LTC3856EFE#PBF Linear Technology, LTC3856EFE#PBF Datasheet - Page 12

• Manufacturer Part Number: LTC3856EFE#PBF
• Description: IC DCDC SWITCH
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LTC3856EFE#PBF

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Voltage - Output: 0.6 ~ 5 V
• 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
• Current - Output: -
• Power - Output: -

operaTion
LTC3856
Main Control Loop
The LTC3856 uses a constant-frequency, current mode
step-down architecture. During normal operation, each
top MOSFET is turned on each cycle when the oscillator
sets the RS latch, and turned off when the main current
comparator, I
current at which I
by the voltage on the I
error amplifier, EA. The V
output voltage feedback signal via the DIFFOUT pin (if
DIFFAMP is used) through the external resistive divider
and is compared to the internal reference voltage. When
the load current increases, it causes a slight decrease in
the V
in turn causes the I
inductor’s average current matches half of the new load
current (assuming the two current sensing resistors are
equal). In Burst Mode operation, after each 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
of the next cycle.
The main control loop is shut down by pulling the RUN pin
low. Releasing RUN allows an internal 1µA current source
to pull up the RUN pin. When the RUN pin reaches 1.22V,
the main control loop is enabled and the IC is powered
up. When the RUN pin is low, all functions are kept in a
controlled state.
INTV
Power for the top and bottom MOSFET drivers and most
other internal circuitry is derived from the INTV
When the EXTV
than 4.7V, an internal 5V linear regulator supplies INTV
power from V
regulator is turned off and an internal switch is turned on
connecting EXTV
power to be derived from a high efficiency external source
such as a switching regulator output. Each top MOSFET
driver is biased from the floating bootstrap capacitor, C

CC
FB
/EXTV
pin voltage relative to the 0.6V reference, which
CMP
CC
IN
CC
Power
. If EXTV
CC
, resets each RS latch. The peak inductor
pin is left open or tied to a voltage less
CMP
. Using the EXTV
(Refer to Functional Diagram)
TH
TH
resets the RS latch is controlled
voltage to increase until each
CC
pin, which is the output of the
FB
is taken above 4.7V, the 5V
pin receives a portion of
CC
REV
pin allows the INTV
, or the beginning
CC
pin.
CC
CC
B
,
which normally recharges during each off cycle through
an external diode when the top MOSFET turns off. If the
input voltage, V
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 plus 100ns every third cycle to allow
C
be present or the IC operates at low frequency during the
dropout transition to ensure C
Shutdown and Start-Up (RUN and TK/SS Pins)
The LTC3856 can be shut down using the RUN pin. Pulling
the RUN pin below 1.22V shuts down the main control loop
for the controller and most internal circuits, including the
INTV
1µA current to pull up the pin and enable the controller.
Alternatively, the RUN pin may be externally pulled up or
driven directly by logic. Be careful not to exceed the ab-
solute maximum rating of 6V on this pin. The start-up of
the controller’s output voltage, V
voltage on the TK/SS pin. When the voltage on the TK/SS
pin is less than the 0.6V internal reference, the LTC3856
regulates the V
of the 0.6V 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 1.25µA
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
rises smoothly from zero to its final 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 RUN pin is pulled low to
disable the controller, or when INTV
undervoltage lockout threshold of 3.2V, the TK/SS pin is
pulled low by an internal MOSFET. When in undervoltage
lockout, all phases of the controller are disabled and the
external MOSFETs are held off.
B
to recharge. However, it is recommended that a load
CC
regulator. Releasing the RUN pin allows an internal
FB
IN
voltage to the TK/SS pin voltage instead
, decreases to a voltage close to V
B
is recharged.
OUT
, is controlled by the
CC
drops below its
OUT
OUT
OUT
3856f
,
,