ltc3850gn-2 Linear Technology Corporation, ltc3850gn-2 Datasheet - Page 10

• Manufacturer Part Number: ltc3850gn-2
• Description: Dual, 2-phase Synchronous Step-down Switching Controller
• Manufacturer: Linear Technology Corporation

OPERATION
LTC3850-2
Main Control Loop
The LTC3850-2 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
current at which I
the voltage on the I
amplifi er EA. The V
signal, which is compared to the internal reference voltage
by the EA. When the load current increases, it causes a
slight decrease in V
in 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 effi ciency external source such
as one of the LTC3850-2 switching regulator outputs.
Each top MOSFET driver is biased from the fl oating boot-
strap capacitor C
off cycle through an external diode when the top MOSFET
turns off. If the input voltage V
close to V
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 third cycle to
allow C
load be present during the drop-out transition to ensure
C
10
B
is recharged.
CC
CC
IN
/EXTV
B
. Using the EXTV
. If EXTV
CC
to recharge. However, it is recommended that a
OUT
pin is left open or tied to a voltage less than
CMP
CC
, the loop may enter dropout and attempt
Power
CC
B
, resets the RS latch. The peak inductor
CMP
, which normally recharges during each
TH
TH
FB
is taken above 4.7V, the 5V regulator is
FB
pin, which is the output of each error
voltage to increase until the average
relative to the 0.8V reference, which
resets the RS latch is controlled by
pin receives the voltage feedback
CC
pin allows the INTV
IN
decreases to a voltage
CC
REV
pin. When
CC
CC
, or the
power
power
Shutdown and Start-Up (RUN1, RUN2 and TK/SS1,
TK/SS2 Pins)
The two channels of the LTC3850-2 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
0.5μA current to pull up the pin and enable that control-
ler. 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 0.8V
internal reference, the LTC3850-2 regulates the V
to the TK/SS pin voltage instead of the 0.8V 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 1.3μA 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 0.8V (and beyond),
the output voltage V
value. Alternatively the TK/SS pin can be used to cause the
start-up of V
this requires connecting to the TK/SS pin an external resistor
divider from the other supply to ground (see the Applica-
tions Information section). When the corresponding RUN
pin is pulled low to disable a controller, or when INTV
drops below its undervoltage lockout threshold of 3V, 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.
Light Load Current Operation (Burst Mode Operation,
Pulse-Skipping, or Continuous Conduction)
The LTC3850-2 can be enabled to enter high effi ciency Burst
Mode operation, constant-frequency pulse-skipping mode,
or forced continuous conduction mode. To select forced
continuous operation, tie the MODE/PLLIN pin to a DC
voltage below 0.8V (e.g., SGND). To select pulse-skipping
OUT
to “track” that of another supply. Typically,
OUT
rises smoothly from zero to its fi nal
FB
voltage
OUT
38502f
CC
CC
is