LTC3770_15 Datasheet, PDF (10/24 Pages) Linear Technology – Fast No RSENSE Step-Down Synchronous Controller with Margining, Tracking and PLL

LTC3770
OPERATION
Main Control Loop
The LTC3770 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 comparator
ICMP trips, restarting the one-shot timer and initiating the
next cycle. Inductor current is determined by sensing the
voltage between the SENSE^– (PGND on G package) and
SENSE⁺ (SW on G package) pins using a sense resistor
or the bottom MOSFET on-resistance . 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 a
reference voltage set by the V_REFIN pin. 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 (0.75V)
to initiate another cycle. Discontinuous mode operation
is disabled by comparator F when the FCB pin is brought
below 0.6V, 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. The nominal
frequency can be adjusted with an external resistor R_ON.
For applications with stringent constant frequency re-
quirements, the LTC3770 can be synchronized with an
external clock. By programming the nominal frequency
of the LTC3770 the same as the external clock frequency,
the LTC3770 behaves as a constant frequency part against
the load and supply variations.
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 after the
internal 25μs power bad mask timer expires. Furthermore,
in an overvoltage condition, M1 is turned off and M2 is
turned on immediately 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 and clamped to 0.9V. This
reduces the inductor valley current level to one tenth of its
maximum value as V_FB approaches 0V. Foldback current
limiting is disabled at start-up.
Pulling the RUN pin low forces the controller into its
shutdown state, turning off both M1 and M2. Forcing a
voltage above 1.5V will turn on the device.
INTV_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 recharged from
INTV_CC through an external Schottky diode DB when the
top MOSFET is turned off. If the input voltage is low and
INTV_CC drops below 3.2V, undervoltage lockout circuitry
prevents the power switches from turning on.
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