LTC3714_15 Datasheet, PDF (10/28 Pages) Linear Technology – Intel Compatible, Wide Operating Range, Step-Down Controller with Internal Op Amp

LTC3714
Operation
Main Control Loop
The LTC3714 is a constant on-time, current mode controller
for DC/DC step-down converters. In normal operation, the
top MOSFET is turned on for a fixed interval determined
by a one-shot timer OST. When the top MOSFET is turned
off, the bottom MOSFET is turned on until the current
comparator I_CMP trips, restarting the one-shot 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-resistance or a
separate sense resistor. The voltage on the I_TH pin sets
the comparator threshold corresponding to inductor val-
ley current. The error amplifier EA adjusts this voltage by
comparing the feedback signal V_FB from the output voltage
with an internal 0.6V reference. The feedback 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_TH voltage 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_REV which then shuts off M2, resulting in
discontinuous operation. Both switches will remain off
with the output capacitor supplying the load current until
the I_TH voltage rises above the zero current level (0.8V)
to initiate another cycle. Continuous synchronous opera-
tion 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 spec-
trum so that it won’t interfere with any neighboring noise
sensitive components.
The operating frequency is determined implicitly by the
top MOSFET on-time and the duty cycle required to main-
tain 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_IN
and V_OUT. The nominal frequency can be adjusted with
an external resistor R_ON.
Overvoltage and undervoltage comparators OV and UV
pull the PGOOD output low if the output feedback volt-
age exits a ±10% window around the regulation point.
Furthermore, 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_FB drops, the buffered current
threshold voltage I_THB is pulled down by clamp Q3 to
a 1V level set by Q2 and Q6. This reduces the inductor
valley current level to one sixth of its maximum value as
V_FB 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 1.2µA current source to charge
up an external soft-start capacitor C_SS. When this voltage
reaches 1.5V, the controller turns on and begins switching,
but with the I_TH voltage clamped at approximately 0.6V
below the RUN/SS voltage. As C_SS continues to charge,
the soft-start current limit is removed.
3714f
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