LTC3853_15 Datasheet, PDF (11/36 Pages) Linear Technology – Triple Output, Multiphase Synchronous Step-Down Controller

LTC3853
OPERATION
Main Control Loop
The LTC3853 is a constant-frequency, current mode
step-down controller with three channels operating 120
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
amplifier, 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 0.8V 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 the reverse current comparator I_REV , or the
beginning of the next cycle.
INTV_CC/EXTV_CC/DRV_CC12 Power
Power for the top and bottom MOSFET drivers of phase 3
and most other internal circuitry is derived from the
INTV_CC pin. DRV_CC12 provides driver power for phase 1
and phase 2. This pin must be externally tied to INTV_CC.
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 de-
rived from a high efficiency external source such as one
of the LTC3853 switching regulator outputs.
Each top MOSFET driver is biased from the floating boot-
strap 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 detec-
tor detects this and forces the top MOSFET off for about
one-twelfth of the clock period every fifth cycle to allow
C_B to recharge. However, it is recommended that there is
always a load be present during the dropout transition to
ensure C_B is recharged.
Shutdown and Start-Up (RUN1, RUN2, RUN3 and TK/
SS1, TK/SS2, TK/SS3 Pins)
The three channels of the LTC3853 can be independently
shut down using the RUN1, RUN2 and RUN3 pins. Pulling
any of these pins below 1.2V shuts down the main control
loop for that controller. Pulling all pins low disables all
three controllers and most internal circuits, including the
INTV_CC regulator. Releasing any RUN pin allows an internal
0.5µA 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, TK/SS2 and
TK/SS3 pins. When the voltage on the TK/SS pin is less
than the 0.8V internal reference, the LTC3853 regulates
the V_FB voltage to the TK/SS pin voltage instead of the
0.8V 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 1.3µA 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 0.8V (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 connect-
ing 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 3.35V, the TK/SS pin
is pulled low by an internal MOSFET. When in undervolt-
age lockout, all controllers are disabled and the external
MOSFETs are held off.
Light Load Current Operation (Burst Mode Operation,
Pulse Skipping or Continuous Conduction)
The LTC3853 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
voltage below 0.8V (e.g., SGND). To select pulse skipping
3853fc
For more information www.linear.com/LTC3853
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