ltc3853euj Linear Technology Corporation, ltc3853euj Datasheet - Page 11

• Manufacturer Part Number: ltc3853euj
• Description: Triple Output, Multiphase Synchronous Step-down Controller
• Manufacturer: Linear Technology Corporation

OPERATION
Main Control Loop
The LTC3853 is a constant-frequency, current mode
step-down controller with three channels operating 120
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 of phase 3
and most other internal circuitry is derived from the
INTV
and phase 2. This pin must be externally tied to INTV
If EXTV
off and an internal switch is turned on connecting EXTV
Using the EXTV
from a high effi ciency external source such as one of the
LTC3853 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
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 every fi fth cycle to allow
C
always a load be present during the drop-out transition
to ensure C
B
to recharge. However, it is recommended that there is
CC
CC
/EXTV
CC
pin. DRV
OUT
is taken above 4.7V, the 5V regulator is turned
B
, the loop may enter dropout and attempt to
CMP
CC
is recharged.
CC
/DRV
CC12
B
, resets the RS latch. The peak inductor
pin allows the INTV
CMP
, which normally recharges during each
TH
TH
FB
FB
CC12
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
provides driver power for phase 1
pin receives the voltage feedback
Power
IN
, decreases to a voltage
CC
power to be derived
REV
, or the
CC
CC
.
.
Shutdown and Start-Up (RUN1, RUN2, RUN3 and
TK/SS1, TK/SS2, TK/SS3 Pins)
The three channels of the LTC3853 can be independently
shut down using the RUN1, RUN2 and RUN3 pins. Pulling
any of these pins below 1.2V shuts down the main control
loop for that controller. Pulling all pins low disables all
three controllers and most internal circuits, including the
INTV
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, TK/SS2 and
TK/SS3 pins. When the voltage on the TK/SS pin is less
than the 0.8V internal reference, the LTC3853 regulates
the V
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
rises smoothly from zero to its fi nal value. Alternatively
the TK/SS pin can be used to cause the start-up of V
to “track” that of another 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
below its undervoltage lockout threshold of 3.35V, the
TK/SS pin is pulled low by an internal MOSFET. When in
undervoltage lockout, all controllers are disabled and the
external MOSFETs are held off.
Light Load Current Operation (Burst Mode Operation,
Pulse Skipping or Continuous Conduction)
The LTC3853 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
CC
FB
regulator. Releasing any RUN pin allows an internal
voltage to the TK/SS pin voltage instead of the
LTC3853
CC
11
OUT
drops
OUT
OUT
3853f
is