LTC3774
10
3774fc
For more information www.linear.com/LTC3774
OPERATION
Main Control Loop
The LTC3774 uses an LTC proprietary current sensing,
current mode step-down architecture. During normal
operation, the top MOSFET is turned on every cycle when
the oscillator sets the RS latch, and turned off when the
main current comparator, I
CMP
, resets the RS latch. The
peak inductor current at which I
CMP
resets the RS latch
is controlled by the voltage on the ITH pin, which is the
output of the error amplifier, EA. The remote sense amplifier
(diffamp) produces a signal equal to the differential voltage
sensed across the output capacitor divided down by the
feedback divider and re-references it to the local IC ground
reference. The error amplifier receives this feedback signal
and compares it to the internal 0.6V reference. When the
load current increases, it causes a slight decrease in the
V
OSNS
+
pin voltage relative to the 0.6V reference, which in
turn causes the ITH voltage to increase until the inductor’s
average current equals the new load current. After the top
MOSFET has turned off, the bottom MOSFET is turned
on until either the inductor current starts to reverse, as
indicated by the reverse current comparator, I
REV
, or the
beginning of the next cycle.
The main control loop is shut down by pulling the RUN
pin low. Releasing RUN allows an internal 1.0µA current
source to pull up the RUN pin. When the RUN pin reaches
1.22V, the main control loop is enabled and the IC is
powered up. When the RUN pin is low, all functions are
kept in a controlled state.
Sensing Signal of Very Low DCR
The LTC3774 employs a unique architecture to enhance
the signal-to-noise ratio that enables it to operate with a
small sense signal of a very low value inductor DCR, 1mΩ
or less, to improve power efficiency, and reduce jitter due
to the switching noise which could corrupt the signal. The
LTC3774 can sense a DCR value as low as 0.2mΩ with
careful PCB layout.The LTC3774 comprises two positive
sense pins, SNSD
+
and SNSA
+
, to acquire signals and
processes them internally to provide the response as with
a DCR sense signal that has a 14dB signal-to-noise ratio
improvement. In the meantime, the current limit threshold
is still a function of the inductor peak current and its DCR
value, and can be accurately set from 10mV to 30mV in
5mV steps with the ILIM pin. The filter time constant,
R1C1, of the SNSD
+
should match the L/DCR of the output
inductor, while the filter at SNSA
+
should have a bandwidth
of five times larger than SNSD
+
, R2•C2 equals R1•C1/5.
Internal Soft-Start
By default, the start-up of the output voltage is normally
controlled by an internal soft-start ramp. The internal soft-
start ramp represents a noninverting input to the error
amplifier. The V
OSNS
+
pin is regulated to the lower of the
error amplifier’s three noninverting inputs (the internal
soft-start ramp, the TK/SS pin or the internal 600mV ref
-
erence). As the ramp voltage rises from 0V to 0.6V over
approximately 600µs,
the output voltage rises smoothly
from its prebiased value to its final set value.
Certain applications can result in the start-up of the con-
verter into a non-zero load voltage, where residual charge
is stored
on the output capacitor at the onset of converter
switching. In order to prevent the output from discharging
under these conditions, the bottom MOSFET is disabled
until soft-start is greater than V
OSNS
+
.
Shutdown and Start-Up (RUN and TK/SS Pins)
The LTC3774 can be shut down using the RUN pin.
Pulling the RUN pin below 1.14V shuts down the main
control loop for the controller and most internal circuits,
including the INTV
CC
regulator. Releasing the RUN pin
allows an internal 1.0µA current to pull up the pin and
enable the controller. Alternatively, the RUN pin may be
externally pulled up or driven directly by logic. Be careful
not to exceed the absolute maximum rating of 6V on this
pin. The start-up of the controller’s output voltage, V
OUT
,
is controlled by the voltage on the TK/SS pin. When the
voltage on the TK/SS pin is less than the 0.6V internal
reference, the LTC3774 regulates the V
OSNS
+
voltage to
the TK/SS pin voltage instead of the 0.6V reference. This
allows the TK/SS pin to be used to program a soft-start
by connecting an external capacitor from the TK/SS pin
to GND. An internal 1.25µA pull-up current charges this
capacitor, creating a voltage ramp on the TK/SS pin. As
the TK/SS voltage rises linearly from 0V to 0.6V (and
beyond), the output voltage, V
OUT
, rises smoothly from
zero to its final value. Alternatively, the TK/SS pin can be
