LTC3855_15 Datasheet, PDF (12/44 Pages) Linear Technology – Dual, Multiphase Synchronous DC/DC Controller with Differential Remote Sense

LTC3855
Operation
Main Control Loop
The LTC3855 is a constant-frequency, current mode step-
down controller with two 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_CMP, resets the RS latch. The peak inductor current at
which I_CMP resets the RS latch is controlled by the voltage
on the I_TH pin, which is the output of each error ampli-
fier EA. The V_FB pin receives the voltage feedback signal,
which is compared to the internal reference voltage by the
EA. When the load current increases, it causes a slight
decrease in V_FB relative to the 0.6V 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 the reverse current comparator I_REV, or the
beginning of the next cycle.
INTV_CC/EXTV_CC Power
Power for the top and bottom MOSFET drivers and most
other internal circuitry is derived from the INTV_CC pin. When
the EXTV_CC pin is left open or tied to a voltage less than
4.7V, an internal 5V linear regulator supplies INTV_CC power
from V_IN. If EXTV_CC is taken above 4.7V, the 5V regulator is
turned off and an internal switch is turned on connecting
EXTV_CC. Using the EXTV_CC pin allows the INTV_CC power
to be derived from a high efficiency external source such
as one of the LTC3855 switching regulator outputs.
Each top MOSFET driver is biased from the floating
bootstrap capacitor C_B, which normally recharges during
each off cycle through an external diode when the top
MOSFET turns off. If the input voltage 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 one-twelfth of the clock period plus 100ns
every third cycle to allow C_B to recharge. However, it is
recommended that a load be present or the IC operates
at low frequency during the drop-out transition to ensure
C_B is recharged.
Shutdown and Start-Up (RUN1, RUN2 and TK/SS1,
TK/SS2 Pins)
The two channels of the LTC3855 can be independently
shut down using the RUN1 and RUN2 pins. Pulling either
of these pins below 1.2V shuts down the main control
loop for that controller. Pulling both pins low disables both
controllers and most internal circuits, including the INTV_CC
regulator. Releasing either RUN pin allows an internal
1µA current to pull up the pin and enable that controller.
Alternatively, the RUN pin may be externally pulled up
or driven directly by logic. Be careful not to exceed the
Absolute Maximum Rating of 6V on this pin.
The start-up of each controller’s output voltage V_OUT is
controlled by the voltage on the TK/SS1 and TK/SS2 pins.
When the voltage on the TK/SS pin is less than the 0.6V
internal reference, the LTC3855 regulates the V_FB voltage
to the TK/SS pin voltage instead of the 0.6V reference. This
allows the TK/SS pin to be used to program the soft-start
period by connecting an external capacitor from the TK/SS
pin to SGND. An internal 1.2µA pull-up current charges
this capacitor, creating a voltage ramp on the TK/SS pin.
As the TK/SS voltage rises linearly from 0V to 0.6V (and
beyond), the output voltage V_OUT rises smoothly from zero
to its final value. Alternatively the TK/SS pin can be used
to cause the start-up of V_OUT to “track” that of another
supply. Typically, this requires connecting to the TK/SS
pin an external resistor divider from the other supply to
ground (see the Applications Information section). When
the corresponding RUN pin is pulled low to disable a
controller, or when INTV_CC drops below its undervoltage
lockout threshold of 3.2V, the TK/SS pin is pulled low by
an internal MOSFET. When in undervoltage lockout, both
controllers are disabled and the external MOSFETs are
held off.
Light Load Current Operation (Burst Mode Operation,
Pulse-Skipping, or Continuous Conduction)
The LTC3855 can be enabled to enter high efficiency Burst
Mode operation, constant-frequency pulse-skipping mode,
or forced continuous conduction mode. To select forced
continuous operation, tie the MODE/PLLIN pin to a DC
3855f
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