LTC3854 LINER [Linear Technology], LTC3854 Datasheet - Page 9

• Manufacturer Part Number: LTC3854
• Description: Small Footprint, Wide VIN Range Synchronous Step-Down DC/DC Controller
• Manufacturer: LINER [Linear Technology]

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
resets the RS latch. The peak inductor current at which
I
the ITH pin, which is the output of the error amplifier EA.
The V
compared to the internal reference voltage by the EA. When
the load current increases, it causes a slight decrease in
V
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
Power for the top and bottom MOSFET drivers and most
other internal circuitry is derived from the INTV
internal 5V low dropout linear regulator supplies INTV
power from V
The top MOSFET driver is biased from a floating bootstrap
capacitor C
an external Schottky diode when the top MOSFET turns
off. If the input voltage V
V
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
CMP
FB
OUT
relative to the 0.8V reference, which in turn causes
, the loop may enter dropout and attempt to turn on
CC
resets the RS latch is controlled by the voltage on
FB
Power
pin receives the voltage feedback signal, which is
B
, which recharges during each off cycle through
IN
.
IN
decreases to a voltage close to
B
to recharge.
CC
pin. An
CMP
CC
,
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
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
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
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
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.
OUT
rises smoothly from zero to the target
CC
regulator.
LTC3854
FB
voltage to
3854fa

OUT