9
LTC1735-1
OPERATIO
U
(Refer to Functional Diagram)
The main control loop is shut down by pulling Pin 2 (RUN/
SS) low. Releasing RUN/SS allows an internal 1.2µA
current source to charge soft-start capacitor C
SS
. When
C
SS
reaches 1.5V, the main control loop is enabled with the
I
TH
voltage clamped at approximately 30% of its maximum
value. As C
SS
continues to charge, I
TH
is gradually re-
leased allowing normal operation to resume. If V
OUT
has
not reached 70% of its final value when C
SS
has charged
to 4.1V, latchoff can be invoked as described in the
Applications Information section.
The internal oscillator can be synchronized to an external
clock applied though a series resistor to the PGOOD pin
and can lock to a frequency between 90% and 130% of its
nominal rate set by capacitor C
OSC
.
An overvoltage comparator OV guards against transient
overshoots (>7.5%) as well as other more serious
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 condition is cleared.
Foldback current limiting for an output shorted to ground
is provided by amplifier A. As V
OSENSE
drops below 0.6V,
the buffered I
TH
input to the current comparator is gradually
pulled down to a 0.86V clamp. This reduces peak inductor
current to about 1/4 of its maximum value.
Low Current Operation
The LTC1735-1 has three low current modes controlled
by the PGOOD pin. Burst Mode operation is selected when
the PGOOD pin is above 0.8V (typically tied through a
resistor to INTV
CC
). During Burst Mode operation, if the
error amplifier drives the I
TH
voltage below 0.86V, the
buffered I
TH
input to the current comparator will be
clamped at 0.86V. The inductor current peak is then held
at approximately 20mV/R
SENSE
(about 1/4 of maximum
output current). If I
TH
then drops below 0.5V, the Burst
Mode comparator B will turn off both MOSFETs to maxi-
mize efficiency. The load current will be supplied solely by
the output capacitor until I
TH
rises above the 60mV
hysteresis of the comparator and switching is resumed.
Burst Mode operation is disabled by comparator F when
the PGOOD pin is brought below 0.8V. This forces
continuous operation and assists in controlling voltage
regulation. If the output voltage is not within 7.5% of its
nominal value the PGOOD open-drain output will be
pulled low and Burst Mode operation will be disabled.
Foldback Current, Short-Circuit Detection
and Short-Circuit Latchoff
The RUN/SS capacitor, C
SS
, is used initially to limit the
inrush current of the switching regulator. After the con-
troller has been started and been given adequate time to
charge up the output capacitors and provide full load
current, C
SS
is used as a short-circuit time-out circuit. If
the output voltage falls to less than 70% of its nominal
output voltage, C
SS
begins discharging on the assumption
that the output is in an overcurrent and/or short-circuit
condition. If the condition lasts for a long enough period
as determined by the size of the C
SS
, the controller will be
shut down until the RUN/SS pin voltage is recycled. This
built-in latchoff can be overridden by providing a current
>5µA at a compliance of 5V to the RUN/SS pin. This
current shortens the soft-start period but also prevents net
discharge of C
SS
during an overcurrent and/or short-
circuit condition. Foldback current limiting is activated
when the output voltage falls below 70% of its nominal
level whether or not the short-circuit latchoff circuit is
enabled.
INTV
CC
/EXTV
CC
Power
Power for the top and bottom MOSFET drivers and most
of the internal circuitry of the LTC1735-1 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. Voltages up
to 7V can be applied to EXTV
CC
for additional gate drive
capability.
To provide clean start-up and to protect the MOSFETs,
undervoltage lockout is used to keep both MOSFETs off
until the input voltage is above 3.5V.
