ltc3608ewkg Linear Technology Corporation, ltc3608ewkg Datasheet - Page 10

• Manufacturer Part Number: ltc3608ewkg
• Description: 18v, 8a Monolithic Synchronous Step-down Dc/dc Converter
• Manufacturer: Linear Technology Corporation

OPERATION
LTC3608
Main Control Loop
The LTC3608 is a high effi ciency monolithic synchronous,
step-down DC/DC converter utilizing a constant on-time,
current mode architecture. It operates from an input volt-
age range of 4V to 18V (20V maximum) and provides a
regulated output voltage at up to 8A of output current. The
internal synchronous power switch increases effi ciency
and eliminates the need for an external Schottky diode. In
normal operation, the top MOSFET is turned on for a fi xed
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
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
corresponding to inductor valley current. The error am-
plifi er EA adjusts this voltage by comparing the feedback
signal V
reference. If the load current increases, it causes a drop
in the feedback voltage relative to the reference. The I
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
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 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
frequency can be adjusted with an external resistor R
10
FB
from the output voltage with an internal 0.6V
REV
which then shuts off M2 (see Func-
TH
pin sets the comparator threshold
TH
voltage rises above the zero
CMP
trips, restarting the
IN
. The nominal
ON
TH
.
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
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
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 fl oating
bootstrap capacitor C
INTV
the top MOSFET is turned off. When the EXTV
grounded, an internal 5V low dropout regulator supplies
the INTV
the internal regulator is turned off, and an internal switch
connects EXTV
source connected to EXTV
ply or a secondary output from the converter, to provide
the INTV
EXTV
low and INTV
circuitry prevents the power switches from turning on.
FB
approaches 0V.
CC
CC
CC
/EXTV
through an external Schottky diode D
for additional gate drive. If the input voltage is
CC
CC
power from V
power. Voltages up to 7V can be applied to
CC
CC
TH
CC
Power
drops below 3.5V, undervoltage lockout
voltage clamped at approximately 0.6V
to INTV
THB
B
. This capacitor is recharged from
is pulled down by clamp Q3 to
CC
FB
IN
CC
. If EXTV
. This allows a high effi ciency
drops, the buffered current
, such as an external 5V sup-
SS
SS
continues to charge,
CC
. When this voltage
rises above 4.7V,
CC
B
pin is
when
3608fa
CC