8
LTC1775
Main Control Loop
The LTC1775 is a constant frequency, current mode
controller for DC/DC step-down converters. In normal
operation, the top MOSFET is turned on when the RS latch
is set by the on-chip oscillator and is turned off when the
current comparator I
1
resets the latch. While the top
MOSFET is turned off, the bottom MOSFET is turned on
until either the inductor current reverses, as determined
by the current reversal comparator I
2
, or the next cycle
begins. Inductor current is measured by sensing the V
DS
potential across the conducting MOSFET. The output of
the appropriate sense amplifier (TA or BA) is selected by
the switch logic and applied to the current comparator.
The voltage on the I
TH
pin sets the comparator threshold
corresponding to peak inductor current. The error ampli-
fier EA adjusts this voltage by comparing the feedback
signal V
FB
from the output voltage with the internal 1.19V
reference. The V
PROG
pin selects whether the feedback
voltage is taken directly from the V
OSENSE
pin or is derived
from an on-chip resistive divider. When 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.
The internal oscillator can be synchronized to an external
clock applied to the SYNC pin and can lock to a frequency
between 100% and 150% of its nominal 150kHz rate.
When the SYNC pin is left open, it is pulled low internally
and the oscillator runs at its normal rate. If this pin is taken
above 1.2V, the oscillator will run at its maximum 225kHz
rate.
Pulling the RUN/SS pin low forces the controller into its
shutdown state and turns off both MOSFETs. Releasing
the RUN/SS pin allows an internal 3µA current source to
charge up an external soft start capacitor C
SS
. When this
voltage reaches 1.4V, the controller begins switching, but
with the I
TH
voltage clamped at approximately 0.8V. As
C
SS
continues to charge, the clamp is raised until full range
operation is restored.
The top MOSFET driver is powered from a floating boot-
strap capacitor C
B
. This capacitor is normally recharged
from INTV
CC
through a diode D
B
when the top MOSFET is
turned off. As V
IN
decreases towards V
OUT
, the converter
will attempt to turn on the top MOSFET continuously
(‘’dropout’’). A dropout counter detects this condition and
forces the top MOSFET to turn off for about 500ns every
tenth cycle to recharge the bootstrap capacitor.
An overvoltage comparator OV guards against transient
overshoots and other conditions that may overvoltage the
output. In this case, the top MOSFET is turned off and the
bottom MOSFET is turned on until the overvoltage condi-
tion is cleared.
Foldback current limiting for an output shorted to ground
is provided by a transconductance amplifer CL. As V
FB
drops below 0.6V, the buffered I
TH
input to the current
comparator is gradually pulled down to a 0.95V clamp.
This reduces peak inductor current to about one fifth of its
maximum value.
Low Current Operation
The LTC1775 is capable of Burst Mode operation at low
load currents. If the error amplifier drives the I
TH
voltage
below 0.95V, the buffered I
TH
input to the current com-
parator will remain clamped at 0.95V. The inductor current
peak is then held at approximately 60mV/R
DS(ON)(TOP)
. If
I
TH
then drops below 0.5V, the Burst Mode comparator B
will turn off both MOSFETs. The load current will be
supplied solely by the output capacitor until I
TH
rises
above the 50mV hysteresis of the comparator and switch-
ing is resumed. Burst Mode operation is disabled by
comparator F when the FCB pin is brought below 1.19V.
This forces continuous operation and can assist second-
ary winding regulation.
INTV
CC
/EXTV
CC
Power
Power for the top and bottom MOSFET drivers and most
of the internal circuitry of the LTC1775 is derived from the
INTV
CC
pin. When the EXTV
CC
pin is left open, an internal
5.2V low dropout regulator supplies the INTV
CC
power
from V
IN
. If EXTV
CC
is raised 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,
such as the primary or a secondary output of the converter
itself, to provide the INTV
CC
power.
OPERATIO
U
