Operation - Linear LTC3875 Datasheet

LTC3875

OPERATION

Main Control Loop
The LTC3875 is a constant frequency, current mode step-
down controller with two channels operating 180° or 240°
out of phase. During normal operation, each top MOSFET
is turned on when the clock for that channel sets the R
latch, and turned off when the main current comparator,
I , resets the R latch. The peak inductor current at
CMP S
which I resets the R latch is controlled by the voltage
CMP S
on the I pin, which is the output of each error amplifier
TH
EA. The remote sense amplifier (DIFFAMP) converts the
sensed differential voltage across the output feedback
resistor divider to an internal voltage (V
SGND. The V signal is then compared to the internal
FB
0.6V reference voltage by the EA. When the load current
increases, it causes a slight decrease in V
the 0.6V reference, which in turn causes the I
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 cur-
rent comparator, I , or the beginning of the next cycle.
REV
INTV /EXTV Power
CC CC
Power for the top and bottom MOSFET drivers and most
other internal circuitry is derived from the INTV
When the EXTV pin is left open or tied to a voltage less
CC
than 4.5V, an internal 5.5V linear regulator supplies INTV
power from V . If EXTV
IN CC
regulator is turned off and an internal switch is turned on
connecting EXTV to INTV
CC
voltage has to be higher than EXTV
has to come before EXTV
CC
current will flow back to V
body diode and potentially damage the device. Using the
EXTV pin allows the INTV
CC
a high efficiency external source.
Each top MOSFET driver is biased from the floating
bootstrap capacitor, C , which normally recharges dur-
B
ing each off cycle through an external diode when the top
MOSFET turns off. If the input voltage, V
a voltage close to V , the loop may enter dropout and
OUT
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
12
) referred to
FB
relative to
FB
voltage
TH
CC
is taken above 4.7V, the 5.5V
. When using EXTV , the V
CC CC
voltage at all time and
CC
is applied. Otherwise, EXTV
through the internal switch's
IN
power to be derived from
CC
, decreases to
IN
For more information
every third cycle to allow C
recommended that a load be present or the IC operates
at low frequency during the drop-out transition to ensure
that C is recharged.
B
S
Shutdown and Start-Up
(RUN1, RUN2 and TK/SS1, TK/SS2 Pins)
The two channels of the LTC3875 can be independently
shut down using the RUN1 and RUN2 pins. Pulling either
of these pins below 1.14V shuts down the main control
loop for that channel. Pulling both pins low disables both
channels and most internal circuits, including the INTV
regulator. Releasing either RUN pin allows an internal
1µA current to pull up the pin and enable the controller.
Alternatively, the RUN pins may be externally pulled up
or driven directly by logic. Be careful not to exceed the
absolute maximum rating of 6V on these pins.
The start-up of each channel's output voltage, V
controlled by the voltage on its TK/SS pin. When the
voltage on the TK/SS pin is less than the 0.6V internal
reference, the LTC3875 regulates the V
TK/SS pin voltage instead of the 0.6V reference. This al-
lows the TK/SS pin to be used to program the soft-start
period by connecting an external capacitor from the TK/SS
pin. 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
CC
beyond), the output voltage V
to its final value. Alternatively the TK/SS pin can be used
to cause the start-up of V
IN
supply. Typically, this requires connecting to the TK/SS
pin an external resistor divider from the other supply to
CC
ground (see the Applications Information section). When
the corresponding RUN pin is pulled low to disable a
controller, or when INTV
lockout threshold of 3.7V, 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.
Internal Soft-Start
By default, the start-up of the output voltage is normally
controlled by an internal soft-start ramp. The internal
soft-start ramp represents one of the noninverting inputs
www.linear.com/LTC3875
to recharge. However, it is
B
voltage to the
FB
rises smoothly from zero
OUT
to "track" that of another
OUT
drops below its undervoltage
CC
CC
, is
OUT
3875fb