LTC3770 Datasheet, PDF (10/24 Pages) Linear Technology – Synchronous Controller with Margining, Tracking and PLL

LTC3770
U
OPERATIO
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 com-
parator I_CMP trips, restarting the one-shot timer and initi-
ating 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 ampli-
fier 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 re-
charged from INTV_CC through an external Schottky diode
D_B 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.
3770f
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