LTC3609 Datasheet, PDF (10/24 Pages) Linear Technology – 32V, 6A Monolithic Synchronous Step-Down DC/DC Converter

LTC3609
OPERATION
Main Control Loop
The LTC3609 is a high efficiency monolithic synchronous,
step-down DC/DC converter utilizing a constant on-time,
current mode architecture. It operates from an input
voltage range of 4V to 32V/36V maximum and provides
a regulated output voltage at up to 6A of output current.
The internal synchronous power switch increases efficiency
and eliminates the need for an external Schottky diode. 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 SW pins using the bottom MOSFET on-resistance.
The voltage on the I_TH pin sets the comparator threshold
corresponding to inductor valley current. The error am-
plifier EA adjusts this voltage by comparing the feedback
signal V_FB from the output voltage with an internal 0.6V
reference. 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 light load, the inductor current can drop to zero and
become negative. This is detected by current reversal
comparator I_REV which then shuts off M2 (see Func-
tional Diagram), 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. Discontinu-
ous 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.
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 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 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.
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 recharged 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 sup-
ply 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.
3609f
10