Datasheet

9
LTC3834-1
38341f
(Refer to Functional Diagram)
OPERATIO
U
Main Control Loop
The LTC3834-1 uses a constant-frequency, current mode
step-down architecture. During normal operation, each
external top MOSFET is turned on when the clock sets the
RS latch, and is turned off when the main current com-
parator, ICMP, resets the RS latch. The peak inductor
current at which ICMP trips and resets the latch is con-
trolled by the voltage on the I
TH
pin, which is the output of
the error amplifier EA. The error amplifier compares the
output voltage feedback signal at the V
FB
pin, (which is
generated with an external resistor divider connected
across the output voltage, V
OUT
, to ground) to the internal
0.800V reference voltage. When the load current increases,
it causes a slight decrease in V
FB
relative to the reference,
which cause the EA to increase the I
TH
voltage until the
average inductor current matches the new load current.
After the top MOSFET is turned off each cycle, the bottom
MOSFET is turned on until either the inductor current
starts to reverse, as indicated by the current comparator
IR, or the beginning of the next clock 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 5.25V low dropout linear regulator supplies INTV
CC
power from V
IN
.
The top MOSFET driver is biased from the floating boot-
strap capacitor C
B
, which normally recharges during each
off cycle through an external 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 detec-
tor detects this and forces the top MOSFET off for about
one twelfth of the clock period every tenth cycle to allow C
B
to recharge.
Shutdown and Start-Up (RUN and TRACK/SS Pins)
The LTC3834-1 can be shut down using the RUN pin.
Pulling this pin below 0.7V shuts down the main control
loop for the controller. A low disables the controller and
most internal circuits, including the INTV
CC
regulator,
at which time the LTC3834-1 draws only 4μA of quies-
cent current.
Releasing the RUN pin allows an internal 0.5μA 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 7V on this pin.
The start-up of the output voltage V
OUT
is controlled by the
voltage on the TRACK/SS pin. When the voltage on the
TRACK/SS pin is less than the 0.8V internal reference, the
LTC3834-1 regulates the V
FB
voltage to the TRACK/SS pin
voltage instead of the 0.8V reference. This allows the
TRACK/SS pin to be used to program a soft-start by
connecting an external capacitor from the TRACK/SS pin
to SGND. An internal 1μA pull-up current charges this
capacitor creating a voltage ramp on the TRACK/SS pin. As
the TRACK/SS voltage rises linearly from 0V to 0.8V (and
beyond), the output voltage V
OUT
rises smoothly from
zero to its final value.
Alternatively the TRACK/SS pin can be used to cause the
start-up of V
OUT
to “track” that of another supply. Typi-
cally, this requires connecting to the TRACK/SS pin an
external resistor divider from the other supply to ground
(see Applications Information section).
When the RUN pin is pulled low to disable the LTC3834-
1, or when V
IN
drops below its undervoltage lockout
threshold of 3.5V, the TRACK/SS pin is pulled low by an
internal MOSFET. When in undervoltage lockout, the
controller is disabled and the external MOSFETs are
held off.