LTC3731H_15 Datasheet, PDF (10/34 Pages) Linear Technology – 3-Phase, 600kHz, Synchronous Buck Switching Regulator Controller

LTC3731H
Operation (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,
I₁, resets each 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 the error amplifier
EA. The EAIN pin receives a portion of output voltage
feedback signal through the external resistive divider and
is compared to the internal reference voltage. When the
load current increases, it causes a slight decrease in the
EAIN pin voltage relative to the 0.6V reference, 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 comparator I₂, or the beginning
of the next cycle.
The top MOSFET drivers are biased from floating bootstrap
capacitor C_B, which is normally recharged through an
external Schottky diode when the top FET is turned off.
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/SS
pin low. Releasing RUN/SS allows an internal 1.5µA
current source to charge soft-start capacitor C_SS. When
C_SS reaches 1.5V, the main control loop is enabled and
the internally buffered I_TH voltage is clamped but allowed
to ramp as the voltage on C_SS continues to ramp. This
“soft-start” clamping prevents abrupt current from being
drawn from the input power source. When the RUN/SS
pin is low, all functions are kept in a controlled state.
The RUN/SS pin is pulled low when the supply input
voltage is below 4V, when the undervoltage lockout pin
(UVADJ) is below 1.2V, or when the IC die temperature
rises above 160°C.
10
Low Current Operation
The FCB pin is a logic input to select between three modes
of operation.
A) Burst Mode 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 – 1.5V 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
synchronous MOSFET(s) when the inductor current goes
negative. 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 compara-
tor 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.
B) Stage Shedding Operation
When the FCB pin is tied to the V_CC pin, Burst Mode opera-
tion is disabled and the forced minimum inductor current
requirement is removed. This provides constant frequency,
discontinuous current operation over the widest possible
output current range. At approximately 10% of maximum
designed load current, the second and third output stages
are shut off and the phase 1 controller alone is active in
discontinuous current mode. This “stage shedding” opti-
mizes 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 stage shedding 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.
3731Hfb