ltc3711 Linear Technology Corporation, ltc3711 Datasheet - Page 9

• Manufacturer Part Number: ltc3711
• Description: 5-bit Adjustable, Wide Operating Range, No Rsense? Step-down Controller
• Manufacturer: Linear Technology Corporation

OPERATIO
Main Control Loop
The LTC3711 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
initiating the next cycle. Inductor current is determined
by sensing the voltage between the PGND and SENSE
pins using either the bottom MOSFET on-resistance or an
optional sense resistor. The voltage on the I
comparator threshold corresponding to inductor valley
current. The error amplifier EA adjusts this voltage by
comparing the feedback signal V
voltage with an internal 0.8V reference. The feedback
voltage is derived from the output voltage by a resistive
divider DAC that is set by the VID code pins VID0-VID4.
If the load current increases, it causes a drop in the
feedback voltage relative to the reference. The I
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
discontinuous operation. Both switches will remain off
with the output capacitor supplying the load current until
the I
initiate another cycle. Discontinuous mode operation is
disabled by comparator F when the FCB pin is brought
below 0.8V, 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
with an external resistor R
IN
TH
and V
voltage rises above the zero current level (0.8V) to
CMP
OUT
REV
trips, restarting the one-shot timer and
. The nominal frequency can be adjusted
U
which then shuts off M2, resulting in
ON
.
FB
from the output
TH
pin sets the
TH
voltage
+
Overvoltage and undervoltage comparators OV and UV
pull the PGOOD output low if the output feedback voltage
exits a
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
level set by Q4 and Q6. This reduces the inductor valley
current level to one sixth of its maximum value as V
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 1.2 A current source to charge
up an external soft-start capacitor C
reaches 1.5V, the controller turns on and begins switch-
ing, but with the I
0.6V below the RUN/SS voltage. As C
charge, 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
floating bootstrap capacitor C
charged from INTV
D
pin is grounded, an internal 5V low dropout regulator
supplies the INTV
above 4.7V, the internal regulator is turned off, and an
internal switch connects EXTV
a high efficiency source connected to EXTV
external 5V supply or a secondary output from the
converter, to provide the INTV
7V can be applied to EXTV
the input voltage is low and INTV
undervoltage lockout circuitry prevents the power
switches from turning on.
B
when the top MOSFET is turned off. When the EXTV
CC
CC
/EXTV
pin. The top MOSFET driver is powered from a
7.5% window around the regulation point.
CC
Power
THB
TH
CC
CC
voltage clamped at approximately
through an external Schottky diode
power from V
is pulled down by clamp Q3 to a 1V
FB
CC
drops, the buffered current
for additional gate drive. If
CC
CC
B
. This capacitor is re-
to INTV
power. Voltages up to
CC
SS
IN
. When this voltage
drops below 3.5V,
. If EXTV
SS
LTC3711
CC
CC
continues to
. This allows
, such as an
CC
rises
9
3711f
CC
FB