range. This constant-frequency operation is not as efficient
as Burst Mode operation, but does provide a lower noise,
constant-frequency operating mode down to approximately
1% of designed maximum output current.
3728fg
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