Datasheet

LTC3827
11
3827ff
OPERATION
Each top MOSFET driver is biased from the fl oating 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 tenth cycle to
allow C
B
to recharge.
Shutdown and Start-Up (RUN1, RUN2 and
TRACK/SS1, TRACK/SS2 Pins)
The two channels of the LTC3827 can be independently
shut down using the RUN1 and RUN2 pins. Pulling either
of these pins below 0.7V 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, and the LTC3827 draws only 8µA of
quiescent current.
Releasing either RUN pin allows an internal 0.5µA 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 7V on this pin.
The start-up of each controller’s output voltage V
OUT
is
controlled by the voltage on the TRACK/SS1 and TRACK/
SS2 pin. When the voltage on the TRACK/SS pin is less
than the 0.8V internal reference, the LTC3827 regulates
the V
FB
voltage to the TRACK/SS pin voltage instead of the
0.8V reference. This allows the TRACK/SS pin to be used
to program a soft-start by connecting an external capacitor
from the TRACK/SS pin to SGND. An internal 1µA pull-up
current charges this capacitor creating a voltage ramp on
the TRACK/SS pin. As the TRACK/SS voltage rises linearly
from 0V to 0.8V (and beyond), the output voltage V
OUT
rises smoothly from zero to its fi nal value.
Alternatively the TRACK/SS pin can be used to cause the
start-up of V
OUT
to “track” that of another supply. Typi-
cally, this requires connecting to the TRACK/SS pin an
(Refer to Functional Diagram)
Main Control Loop
The LTC3827 uses a constant frequency, current mode
step-down architecture with the two controller channels
operating 180 degrees out of phase. During normal op-
eration, each external top MOSFET is turned on when the
clock for that channel sets the RS latch, and is turned off
when the main current comparator, I
CMP
, resets the RS
latch. The peak inductor current at which I
CMP
trips and
resets the latch is controlled by the voltage on the I
TH
pin,
which is the output of the error amplifi er EA. The error
amplifi er compares the output voltage feedback signal at
the V
FB
pin, (which is generated with an external resis-
tor divider connected across the output voltage, V
OUT
, to
ground) to the internal 0.800V reference voltage. When the
load current increases, it causes a slight decrease in V
FB
relative to the reference, which causes the EA to increase
the I
TH
voltage until the average inductor current matches
the new load current.
After the top MOSFET is turned off each cycle, the bottom
MOSFET is turned on until either the inductor current starts
to reverse, as indicated by the current comparator IR, or
the beginning of the next clock 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 5.25V low dropout linear regulator
supplies INTV
CC
power from V
IN
. If EXTV
CC
is taken above
4.7V, the 5.25V regulator is turned off and a 7.5V low
dropout linear regulator is enabled that supplies INTV
CC
power from EXTV
CC
. If EXTV
CC
is less than 7.5V (but
greater than 4.7V), the 7.5V regulator is in dropout and
INTV
CC
is approximately equal to EXTV
CC
. When EXTV
CC
is greater than 7.5V (up to an absolute maximum rating
of 10V), INTV
CC
is regulated to 7.5V. 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 LTC3827
switching regulator outputs.