LTC3865 Datasheet, PDF (12/36 Pages) Linear Technology – Dual, 2-Phase Synchronous DC/DC Controller with Pin Selectable Outputs

LTC3865/LTC3865-1
Operation
Main Control Loop
The LTC3865/LTC3865-1 are constant-frequency, current
mode step-down controllers 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
amplifier, EA (refer to the Functional Diagram). V_FB is the
voltage feedback signal, which is compared to the internal
reference voltage by the EA. When the load current increas-
es, it causes a slight decrease in feedback voltage 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 cur-
rent 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 LTC3865/LTC3865-1 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
detector detects this and forces the top MOSFET off for
about one-twelfth of the clock period every third cycle to
allow C_B to recharge. However, it is recommended that a
load be present 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 LTC3865/LTC3865-1 can be in-
dependently 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 LTC3865 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 1.3µA 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 0.6V (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 sup-
ply 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 3.3V, 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 LTC3865/LTC3865-1 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
3865f
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