LTC3611EWP LINER [Linear Technology], LTC3611EWP Datasheet - Page 10

• Manufacturer Part Number: LTC3611EWP
• Description: 10A, 32V Monolithic Synchronous Step-Down DC/DC Converter
• Manufacturer: LINER [Linear Technology]

OPERATION
LTC3611
Main Control Loop
The LTC3611 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 4.5V to 32V and provides a regulated output voltage
at up to 10A 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
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
to inductor valley current. The error amplifier EA adjusts
this voltage by comparing the feedback signal V
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
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
tional Diagram), resulting in discontinuous operation. Both
switches will remain off with the output capacitor supplying
the load current until the I
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 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
The nominal frequency can be adjusted with an external
resistor R
10
TH
pin sets the comparator threshold corresponding
ON
.
CMP
REV
trips, restarting the one-shot timer and
which then shuts off M2 (see Func-
TH
voltage rises above the zero
TH
voltage then
FB
from
IN
.
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
threshold voltage I
a 1V level set by Q4 and Q6. This reduces the inductor
valley current level to one sixth of its maximum value as
V
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
reaches 1.5V, the controller turns on and begins switching,
but with the I
below the RUN/SS voltage. As C
the soft-start current limit is removed.
INTV
Power for the top and bottom MOSFET drivers and most of
the internal controller circuitry is derived from the INTV
pin. The top MOSFET driver is powered from a floating
bootstrap capacitor C
INTV
MOSFET is turned off. When the EXTV
an internal 5V low dropout regulator supplies the INTV
power from V
regulator is turned off, and an internal switch connects
EXTV
connected to EXTV
a secondary output from the converter, to provide the
INTV
for additional gate drive. If the input voltage is low and
INTV
prevents the power switches from turning on.
Pulling the RUN/SS pin low forces the controller into its
FB
approaches 0V.
CC
CC
CC
CC
CC
/EXTV
through an external Schottky diode D
power. Voltages up to 7V can be applied to EXTV
drops below 3.5V, undervoltage lockout circuitry
to INTV
CC
IN
TH
. If EXTV
Power
CC
voltage clamped at approximately 0.6V
. This allows a high efficiency source
THB
CC
B
, such as an external 5V supply or
. This capacitor is recharged from
is pulled down by clamp Q3 to
CC
FB
rises above 4.7V, the internal
drops, the buffered current
SS
SS
continues to charge,
. When this voltage
CC
pin is grounded,
B
when the top
3611fb
CC
CC
CC