ltc3869gn-2 Linear Technology Corporation, ltc3869gn-2 Datasheet - Page 10

• Manufacturer Part Number: ltc3869gn-2
• Description: Ltc3869/ltc3869-2 - Dual, 2-phase Synchronous Step-down Dc/dc Controllers
• Manufacturer: Linear Technology Corporation

OPERATION
LTC3869/LTC3869-2
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
which I
on the I
fier EA. The V
which is compared to the internal reference voltage by the
EA. When the load current increases, it causes a slight
decrease in V
turn causes the I
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
beginning of the next cycle.
INTV
Power for the top and bottom MOSFET drivers and most
other internal circuitry is derived from the INTV
the EXTV
4.7V, an internal 5V linear regulator supplies INTV
from V
turned off and an internal switch is turned on connecting
EXTV
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 boot-
strap capacitor C
off cycle through an external diode when the top MOSFET
turns off. If the input voltage V
close to V
turn on the top MOSFET continuously. The dropout detec-
tor detects this and forces the top MOSFET off for about
one-twelfth of the clock period plus 100ns every third
10
CMP
, resets the RS latch. The peak inductor current at
CC
CC
IN
CMP
/EXTV
. Using the EXTV
TH
. If EXTV
CC
OUT
pin, which is the output of each error ampli-
resets the RS latch is controlled by the voltage
pin is left open or tied to a voltage less than
CC
, the loop may enter dropout and attempt to
FB
FB
Power
pin receives the voltage feedback signal,
CC
relative to the 0.6V reference, which in
B
TH
, which normally recharges during each
is taken above 4.7V, the 5V regulator is
voltage to increase until the average
CC
pin allows the INTV
IN
decreases to a voltage
CC
REV
pin. When
CC
CC
, or the
power
power
cycle to allow C
that a load be present or the IC operates at low frequency
during the drop-out transition to ensure C
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
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
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
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
to its final value. Alternatively the TK/SS pin can be used
to cause the start-up of V
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
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.
B
to recharge. However, it is recommended
CC
OUT
drops below its undervoltage
OUT
to “track” that of another
rises smoothly from zero
B
is recharged.
FB
voltage
OUT
3869f
CC
is