LTC3854_15 Datasheet, PDF (9/28 Pages) Linear Technology – Small Footprint, Wide VIN Range Synchronous Step-Down DC/DC Controller

LTC3854
OPERATION
Main Control Loop
The LTC3854 is a constant-frequency, peak current mode
step-down controller. During normal operation, the top
MOSFET is turned on when the clock sets the RS latch,
and is turned off when the main current comparator, I_CMP,
resets the RS latch. The peak inductor current at which
I_CMP resets the RS latch is controlled by the voltage on
the ITH pin, which is the output of the error amplifier EA.
The V_FB pin receives the voltage feedback signal, which is
compared to the internal reference voltage by the EA. When
the load current increases, it causes a slight decrease in
V_FB relative to the 0.8V reference, which in turn causes
the ITH voltage to increase until the average inductor cur-
rent matches the new load current. After the top MOSFET
has turned off, the bottom MOSFET is turned on until the
beginning of the next cycle.
INTV_CC Power
Power for the top and bottom MOSFET drivers and most
other internal circuitry is derived from the INTV_CC pin. An
internal 5V low dropout linear regulator supplies INTV_CC
power from V_IN.
The top MOSFET driver is biased from a floating bootstrap
capacitor C_B , which recharges during each off cycle through
an external Schottky diode when the top MOSFET turns
off. If the input voltage V_IN decreases to a voltage close to
V_OUT, the loop may enter dropout and attempt to turn on
the top MOSFET continuously. The dropout detector then
forces the top MOSFET off for 1/10 of the clock period
every fourth cycle to allow C_B to recharge.
Shutdown and Start-Up (RUN/SS)
The LTC3854 is shut down using the RUN/SS pin. Pulling
this pin below 1.2V disables the controller and most of the
internal circuitry, including the INTV_CC regulator.
However, for RUN/SS>0.8V the internal bandgap is
functional and the input current will be greater than the
minimum shutdown current. To keep the part in a true
shutdown mode the RUN/SS pin should be held below 0.4V.
Releasing RUN/SS pin allows an internal 1.25µA current to
pull up the pin and enable the controller. Alternatively, the
RUN/SS 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 the controller’s output voltage V_OUT
is governed by the voltage on the RUN/SS pin until
RUN/SS > 2V.
When the voltage on the RUN/SS pin is greater than 1.2V
and less than 2V the LTC3854 regulates the V_FB voltage to
1.2V below the RUN/SS pin voltage. The RUN/SS pin pro-
grams the soft-start period through an external capacitor
from the RUN/SS pin to GND. An internal 1.25µA pull-up
current charges this capacitor creating a voltage ramp on
the RUN/SS pin. As the RUN/SS voltage rises linearly from
1.2V to 2V, V_OUT rises smoothly from zero to the target
output voltage. When the LTC3854 is in undervoltage
lockout the external MOSFETs are held off.
Frequency of Operation
The LTC3854 operates at a fixed frequency of 400kHz.
Output Overvoltage Protection
An overvoltage comparator, OV, guards against transient
overshoots (>10%) as well as other more serious condi-
tions that may overvoltage the output. In such cases, the
top MOSFET is turned off and the bottom MOSFET is turned
on until the overvoltage condition is cleared.
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