LTC3720 Datasheet, PDF (9/24 Pages) Linear Technology – Single Phase VRM8.5 Current Mode Step-Down Controller

U
OPERATIO (Refer to Functional Diagram)
Main Control Loop
The LTC3720 is a 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 com-
parator I_CMP trips, restarting the one-shot timer and
initiating the next cycle. Inductor current is determined
by sensing the voltage between the SENSE^– 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
valley current. The error amplifier EA adjusts this voltage
by comparing the feedback signal V_FB from the output
voltage with an internal 0.8V 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. Discontinuous mode operation is
disabled by comparator F when the FCB pin is brought
below 0.8V, forcing continuous synchronous operation.
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_IN and V_OUT. The nominal frequency can be adjusted
with an external resistor R_ON.
LTC3720
Overvoltage and undervoltage comparators OV and UV
pull the PGOOD output low if the output feedback voltage
exits a ±7.5% 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 Q4 and Q6. This reduces the inductor valley
current level to one sixth of its maximum value as V_FB
approaches 0V.
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 switch-
ing, 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.
INTV_CC/EXTV_CC Power
Power for the top and bottom MOSFET drivers and most
of the internal controller circuitry is derived from the
INTV_CC pin. The top MOSFET driver is powered from a
floating bootstrap capacitor C_B. This capacitor is re-
charged from INTV_CC through an external Schottky diode
D_B when the top MOSFET is turned off. When the EXTV_CC
pin is grounded, an internal 5V low dropout regulator
supplies the INTV_CC power from V_IN. If EXTV_CC rises
above 4.7V, the internal regulator is turned off, and an
internal switch connects EXTV_CC to INTV_CC. This allows
a high efficiency source connected to EXTV_CC, such as an
external 5V supply or a secondary output from the
converter, to provide the INTV_CC power. Voltages up to
7V can be applied to EXTV_CC for additional gate drive. If
the input voltage is low and INTV_CC drops below 3.5V,
undervoltage lockout circuitry prevents the power
switches from turning on.
3720f
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