LTC3867
11
3867f
OPERATION
The V
FB
pin receives this feedback signal and compares
it to the internal 0.6V reference. When the load current
increases, it causes a slight decrease in the V
FB
pin voltage
relative to the 0.6V reference, which in turn causes the
I
TH
voltage to increase until the inductor’s average current
equals 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.
The main control loop is shut down by pulling the RUN
pin low. Releasing RUN allows an internal 1.0µA current
source to pull up the RUN pin. When the RUN pin reaches
1.22V, the main control loop is enabled and the IC is pow-
ered up. When the RUN pin is low, all functions are kept
in a controlled state.
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 5.3V linear regulator supplies INTV
CC
power from V
IN
. If EXTV
CC
is taken above 4.7V, the 5.3V
regulator is turned off and an internal switch is turned on
connecting EXTV
CC
to INTV
CC
. Using the EXTV
CC
pin al-
lows the INTV
CC
power to be derived from a high efficiency
external source such as a switching regulator output. The
top MOSFET driver is biased from the floating bootstrap
capacitor, C
B
, which normally recharges during the 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 50ns every fifth
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 dropout transition to ensure C
B
is recharged.
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 a noninverting input to the
error amplifier. The FB pin is regulated to the lower of the
error amplifier’s three noninverting inputs (the internal
soft-start ramp, the TK/SS pin or the internal 600mV ref-
erence). As the ramp voltage rises from 0V to 0.6V over
approximately 600µs, the output voltage rises smoothly
from its prebiased value to its final set value.
Certain applications can result in the start-up of the con-
verter into a non-zero load voltage, where residual charge
is stored on the output capacitor at the onset of converter
switching. In order to prevent the output from discharging
under these conditions, the bottom MOSFET is disabled
until soft-start is greater than V
FB
.
Shutdown and Start-Up (RUN and TK/SS Pins)
The LTC3867 can be shut down using the RUN pin. Pulling
the RUN pin below 1.22V shuts down the main control loop
for the controller and most internal circuits, including the
INTV
CC
regulator. Releasing the RUN pin allows an internal
1.0µA current to pull up the pin and enable the controller.
Alternatively, the RUN pin may be externally pulled up or
driven directly by logic. Be careful not to exceed the ab-
solute maximum rating of 6V on this pin. The start-up of
the controller’s output voltage, V
OUT
, is controlled by the
voltage on the TK/SS pin. When the voltage on the TK/SS
pin is less than the 0.6V internal reference, the LTC3867
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 1.25µA 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 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 RUN pin is pulled low to disable the
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, the
controller is disabled and the external MOSFETs are held off.
