Datasheet

LTC3869/LTC3869-2
11
38692fa
For more information www.linear.com/LTC3869
operaTion
Main Control Loop
The LTC3869 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 LTC3869 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 LTC3869 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 LTC3869 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.