LTC3856 Datasheet, PDF (12/40 Pages) Linear Technology – 2-Phase Synchronous Step-Down DC/DC Controller with Diffamp

LTC3856
Operation (Refer to Functional Diagram)
Main Control Loop
The LTC3856 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_CMP , resets each 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 the
error amplifier, EA. The V_FB pin receives a portion of
output voltage feedback signal via the DIFFOUT pin (if
DIFFAMP is used) 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 V_FB 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 half of the new load
current (assuming the two current sensing resistors are
equal). In Burst Mode operation, after each 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.
The main control loop is shut down by pulling the RUN pin
low. Releasing RUN allows an internal 1µA current source
to pull up the RUN pin. When the RUN pin reaches 1.22V,
the main control loop is enabled and the IC is powered
up. When the RUN pin is low, all functions are kept in a
controlled state.
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 a switching regulator output. 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
dropout transition to ensure C_B is recharged.
Shutdown and Start-Up (RUN and TK/SS Pins)
The LTC3856 can be shut down using the RUN pin. Pulling
the RUN pin below 1.22V shuts down the main control loop
for the controller and most internal circuits, including the
INTV_CC regulator. Releasing the RUN pin allows an internal
1µA current to pull up the pin and enable the controller.
Alternatively, the RUN pin may be externally pulled up or
driven directly by logic. Be careful not to exceed the ab-
solute maximum rating of 6V on this pin. The start-up of
the controller’s output voltage, V_OUT , is controlled by the
voltage on the TK/SS pin. When the voltage on the TK/SS
pin is less than the 0.6V internal reference, the LTC3856
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 a soft-start by connecting an external
capacitor from the TK/SS pin to SGND. An internal 1.25µA
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 RUN pin is pulled low to
disable the 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, all phases of the controller are disabled and the
external MOSFETs are held off.
3856f
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