LTC3855IFE#PBF Linear Technology, LTC3855IFE#PBF Datasheet - Page 12

• Manufacturer Part Number: LTC3855IFE#PBF
• Description: IC CTLR DC/DC MULTIPHASE 38SSOP
• Manufacturer: Linear Technology
• Series: PolyPhase®r
• Type: Step-Down (Buck)r

Specifications of LTC3855IFE#PBF

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 2
• Voltage - Output: 0.6 ~ 3.3 V, 0.6 ~ 12.5 V
• Current - Output: 25A
• Frequency - Switching: 250kHz ~ 770kHz
• Voltage - Input: 4.5 ~ 38 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 38-TSSOP Exposed Pad, 38-eTSSOP, 38-HTSSOP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -

operaTion
LTC3855
Main Control Loop
The LTC3855 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 LTC3855 switching regulator outputs.
Each top MOSFET driver is biased from the floating
bootstrap capacitor C
each off cycle through an external diode when the top
MOSFET turns off. If the input voltage V
a voltage close to V
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 100ns
every third cycle to allow C
recommended that a load be present or the IC operates
at low frequency during the drop-out transition to ensure
C

CMP
B
is recharged.
, resets the RS latch. The peak inductor current at
CC
CC
IN
CMP
/EXTV
. Using the EXTV
TH
. If EXTV
CC
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
FB
FB
Power
pin receives the voltage feedback signal,
CC
relative to the 0.6V reference, which in
TH
is taken above 4.7V, the 5V regulator is
OUT
voltage to increase until the average
B
, which normally recharges during
, the loop may enter dropout and
CC
pin allows the INTV
B
to recharge. However, it is
IN
decreases to
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 LTC3855 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 LTC3855 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.
Light Load Current Operation (Burst Mode Operation,
Pulse-Skipping, or Continuous Conduction)
The LTC3855 can be enabled to enter high efficiency 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
OUT
drops below its undervoltage
OUT
to “track” that of another
rises smoothly from zero
FB
voltage
OUT
3855f
CC
is