LTC3613 Datasheet, PDF (11/36 Pages) Linear Technology – 24V, 15A Monolithic Step Down Regulator

LTC3613
OPERATION (Refer to Functional Diagram)
and turns the bottom MOSFET off immediately and the
top MOSFET on. Again in order to avoid shoot-through
current there is a small dead time delay before the top
MOSFET turns on.
The voltage on the ITH pin sets the I_CMP valley threshold
point. The error amplifier, EA, adjusts this ITH voltage
by comparing the differential feedback signal, V_OSNS+ −
V_OSNS–, to a 0.6V internal reference voltage. Consequently,
the LTC3613 regulates the output voltage by forcing the
differential feedback voltage to be equal to the 0.6V internal
reference. The difference amplifier, DA, converts the dif-
ferential feedback signal to a single-ended input for the
EA. If the load current increases, it causes a drop in the
differential feedback voltage relative to the reference. The
EA forces ITH voltage to rise until the average inductor
current again matches the load current.
Differential Output Sensing
The output voltage is resistively divided externally to create
a feedback voltage for the controller. The internal difference
amplifier, DA, senses this feedback voltage along with the
output’s remote ground reference to create a differential
feedback voltage. This scheme overcomes any ground
offsets between local ground and remote output ground,
resulting in a more accurate output voltage. The LTC3613
allows for remote output ground deviations as much as
±500mV with respect to local ground.
INTV_CC/EXTV_CC Power
Power for the top and bottom MOSFET drivers and most
other internal circuitry is derived from the INTV_CC pin. Power
on the INTV_CC pin is derived in two ways: if the EXTV_CC
pin is below 4.6V, then an internal 5.3V low dropout linear
regulator, LDO, supplies INTV_CC power from PV_IN; if the
EXTV_CC pin is tied to an external source larger than 4.6V,
then the LDO is shut down and an internal switch shorts
the EXTV_CC pin to the INTV_CC pin, thereby powering the
INTV_CC pin with the external source and helping to increase
overall efficiency and decrease internal self heating through
power dissipated in the LDO. This external power source
could be the output of the step-down switching regulator
itself if the output is programmed to higher than 4.6V.
The top MOSFET driver is biased from the floating boot-
strap capacitor, C_B, which normally recharges during
each off cycle through an external Schottky diode when
the top MOSFET turns off. If the V_IN voltage is low and
INTV_CC drops below 3.65V, undervoltage lockout circuitry
disables the external MOSFET driver and prevents the
power switches from turning on.
Shutdown and Start-Up
The LTC3613 can be shut down using the RUN pin. Pull-
ing this pin below 1.2V prevents switching, and less than
0.75V disables most of the internal bias circuitry, including
the INTV_CC regulator. When RUN is less than 0.75V, the
shutdown I_Q is about 15μA. Pulling the RUN pin between
0.75V and 1.2V enables the controller into a standby mode
where all internal circuitry is powered-up except for the
MOSFET driver. The standby I_Q is about 2mA. Releasing
the RUN pin from ground allows an internal 1.3μA current
to pull the pin above 1.2V and fully enable the controller
including the MOSFET driver. 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. When pulled up by a resistor to an external
voltage, the RUN pin will sink about 35μA of current before
reaching 6V. If the external voltage is above 6V (e.g., V_IN),
select a large enough resistor value so that the voltage on
RUN will not exceed 6V.
The start-up of the output voltage, V_OUT, is controlled by
the voltage on the TRACK/SS pin. When the voltage on
the TRACK/SS pin is less than the 0.6V internal reference,
the LTC3613 regulates the differential feedback voltage to
the TRACK/SS voltage instead of the 0.6V reference. This
allows the TRACK/SS pin to be used for programming a
ramp-up time for V_OUT by connecting an external capacitor
from the TRACK/SS pin to SGND. An internal 1μA pull-up
current charges this capacitor, creating a voltage ramp on
the TRACK/SS pin. As the TRACK/SS voltage rises from
0V to 0.6V (and beyond), the LTC3613 forces the output
voltage, V_OUT, to ramp up smoothly to its final value.
Alternatively, the TRACK/SS pin can be used to track the
start-up of V_OUT to another external supply as in a master
slave configuration. Typically, this requires connecting a
resistor divider from the master supply to the TRACK/SS
pin (see Soft-Start and Tracking).
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