LTC3865/LTC3865-1
12
3865fb
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 amplifi er, 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
increases, 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 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 effi ciency external
source such as one of the LTC3865/LTC3865-1 switching
regulator outputs.
Each top MOSFET driver is biased from the fl oating
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 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
independently shut down using the RUN1 and RUN2 pins.
Pulling either of these pins below 1.22V 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 fi nal 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
effi ciency Burst Mode operation, constant-frequency pulse-
skipping mode, or forced continuous conduction mode. To
select forced continuous operation, tie the MODE/PLLIN
