LTC3714 Linear, LTC3714 Datasheet - Page 10

• Manufacturer Part Number: LTC3714
• Description: Wide Operating Range / Step-Down Controller with Internal Op Amp
• Manufacturer: Linear

OPERATIO
LTC3714
Main Control Loop
The LTC3714 is a constant on-time, current mode control-
ler for DC/DC step-down converters. In normal operation,
the top MOSFET is turned on for a fixed interval deter-
mined by a one-shot timer OST. When the top MOSFET is
turned off, the bottom MOSFET is turned on until the
current comparator I
timer and initiating the next cycle. Inductor current is
determined by sensing the voltage between the PGND and
SENSE pins using either the bottom MOSFET on-resis-
tance or a separate sense resistor. The voltage on the I
pin sets the comparator threshold corresponding to in-
ductor valley current. The error amplifier EA adjusts this
voltage by comparing the feedback signal V
output voltage with an internal 0.6V reference. The feed-
back voltage is derived from the output voltage by a
resistive divider DAC that is set by the VID code pins VID0-
VID4. If the load current increases, it causes a drop in the
feedback voltage relative to the reference. The I
then rises until the average inductor current again matches
the load current.
At low load currents, the inductor current can drop to zero
and become negative. This is detected by current reversal
comparator I
discontinuous operation. Both switches will remain off
with the output capacitor supplying the load current until
the I
initiate another cycle. Continuous synchronous operation
can be forced in the LTC3714 by bringing the FCB pin
below 0.6V. The benefit of forced continuous operation is
lower output voltage ripple, faster transient response to
current load steps and a much quieter frequency spectrum
so that it won’t interfere with any neighboring noise
sensitive components.
10
TH
voltage rises above the zero current level (0.8V) to
REV
U
which then shuts off M2, resulting in
CMP
trips, restarting the one-shot
FB
TH
from the
voltage
TH
The operating frequency is determined implicitly by the
top MOSFET on-time and the duty cycle required to
maintain regulation. The one-shot timer generates an on-
time that is proportional to the ideal duty cycle, thus
holding frequency approximately constant with changes
in V
with an external resistor R
Overvoltage and undervoltage comparators OV and UV
pull the PGOOD output low if the output feedback voltage
exits a 10% window around the regulation point. Further-
more, in an overvoltage condition, M1 is turned off and M2
is turned on and held on until the overvoltage condition
clears.
Foldback current limiting is provided if the output is
shorted to ground. As V
threshold voltage I
level set by Q2 and Q6. This reduces the inductor valley
current level to one sixth of its maximum value as V
approaches ground.
Pulling the RUN/SS pin low forces the controller into its
shutdown state, turning off both M1 and M2. Releasing
the pin allows an internal 1.2 A current source to charge
up an external soft-start capacitor C
reaches 1.5V, the controller turns on and begins switch-
ing, but with the I
0.6V below the RUN/SS voltage. As C
charge, the soft-start current limit is removed.
IN
and V
OUT
. The nominal frequency can be adjusted
THB
TH
voltage clamped at approximately
is pulled down by clamp Q3 to a 1V
FB
ON
drops, the buffered current
.
SS
. When this voltage
SS
continues to
3714f
FB